From cb002400b9edb3f04032de389923eab6ef176a1a Mon Sep 17 00:00:00 2001
From: James Graham © Copyright 2004-2011 Apple Computer, Inc.,
- Mozilla Foundation, and Opera Software ASA. You are granted a license to use, reproduce
- and create derivative works of this document. Parts of this specification are © Copyright 2004-2014 Apple Inc., Mozilla Foundation, and Opera Software ASA. You are granted a license to use, reproduce and create derivative works of this document. Support: canvasChrome for Android 59+Chrome 4+UC Browser for Android 11.4+iOS Safari 3.2+Firefox 3.6+IE 9+Samsung Internet 4+Opera Mini (limited) all+Safari 4+Android Browser 3+Opera 9+Edge 12+ Source: caniuse.com The Authors should not use the When authors use the In interactive visual media, if scripting is enabled for the
- The In non-interactive, static, visual media, if the
- Authors should not use the In non-visual media, and in visual media if scripting is disabled for the
- When authors use the When a The In interactive visual media, if scripting is enabled for
+ the The intrinsic dimensions of the In non-interactive, static, visual media, if the In non-visual media, and in visual media if scripting is
+ disabled for the The size of the coordinate space does not necessarily represent
- the size of the actual bitmap that the user agent will use
- internally or during rendering. On high-definition displays, for
- instance, the user agent may internally use a bitmap with two device
- pixels per unit in the coordinate space, so that the rendering
- remains at high quality throughout. When a When the An element whose nearest When the canvas is initialized, its bitmap must be cleared to
- transparent black. The The The intrinsic dimensions of the The user agent must use a square pixel density consisting of one pixel of image data per
+ coordinate space unit for the bitmaps of a A The bitmaps of A When its canvas context mode is none, a When its canvas context mode is placeholder, a When a Whenever the Action Follow the steps to set bitmap
+ dimensions to the numeric values of the Follow the behavior defined in the WebGL specification. [WEBGL] If the context's bitmap
+ mode is set to blank,
+ run the steps to set an Do nothing. Do nothing. The Returns an object that exposes an API for drawing on the canvas. The first argument
+ specifies the desired API, either " This specification defines the " Returns null if the given context ID is not supported, or if the canvas has already been
+ initialized with another context type (e.g. trying to get a " The * For example, the " Returns a The first argument, if provided, controls the type of the image to be returned (e.g. PNG or
+ JPEG). The default is " When trying to use types other than " Creates a The second argument, if provided, controls the type of the image to be returned (e.g. PNG or
+ JPEG). The default is " Returns a newly created The If this If this Let file be a
+ serialization of this If file is null then return " The If this If this Otherwise, let result be a Return, but continue running these steps in parallel. Queue a task to invoke
+ the The If this Let offscreenCanvas be a new Set the placeholder Set this Return offscreenCanvas. To maintain compatibility with existing Web content, user agents need to
+ enumerate methods defined in Returns a If the Returns the A The output bitmap has an origin-clean flag, which can be set to true or false.
+ Initially, when one of these bitmaps is created, its origin-clean flag must be set to true. The Thus, the bitmap of such a context starts off as fully-opaque black instead of
+ fully-transparent black; The The size of the coordinate space does not necessarily represent the size of the actual bitmaps
+ that the user agent will use internally or during rendering. On high-definition displays, for
+ instance, the user agent may internally use bitmaps with two device pixels per unit in the
+ coordinate space, so that the rendering remains at high quality throughout. Anti-aliasing can
+ similarly be implemented using over-sampling with bitmaps of a higher resolution than the final
+ image on the display. The 2D context creation algorithm, which is passed a target (a
+ If the algorithm was passed some arguments, then let arg be the first such
+ argument. Otherwise, let arg be undefined. Let settings be the result of coercing the arg context arguments for 2D. Create a new Initialize its Let the new Set bitmap dimensions to the
+ numeric values of target's Process each of the members of settings as follows: Return the new When a user agent is required to coerce context arguments for 2D, it must run these
+ steps: Let input be the argument to coerce. Let jsval be the result of converting
+ input to a JavaScript value. (This can throw an exception.) Let dict be the result of converting
+ jsval to the dictionary type Return dict. When the user agent is to set bitmap
+ dimensions to width and height, it must run these steps: Resize the output bitmap to the new width and height
+ and clear it to fully transparent black. Let canvas be the If the numeric value of canvas's If the numeric value of canvas's Only one square appears to be drawn in the following example: When the user agent is to run the unbinding
+ steps for a rendering context, it must run these steps: Returns an object that exposes an API for drawing on the
- canvas. The first argument specifies the desired API. Subsequent
- arguments are handled by that API. Clear the Set the Let the When the user agent is to run the binding
+ steps to bind the rendering context to the This specification defines the " Resize the Set the Let the The list of defined contexts is given on the WHATWG Wiki
- CanvasContexts page. [WHATWGWIKI]
+ The HTML
- Living Standard — Last Updated 8 August 2011
- HTML
Living Standard — Last Updated 31 August 2017
+
+
- 4.8.11 The
+
+ canvas
element4.12.5 The
canvas
element
- a
elements, button
elements, input
elements whose type
attribute are in the Checkbox or Radio Button states, and input
elements that are buttons.width
height
interface HTMLCanvasElement : HTMLElement {
- attribute unsigned long width;
- attribute unsigned long height;
+
a
elements, img
elements with
+ usemap
attributes, button
elements,
+ input
elements whose type
attribute are in
+ the Checkbox or Radio Button states, input
elements that are
+ buttons, select
elements with a multiple
attribute or a display size greater than 1, and elements that would not be
+ interactive content except for having the tabindex
attribute specified.width
— Horizontal dimensionheight
— Vertical dimensiontypedef (CanvasRenderingContext2D or WebGLRenderingContext) RenderingContext;
- DOMString toDataURL(in optional DOMString type, in any... args);
- void toBlob(in FileCallback? callback, in optional DOMString type, in any... args);
+[Exposed=Window,
+ HTMLConstructor]
+interface HTMLCanvasElement : HTMLElement {
+ [CEReactions] attribute unsigned long width;
+ [CEReactions] attribute unsigned long height;
- object? getContext(in DOMString contextId, in any... args);
-};
- canvas
element provides scripts with a
- resolution-dependent bitmap canvas, which can be used for rendering
- graphs, game graphics, or other visual images on the fly.canvas
element in a
- document when a more suitable element is available. For example, it
- is inappropriate to use a canvas
element to render a
- page heading: if the desired presentation of the heading is
- graphically intense, it should be marked up using appropriate
- elements (typically h1
) and then styled using CSS and
- supporting technologies such as XBL.canvas
element, they must also
- provide content that, when presented to the user, conveys
- essentially the same function or purpose as the bitmap canvas. This
- content may be placed as content of the canvas
- element. The contents of the canvas
element, if any,
- are the element's fallback content.canvas
element, and if support for canvas
- elements has been enabled, the canvas
element
- represents embedded content consisting of
- a dynamically created image.canvas
element provides scripts with a resolution-dependent bitmap canvas,
+ which can be used for rendering graphs, game graphics, art, or other visual images on the fly.canvas
element has been previously painted on (e.g. if
- the page was viewed in an interactive visual medium and is now being
- printed, or if some script that ran during the page layout process
- painted on the element), then the canvas
element
- represents embedded content with the
- current image and size. Otherwise, the element represents its
- fallback content instead.canvas
element in a document when a more suitable
+ element is available. For example, it is inappropriate to use a canvas
element to
+ render a page heading: if the desired presentation of the heading is graphically intense, it
+ should be marked up using appropriate elements (typically h1
) and then styled using
+ CSS and supporting technologies such as Web Components.canvas
element or if support for canvas
- elements has been disabled, the canvas
element
- represents its fallback content
- instead.canvas
element, they must also provide content that, when
+ presented to the user, conveys essentially the same function or purpose as the
+ canvas
's bitmap. This content may be placed as content of the canvas
+ element. The contents of the canvas
element, if any, are the element's fallback
+ content.canvas
element represents
- embedded content, the user can still focus descendants
- of the canvas
element (in the fallback
- content). When an element is focused, it is the target of
- keyboard interaction events (even though the element itself is not
- visible). This allows authors to make an interactive canvas
- keyboard-accessible: authors should have a one-to-one mapping of
- interactive regions to focusable elements in the fallback
- content. (Focus has no effect on mouse interaction
- events.) [DOMEVENTS]
- canvas
element has two attributes to control the
- size of the coordinate space: width
and height
. These
- attributes, when specified, must have values that are valid non-negative
- integers. The rules for parsing
- non-negative integers must be used to obtain their numeric
- values. If an attribute is missing, or if parsing its value returns
- an error, then the default value must be used instead. The
- width
attribute defaults to
- 300, and the height
- attribute defaults to 150.canvas
element, and if support for canvas
elements has been enabled,
+ then the canvas
element represents embedded content
+ consisting of a dynamically created image, the element's bitmap.canvas
element equal
- the size of the coordinate space, with the numbers interpreted in
- CSS pixels. However, the element can be sized arbitrarily by a
- style sheet. During rendering, the image is scaled to fit this layout
- size.canvas
element has been
+ previously associated with a rendering context (e.g. if the page was viewed in an interactive
+ visual medium and is now being printed, or if some script that ran during the page layout process
+ painted on the element), then the canvas
element represents
+ embedded content with the element's current bitmap and size. Otherwise, the element
+ represents its fallback content instead.canvas
element or if support for canvas
elements
+ has been disabled, the canvas
element represents its fallback
+ content instead.canvas
element represents embedded content, the
+ user can still focus descendants of the canvas
element (in the fallback
+ content). When an element is focused, it is the target of keyboard interaction
+ events (even though the element itself is not visible). This allows authors to make an interactive
+ canvas keyboard-accessible: authors should have a one-to-one mapping of interactive regions to focusable areas in the fallback content. (Focus has no
+ effect on mouse interaction events.) [UIEVENTS]canvas
element is created, and subsequently
- whenever the width
and height
attributes are set (whether
- to a new value or to the previous value), the bitmap and any
- associated contexts must be cleared back to their initial state and
- reinitialized with the newly specified coordinate space
- dimensions.canvas
element ancestor is being rendered
+ and represents embedded content is an element that is being used as
+ relevant canvas fallback content.
- width
and
- height
IDL
- attributes must reflect the respective content
- attributes of the same name, with the same defaults.canvas
element has two attributes to control the size of the element's bitmap:
+ width
and height
. These attributes, when specified, must have
+ values that are valid non-negative integers. The rules for parsing non-negative integers must be used to obtain their
+ numeric values. If an attribute is missing, or if parsing its value returns an error, then the
+ default value must be used instead. The width
+ attribute defaults to 300, and the height
attribute
+ defaults to 150.canvas
element when it
+ represents embedded content are equal to the dimensions of the
+ element's bitmap.canvas
and its rendering contexts.canvas
element can be sized arbitrarily by a style sheet, its
+ bitmap is then subject to the 'object-fit' CSS property.
+
+ canvas
elements, the bitmaps of ImageBitmap
objects,
+ as well as some of the bitmaps of rendering contexts, such as those described in the sections on
+ the CanvasRenderingContext2D
and ImageBitmapRenderingContext
objects
+ below, have an origin-clean flag, which can be
+ set to true or false. Initially, when the canvas
element or ImageBitmap
+ object is created, its bitmap's origin-clean
+ flag must be set to true.canvas
element can have a rendering context bound to it. Initially, it does not
+ have a bound rendering context. To keep track of whether it has a rendering context or not, and
+ what kind of rendering context it is, a canvas
also has a canvas context mode, which is initially none but can be changed to either placeholder, 2d, bitmaprenderer, or webgl by algorithms defined in this specification.canvas
element has no rendering context,
+ and its bitmap must be fully transparent black with an intrinsic width equal to the
+ numeric value of the element's width
attribute and an
+ intrinsic height equal to the numeric value of the element's height
attribute, those values being interpreted in CSS pixels, and being updated as the attributes are set, changed, or
+ removed.canvas
element has no
+ rendering context. It serves as a placeholder for an OffscreenCanvas
object, and
+ the content of the canvas
element is updated by calling the commit()
method of the OffscreenCanvas
+ object's rendering context.canvas
element represents embedded content, it provides a
+ paint source whose width is the element's intrinsic width, whose height
+ is the element's intrinsic height, and whose appearance is the element's bitmap.width
and height
content attributes are set, removed, changed, or
+ redundantly set to the value they already have, then the user agent must perform the action
+ from the row of the following table that corresponds to the canvas
element's context mode.
+
+
+
+
+
+
+
+ width
+ and height
content attributes.
+
+
+
+
+
+ ImageBitmapRenderingContext
's output bitmap,
+ passing the canvas
element's rendering context.
+
+
+
+
+
+ width
and height
IDL attributes must reflect the
+ respective content attributes of the same name, with the same defaults.
+
+
+
+
+
+
+ getContext
(contextId [, ... ] )2d
", "bitmaprenderer
" or "webgl
". Subsequent arguments are handled by that API.2d
" and "bitmaprenderer
" contexts below. There is also a
+ specification that defines a "webgl
" context.
+ [WEBGL]2d
" context after getting a "webgl
" context).getContext(contextId,
+ arguments...)
method of the canvas
element, when invoked,
+ must run the steps in the cell of the following table whose column header describes the
+ canvas
element's canvas context
+ mode and whose row header describes the method's first argument.
+
+
+ none
+ 2d
+ bitmaprenderer
+ webgl
+ placeholder
+ " 2d
"
+
+
+ Follow the 2D context creation algorithm defined in the section below, passing
+ it the canvas
element and the method's arguments..., to obtain a
+ CanvasRenderingContext2D
object; if this does not throw an exception, then
+ set the canvas
element's context
+ mode to 2d, and return the
+ CanvasRenderingContext2D
object.
+
+
+ Return the same object as was returned the last time the method was invoked with this same
+ first argument.
+
+ Return null.
+
+ Return null.
+
+ Throw an " InvalidStateError
" DOMException
.
+
+ " bitmaprenderer
"
+
+
+ Follow the ImageBitmapRenderingContext
creation algorithm defined
+ in the section below, passing it the canvas
element and the method's
+ arguments..., to obtain an ImageBitmapRenderingContext
object; then
+ set the canvas
element's context
+ mode to bitmaprenderer, and return the
+ ImageBitmapRenderingContext
object.
+
+
+ Return null.
+
+ Return the same object as was returned the last time the method was invoked with this same
+ first argument.
+
+ Return null.
+
+ Throw an " InvalidStateError
" DOMException
.
+
+ " webgl
", if the user agent supports the WebGL feature in its current configuration
+
+
+ Follow the instructions given in the WebGL specification's Context Creation section to
+ obtain either a WebGLRenderingContext
or null; if the returned value is null,
+ then return null and abort these steps, otherwise, set the canvas
element's
+ context mode to webgl, and return the WebGLRenderingContext
+ object [WEBGL]
+
+
+ Return null.
+
+ Return null.
+
+ Return the same object as was returned the last time the method was invoked with this same
+ first argument.
+
+ Throw an " InvalidStateError
" DOMException
.
+
+ An unsupported value*
+
+ Return null.
+
+ Return null.
+
+ Return null.
+
+ Return null.
+
+ Throw an " InvalidStateError
" DOMException
.
+
+ webgl
"
+ value in the case of a user agent having exhausted the graphics hardware's abilities and having no
+ software fallback implementation.
+
+
+
+
+
+
+
+ toDataURL
( [ type [, quality ] ] )data:
URL for the image in the
+ canvas.image/png
"; that type is also used if the given type isn't
+ supported. The second argument applies if the type is an image format that supports variable
+ quality (such as "image/jpeg
"), and is a number in the range 0.0 to 1.0 inclusive
+ indicating the desired quality level for the resulting image.image/png
", authors can check if the image
+ was really returned in the requested format by checking to see if the returned string starts
+ with one of the exact strings "data:image/png,
" or "data:image/png;
". If it does, the image is PNG, and thus the requested type was
+ not supported. (The one exception to this is if the canvas has either no height or no width, in
+ which case the result might simply be "data:,
".)toBlob
(callback [, type [, quality ] ] )Blob
object representing a file containing the image in the canvas,
+ and invokes a callback with a handle to that object.image/png
"; that type is also used if the given type isn't
+ supported. The third argument applies if the type is an image format that supports variable
+ quality (such as "image/jpeg
"), and is a number in the range 0.0 to 1.0 inclusive
+ indicating the desired quality level for the resulting image.transferControlToOffscreen
()OffscreenCanvas
object that uses the
+ canvas
element as a placeholder. Once the canvas
element has become a
+ placeholder for an OffscreenCanvas
object, its intrinsic size can no longer be
+ changed, and it cannot have a rendering context. The content of the placeholder canvas is
+ updated by calling the commit()
method of the
+ OffscreenCanvas
object's rendering context.
+ toDataURL(type,
+ quality)
method, when invoked, must run these steps:
+
+ canvas
element's bitmap's origin-clean flag is set to false, then throw a
+ "SecurityError
" DOMException
and abort these steps.canvas
element's bitmap has no pixels (i.e. either its horizontal
+ dimension or its vertical dimension is zero) then return the string "data:,
" and abort these steps. (This is the shortest data:
URL; it represents the empty string in a text/plain
resource.)canvas
element's bitmap as a file, passing
+ type and quality if they were given.data:,
".toBlob(callback, type,
+ quality)
method, when invoked, must run these steps:
+
+ canvas
element's bitmap's origin-clean flag is set to false, then throw a
+ "SecurityError
" DOMException
and abort these steps.canvas
element's bitmap has no pixels (i.e. either its horizontal
+ dimension or its vertical dimension is zero) then let result be null.Blob
object representing a serialization of this canvas
element's
+ bitmap as a file, passing type and quality if they were given. [FILEAPI]BlobCallback
callback with result as its argument. The
+ task source for this task is the
+ canvas blob serialization task
+ source.transferControlToOffscreen()
method,
+ when invoked, must run these steps:
+
+
+
+
+
+ canvas
element's context
+ mode is not set to none, throw an
+ "InvalidStateError
" DOMException
and abort these
+ steps.OffscreenCanvas
object with its
+ width and height equal to the values of the width
+ and height
content attributes of this
+ canvas
element.canvas
+ element of offscreenCanvas to be a weak reference to this canvas
+ element.canvas
element's context
+ mode to placeholder.4.12.5.1 The 2D rendering context
+
+
+
+ typedef (HTMLImageElement or
+ SVGImageElement) HTMLOrSVGImageElement;
+
+typedef (HTMLOrSVGImageElement or
+ HTMLVideoElement or
+ HTMLCanvasElement or
+ ImageBitmap or
+ OffscreenCanvas) CanvasImageSource;
+
+enum CanvasFillRule { "nonzero", "evenodd" };
+
+dictionary CanvasRenderingContext2DSettings {
+ boolean alpha = true;
+};
+
+enum ImageSmoothingQuality { "low", "medium", "high" };
+
+interface CanvasRenderingContext2D {
+ // back-reference to the canvas
+ readonly attribute HTMLCanvasElement canvas;
+};
+CanvasRenderingContext2D implements CanvasState;
+CanvasRenderingContext2D implements CanvasTransform;
+CanvasRenderingContext2D implements CanvasCompositing;
+CanvasRenderingContext2D implements CanvasImageSmoothing;
+CanvasRenderingContext2D implements CanvasFillStrokeStyles;
+CanvasRenderingContext2D implements CanvasShadowStyles;
+CanvasRenderingContext2D implements CanvasFilters;
+CanvasRenderingContext2D implements CanvasRect;
+CanvasRenderingContext2D implements CanvasDrawPath;
+CanvasRenderingContext2D implements CanvasUserInterface;
+CanvasRenderingContext2D implements CanvasText;
+CanvasRenderingContext2D implements CanvasDrawImage;
+CanvasRenderingContext2D implements CanvasImageData;
+CanvasRenderingContext2D implements CanvasPathDrawingStyles;
+CanvasRenderingContext2D implements CanvasTextDrawingStyles;
+CanvasRenderingContext2D implements CanvasPath;
+
+[NoInterfaceObject, Exposed=(Window,Worker)]
+interface CanvasState {
+ // state
+ void save(); // push state on state stack
+ void restore(); // pop state stack and restore state
+};
+
+[NoInterfaceObject, Exposed=(Window,Worker)]
+interface CanvasTransform {
+ // transformations (default transform is the identity matrix)
+ void scale(unrestricted double x, unrestricted double y);
+ void rotate(unrestricted double angle);
+ void translate(unrestricted double x, unrestricted double y);
+ void transform(unrestricted double a, unrestricted double b, unrestricted double c, unrestricted double d, unrestricted double e, unrestricted double f);
+
+ [NewObject] DOMMatrix getTransform();
+ void setTransform(unrestricted double a, unrestricted double b, unrestricted double c, unrestricted double d, unrestricted double e, unrestricted double f);
+ void setTransform(optional DOMMatrix2DInit transform);
+ void resetTransform();
+
+};
+
+[NoInterfaceObject, Exposed=(Window,Worker)]
+interface CanvasCompositing {
+ // compositing
+ attribute unrestricted double globalAlpha; // (default 1.0)
+ attribute DOMString globalCompositeOperation; // (default source-over)
+};
+
+[NoInterfaceObject, Exposed=(Window,Worker)]
+interface CanvasImageSmoothing {
+ // image smoothing
+ attribute boolean imageSmoothingEnabled; // (default true)
+ attribute ImageSmoothingQuality imageSmoothingQuality; // (default low)
+
+};
+
+[NoInterfaceObject, Exposed=(Window,Worker)]
+interface CanvasFillStrokeStyles {
+ // colors and styles (see also the CanvasPathDrawingStyles and CanvasTextDrawingStyles interfaces)
+ attribute (DOMString or CanvasGradient or CanvasPattern) strokeStyle; // (default black)
+ attribute (DOMString or CanvasGradient or CanvasPattern) fillStyle; // (default black)
+ CanvasGradient createLinearGradient(double x0, double y0, double x1, double y1);
+ CanvasGradient createRadialGradient(double x0, double y0, double r0, double x1, double y1, double r1);
+ CanvasPattern? createPattern(CanvasImageSource image, [TreatNullAs=EmptyString] DOMString repetition);
+
+};
+
+[NoInterfaceObject, Exposed=(Window,Worker)]
+interface CanvasShadowStyles {
+ // shadows
+ attribute unrestricted double shadowOffsetX; // (default 0)
+ attribute unrestricted double shadowOffsetY; // (default 0)
+ attribute unrestricted double shadowBlur; // (default 0)
+ attribute DOMString shadowColor; // (default transparent black)
+};
+
+[NoInterfaceObject, Exposed=(Window,Worker)]
+interface CanvasFilters {
+ // filters
+ attribute DOMString filter; // (default "none")
+};
+
+[NoInterfaceObject, Exposed=(Window,Worker)]
+interface CanvasRect {
+ // rects
+ void clearRect(unrestricted double x, unrestricted double y, unrestricted double w, unrestricted double h);
+ void fillRect(unrestricted double x, unrestricted double y, unrestricted double w, unrestricted double h);
+ void strokeRect(unrestricted double x, unrestricted double y, unrestricted double w, unrestricted double h);
+};
+
+[NoInterfaceObject, Exposed=(Window,Worker)]
+interface CanvasDrawPath {
+ // path API (see also CanvasPath)
+ void beginPath();
+ void fill(optional CanvasFillRule fillRule = "nonzero");
+ void fill(Path2D path, optional CanvasFillRule fillRule = "nonzero");
+ void stroke();
+ void stroke(Path2D path);
+ void clip(optional CanvasFillRule fillRule = "nonzero");
+ void clip(Path2D path, optional CanvasFillRule fillRule = "nonzero");
+ void resetClip();
+ boolean isPointInPath(unrestricted double x, unrestricted double y, optional CanvasFillRule fillRule = "nonzero");
+ boolean isPointInPath(Path2D path, unrestricted double x, unrestricted double y, optional CanvasFillRule fillRule = "nonzero");
+ boolean isPointInStroke(unrestricted double x, unrestricted double y);
+ boolean isPointInStroke(Path2D path, unrestricted double x, unrestricted double y);
+};
+
+[Exposed=Window,
+ NoInterfaceObject]
+interface CanvasUserInterface {
+ void drawFocusIfNeeded(Element element);
+ void drawFocusIfNeeded(Path2D path, Element element);
+ void scrollPathIntoView();
+ void scrollPathIntoView(Path2D path);
+};
+
+[Exposed=Window,
+ NoInterfaceObject]
+interface CanvasText {
+ // text (see also the CanvasPathDrawingStyles and CanvasTextDrawingStyles interfaces)
+ void fillText(DOMString text, unrestricted double x, unrestricted double y, optional unrestricted double maxWidth);
+ void strokeText(DOMString text, unrestricted double x, unrestricted double y, optional unrestricted double maxWidth);
+ TextMetrics measureText(DOMString text);
+};
+
+[NoInterfaceObject, Exposed=(Window,Worker)]
+interface CanvasDrawImage {
+ // drawing images
+ void drawImage(CanvasImageSource image, unrestricted double dx, unrestricted double dy);
+ void drawImage(CanvasImageSource image, unrestricted double dx, unrestricted double dy, unrestricted double dw, unrestricted double dh);
+ void drawImage(CanvasImageSource image, unrestricted double sx, unrestricted double sy, unrestricted double sw, unrestricted double sh, unrestricted double dx, unrestricted double dy, unrestricted double dw, unrestricted double dh);
+};
+
+[NoInterfaceObject, Exposed=(Window,Worker)]
+interface CanvasImageData {
+ // pixel manipulation
+ ImageData createImageData(long sw, long sh);
+ ImageData createImageData(ImageData imagedata);
+ ImageData getImageData(long sx, long sy, long sw, long sh);
+ void putImageData(ImageData imagedata, long dx, long dy);
+ void putImageData(ImageData imagedata, long dx, long dy, long dirtyX, long dirtyY, long dirtyWidth, long dirtyHeight);
+};
+
+enum CanvasLineCap { "butt", "round", "square" };
+enum CanvasLineJoin { "round", "bevel", "miter" };
+enum CanvasTextAlign { "start", "end", "left", "right", "center" };
+enum CanvasTextBaseline { "top", "hanging", "middle", "alphabetic", "ideographic", "bottom" };
+enum CanvasDirection { "ltr", "rtl", "inherit" };
+
+[NoInterfaceObject, Exposed=(Window,Worker)]
+interface CanvasPathDrawingStyles {
+ // line caps/joins
+ attribute unrestricted double lineWidth; // (default 1)
+ attribute CanvasLineCap lineCap; // (default "butt")
+ attribute CanvasLineJoin lineJoin; // (default "miter")
+ attribute unrestricted double miterLimit; // (default 10)
+
+ // dashed lines
+ void setLineDash(sequence<unrestricted double> segments); // default empty
+ sequence<unrestricted double> getLineDash();
+ attribute unrestricted double lineDashOffset;
+};
+
+[Exposed=Window,
+ NoInterfaceObject]
+interface CanvasTextDrawingStyles {
+ // text
+ attribute DOMString font; // (default 10px sans-serif)
+ attribute CanvasTextAlign textAlign; // (default: "start")
+ attribute CanvasTextBaseline textBaseline; // (default: "alphabetic")
+ attribute CanvasDirection direction; // (default: "inherit")
+};
+
+[NoInterfaceObject, Exposed=(Window,Worker)]
+interface CanvasPath {
+ // shared path API methods
+ void closePath();
+ void moveTo(unrestricted double x, unrestricted double y);
+ void lineTo(unrestricted double x, unrestricted double y);
+ void quadraticCurveTo(unrestricted double cpx, unrestricted double cpy, unrestricted double x, unrestricted double y);
+ void bezierCurveTo(unrestricted double cp1x, unrestricted double cp1y, unrestricted double cp2x, unrestricted double cp2y, unrestricted double x, unrestricted double y);
+ void arcTo(unrestricted double x1, unrestricted double y1, unrestricted double x2, unrestricted double y2, unrestricted double radius);
+ void arcTo(unrestricted double x1, unrestricted double y1, unrestricted double x2, unrestricted double y2, unrestricted double radiusX, unrestricted double radiusY, unrestricted double rotation);
+ void rect(unrestricted double x, unrestricted double y, unrestricted double w, unrestricted double h);
+ void arc(unrestricted double x, unrestricted double y, unrestricted double radius, unrestricted double startAngle, unrestricted double endAngle, optional boolean anticlockwise = false);
+ void ellipse(unrestricted double x, unrestricted double y, unrestricted double radiusX, unrestricted double radiusY, unrestricted double rotation, unrestricted double startAngle, unrestricted double endAngle, optional boolean anticlockwise = false);
+};
+
+[Exposed=(Window,Worker)]
+interface CanvasGradient {
+ // opaque object
+ void addColorStop(double offset, DOMString color);
+};
+
+[Exposed=(Window,Worker)]
+interface CanvasPattern {
+ // opaque object
+ void setTransform(optional DOMMatrix2DInit transform);
+};
+
+[Exposed=Window]
+interface TextMetrics {
+ // x-direction
+ readonly attribute double width; // advance width
+ readonly attribute double actualBoundingBoxLeft;
+ readonly attribute double actualBoundingBoxRight;
+
+ // y-direction
+ readonly attribute double fontBoundingBoxAscent;
+ readonly attribute double fontBoundingBoxDescent;
+ readonly attribute double actualBoundingBoxAscent;
+ readonly attribute double actualBoundingBoxDescent;
+ readonly attribute double emHeightAscent;
+ readonly attribute double emHeightDescent;
+ readonly attribute double hangingBaseline;
+ readonly attribute double alphabeticBaseline;
+ readonly attribute double ideographicBaseline;
+};
+
+[Constructor(unsigned long sw, unsigned long sh),
+ Constructor(Uint8ClampedArray data, unsigned long sw, optional unsigned long sh),
+ Exposed=(Window,Worker),
+ Serializable]
+interface ImageData {
+ readonly attribute unsigned long width;
+ readonly attribute unsigned long height;
+ readonly attribute Uint8ClampedArray data;
+};
+
+[Constructor,
+ Constructor(Path2D path),
+ Constructor(sequence<Path2D> paths, optional CanvasFillRule fillRule = "nonzero"),
+ Constructor(DOMString d),
+ Exposed=(Window,Worker)]
+interface Path2D {
+ void addPath(Path2D path, optional DOMMatrix2DInit transform);
+};
+Path2D implements CanvasPath;
+
+
+
+ CanvasUserInterface
immediately after the stroke()
method on CanvasRenderingContext2D
+ objects.
+
+
+
+ getContext
('2d' [, { [ alpha
: false ] } ] )CanvasRenderingContext2D
object that is permanently bound to a
+ particular canvas
element.alpha
setting is
+ provided and set to false, then the canvas is forced to always be opaque.canvas
canvas
element.CanvasRenderingContext2D
object has an output bitmap that
+ is initialized when the object is created.CanvasRenderingContext2D
object also has an alpha flag, which can be set to true or false. Initially,
+ when the context is created, its alpha flag must be
+ set to true. When a CanvasRenderingContext2D
object has its alpha flag set to false, then its alpha channel must be
+ fixed to 1.0 (fully opaque) for all pixels, and attempts to change the alpha component of any
+ pixel must be silently ignored.clearRect()
always
+ results in fully-opaque black pixels, every fourth byte from getImageData()
is always 255, the putImageData()
method effectively ignores every
+ fourth byte in its input, and so on. However, the alpha component of styles and images drawn
+ onto the canvas are still honoured up to the point where they would impact the output
+ bitmap's alpha channel; for instance, drawing a 50% transparent white square on a freshly
+ created output bitmap with its alpha
+ flag set to false will result in a fully-opaque gray square.
+
+ CanvasRenderingContext2D
2D rendering context represents a flat linear
+ Cartesian surface whose origin (0,0) is at the top left corner, with the coordinate space having
+ x values increasing when going right, and y values increasing when going
+ down. The x-coordinate of the right-most edge is equal to the width of the rendering
+ context's output bitmap in CSS pixels; similarly, the
+ y-coordinate of the bottom-most edge is equal to the height of the rendering context's
+ output bitmap in CSS pixels.
+
+ canvas
element) and optionally some arguments, consists of running the following
+ steps:
+
+ CanvasRenderingContext2D
object.canvas
attribute to point to
+ target.CanvasRenderingContext2D
object's output bitmap be
+ the same bitmap as target's bitmap (so that they are shared).width
and
+ height
content attributes.
+
+ alpha
CanvasRenderingContext2D
object's alpha flag to false.CanvasRenderingContext2D
object.
+
+
+
+
+
+ CanvasRenderingContext2DSettings
. (This can
+ throw an exception.)
+
+ canvas
element to which the rendering
+ context's canvas
attribute was initialized.width
+ content attribute differs from width, then set canvas's width
content attribute to the shortest possible string
+ representing width as a valid non-negative integer.height
+ content attribute differs from height, then set canvas's height
content attribute to the shortest possible string
+ representing height as a valid non-negative integer. // canvas is a reference to a <canvas> element
- var context = canvas.getContext('2d');
- context.fillRect(0,0,50,50);
- canvas.setAttribute('width', '300'); // clears the canvas
- context.fillRect(0,100,50,50);
- canvas.width = canvas.width; // clears the canvas
- context.fillRect(100,0,50,50); // only this square remains
+ // canvas is a reference to a <canvas> element
+var context = canvas.getContext('2d');
+context.fillRect(0,0,50,50);
+canvas.setAttribute('width', '300'); // clears the canvas
+context.fillRect(0,100,50,50);
+canvas.width = canvas.width; // clears the canvas
+context.fillRect(100,0,50,50); // only this square remains
getContext
(contextId [, ... ])
-
-
+ CanvasRenderingContext2D
object's output bitmap to a
+ transparent black.CanvasRenderingContext2D
object's origin-clean flag to true.CanvasRenderingContext2D
object have no output
+ bitmap.canvas
element target, it
+ must run these steps:2d
" context below. There is also
- a specification that defines a "webgl
" context. [WEBGL]
-
+ CanvasRenderingContext2D
object's output bitmap to
+ the dimensions of target's bitmap and clear it to fully transparent black.CanvasRenderingContext2D
object's origin-clean flag to true.CanvasRenderingContext2D
object's output bitmap be target's bitmap.
- canvas
attribute must return the
+ value it was initialized to when the object was created.
Returns null if the given context ID is not supported or if the
- canvas has already been initialized with some other (incompatible)
- context type (e.g. trying to get a "2d
" context after getting a
- "webgl
" context).
Whenever the CSS value currentColor
is used as a color in the CanvasRenderingContext2D
+ API, the computed value of the currentColor
keyword is
+ the computed value of the 'color' property on the canvas
+ element at the time that the color is specified (e.g. when the appropriate attribute is set, or
+ when the method is called; not when the color is rendered or otherwise used). If the computed
+ value of the 'color' property is undefined for a particular case (e.g. because the element is
+ not in a document), then the computed value of the 'color' property
+ for the purposes of determining the computed value of the currentColor
+ keyword is fully opaque black. [CSSCOLOR]
In the case of addColorStop()
on
+ CanvasGradient
, the "computed value of the 'color'
+ property" for the purposes of determining the computed value of the currentColor
keyword is always fully opaque black (there is no associated
+ element). [CSSCOLOR]
A canvas
element can have a primary
- context, which is the first context to have been obtained for
- that element. When created, a canvas
element must not
- have a primary context.
This is because CanvasGradient
objects are
+ canvas
-neutral — a CanvasGradient
object created by one
+ canvas
can be used by another, and there is therefore no way to know which is the
+ "element in question" at the time that the color is specified.
Similar concerns exist with font-related properties; the rules for those are + described in detail in the relevant section below.
-The getContext(contextId, args...)
- method of the canvas
element, when invoked, must run
- the following steps:
Let contextId be the first argument to - the method.
The CanvasFillRule
enumeration is used to select the fill rule
+ algorithm by which to determine if a point is inside or outside a path.
If contextId is not the name of a context - supported by the user agent, return null and abort these - steps.
+The value "nonzero
" value
+ indicates the non-zero winding rule, wherein
-
An example of this would be a user agent that
- theoretically supports the "webgl
" 3D context, in the case
- where the platform does not have hardware support for OpenGL and
- the user agent does not have a software OpenGL implementation.
- Despite the user agent recognising the "webgl
" name, it would return
- null at this step because that context is not, in practice,
- supported at the time of the call.
If the element has a primary context and that - context's entry in the WHATWG Wiki - CanvasContexts page does not list contextId - as a context with which it is compatible, return null and abort - these steps. [WHATWGWIKI]
If the element does not have a primary context, - let the element's primary context be contextId.
The "evenodd
" value indicates
+ the even-odd rule, wherein
-
If the getContext()
method has
- already been invoked on this element for the same contextId, return the same object as was returned
- that time, and abort these steps. The additional arguments are
- ignored.
Return a new object for contextId, as defined by the specification - given for contextId's entry in the WHATWG Wiki - CanvasContexts page. [WHATWGWIKI]
New context types may be registered in the WHATWG Wiki - CanvasContexts page. [WHATWGWIKI]
+If a point is not outside a shape, it is inside the shape.
-Anyone is free to edit the WHATWG Wiki CanvasContexts page at any - time to add a new context type. These new context types must be - specified with the following information:
-The value of contextID that will return - the object for the new API.
The ImageSmoothingQuality
enumeration is used to express a preference for the
+ interpolation quality to use when smoothing images.
The "low
" value
+ indicates a preference for a low level of image interpolation quality. Low-quality image
+ interpolation may be more computationally efficient than higher settings.
The "medium
" value
+ indicates a preference for a medium level of image interpolation quality.
A link to a formal specification of the context type's - API. It could be another page on the Wiki, or a link to an external - page. If the type does not have a formal specification, an informal - description can be substituted until such time as a formal - specification is available.
The "high
" value
+ indicates a preference for a high level of image interpolation quality. High-quality image
+ interpolation may be more computationally expensive than lower settings.
Bilinear scaling is an example of a relatively fast, lower-quality image-smoothing + algorithm. Bicubic or Lanczos scaling are examples of image-smoothing algorithms that produce + higher-quality output. This specification does not mandate that specific interpolation algorithms + be used.
-The list of context types that are compatible with this one - (i.e. that operate on the same underlying bitmap). This list must - be transitive and symmetric; if one context type is defined as - compatible with another, then all types it is compatible with must - be compatible with all types the other is compatible with.
Vendors may also define experimental contexts using the syntax
- vendorname-context
, for example,
- moz-3d
. Such contexts should be registered in the
- WHATWG Wiki CanvasContexts page.
This section is non-normative.
-The output bitmap, when it is not directly displayed by the user agent,
+ implementations can, instead of updating this bitmap, merely remember the sequence of drawing
+ operations that have been applied to it until such time as the bitmap's actual data is needed
+ (for example because of a call to drawImage()
, or
+ the createImageBitmap()
factory method). In many
+ cases, this will be more memory efficient.
toDataURL
( [ type, ... ])The bitmap of a canvas
element is the one bitmap that's pretty much always going
+ to be needed in practice. The output bitmap of a rendering context, when it has one,
+ is always just an alias to a canvas
element's bitmap.
Additional bitmaps are sometimes needed, e.g. to enable fast drawing when the canvas is being + painted at a different size than its intrinsic size, + or to enable double buffering so that graphics updates, like page scrolling for example, can be + processed concurrently while canvas draw commands are being executed.
-Returns a data:
URL for the image in the canvas.
The first argument, if provided, controls the type of the image
- to be returned (e.g. PNG or JPEG). The default is image/png
; that type is also used if the given
- type isn't supported. The other arguments are specific to the
- type, and control the way that the image is generated, as given in
- the table below.
Objects that implement the CanvasState
interface maintain a stack of drawing
+ states. Drawing states consist of:
When trying to use types other than "image/png
",
- authors can check if the image was really returned in the
- requested format by checking to see if the returned string starts
- with one of the exact strings "data:image/png,
" or "data:image/png;
". If it does, the image is PNG,
- and thus the requested type was not supported. (The one exception
- to this is if the canvas has either no height or no width, in
- which case the result might simply be "data:,
".)
strokeStyle
, fillStyle
, globalAlpha
, lineWidth
, lineCap
, lineJoin
, miterLimit
, lineDashOffset
, shadowOffsetX
, shadowOffsetY
, shadowBlur
, shadowColor
, filter
, globalCompositeOperation
, font
, textAlign
, textBaseline
, direction
, imageSmoothingEnabled
, imageSmoothingQuality
.The current default path and the rendering context's bitmaps are not
+ part of the drawing state. The current default path is persistent, and can only be
+ reset using the beginPath()
method. The bitmaps
+ depend on whether and how the rendering context is bound to a canvas
element.
toBlob
(callback [, type, ... ])Creates a Blob
object representing a file
- containing the image in the canvas, and invokes a callback with a
- handle to that object.
The second argument, if provided, controls the type of the
- image to be returned (e.g. PNG or JPEG). The default is image/png
; that type is also used if the given
- type isn't supported. The other arguments are specific to the
- type, and control the way that the image is generated, as given in
- the table below.
The toDataURL()
method
- must run the following steps:
If the canvas has no pixels (i.e. either its horizontal
- dimension or its vertical dimension is zero) then return the string
- "data:,
" and abort these steps. (This is the
- shortest data:
- URL; it represents the empty string in a text/plain
resource.)
Let file be a serialization of the - image as a file, using the method's arguments (if any) as - the arguments.
The toBlob()
method
- must run the following steps:
Let callback be the first - argument.
Let arguments be the second and - subsequent arguments to the method, if any.
Let file be a serialization of the - image as a file, using arguments.
Return, but continue running these steps - asynchronously.
If callback is null, abort these - steps.
Queue a task to invoke the
- FileCallback
callback with a
- Blob
object representing file as
- its argument. The task source for this task is the
- canvas blob serialization task source. [FILESYSTEMAPI] [FILEAPI]
When a user agent is to create a serialization of the image - as a file, optionally with some given arguments, it must create an image file in the format - given by the first value of arguments, or, if - there are no arguments, in the PNG format. [PNG]
- -If arguments is not empty, the first value - must be interpreted as a MIME type - giving the format to use. If the type has any parameters, it must be - treated as not supported.
- -For example, the value "image/png
" would
- mean to generate a PNG image, the value "image/jpeg
"
- would mean to generate a JPEG image, and the value
- "image/svg+xml
" would mean to generate an SVG image
- (which would probably require that the implementation actually keep
- enough information to reliably render an SVG image from the canvas).
User agents must support PNG ("image/png
"). User
- agents may support other types. If the user agent does not support
- the requested type, it must create the file using the PNG format. [PNG]
User agents must convert the provided type to ASCII lowercase - before establishing if they support that type.
- -For image types that do not support an alpha channel, the - serialized image must be the canvas image composited onto a solid - black background using the source-over operator.
- -If the first argument in arguments gives a - type corresponding to one of the types given in the first column of - the following table, and the user agent supports that type, then the - subsequent arguments, if any, must be treated as described in the - second cell of that row.
- -Type | Other arguments | Reference - |
---|---|---|
image/jpeg
- | The second argument, if it is a - number in the range 0.0 to 1.0 inclusive, must - be treated as the desired quality level. If it is not a number or is outside that range, the - user agent must use its default value, as if the argument had - been omitted. - | [JPEG] - |
For the purposes of these rules, an argument is considered to be
- a number if it is converted to an IDL double value by the rules for
- handling arguments of type any
in the Web IDL
- specification. [WEBIDL]
Other arguments must be ignored and must not cause the user agent - to raise an exception. A future version of this specification will - probably define other parameters to be passed to these methods to - allow authors to more carefully control compression settings, image - metadata, etc.
- -This specification defines the 2d
context type, whose
- API is implemented using the CanvasRenderingContext2D
- interface.
When the getContext()
- method of a canvas
element is to return a new object for the contextId 2d
, the user agent must return a
- new CanvasRenderingContext2D
object. Any additional
- arguments are ignored.
The 2D context represents a flat Cartesian surface whose origin - (0,0) is at the top left corner, with the coordinate space having - x values increasing when going right, and y values increasing when going down.
- -interface CanvasRenderingContext2D { - - // back-reference to the canvas - readonly attribute HTMLCanvasElement canvas; - - // state - void save(); // push state on state stack - void restore(); // pop state stack and restore state - - // transformations (default transform is the identity matrix) - void scale(in double x, in double y); - void rotate(in double angle); - void translate(in double x, in double y); - void transform(in double a, in double b, in double c, in double d, in double e, in double f); - void setTransform(in double a, in double b, in double c, in double d, in double e, in double f); - - // compositing - attribute double globalAlpha; // (default 1.0) - attribute DOMString globalCompositeOperation; // (default source-over) - - // colors and styles - attribute any strokeStyle; // (default black) - attribute any fillStyle; // (default black) - CanvasGradient createLinearGradient(in double x0, in double y0, in double x1, in double y1); - CanvasGradient createRadialGradient(in double x0, in double y0, in double r0, in double x1, in double y1, in double r1); - CanvasPattern createPattern(in HTMLImageElement image, in DOMString repetition); - CanvasPattern createPattern(in HTMLCanvasElement image, in DOMString repetition); - CanvasPattern createPattern(in HTMLVideoElement image, in DOMString repetition); - - // line caps/joins - attribute double lineWidth; // (default 1) - attribute DOMString lineCap; // "butt", "round", "square" (default "butt") - attribute DOMString lineJoin; // "round", "bevel", "miter" (default "miter") - attribute double miterLimit; // (default 10) - - // shadows - attribute double shadowOffsetX; // (default 0) - attribute double shadowOffsetY; // (default 0) - attribute double shadowBlur; // (default 0) - attribute DOMString shadowColor; // (default transparent black) - - // rects - void clearRect(in double x, in double y, in double w, in double h); - void fillRect(in double x, in double y, in double w, in double h); - void strokeRect(in double x, in double y, in double w, in double h); - - // path API - void beginPath(); - void closePath(); - void moveTo(in double x, in double y); - void lineTo(in double x, in double y); - void quadraticCurveTo(in double cpx, in double cpy, in double x, in double y); - void bezierCurveTo(in double cp1x, in double cp1y, in double cp2x, in double cp2y, in double x, in double y); - void arcTo(in double x1, in double y1, in double x2, in double y2, in double radius); - void rect(in double x, in double y, in double w, in double h); - void arc(in double x, in double y, in double radius, in double startAngle, in double endAngle, in optional boolean anticlockwise); - void fill(); - void stroke(); - void drawSystemFocusRing(in Element element); - boolean drawCustomFocusRing(in Element element); - void scrollPathIntoView(); - void clip(); - boolean isPointInPath(in double x, in double y); - - // text - attribute DOMString font; // (default 10px sans-serif) - attribute DOMString textAlign; // "start", "end", "left", "right", "center" (default: "start") - attribute DOMString textBaseline; // "top", "hanging", "middle", "alphabetic", "ideographic", "bottom" (default: "alphabetic") - void fillText(in DOMString text, in double x, in double y, in optional double maxWidth); - void strokeText(in DOMString text, in double x, in double y, in optional double maxWidth); - TextMetrics measureText(in DOMString text); - - // drawing images - void drawImage(in HTMLImageElement image, in double dx, in double dy); - void drawImage(in HTMLImageElement image, in double dx, in double dy, in double dw, in double dh); - void drawImage(in HTMLImageElement image, in double sx, in double sy, in double sw, in double sh, in double dx, in double dy, in double dw, in double dh); - void drawImage(in HTMLCanvasElement image, in double dx, in double dy); - void drawImage(in HTMLCanvasElement image, in double dx, in double dy, in double dw, in double dh); - void drawImage(in HTMLCanvasElement image, in double sx, in double sy, in double sw, in double sh, in double dx, in double dy, in double dw, in double dh); - void drawImage(in HTMLVideoElement image, in double dx, in double dy); - void drawImage(in HTMLVideoElement image, in double dx, in double dy, in double dw, in double dh); - void drawImage(in HTMLVideoElement image, in double sx, in double sy, in double sw, in double sh, in double dx, in double dy, in double dw, in double dh); - - // pixel manipulation - ImageData createImageData(in double sw, in double sh); - ImageData createImageData(in ImageData imagedata); - ImageData getImageData(in double sx, in double sy, in double sw, in double sh); - void putImageData(in ImageData imagedata, in double dx, in double dy); - void putImageData(in ImageData imagedata, in double dx, in double dy, in double dirtyX, in double dirtyY, in double dirtyWidth, in double dirtyHeight); -}; - -interface CanvasGradient { - // opaque object - void addColorStop(in double offset, in DOMString color); -}; - -interface CanvasPattern { - // opaque object -}; - -interface TextMetrics { - readonly attribute double width; -}; - -interface ImageData { - readonly attribute unsigned long width; - readonly attribute unsigned long height; - readonly attribute CanvasPixelArray data; -}; - -interface CanvasPixelArray { - readonly attribute unsigned long length; - getter octet (in unsigned long index); - setter void (in unsigned long index, [Clamp] in octet value); -};- -
The canvas
- attribute must return the canvas
element that the
- context paints on.
Except where otherwise specified, for the 2D context interface, - any method call with a numeric argument whose value is infinite or a - NaN value must be ignored.
- - - -Whenever the CSS value currentColor
is used
- as a color in this API, the "computed value of the 'color' property"
- for the purposes of determining the computed value of the currentColor
keyword is the computed value of the
- 'color' property on the element in question at the time that the
- color is specified (e.g. when the appropriate attribute is set, or
- when the method is called; not when the color is rendered or
- otherwise used). If the computed value of the 'color' property is
- undefined for a particular case (e.g. because the element is not
- in a Document
), then the "computed value
- of the 'color' property" for the purposes of determining the
- computed value of the currentColor
keyword is
- fully opaque black. [CSSCOLOR]
In the case of addColorStop()
on
- CanvasGradient
, the "computed value of the 'color'
- property" for the purposes of determining the computed value of the
- currentColor
keyword is always fully opaque
- black (there is no associated element). [CSSCOLOR]
This is because CanvasGradient
objects
- are canvas
-neutral — a
- CanvasGradient
object created by one
- canvas
can be used by another, and there is therefore
- no way to know which is the "element in question" at the time that
- the color is specified.
Each context maintains a stack of drawing states. Drawing states consist of:
- -strokeStyle
, fillStyle
, globalAlpha
, lineWidth
, lineCap
, lineJoin
, miterLimit
, shadowOffsetX
, shadowOffsetY
, shadowBlur
, shadowColor
, globalCompositeOperation
, font
, textAlign
, textBaseline
.The current path and the current bitmap are not part
- of the drawing state. The current path is persistent, and can only
- be reset using the beginPath()
method. The
- current bitmap is a property of the canvas, not the context.
save
()The save()
- method must push a copy of the current drawing state onto the
- drawing state stack.
The save()
method, when invoked, must push
+ a copy of the current drawing state onto the drawing state stack.
The restore()
method
- must pop the top entry in the drawing state stack, and reset the
- drawing state it describes. If there is no saved state, the method
- must do nothing.
The restore()
method, when invoked,
+ must pop the top entry in the drawing state stack, and reset the drawing state it describes. If
+ there is no saved state, then the method must do nothing.
When the user agent is to reset the rendering context to its default state, it must + clear the drawing state stack and everything that drawing state consists of to + initial values.
-The transformation matrix is applied to coordinates when creating - shapes and paths.
+When the context is created, the transformation matrix must - initially be the identity transform. It may then be adjusted using - the transformation methods.
- -The transformations must be performed in reverse order. For - instance, if a scale transformation that doubles the width is - applied, followed by a rotation transformation that rotates drawing - operations by a quarter turn, and a rectangle twice as wide as it is - tall is then drawn on the canvas, the actual result will be a - square.
- -scale
(x, y)Changes the transformation matrix to apply a scaling transformation with the given characteristics.
- -rotate
(angle)Changes the transformation matrix to apply a rotation transformation with the given characteristics. The angle is in radians.
- -translate
(x, y)Changes the transformation matrix to apply a translation transformation with the given characteristics.
- -transform
(a, b, c, d, e, f)Changes the transformation matrix to apply the matrix given by the arguments as described below.
- -setTransform
(a, b, c, d, e, f)Changes the transformation matrix to the matrix given by the arguments as described below.
- -The scale(x, y)
method must
- add the scaling transformation described by the arguments to the
- transformation matrix. The x argument represents
- the scale factor in the horizontal direction and the y argument represents the scale factor in the
- vertical direction. The factors are multiples.
The rotate(angle)
method must add the rotation
- transformation described by the argument to the transformation
- matrix. The angle argument represents a
- clockwise rotation angle expressed in radians.
The translate(x, y)
method must
- add the translation transformation described by the arguments to the
- transformation matrix. The x argument represents
- the translation distance in the horizontal direction and the y argument represents the translation distance in the
- vertical direction. The arguments are in coordinate space units.
The transform(a, b, c, d, e, f)
method must replace the current
- transformation matrix with the result of multiplying the current
- transformation matrix with the matrix described by:
a | -c | -e | -
b | -d | -f | -
0 | -0 | -1 | -
The arguments a, b, c, d, e, and f are sometimes called - m11, m12, m21, m22, dx, - and dy or m11, m21, m12, m22, dx, and dy. Care should be taken in particular with the order - of the second and third arguments (b and c) as their order varies from API to API and APIs - sometimes use the notation m12/m21 and sometimes m21/m12 for those positions.
- -The setTransform(a, b, c, d, e,
- f)
method must reset the current
- transform to the identity matrix, and then invoke the transform(a, b, c, d, e,
- f)
method with the same arguments.
globalAlpha
[ = value ]Returns the current alpha value applied to rendering operations.
- -Can be set, to change the alpha value. Values outside of the - range 0.0 .. 1.0 are ignored.
- -globalCompositeOperation
[ = value ]Returns the current composition operation, from the list below.
- -Can be set, to change the composition operation. Unknown values - are ignored.
- -All drawing operations are affected by the global compositing
- attributes, globalAlpha
and globalCompositeOperation
.
The globalAlpha
- attribute gives an alpha value that is applied to shapes and images
- before they are composited onto the canvas. The value must be in the
- range from 0.0 (fully transparent) to 1.0 (no additional
- transparency). If an attempt is made to set the attribute to a value
- outside this range, including Infinity and Not-a-Number (NaN)
- values, the attribute must retain its previous value. When the
- context is created, the globalAlpha
attribute must
- initially have the value 1.0.
The globalCompositeOperation
- attribute sets how shapes and images are drawn onto the existing
- bitmap, once they have had globalAlpha
and the
- current transformation matrix applied. It must be set to a value
- from the following list. In the descriptions below, the source
- image, A, is the shape or image being rendered,
- and the destination image, B, is the current
- state of the bitmap.
source-atop
source-in
source-out
source-over
(default)destination-atop
source-atop
but using the
- destination image instead of the source image and vice versa.destination-in
source-in
but using the destination
- image instead of the source image and vice versa.destination-out
source-out
but using the destination
- image instead of the source image and vice versa.destination-over
source-over
but using the
- destination image instead of the source image and vice versa.lighter
copy
xor
vendorName-operationName
The operators in the above list must be treated as described by - the Porter-Duff operator given at the start of their description - (e.g. A over B). They are to - be applied as part of the drawing model, at which point the - clipping region is also applied. (Without a clipping - region, these operators act on the whole bitmap with every - operation.) [PORTERDUFF]
- -These values are all case-sensitive — they must be used - exactly as shown. User agents must not recognize values that are not - a case-sensitive match for one of the values given - above.
- -On setting, if the user agent does not recognize the specified
- value, it must be ignored, leaving the value of globalCompositeOperation
- unaffected.
When the context is created, the globalCompositeOperation
- attribute must initially have the value
- source-over
.
strokeStyle
[ = value ]Returns the current style used for stroking shapes.
- -Can be set, to change the stroke style.
- -The style can be either a string containing a CSS color, or a
- CanvasGradient
or CanvasPattern
- object. Invalid values are ignored.
fillStyle
[ = value ]Returns the current style used for filling shapes.
- -Can be set, to change the fill style.
- -The style can be either a string containing a CSS color, or a
- CanvasGradient
or CanvasPattern
- object. Invalid values are ignored.
The strokeStyle
- attribute represents the color or style to use for the lines around
- shapes, and the fillStyle
- attribute represents the color or style to use inside the
- shapes.
Both attributes can be either strings,
- CanvasGradient
s, or CanvasPattern
s. On
- setting, strings must be parsed as CSS <color> values and the color
- assigned, and CanvasGradient
and
- CanvasPattern
objects must be assigned themselves. [CSSCOLOR] If the value is a string but
- cannot be parsed as a CSS <color> value, or is
- neither a string, a CanvasGradient
, nor a
- CanvasPattern
, then it must be ignored, and the
- attribute must retain its previous value.
When set to a CanvasPattern
or
- CanvasGradient
object, the assignment is
- live, meaning that changes made to the object after the
- assignment do affect subsequent stroking or filling of shapes.
On getting, if the value is a color, then the serialization of the color
- must be returned. Otherwise, if it is not a color but a
- CanvasGradient
or CanvasPattern
, then the
- respective object must be returned. (Such objects are opaque and
- therefore only useful for assigning to other attributes or for
- comparison to other gradients or patterns.)
The serialization of a color for a color value is a
- string, computed as follows: if it has alpha equal to 1.0, then the
- string is a lowercase six-digit hex value, prefixed with a "#"
- character (U+0023 NUMBER SIGN), with the first two digits
- representing the red component, the next two digits representing the
- green component, and the last two digits representing the blue
- component, the digits being in the range 0-9 a-f (U+0030 to U+0039
- and U+0061 to U+0066). Otherwise, the color value has alpha less
- than 1.0, and the string is the color value in the CSS rgba()
functional-notation format: the literal
- string rgba
(U+0072 U+0067 U+0062 U+0061)
- followed by a U+0028 LEFT PARENTHESIS, a base-ten integer in the
- range 0-255 representing the red component (using digits 0-9, U+0030
- to U+0039, in the shortest form possible), a literal U+002C COMMA
- and U+0020 SPACE, an integer for the green component, a comma and a
- space, an integer for the blue component, another comma and space, a
- U+0030 DIGIT ZERO, if the alpha value is greater than zero then a
- U+002E FULL STOP (representing the decimal point), if the alpha
- value is greater than zero then one or more digits in the range 0-9
- (U+0030 to U+0039) representing the fractional part of the alpha
- value, and finally a U+0029 RIGHT PARENTHESIS.
When the context is created, the strokeStyle
and fillStyle
attributes must
- initially have the string value #000000
.
There are two types of gradients, linear gradients and radial
- gradients, both represented by objects implementing the opaque
- CanvasGradient
interface.
Once a gradient has been created (see below), - stops are placed along it to define how the colors are distributed - along the gradient. The color of the gradient at - each stop is the color specified for that stop. Between each such - stop, the colors and the alpha component must be linearly - interpolated over the RGBA space without premultiplying the alpha - value to find the color to use at that offset. Before the first - stop, the color must be the color of the first stop. After the last - stop, the color must be the color of the last stop. When there are - no stops, the gradient is transparent black.
- -addColorStop
(offset, color)Adds a color stop with the given color to the gradient at the - given offset. 0.0 is the offset at one end of the gradient, 1.0 is - the offset at the other end.
- -Throws an INDEX_SIZE_ERR
exception if the offset
- is out of range. Throws a SYNTAX_ERR
exception if the
- color cannot be parsed.
createLinearGradient
(x0, y0, x1, y1)Returns a CanvasGradient
object that represents a
- linear gradient that paints along the line given by the
- coordinates represented by the arguments.
If any of the arguments are not finite numbers, throws a
- NOT_SUPPORTED_ERR
exception.
createRadialGradient
(x0, y0, r0, x1, y1, r1)Returns a CanvasGradient
object that represents a
- radial gradient that paints along the cone given by the circles
- represented by the arguments.
If any of the arguments are not finite numbers, throws a
- NOT_SUPPORTED_ERR
exception. If either of the radii
- are negative, throws an INDEX_SIZE_ERR
exception.
The addColorStop(offset, color)
- method on the CanvasGradient
interface adds a new stop
- to a gradient. If the offset is less than 0,
- greater than 1, infinite, or NaN, then an
- INDEX_SIZE_ERR
exception must be raised. If the color cannot be parsed as a CSS <color>
- value, then a SYNTAX_ERR
exception must be
- raised. Otherwise, the gradient must have a new stop placed, at
- offset offset relative to the whole gradient,
- and with the color obtained by parsing color as
- a CSS <color> value. If multiple stops are added at the same
- offset on a gradient, they must be placed in the order added, with
- the first one closest to the start of the gradient, and each
- subsequent one infinitesimally further along towards the end point
- (in effect causing all but the first and last stop added at each
- point to be ignored).
The createLinearGradient(x0, y0, x1,
- y1)
method takes four arguments
- that represent the start point (x0, y0) and end point (x1, y1) of the gradient. If any of the arguments to createLinearGradient()
- are infinite or NaN, the method must raise a
- NOT_SUPPORTED_ERR
exception. Otherwise, the method must
- return a linear CanvasGradient
initialized with the
- specified line.
Linear gradients must be rendered such that all points on a line - perpendicular to the line that crosses the start and end points have - the color at the point where those two lines cross (with the colors - coming from the interpolation and - extrapolation described above). The points in the linear - gradient must be transformed as described by the current transformation - matrix when rendering.
- -If x0 = x1 and y0 = y1, then - the linear gradient must paint nothing.
- -The createRadialGradient(x0, y0, r0,
- x1, y1, r1)
method takes six arguments, the
- first three representing the start circle with origin (x0, y0) and radius r0, and the last three representing the end circle
- with origin (x1, y1) and
- radius r1. The values are in coordinate space
- units. If any of the arguments are infinite or NaN, a
- NOT_SUPPORTED_ERR
exception must be raised. If either
- of r0 or r1 are negative, an
- INDEX_SIZE_ERR
exception must be raised. Otherwise,
- the method must return a radial CanvasGradient
- initialized with the two specified circles.
Radial gradients must be rendered by following these steps:
- -If x0 = x1 and y0 = y1 and r0 = r1, then the radial gradient must - paint nothing. Abort these steps.
Let x(ω) = (x1-x0)ω + x0
- -Let y(ω) = (y1-y0)ω + y0
- -Let r(ω) = (r1-r0)ω + r0
- -Let the color at ω be the color at - that position on the gradient (with the colors coming from the interpolation and extrapolation - described above).
- -For all values of ω where r(ω) > 0, - starting with the value of ω nearest to - positive infinity and ending with the value of ω nearest to negative infinity, draw the - circumference of the circle with radius r(ω) at position (x(ω), y(ω)), with the color at ω, but only painting on the parts of the - canvas that have not yet been painted on by earlier circles in this - step for this rendering of the gradient.
This effectively creates a cone, touched by the two - circles defined in the creation of the gradient, with the part of - the cone before the start circle (0.0) using the color of the first - offset, the part of the cone after the end circle (1.0) using the - color of the last offset, and areas outside the cone untouched by - the gradient (transparent black).
- -The points in the radial gradient must be transformed as - described by the current - transformation matrix when rendering.
- -Gradients must be painted only where the relevant stroking or - filling effects requires that they be drawn.
- - - -Patterns are represented by objects implementing the opaque
- CanvasPattern
interface.
createPattern
(image, repetition)Returns a CanvasPattern
object that uses the given image
- and repeats in the direction(s) given by the repetition argument.
The allowed values for repetition are repeat
(both directions), repeat-x
(horizontal only), repeat-y
(vertical only), and no-repeat
(neither). If the repetition argument is empty, the value repeat
is used.
If the image has no image data, throws an
- INVALID_STATE_ERR
exception. If the second argument
- isn't one of the allowed values, throws a SYNTAX_ERR
- exception. If the image isn't yet fully decoded, then the method
- returns null.
To create objects of this type, the createPattern(image, repetition)
- method is used. The first argument gives the image to use as the
- pattern (either an HTMLImageElement
,
- HTMLCanvasElement
, or HTMLVideoElement
- object). Modifying this image after calling the createPattern()
method
- must not affect the pattern. The second argument must be a string
- with one of the following values: repeat
,
- repeat-x
, repeat-y
,
- no-repeat
. If the empty string is specified,
- repeat
must be assumed. If an unrecognized value
- is given, then the user agent must raise a SYNTAX_ERR
- exception. User agents must recognize the four values described above
- exactly (e.g. they must not do case folding). Except as specified
- below, the method must return a CanvasPattern
object
- suitably initialized.
The image argument is an instance of either
- HTMLImageElement
, HTMLCanvasElement
, or
- HTMLVideoElement
.
If the image argument is an
- HTMLImageElement
object that is not fully decodable, or if the image argument is an HTMLVideoElement
- object whose readyState
- attribute is either HAVE_NOTHING
or HAVE_METADATA
, then the
- implementation must return null.
If the image argument is an
- HTMLCanvasElement
object with either a horizontal
- dimension or a vertical dimension equal to zero, then the
- implementation must raise an INVALID_STATE_ERR
- exception.
Patterns must be painted so that the top left of the first image
- is anchored at the origin of the coordinate space, and images are
- then repeated horizontally to the left and right (if the
- repeat-x
string was specified) or vertically up and
- down (if the repeat-y
string was specified) or in all
- four directions all over the canvas (if the repeat
- string was specified). The images are not scaled by this process;
- one CSS pixel of the image must be painted on one coordinate space
- unit. Of course, patterns must actually be painted only where the
- stroking or filling effect requires that they be drawn, and are
- affected by the current transformation matrix.
If the original image data is a bitmap image, the value painted - at a point in the area of the repetitions is computed by filtering - the original image data. The user agent may use any filtering - algorithm (for example bilinear interpolation or nearest-neighbor). - When the filtering algorithm requires a pixel value from outside the - original image data, it must instead use the value from wrapping the - pixel's coordinates to the original image's dimensions. (That is, - the filter uses 'repeat' behavior, regardless of the value of - repetition.) - - -
When the createPattern()
method
- is passed an animated image as its image
- argument, the user agent must use the poster frame of the animation,
- or, if there is no poster frame, the first frame of the
- animation.
When the image argument is an
- HTMLVideoElement
, then the frame at the current
- playback position must be used as the source image, and the
- source image's dimensions must be the intrinsic width and
- intrinsic height
- of the media resource (i.e. after any aspect-ratio
- correction has been applied).
lineWidth
[ = value ]lineWidth
[ = value ]lineWidth
[ = value ]Returns the current line width.
-Can be set, to change the line width. Values that are not - finite values greater than zero are ignored.
+Can be set, to change the line width. Values that are not finite values greater than zero are + ignored.
-lineCap
[ = value ]lineCap
[ = value ]lineCap
[ = value ]Returns the current line cap style.
Can be set, to change the line cap style.
-The possible line cap styles are butt
,
- round
, and square
. Other values are
- ignored.
The possible line cap styles are "butt
", "round
", and "square
". Other values are ignored.
lineJoin
[ = value ]lineJoin
[ = value ]lineJoin
[ = value ]Returns the current line join style.
Can be set, to change the line join style.
-The possible line join styles are bevel
,
- round
, and miter
. Other values are
- ignored.
The possible line join styles are "bevel
", "round
", and "miter
". Other values are ignored.
miterLimit
[ = value ]miterLimit
[ = value ]miterLimit
[ = value ]Returns the current miter limit ratio.
-Can be set, to change the miter limit ratio. Values that are - not finite values greater than zero are ignored.
+Can be set, to change the miter limit ratio. Values that are not finite values greater than + zero are ignored.
-setLineDash
(segments)setLineDash
(segments)Sets the current line dash pattern (as used when stroking). The argument is a list of + distances for which to alternately have the line on and the line off.
-The lineWidth
- attribute gives the width of lines, in coordinate space units. On
- getting, it must return the current value. On setting, zero,
- negative, infinite, and NaN values must be ignored, leaving the
- value unchanged; other values must change the current value to the
- new value.
getLineDash
()getLineDash
()When the context is created, the lineWidth
attribute must
- initially have the value 1.0
.
Returns a copy of the current line dash pattern. The array returned will always have an even + number of entries (i.e. the pattern is normalized).
-The lineCap
attribute
- defines the type of endings that UAs will place on the end of
- lines. The three valid values are butt
,
- round
, and square
. The butt
- value means that the end of each line has a flat edge perpendicular
- to the direction of the line (and that no additional line cap is
- added). The round
value means that a semi-circle with
- the diameter equal to the width of the line must then be added on to
- the end of the line. The square
value means that a
- rectangle with the length of the line width and the width of half
- the line width, placed flat against the edge perpendicular to the
- direction of the line, must be added at the end of each line.
lineDashOffset
lineDashOffset
On getting, it must return the current value. On setting, if the
- new value is one of the literal strings butt
,
- round
, and square
, then the current value
- must be changed to the new value; other values must ignored, leaving
- the value unchanged.
Returns the phase offset (in the same units as the line dash pattern).
-When the context is created, the lineCap
attribute must
- initially have the value butt
.
Can be set, to change the phase offset. Values that are not finite values are ignored.
-The lineJoin
- attribute defines the type of corners that UAs will place where two
- lines meet. The three valid values are bevel
,
- round
, and miter
.
On getting, it must return the current value. On setting, if the
- new value is one of the literal strings bevel
,
- round
, and miter
, then the current value
- must be changed to the new value; other values must be ignored,
- leaving the value unchanged.
When the context is created, the lineJoin
attribute must
- initially have the value miter
.
Objects that implement the CanvasPathDrawingStyles
interface have attributes and
+ methods (defined in this section) that control how lines are treated by the object.
A join exists at any point in a subpath shared by two consecutive - lines. When a subpath is closed, then a join also exists at its - first point (equivalent to its last point) connecting the first and - last lines in the subpath.
+The lineWidth
attribute gives the
+ width of lines, in coordinate space units. On getting, it must return the current value. On
+ setting, zero, negative, infinite, and NaN values must be ignored, leaving the value unchanged;
+ other values must change the current value to the new value.
In addition to the point where the join occurs, two additional - points are relevant to each join, one for each line: the two corners - found half the line width away from the join point, one - perpendicular to each line, each on the side furthest from the other - line.
+When the object implementing the CanvasPathDrawingStyles
interface is created, the
+ lineWidth
attribute must initially have the value
+ 1.0.
A filled triangle connecting these two opposite corners with a
- straight line, with the third point of the triangle being the join
- point, must be rendered at all joins. The lineJoin
attribute controls
- whether anything else is rendered. The three aforementioned values
- have the following meanings:
The bevel
value means that this is all that is
- rendered at joins.
The lineCap
attribute defines the type
+ of endings that UAs will place on the end of lines. The three valid values are "butt
", "round
", and "square
".
The round
value means that a filled arc connecting
- the two aforementioned corners of the join, abutting (and not
- overlapping) the aforementioned triangle, with the diameter equal to
- the line width and the origin at the point of the join, must be
- rendered at joins.
On getting, it must return the current value. On setting, the current value must be changed + to the new value.
-The miter
value means that a second filled triangle
- must (if it can given the miter length) be rendered at the join,
- with one line being the line between the two aforementioned corners,
- abutting the first triangle, and the other two being continuations of
- the outside edges of the two joining lines, as long as required to
- intersect without going over the miter length.
When the object implementing the CanvasPathDrawingStyles
interface is created, the
+ lineCap
attribute must initially have the value
+ "butt
".
The miter length is the distance from the point where the join - occurs to the intersection of the line edges on the outside of the - join. The miter limit ratio is the maximum allowed ratio of the - miter length to half the line width. If the miter length would cause - the miter limit ratio to be exceeded, this second triangle must not - be rendered.
+The miter limit ratio can be explicitly set using the miterLimit
- attribute. On getting, it must return the current value. On setting,
- zero, negative, infinite, and NaN values must be ignored, leaving
- the value unchanged; other values must change the current value to
- the new value.
The lineJoin
attribute defines the type
+ of corners that UAs will place where two lines meet. The three valid values are
+ "bevel
", "round
", and "miter
".
When the context is created, the miterLimit
attribute must
- initially have the value 10.0
.
On getting, it must return the current value. On setting, the current value must be changed + to the new value.
- +When the lineJoin
attribute has the value "miter
", strokes use the miter limit ratio to decide how to render joins. The
+ miter limit ratio can be explicitly set using the miterLimit
attribute. On getting, it must return
+ the current value. On setting, zero, negative, infinite, and NaN values must be ignored, leaving
+ the value unchanged; other values must change the current value to the new value.
When the object implementing the CanvasPathDrawingStyles
interface is created, the
+ miterLimit
attribute must initially have the value
+ 10.0.
Each CanvasPathDrawingStyles
object has a dash list, which is either
+ empty or consists of an even number of non-negative numbers. Initially, the dash list
+ must be empty.
The setLineDash()
method, when
+ invoked, must run these steps:
Let a be the argument.
If any value in a is not finite (e.g. an Infinity or a NaN value), or + if any value is negative (less than zero), then abort these steps (without throwing an exception; + user agents could show a message on a developer console, though, as that would be helpful for + debugging).
If the number of elements in a is odd, then let a + be the concatenation of two copies of a.
Let the object's dash list be a.
When the getLineDash()
method is
+ invoked, it must return a sequence whose values are the values of the object's dash
+ list, in the same order.
It is sometimes useful to change the "phase" of the dash pattern, e.g. to achieve a "marching
+ ants" effect. The phase can be set using the lineDashOffset
attribute. On getting, it must
+ return the current value. On setting, infinite and NaN values must be ignored, leaving the value
+ unchanged; other values must change the current value to the new value.
When the object implementing the CanvasPathDrawingStyles
interface is created, the
+ lineDashOffset
attribute must initially have
+ the value 0.0.
When a user agent is to trace a path, given an object style
+ that implements the CanvasPathDrawingStyles
interface, it must run the following
+ algorithm. This algorithm returns a new path.
Let path be a copy of the path being traced.
Prune all zero-length line segments from path.
Remove from path any subpaths containing no lines (i.e. subpaths with + just one point).
Replace each point in each subpath of path other than the first point + and the last point of each subpath by a join that joins the line leading to that point to + the line leading out of that point, such that the subpaths all consist of two points (a starting + point with a line leading out of it, and an ending point with a line leading into it), one or + more lines (connecting the points and the joins), and zero or more joins (each connecting one + line to another), connected together such that each subpath is a series of one or more lines with + a join between each one and a point on each end.
Add a straight closing line to each closed subpath in path connecting + the last point and the first point of that subpath; change the last point to a join (from the + previously last line to the newly added closing line), and change the first point to a join (from + the newly added closing line to the first line).
+ +If the styles dash list is empty, then jump to the step + labeled convert.
Let pattern width be the concatenation of all the entries of the styles dash list, in coordinate space units.
+ +For each subpath subpath in path, run the + following substeps. These substeps mutate the subpaths in path in + vivo.
+ +Let subpath width be the length of all the lines of subpath, in coordinate space units.
+ +Let offset be the value of the styles lineDashOffset
, in coordinate space
+ units.
While offset is greater than pattern width, + decrement it by pattern width.
+ +While offset is less than zero, increment it by pattern + width.
+ +Define L to be a linear coordinate line defined along all lines in + subpath, such that the start of the first line in the subpath is defined + as coordinate 0, and the end of the last line in the subpath is defined as coordinate subpath width.
Let position be zero minus offset.
Let index be 0.
Let current state be off (the other states being on + and zero-on).
Dash on: Let segment length be + the value of the styles dash + list's indexth entry.
Increment position by segment length.
If position is greater than subpath width, + then end these substeps for this subpath and start them again for the next subpath; if there + are no more subpaths, then jump to the step labeled convert instead.
If segment length is non-zero, then let current state be + on.
Increment index by one.
Dash off: Let segment + length be the value of the styles dash list's indexth entry.
Let start be the offset position on L.
Increment position by segment length.
If position is less than zero, then jump to the step labeled + post-cut.
If start is less than zero, then let start be + zero.
If position is greater than subpath width, + then let end be the offset subpath width on L. Otherwise, let end be the offset position on L.
Jump to the first appropriate step:
+ +Do nothing, just continue to the next step.
+ +Cut the line on which end finds itself short at end and place a point there, cutting the subpath that it was in in two; + remove all line segments, joins, points, and subpaths that are between start and end; and finally place a single point at start with no lines connecting to it.
+ +The point has a directionality for the purposes of drawing line caps (see below). + The directionality is the direction that the original line had at that point (i.e. when L was defined above).
+ +Cut the line on which start finds itself into two at start and place a point there, cutting the subpath that it was in in two, and + similarly cut the line on which end finds itself short at end and place a point there, cutting the subpath that it was in in + two, and then remove all line segments, joins, points, and subpaths that are between start and end.
+ +If start and end are the same point, then this + results in just the line being cut in two and two points being inserted there, with nothing + being removed, unless a join also happens to be at that point, in which case the join must + be removed.
+ +Post-cut: If position is greater than subpath width, then jump to the step labeled convert.
If segment length is greater than zero, then let + positioned-at-on-dash be false.
Increment index by one. If it is equal to the number of entries in + the styles dash list, then let index be + 0.
Return to the step labeled dash on.
Convert: This is the step that converts the path to a new path that represents its + stroke.
+ +Create a new path that describes the edge of the areas
+ that would be covered if a straight line of length equal to the styles
+ lineWidth
was swept along each subpath in path while being kept at an angle such that the line is orthogonal to the path
+ being swept, replacing each point with the end cap necessary to satisfy the styles lineCap
attribute as
+ described previously and elaborated below, and replacing each join with the join necessary to
+ satisfy the styles lineJoin
+ type, as defined below.
Caps: Each point has a flat edge perpendicular to the direction of the line
+ coming out of it. This is then augmented according to the value of the styles lineCap
. The "butt
" value means that no additional line cap is added. The "round
" value means that a semi-circle with the diameter equal to the styles lineWidth
width must
+ additionally be placed on to the line coming out of each point. The "square
" value means that a rectangle with the length of the styles lineWidth
width and the
+ width of half the styles lineWidth
width, placed flat against the edge
+ perpendicular to the direction of the line coming out of the point, must be added at each
+ point.
Points with no lines coming out of them must have two caps placed back-to-back as if it was + really two points connected to each other by an infinitesimally short straight line in the + direction of the point's directionality (as defined above).
+ +Joins: In addition to the point where a join occurs, two additional points + are relevant to each join, one for each line: the two corners found half the line width away + from the join point, one perpendicular to each line, each on the side furthest from the other + line.
+ +A triangle connecting these two opposite corners with a straight line, with the third point
+ of the triangle being the join point, must be added at all joins. The lineJoin
attribute controls whether anything else is
+ rendered. The three aforementioned values have the following meanings:
The "bevel
" value means that this is all that is rendered at
+ joins.
The "round
" value means that an arc connecting the two aforementioned
+ corners of the join, abutting (and not overlapping) the aforementioned triangle, with the
+ diameter equal to the line width and the origin at the point of the join, must be added at
+ joins.
The "miter
" value means that a second triangle must (if it can given
+ the miter length) be added at the join, with one line being the line between the two
+ aforementioned corners, abutting the first triangle, and the other two being continuations of
+ the outside edges of the two joining lines, as long as required to intersect without going over
+ the miter length.
The miter length is the distance from the point where the join occurs to the intersection of
+ the line edges on the outside of the join. The miter limit ratio is the maximum allowed ratio of
+ the miter length to half the line width. If the miter length would cause the miter limit ratio
+ (as set by the style miterLimit
attribute) to be exceeded, then this second
+ triangle must not be added.
The subpaths in the newly created path must be oriented such that for any point, the number + of times a half-infinite straight line drawn from that point crosses a subpath is even if and + only if the number of times a half-infinite straight line drawn from that same point crosses a + subpath going in one direction is equal to the number of times it crosses a subpath going in the + other direction.
+ +Return the newly created path.
font
[ = value ]font
[ = value ]Returns the current font settings.
+ +Can be set, to change the font. The syntax is the same as for the CSS 'font' + property; values that cannot be parsed as CSS font values are ignored.
+ +Relative keywords and lengths are computed relative to the font of the canvas
+ element.
textAlign
[ = value ]textAlign
[ = value ]Returns the current text alignment settings.
+ +Can be set, to change the alignment. The possible values are and their meanings are given
+ below. Other values are ignored. The default is "start
".
textBaseline
[ = value ]textBaseline
[ = value ]Returns the current baseline alignment settings.
+ +Can be set, to change the baseline alignment. The possible values and their meanings are
+ given below. Other values are ignored. The default is "alphabetic
".
direction
[ = value ]direction
[ = value ]Returns the current directionality.
+ +Can be set, to change the directionality. The possible values and their meanings are given
+ below. Other values are ignored. The default is "inherit
".
Objects that implement the CanvasTextDrawingStyles
interface have attributes
+ (defined in this section) that control how text is laid out (rasterized or outlined) by the
+ object. Such objects can also have a font style source object. For
+ CanvasRenderingContext2D
objects, this is the canvas
element
+ referenced by the context's canvas property.
The font
IDL attribute, on setting, must
+ be parsed as a CSS <'font'>
+ value (but without supporting property-independent style sheet syntax like 'inherit'), and
+ the resulting font must be assigned to the context, with the 'line-height' component
+ forced to 'normal', with the 'font-size' component converted to CSS pixels, and with system fonts being computed to explicit values. If the
+ new value is syntactically incorrect (including using property-independent style sheet syntax like
+ 'inherit' or 'initial'), then it must be ignored, without assigning a new font value. [CSS]
Font family names must be interpreted in the context of the font style source
+ object when the font is to be used; any fonts embedded using @font-face
or loaded using FontFace
objects that are visible to the
+ font style source object must therefore be available once they are loaded. (Each font style source
+ object has a font source, which determines what fonts are available.) If a font
+ is used before it is fully loaded, or if the font style source object does not have
+ that font in scope at the time the font is to be used, then it must be treated as if it was an
+ unknown font, falling back to another as described by the relevant CSS specifications. [CSSFONTS] [CSSFONTLOAD]
On getting, the font
attribute must return the serialized form of the current font of the context (with
+ no 'line-height' component). [CSSOM]
For example, after the following statement:
+ +context.font = 'italic 400 12px/2 Unknown Font, sans-serif';+ +
...the expression context.font
would evaluate to the string "italic 12px "Unknown Font", sans-serif
". The "400"
+ font-weight doesn't appear because that is the default value. The line-height doesn't appear
+ because it is forced to "normal", the default value.
When the object implementing the CanvasTextDrawingStyles
interface is created, the
+ font of the context must be set to 10px sans-serif. When the 'font-size' component is
+ set to lengths using percentages, 'em' or 'ex' units, or the 'larger' or
+ 'smaller' keywords, these must be interpreted relative to the computed value of the
+ 'font-size' property of the font style source object at the time that
+ the attribute is set, if it is an element. When the 'font-weight' component is set to
+ the relative values 'bolder' and 'lighter', these must be interpreted relative to the
+ computed value of the 'font-weight' property of the font style
+ source object at the time that the attribute is set, if it is an element. If the computed values are undefined for a particular case (e.g. because
+ the font style source object is not an element or is not being
+ rendered), then the relative keywords must be interpreted relative to the normal-weight
+ 10px sans-serif default.
The textAlign
IDL attribute, on
+ getting, must return the current value. On setting, the current value must be changed to the new
+ value. When the object implementing the CanvasTextDrawingStyles
interface is
+ created, the textAlign
attribute must initially
+ have the value start
.
The textBaseline
IDL attribute, on
+ getting, must return the current value. On setting, the current value must be changed to the new
+ value. When the object implementing the CanvasTextDrawingStyles
interface is
+ created, the textBaseline
attribute must
+ initially have the value alphabetic
.
The direction
IDL attribute, on
+ getting, must return the current value. On setting, the current value must be changed to the new
+ value. When the object implementing the CanvasTextDrawingStyles
interface is
+ created, the direction
attribute must initially
+ have the value "inherit
".
The textAlign
attribute's allowed keywords are
+ as follows:
start
+ Align to the start edge of the text (left side in left-to-right text, right side in + right-to-left text).
end
+ Align to the end edge of the text (right side in left-to-right text, left side in + right-to-left text).
left
+ Align to the left.
right
+ Align to the right.
center
+ Align to the center.
The textBaseline
+ attribute's allowed keywords correspond to alignment points in the
+ font:
The keywords map to these alignment points as follows:
+ +top
+ hanging
+ middle
+ alphabetic
+ ideographic
+ bottom
+ The direction
attribute's allowed keywords are
+ as follows:
ltr
+
+ Treat input to the text preparation algorithm as left-to-right text.
rtl
+
+ Treat input to the text preparation algorithm as right-to-left text.
inherit
+
+ Default to the directionality of the canvas
element or Document
+ as appropriate.
The text preparation algorithm is as follows. It takes as input a string text
+ , a CanvasTextDrawingStyles
object target, and an optional length
+ maxWidth. It returns an array of glyph shapes, each positioned on a common coordinate
+ space, a physical alignment whose value is one of left, right, and
+ center, and an inline box. (Most callers of this algorithm ignore the
+ physical alignment and the inline box.)
If maxWidth was provided but is less than or equal to zero or equal to NaN, + then return an empty array.
Replace all ASCII whitespace in text with U+0020 SPACE + characters.
Let font be the current font of target, as given
+ by that object's font
attribute.
Apply the appropriate step from the following list to determine the value of direction:
+ +direction
attribute has the value "ltr
"direction
attribute has the value "rtl
"Document
with a non-null document elementForm a hypothetical infinitely-wide CSS line box containing a single + inline box containing the text text, with all the properties at their + initial values except the 'font' property of the inline box set to + font, the 'direction' property of the inline box set to + direction, and the 'white-space' property set to 'pre'. [CSS]
If maxWidth was provided and the hypothetical width of the + inline box in the hypothetical line box is greater than + maxWidth CSS pixels, then change font to have a + more condensed font (if one is available or if a reasonably readable one can be synthesized by + applying a horizontal scale factor to the font) or a smaller font, and return to the previous + step.
The anchor point is a point on the inline box, and the physical
+ alignment is one of the values left, right, and center. These
+ variables are determined by the textAlign
and
+ textBaseline
values as follows:
Horizontal position:
+ +textAlign
is left
textAlign
is start
and direction is
+ 'ltr'textAlign
is end
and direction is 'rtl'textAlign
is right
textAlign
is end
and direction is 'ltr'textAlign
is start
and direction is
+ 'rtl'textAlign
is center
Vertical position:
+ +textBaseline
is top
textBaseline
is hanging
textBaseline
is middle
textBaseline
is alphabetic
textBaseline
is ideographic
textBaseline
is bottom
Let result be an array constructed by iterating over each glyph in the + inline box from left to right (if any), adding to the array, for each glyph, the + shape of the glyph as it is in the inline box, positioned on a coordinate space + using CSS pixels with its origin is at the anchor + point.
+ +Return result, physical alignment, and the inline + box.
All drawing operations are affected by the four global shadow - attributes.
+Objects that implement the CanvasPath
interface have a path. A path has a list of zero or
+ more subpaths. Each subpath consists of a list of one or more points, connected by straight or
+ curved line segments, and a flag indicating whether the subpath is closed or not. A
+ closed subpath is one where the last point of the subpath is connected to the first point of the
+ subpath by a straight line. Subpaths with only one point are ignored when painting the path.
shadowColor
[ = value ]Returns the current shadow color.
- -Can be set, to change the shadow color. Values that cannot be parsed as CSS colors are ignored.
- -shadowOffsetX
[ = value ]shadowOffsetY
[ = value ]Returns the current shadow offset.
- -Can be set, to change the shadow offset. Values that are not finite numbers are ignored.
- -shadowBlur
[ = value ]Returns the current level of blur applied to shadows.
- -Can be set, to change the blur level. Values that are not finite numbers greater than or equal to zero are ignored.
- -The shadowColor
- attribute sets the color of the shadow.
When the context is created, the shadowColor
attribute
- initially must be fully-transparent black.
On getting, the serialization of the color must be returned.
- -On setting, the new value must be parsed as a CSS - <color> value and the color assigned. If the value - cannot be parsed as a CSS <color> value then it must be - ignored, and the attribute must retain its previous value. [CSSCOLOR]
- -The shadowOffsetX
- and shadowOffsetY
- attributes specify the distance that the shadow will be offset in
- the positive horizontal and positive vertical distance
- respectively. Their values are in coordinate space units. They are
- not affected by the current transformation matrix.
When the context is created, the shadow offset attributes must
- initially have the value 0
.
On getting, they must return their current value. On setting, the - attribute being set must be set to the new value, except if the - value is infinite or NaN, in which case the new value must be - ignored.
- -The shadowBlur
- attribute specifies the level of the blurring effect. (The units do
- not map to coordinate space units, and are not affected by the
- current transformation matrix.)
When the context is created, the shadowBlur
attribute must
- initially have the value 0
.
On getting, the attribute must return its current value. On - setting the attribute must be set to the new value, except if the - value is negative, infinite or NaN, in which case the new value must - be ignored.
- -Shadows are only drawn
- if the opacity component of the alpha component of the color
- of shadowColor
is
- non-zero and either the shadowBlur
is non-zero, or
- the shadowOffsetX
- is non-zero, or the shadowOffsetY
is
- non-zero.
It is likely that this will change: browser - vendors have indicated an interest in changing the processing model - for shadows such that they only draw when the composition operator - is "source-over" (the default). Read - more...
- -When shadows are drawn, they must be rendered as follows:
- -Let A be an infinite transparent black - bitmap on which the source image for which a shadow is being - created has been rendered.
Let B be an infinite transparent black - bitmap, with a coordinate space and an origin identical to A.
Copy the alpha channel of A to B, offset by shadowOffsetX
in the
- positive x direction, and shadowOffsetY
in the
- positive y direction.
If shadowBlur
is greater than
- 0:
Let σ be half the value of
- shadowBlur
.
Perform a 2D Gaussian Blur on B, - using σ as the standard deviation.
-User agents may limit values of σ to - an implementation-specific maximum value to avoid exceeding - hardware limitations during the Gaussian blur operation.
- -Set the red, green, and blue components of every pixel in
- B to the red, green, and blue components
- (respectively) of the color of shadowColor
.
Multiply the alpha component of every pixel in B by the alpha component of the color of shadowColor
.
The shadow is in the bitmap B, and is - rendered as part of the drawing model described below.
If the current composition operation is copy
, shadows effectively won't render
- (since the shape will overwrite the shadow).
There are three methods that immediately draw rectangles to the - bitmap. They each take four arguments; the first two give the x and y coordinates of the top - left of the rectangle, and the second two give the width w and height h of the rectangle, - respectively.
- -The current - transformation matrix must be applied to the following four - coordinates, which form the path that must then be closed to get the - specified rectangle: (x, y), (x+w, y), - (x+w, - y+h), - (x, y+h).
- -Shapes are painted without affecting the current path, and are
- subject to the clipping region,
- and, with the exception of clearRect()
, also shadow effects, global alpha, and global composition
- operators.
clearRect
(x, y, w, h)Clears all pixels on the canvas in the given rectangle to transparent black.
- -fillRect
(x, y, w, h)Paints the given rectangle onto the canvas, using the current fill style.
- -strokeRect
(x, y, w, h)Paints the box that outlines the given rectangle onto the canvas, using the current stroke style.
- -The clearRect(x, y, w, h)
method must clear the pixels in the
- specified rectangle that also intersect the current clipping region
- to a fully transparent black, erasing any previous image. If either
- height or width are zero, this method has no effect.
The fillRect(x, y, w, h)
method must paint the specified
- rectangular area using the fillStyle
. If either height
- or width are zero, this method has no effect.
The strokeRect(x, y, w, h)
method must stroke the specified
- rectangle's path using the strokeStyle
, lineWidth
, lineJoin
, and (if
- appropriate) miterLimit
attributes. If
- both height and width are zero, this method has no effect, since
- there is no path to stroke (it's a point). If only one of the two is
- zero, then the method will draw a line instead (the path for the
- outline is just a straight line along the non-zero dimension).
The context always has a current path. There is only one current - path, it is not part of the drawing state.
- -A path has a list of zero or more subpaths. Each - subpath consists of a list of one or more points, connected by - straight or curved lines, and a flag indicating whether the subpath - is closed or not. A closed subpath is one where the last point of - the subpath is connected to the first point of the subpath by a - straight line. Subpaths with fewer than two points are ignored when - painting the path.
- -beginPath
()Resets the current path.
- -moveTo
(x, y)Paths have a need new subpath flag. When this + flag is set, certain APIs create a new subpath rather than extending the previous one. When a + path is created, its need new subpath flag must be + set.
+ +When an object implementing the CanvasPath
interface is created, its path must be initialized to zero subpaths.
closePath
()closePath
()closePath
()Marks the current subpath as closed, and starts a new subpath with a point the same as the + start and end of the newly closed subpath.
-lineTo
(x, y)lineTo
(x, y)Marks the current subpath as closed, and starts a new subpath with a point the same as the start and end of the newly closed subpath.
- -lineTo
(x, y)Adds the given point to the current subpath, connected to the previous one by a straight line.
- -quadraticCurveTo
(cpx, cpy, x, y)Adds the given point to the current subpath, connected to the previous one by a quadratic Bézier curve with the given control point.
- -bezierCurveTo
(cp1x, cp1y, cp2x, cp2y, x, y)Adds the given point to the current subpath, connected to the previous one by a cubic Bézier curve with the given control points.
- -arcTo
(x1, y1, x2, y2, radius)Adds an arc with the given control points and radius to the - current subpath, connected to the previous point by a straight +
Adds the given point to the current subpath, connected to the previous one by a straight line.
-Throws an INDEX_SIZE_ERR
exception if the given
+
quadraticCurveTo
(cpx, cpy, x, y)quadraticCurveTo
(cpx, cpy, x, y)Adds the given point to the current subpath, connected to the previous one by a quadratic + Bézier curve with the given control point.
+ +bezierCurveTo
(cp1x, cp1y, cp2x, cp2y, x, y)bezierCurveTo
(cp1x, cp1y, cp2x, cp2y, x, y)Adds the given point to the current subpath, connected to the previous one by a cubic + Bézier curve with the given control points.
+ +arcTo
(x1, y1, x2, y2, radiusX [, radiusY, rotation ] )arcTo
(x1, y1, x2, y2, radiusX [, radiusY, rotation ] )Adds an arc with the given control points and radius to the current subpath, connected to the + previous point by a straight line.
+ +If two radii are provided, then the first controls the width of the arc's ellipse, and the + second controls the height. If only one is provided, or if they are the same, then the arc is + from a circle. In the case of an ellipse, the rotation argument controls the clockwise + inclination of the ellipse relative to the x-axis.
+ +Throws an "IndexSizeError
" DOMException
if the given
radius is negative.
arc
(x, y, radius, startAngle, endAngle [, anticlockwise ] )Adds points to the subpath such that the arc described by the - circumference of the circle described by the arguments, starting - at the given start angle and ending at the given end angle, going - in the given direction (defaulting to clockwise), is added to the - path, connected to the previous point by a straight line.
+arc
(x, y, radius, startAngle, endAngle [, anticlockwise ] )arc
(x, y, radius, startAngle, endAngle [, anticlockwise ] )Throws an INDEX_SIZE_ERR
exception if the given
+
Adds points to the subpath such that the arc described by the circumference of the circle + described by the arguments, starting at the given start angle and ending at the given end angle, + going in the given direction (defaulting to clockwise), is added to the path, connected to the + previous point by a straight line.
+ +Throws an "IndexSizeError
" DOMException
if the given
radius is negative.
rect
(x, y, w, h)ellipse
(x, y, radiusX, radiusY, rotation, startAngle, endAngle [, anticlockwise] )ellipse
(x, y, radiusX, radiusY, rotation, startAngle, endAngle [, anticlockwise] )Adds points to the subpath such that the arc described by the circumference of the ellipse + described by the arguments, starting at the given start angle and ending at the given end angle, + going in the given direction (defaulting to clockwise), is added to the path, connected to the + previous point by a straight line.
+ +Throws an "IndexSizeError
" DOMException
if the given
+ radius is negative.
rect
(x, y, w, h)rect
(x, y, w, h)Adds a new closed subpath to the path, representing the given rectangle.
-The following methods allow authors to manipulate the paths
+ of objects implementing the CanvasPath
interface.
For objects implementing the CanvasDrawPath
and CanvasTransform
+ interfaces, the points passed to the methods, and the resulting lines added to current
+ default path by these methods, must be transformed according to the current transformation matrix before being added to
+ the path.
The moveTo(x, y)
+ method, when invoked, must run these steps:
If either of the arguments are infinite or NaN, then abort these steps.
Create a new subpath with the specified point as its first (and only) point.
When the user agent is to ensure there is a subpath for a coordinate (x,
+ y) on a path, the user agent must check to see if
+ the path has its need new subpath flag set. If it
+ does, then the user agent must create a new subpath with the point (x, y) as
+ its first (and only) point, as if the moveTo()
method
+ had been called, and must then unset the path's need new
+ subpath flag.
The closePath()
method, when invoked,
+ must do nothing if the object's path has no subpaths. Otherwise, it must mark the last subpath as
+ closed, create a new subpath whose first point is the same as the previous subpath's first point,
+ and finally add this new subpath to the path.
If the last subpath had more than one point in its list of points, then this is
+ equivalent to adding a straight line connecting the last point back to the first point, thus
+ "closing" the shape, and then repeating the last (possibly implied) moveTo()
call.
New points and the lines connecting them are added to subpaths using the methods described + below. In all cases, the methods only modify the last subpath in the object's path.
+ + +The lineTo(x, y)
+ method, when invoked, must run these steps:
If either of the arguments are infinite or NaN, then abort these steps.
If the object's path has no subpaths, then ensure there is a subpath for + (x, y).
Otherwise, connect the last point in the subpath to the given point (x, + y) using a straight line, and then add the given point (x, y) to + the subpath.
The quadraticCurveTo(cpx,
+ cpy, x, y)
method, when invoked, must run these
+ steps:
If any of the arguments are infinite or NaN, then abort these steps.
Ensure there is a subpath for (cpx, + cpy)
Connect the last point in the subpath to the given point (x, y) + using a quadratic Bézier curve with control point (cpx, cpy). [BEZIER]
Add the given point (x, y) to the subpath.
fill
()The bezierCurveTo(cp1x,
+ cp1y, cp2x, cp2y, x, y)
+ method, when invoked, must run these steps:
If any of the arguments are infinite or NaN, then abort these steps.
Ensure there is a subpath for (cp1x, + cp1y).
Connect the last point in the subpath to the given point (x, y) + using a cubic Bézier curve with control points (cp1x, cp1y) and + (cp2x, cp2y). [BEZIER]
Add the point (x, y) to the subpath.
Fills the subpaths with the current fill style.
+The arcTo(x1, y1, x2,
+ y2, radiusX, radiusY, rotation)
method,
+ when invoked, must run these steps:
stroke
()If any of the arguments are infinite or NaN, then abort these steps.
Ensure there is a subpath for (x1, + y1).
If either radiusX or radiusY are negative, then throw an
+ "IndexSizeError
" DOMException
.
If radiusY is omitted, then set radiusY to + radiusX.
Let the point (x0, y0) be the last point in the subpath, transformed + by the inverse of the current transformation + matrix (so that it is in the same coordinate system as the points passed to the + method).
If the point (x0, y0) is equal to the point (x1, + y1), or if the point (x1, y1) is equal to the point + (x2, y2), or if both radiusX and radiusY are zero, + then add the point (x1, y1) to the subpath, and connect that point to the + previous point (x0, y0) by a straight line.
Otherwise, if the points (x0, y0), (x1, y1), + and (x2, y2) all lie on a single straight line, then add the point + (x1, y1) to the subpath, and connect that point to the previous point + (x0, y0) by a straight line.
Otherwise, let The Arc be the shortest arc given by circumference of the + ellipse that has radius radiusX on the major axis and radius radiusY on the + minor axis, and whose semi-major axis is rotated rotation radians clockwise from the + positive x-axis, and that has one point tangent to the half-infinite line that crosses the point + (x0, y0) and ends at the point (x1, y1), and that has + a different point tangent to the half-infinite line that ends at the point (x1, + y1) and crosses the point (x2, y2). The points at which this + ellipse touches these two lines are called the start and end tangent points respectively. Connect + the point (x0, y0) to the start tangent point by a straight line, adding + the start tangent point to the subpath, and then connect the start tangent point to the end + tangent point by The Arc, adding the end tangent point to the subpath.
Strokes the subpaths with the current stroke style.
+The arc(x, y,
+ radius, startAngle, endAngle, anticlockwise)
and ellipse(x, y, radiusX, radiusY, rotation, startAngle, endAngle, anticlockwise)
methods draw arcs.
The arc()
method is equivalent to the ellipse()
method in the case where the two radii are equal.
+ When the arc()
method is invoked, it must act as if the
+ ellipse()
method had been invoked with the radiusX and radiusY arguments set to the value of the radius argument, the rotation argument set to zero, and the
+ other arguments set to the same values as their identically named arguments on the arc()
method.
drawSystemFocusRing
(element)The ellipse()
method, when invoked, must run these
+ steps:
If any of the arguments are infinite or NaN, then abort these steps.
If either radiusX or radiusY are negative, then throw an
+ "IndexSizeError
" DOMException
.
If the object's path has any subpaths, then add a straight line from the last point in the + subpath to the start point of the arc.
Add the start and end points of the arc to the subpath, and connect them with an arc. The arc + and its start and end points are defined as follows:
-If the given element is focused, draws a focus ring around the - current path, following the platform conventions for focus - rings.
+Consider an ellipse that has its origin at (x, y), that has a + major-axis radius radiusX and a minor-axis radius radiusY, and that is + rotated about its origin such that its semi-major axis is inclined rotation radians + clockwise from the x-axis.
-If anticlockwise is false and endAngle-startAngle is equal to or greater than 2π, or, if anticlockwise is true and startAngle-endAngle is equal to or greater than 2π, then the arc is the whole circumference of this ellipse, and the point + at startAngle along this circle's circumference, measured in radians clockwise from + the ellipse's semi-major axis, acts as both the start point and the end point.
-drawCustomFocusRing
(element)Otherwise, the points at startAngle and endAngle along this circle's + circumference, measured in radians clockwise from the ellipse's semi-major axis, are the start + and end points respectively, and the arc is the path along the circumference of this ellipse + from the start point to the end point, going anti-clockwise if anticlockwise is true, + and clockwise otherwise. Since the points are on the ellipse, as opposed to being simply angles + from zero, the arc can never cover an angle greater than 2π + radians.
-Even if the arc covers the entire circumference of the ellipse and there are no
+ other points in the subpath, the path is not closed unless the closePath()
method is appropriately invoked.
If the given element is focused, and the user has configured - his system to draw focus rings in a particular manner (for - example, high contrast focus rings), draws a focus ring around the - current path and returns false.
+Otherwise, returns true if the given element is focused, and - false otherwise. This can thus be used to determine when to draw a - focus ring (see the - example below).
+The rect(x, y, w,
+ h)
method, when invoked, must run these steps:
If any of the arguments are infinite or NaN, then abort these steps.
Create a new subpath containing just the four points (x, y), + (x+w, y), (x+w, + y+h), (x, y+h), in that order, with those + four points connected by straight lines.
Mark the subpath as closed.
Create a new subpath with the point (x, y) as the only point in the + subpath.
scrollPathIntoView
()Scrolls the current path into view. This is especially useful - on devices with small screens, where the whole canvas might not be - visible at once.
-clip
()Path2D
objectsPath2D
objects can be used to declare paths that are then later used on
+ objects implementing the CanvasDrawPath
interface. In addition to many of the APIs
+ described in earlier sections, Path2D
objects have methods to combine paths, and to
+ add text to paths.
Further constrains the clipping region to the given path.
+Path2D
()Creates a new empty Path2D
object.
isPointInPath
(x, y)Path2D
(path)Creates a new Path2D
object that is a copy of the argument.
Returns true if the given point is in the current path.
+Path2D
(paths [, fillRule ] )Creates a new Path2D
object that describes a path that outlines the given paths, using the given fill rule.
Path2D
(d)Initially, the context's path must have zero subpaths.
+Creates a new path with the path described by the argument, interpreted as SVG path data. [SVG]
-The points and lines added to the path by these methods must be - transformed according to the current transformation - matrix as they are added.
+addPath
(path [, transform ] )Adds to the path the path given by the argument.
-The beginPath()
- method must empty the list of subpaths so that the context once
- again has zero subpaths.
The moveTo(x, y)
method must
- create a new subpath with the specified point as its first (and
- only) point.
The Path2D()
constructor, when invoked, must return a
+ newly created Path2D
object.
When the user agent is to ensure there is a subpath
- for a coordinate (x, y), the
- user agent must check to see if the context has any subpaths, and if
- it does not, then the user agent must create a new subpath with the
- point (x, y) as its first
- (and only) point, as if the moveTo()
method had been
- called.
The Path2D(path)
+ constructor, when invoked, must return a newly created Path2D
object, to which the
+ subpaths of the argument are added. (In other words, it returns a copy of the argument.)
The closePath()
- method must do nothing if the context has no subpaths. Otherwise, it
- must mark the last subpath as closed, create a new subpath whose
- first point is the same as the previous subpath's first point, and
- finally add this new subpath to the path.
If the last subpath had more than one point in its
- list of points, then this is equivalent to adding a straight line
- connecting the last point back to the first point, thus "closing"
- the shape, and then repeating the last (possibly implied) moveTo()
call.
The Path2D(paths,
+ fillRule)
constructor, when invoked, must run these steps:
New points and the lines connecting them are added to subpaths - using the methods described below. In all cases, the methods only - modify the last subpath in the context's paths.
+Run the appropriate step from the following list, based on the constructor's second + argument:
+The lineTo(x, y)
method must
- ensure there is a subpath for (x, y) if the context has
- no subpaths. Otherwise, it must connect the last point in the
- subpath to the given point (x, y) using a straight line, and must then add the given
- point (x, y) to the
- subpath.
Let merged path be a path that + consists of a set of non-overlapping subpaths that exactly outline the points from which, in + any of the paths provided in the constructor's first + argument, the number of times a half-infinite straight line drawn from that point crosses a + subpath going in one direction is not equal to the number of times it crosses a subpath going + in the other direction.
+The quadraticCurveTo(cpx, cpy, x,
- y)
method must ensure there
- is a subpath for (cpx,
- cpy), and then must connect the last
- point in the subpath to the given point (x, y) using a quadratic Bézier curve with control
- point (cpx, cpy), and must
- then add the given point (x, y) to the subpath. [BEZIER]
Let merged path be a path that + consists of a set of non-overlapping subpaths that exactly outline the points from which, in + any of the paths provided in the constructor's first + argument, the number of times a half-infinite straight line drawn from that point crosses that + path is odd.
+The bezierCurveTo(cp1x, cp1y, cp2x, cp2y, x, y)
method must
- ensure there is a subpath for (cp1x, cp1y), and then must
- connect the last point in the subpath to the given point (x, y) using a cubic Bézier
- curve with control points (cp1x, cp1y) and (cp2x, cp2y). Then, it must add the point (x, y) to the subpath. [BEZIER]
The subpaths in merged path must be oriented such that for any point, + the number of times a half-infinite straight line drawn from that point crosses a subpath is + even if and only if the number of times a half-infinite straight line drawn from that same point + crosses a subpath going in one direction is equal to the number of times it crosses a subpath + going in the other direction.
-The arcTo(x1, y1, x2,
- y2, radius)
- method must first ensure there is a subpath for (x1, y1). Then, the behavior depends on the
- arguments and the last point in the subpath, as described below.
Add all the subpaths in merged path to the Path2D
+ object.
Set the Path2D
object's need new subpath flag.
Negative values for radius must cause the
- implementation to raise an INDEX_SIZE_ERR
- exception.
Let the point (x0, y0) be - the last point in the subpath.
- -If the point (x0, y0) is - equal to the point (x1, y1), - or if the point (x1, y1) is - equal to the point (x2, y2), - or if the radius radius is zero, then the method - must add the point (x1, y1) - to the subpath, and connect that point to the previous point (x0, y0) by a straight line.
- -Otherwise, if the points (x0, y0), (x1, y1), and (x2, y2) all lie on a single straight line, then the - method must add the point (x1, y1) to the subpath, and connect that point to the - previous point (x0, y0) by a - straight line.
- -Otherwise, let The Arc be the shortest arc - given by circumference of the circle that has radius radius, and that has one point tangent to the - half-infinite line that crosses the point (x0, - y0) and ends at the point (x1, y1), and that has a different - point tangent to the half-infinite line that ends at the point (x1, y1) and crosses the point - (x2, y2). The points at - which this circle touches these two lines are called the start and - end tangent points respectively. The method must connect the point - (x0, y0) to the start - tangent point by a straight line, adding the start tangent point to - the subpath, and then must connect the start tangent point to the - end tangent point by The Arc, adding the end - tangent point to the subpath.
- -The arc(x, y, radius,
- startAngle, endAngle, anticlockwise)
method draws an arc. If
- the context has any subpaths, then the method must add a straight
- line from the last point in the subpath to the start point of the
- arc. In any case, it must draw the arc between the start point of
- the arc and the end point of the arc, and add the start and end
- points of the arc to the subpath. The arc and its start and end
- points are defined as follows:
Consider a circle that has its origin at (x, - y) and that has radius radius. The points at startAngle - and endAngle along this circle's circumference, - measured in radians clockwise from the positive x-axis, are the - start and end points respectively.
- -If the anticlockwise argument is omitted or - false and endAngle-startAngle is equal to or greater than 2π, or, if the anticlockwise - argument is true and startAngle-endAngle is - equal to or greater than 2π, then the arc - is the whole circumference of this circle.
- -Otherwise, the arc is the path along the circumference of this - circle from the start point to the end point, going anti-clockwise - if the anticlockwise argument is true, and - clockwise otherwise. Since the points are on the circle, as opposed - to being simply angles from zero, the arc can never cover an angle - greater than 2π radians. If the two points are the - same, or if the radius is zero, then the arc is defined as being of - zero length in both directions.
- -Negative values for radius must cause the
- implementation to raise an INDEX_SIZE_ERR
- exception.
The rect(x, y, w, h)
method must create a new subpath
- containing just the four points (x, y), (x+w,
- y), (x+w, y+h),
- (x, y+h), with those four points connected by straight
- lines, and must then mark the subpath as closed. It must then create
- a new subpath with the point (x, y) as the only point in the subpath.
The fill()
- method must fill all the subpaths of the current path, using
- fillStyle
, and using
- the non-zero winding number rule. Open subpaths must be implicitly
- closed when being filled (without affecting the actual
- subpaths).
Thus, if two overlapping but otherwise independent - subpaths have opposite windings, they cancel out and result in no - fill. If they have the same winding, that area just gets painted - once.
- -The stroke()
method
- must calculate the strokes of all the subpaths of the current path,
- using the lineWidth
,
- lineCap
, lineJoin
, and (if
- appropriate) miterLimit
attributes, and
- then fill the combined stroke area using the strokeStyle
- attribute.
Since the subpaths are all stroked as one, - overlapping parts of the paths in one stroke operation are treated - as if their union was what was painted.
- -Paths, when filled or stroked, must be painted without affecting - the current path, and must be subject to shadow effects, global alpha, the clipping region, and global composition - operators. (Transformations affect the path when the path is - created, not when it is painted, though the stroke style is - still affected by the transformation during painting.)
- -Zero-length line segments must be pruned before stroking a - path. Empty subpaths must be ignored.
- -The drawSystemFocusRing(element)
method, when invoked, must run
- the following steps:
If element is not focused or is not a - descendant of the element with whose context the method is - associated, then abort these steps.
If the user has requested the use of particular focus rings - (e.g. high-contrast focus rings), or if the element would have a focus ring drawn around it, - then draw a focus ring of the appropriate style along the path, - following platform conventions, and abort these steps.
- -Some platforms only draw focus rings around
- elements that have been focused from the keyboard, and not those
- focused from the mouse. Other platforms simply don't draw focus
- rings around some elements at all unless relevant accessibility
- features are enabled. This API is intended to follow these
- conventions. User agents that implement distinctions based on the
- manner in which the element was focused are encouraged to classify
- focus driven by the focus()
method
- based on the kind of user interaction event from which the call
- was triggered (if any).
The focus ring should not be subject to the shadow effects, the global alpha, or the global composition - operators, but should be subject to the clipping region.
- -Optionally, inform the user that the - focus is at the location given by the path. User agents may wait - until the next time the event loop reaches its - "update the rendering" step to optionally inform the user.
- -The drawCustomFocusRing(element)
method, when invoked, must run
- the following steps:
If element is not focused or is not a - descendant of the element with whose context the method is - associated, then return false and abort these steps.
If the user has requested the use of particular focus rings - (e.g. high-contrast focus rings), then draw a focus ring of the - appropriate style along the path, return false, and abort these - steps.
- -The focus ring should not be subject to the shadow effects, the global alpha, or the global composition - operators, but should be subject to the clipping region.
- -Optionally, inform the user that the - focus is at the location given by the path. User agents may wait - until the next time the event loop reaches its - "update the rendering" step to optionally inform the user.
- -Return true.
The scrollPathIntoView()
- method, when invoked, must run the following steps:
Let notional child be a hypothetical
- element that is a rendered child of the canvas
element
- whose dimensions are exactly the rectangle of the bounding box of
- the current path.
Scroll notional child into view with the align to top flag set.
- -Optionally, inform the user that the - caret and/or selection cover the specified - rectangle of the canvas. User agents may wait until the next - time the event loop reaches its "update the rendering" - step to optionally inform the user.
"Inform the user", as used in this - section, could mean calling a system accessibility API, which would - notify assistive technologies such as magnification tools. To - properly drive magnification based on a focus change, a system - accessibility API driving a screen magnifier needs the bounds for - the newly focused object. The methods above are intended to enable - this by allowing the user agent to report the bounding box of the - path used to render the focus ring as the bounds of the element element passed as an argument, if that - element is focused, and the bounding box of the area to which the - user agent is scrolling as the bounding box of the current - selection.
+The Path2D(d)
+ constructor, when invoked, must run these steps:
The clip()
- method must create a new clipping region by calculating
- the intersection of the current clipping region and the area
- described by the current path, using the non-zero winding number
- rule. Open subpaths must be implicitly closed when computing the
- clipping region, without affecting the actual subpaths. The new
+
Parse and interpret the d argument according to the SVG specification's + rules for path data, thus obtaining an SVG path. [SVG]
+ +The resulting path could be empty. SVG defines error handling rules for parsing + and applying path data.
+ +Let (x, y) be the last point in the SVG + path.
Create a new Path2D
object and add all the subpaths in the SVG path, if any,
+ to that Path2D
object.
Create a new subpath in the Path2D
object with (x, y) as the only point in the subpath.
Return the Path2D
object as the constructed object.
The addPath(b,
+ transform)
method, when invoked on a Path2D
object
+ a, must run these steps:
If the Path2D
object b has no subpaths, then abort these
+ steps.
Create a copy of all the subpaths in b. Let this copy be known as c.
Let matrix be the result of creating a DOMMatrix
from a dictionary
+ transform, with ignore3D set to true.
Transform all the coordinates and lines in c by the transform matrix + matrix.
Let (x, y) be the last point in the last subpath + of c.
Add all the subpaths in c to a.
Create a new subpath in a with (x, y) as the only point in the subpath.
Spec bugs: 28333
Objects that implement the CanvasTransform
interface have a current
+ transformation matrix, as well as methods (described in this section) to manipulate it. When
+ an object implementing the CanvasTransform
interface is created, its transformation
+ matrix must be initialized to the identity transform.
The current transformation matrix is applied to coordinates when creating the
+ current default path, and when painting text, shapes, and Path2D
+ objects, on objects implementing the CanvasTransform
interface.
Most of the API uses DOMMatrix
objects rather than this API. This API
+ remains mostly for historical reasons.
The transformations must be performed in reverse order.
+ +For instance, if a scale transformation that doubles the width is applied to the + canvas, followed by a rotation transformation that rotates drawing operations by a quarter turn, + and a rectangle twice as wide as it is tall is then drawn on the canvas, the actual result will be + a square.
+ + + +scale
(x, y)Changes the current transformation matrix to apply a scaling transformation + with the given characteristics.
+ +rotate
(angle)Changes the current transformation matrix to apply a rotation transformation + with the given characteristics. The angle is in radians.
+ +translate
(x, y)Changes the current transformation matrix to apply a translation transformation + with the given characteristics.
+ +transform
(a, b, c, d, e, f)Changes the current transformation matrix to apply the matrix given by the + arguments as described below.
+ +getTransform
()Returns a copy of the current transformation matrix, as a newly created
+ DOMMatrix
object.
setTransform
(a, b, c, d, e, f)Changes the current transformation matrix to the matrix given by the + arguments as described below.
+ +setTransform
(transform)Changes the current transformation matrix to the matrix represented by
+ the passed DOMMatrix2DInit
dictionary.
resetTransform
()Changes the current transformation matrix to the identity transform.
+ +The scale(x, y)
+ method, when invoked, must run these steps:
If either of the arguments are infinite or NaN, then abort these steps.
Add the scaling transformation described by the arguments to the current + transformation matrix. The x argument represents the scale factor in the + horizontal direction and the y argument represents the scale factor in the vertical + direction. The factors are multiples.
The rotate(angle)
method,
+ when invoked, must run these steps:
If angle is infinite or NaN, then abort these steps.
Add the rotation transformation described by the argument to the current + transformation matrix. The angle argument represents a clockwise rotation angle + expressed in radians.
The translate(x,
+ y)
method, when invoked, must run these steps:
If either of the arguments are infinite or NaN, then abort these steps.
Add the translation transformation described by the arguments to the current + transformation matrix. The x argument represents the translation distance in + the horizontal direction and the y argument represents the translation distance in the + vertical direction. The arguments are in coordinate space units.
The transform(a, b,
+ c, d, e, f)
method, when invoked, must
+ run these steps:
If any of the arguments are infinite or NaN, then abort these steps.
Replace the current transformation matrix with the result of multiplying the + current transformation matrix with the matrix described by:
a | c | e |
b | d | f |
0 | 0 | 1 |
The arguments a, b, c, d, + e, and f are sometimes called m11, m12, + m21, m22, dx, and dy or m11, + m21, m12, m22, dx, and dy. Care ought to be + taken in particular with the order of the second and third arguments (b and + c) as their order varies from API to API and APIs sometimes use the notation + m12/m21 and sometimes m21/m12 for those positions.
+ + + +The getTransform()
method, when
+ invoked, must return a newly created DOMMatrix
representing a copy of the
+ current transformation matrix matrix of the context.
This returned object is not live, so updating it will not affect the
+ current transformation matrix, and updating the current transformation
+ matrix will not affect an already returned DOMMatrix
.
The setTransform(a, b,
+ c, d, e, f)
method, when invoked, must
+ run these steps:
If any of the arguments are infinite or NaN, then abort these steps.
Reset the current transformation matrix to the identity matrix.
Invoke the transform(a, b, c,
+ d, e, f)
method with the same arguments.
The setTransform(transform)
method, when invoked,
+ must reset the current transformation matrix to the value represented by the result
+ of creating a DOMMatrix
from a
+ dictionary transform, with ignore3D set to true.
The resetTransform()
method,
+ when invoked, must reset the current transformation matrix to the identity
+ matrix.
Some methods on the CanvasDrawImage
and CanvasFillStrokeStyles
+ interfaces take the union type CanvasImageSource
as an argument.
This union type allows objects implementing any of the following interfaces to be used as image + sources:
+ +HTMLOrSVGImageElement
(img
or SVG image
+ elements)HTMLVideoElement
(video
elements)HTMLCanvasElement
(canvas
elements)ImageBitmap
Although not formally specified as such, SVG image
+ elements are expected to be implemented nearly identical to img
elements. That is,
+ SVG image
elements share the fundamental concepts and features of
+ img
elements.
The ImageBitmap
interface can be created from a number of other
+ image-representing types, including ImageData
.
When a user agent is required to check the usability of the image
+ argument, where image is a CanvasImageSource
object, the
+ user agent must run these steps, which return either good, bad, or
+ aborted:
If image is an HTMLOrSVGImageElement
object that is in the broken state, then throw an
+ "InvalidStateError
" DOMException
, return aborted,
+ and abort these steps.
If image is an HTMLOrSVGImageElement
object that is not fully decodable, or if image is an
+ HTMLVideoElement
object whose readyState
+ attribute is either HAVE_NOTHING
or HAVE_METADATA
, then return bad and abort these
+ steps.
If image is an HTMLOrSVGImageElement
object with an
+ intrinsic width or intrinsic height (or both) equal to zero, then
+ return bad and abort these steps.
If image is an HTMLCanvasElement
object with either a horizontal
+ dimension or a vertical dimension equal to zero, then return bad and abort these
+ steps.
If image is an ImageBitmap
object with its
+ [[Detached]] internal slot value set to true, then throw an
+ "InvalidStateError
" DOMException
, return aborted,
+ and abort these steps.
Return good.
When a CanvasImageSource
object represents an HTMLOrSVGImageElement
,
+ the element's image must be used as the source image.
Specifically, when a CanvasImageSource
object represents an animated image in an
+ HTMLOrSVGImageElement
, the user agent must use the default image of the animation
+ (the one that the format defines is to be used when animation is not supported or is disabled),
+ or, if there is no such image, the first frame of the animation, when rendering the image for
+ CanvasRenderingContext2D
APIs.
When a CanvasImageSource
object represents an HTMLVideoElement
, then
+ the frame at the current playback position when the method with the argument is
+ invoked must be used as the source image when rendering the image for
+ CanvasRenderingContext2D
APIs, and the source image's dimensions must be the intrinsic width and intrinsic height of the media resource
+ (i.e. after any aspect-ratio correction has been applied).
When a CanvasImageSource
object represents an HTMLCanvasElement
, the
+ element's bitmap must be used as the source image.
When a CanvasImageSource
object represents an element that is being
+ rendered and that element has been resized, the original image data of the source image
+ must be used, not the image as it is rendered (e.g. width
and
+ height
attributes on the source element have no effect on how
+ the object is interpreted when rendering the image for CanvasRenderingContext2D
+ APIs).
When a CanvasImageSource
object represents an ImageBitmap
, the
+ object's bitmap image data must be used as the source image.
The image argument is not origin-clean if it is an
+ HTMLOrSVGImageElement
or HTMLVideoElement
whose origin is
+ not the same as the origin specified by the entry settings
+ object, or if it is an HTMLCanvasElement
whose bitmap's origin-clean flag is false.
fillStyle
[ = value ]Returns the current style used for filling shapes.
+ +Can be set, to change the fill style.
+ +The style can be either a string containing a CSS color, or a CanvasGradient
or
+ CanvasPattern
object. Invalid values are ignored.
strokeStyle
[ = value ]Returns the current style used for stroking shapes.
+ +Can be set, to change the stroke style.
+ +The style can be either a string containing a CSS color, or a CanvasGradient
or
+ CanvasPattern
object. Invalid values are ignored.
Objects that implement the CanvasFillStrokeStyles
interface have attributes and
+ methods (defined in this section) that control how shapes are treated by the object.
The fillStyle
attribute represents the
+ color or style to use inside shapes, and the strokeStyle
attribute represents the color
+ or style to use for the lines around the shapes.
Both attributes can be either strings, CanvasGradient
s, or
+ CanvasPattern
s. On setting, strings must be parsed as CSS <color> values and the color assigned, and
+ CanvasGradient
and CanvasPattern
objects must be assigned themselves. [CSSCOLOR] If the value is a string but cannot be parsed as a CSS
+ <color> value, then it must be ignored, and the attribute must retain its previous
+ value.
+
+ If the new value is a CanvasPattern
object that is marked as not origin-clean, then the
+ CanvasRenderingContext2D
's origin-clean flag must be set to false.
+
+
When set to a CanvasPattern
or CanvasGradient
object, the assignment
+ is live, meaning that changes made to the object after the assignment do affect
+ subsequent stroking or filling of shapes.
On getting, if the value is a color, then the serialization of the color must be returned. Otherwise, if it is not a color but a
+ CanvasGradient
or CanvasPattern
, then the respective object must be
+ returned. (Such objects are opaque and therefore only useful for assigning to other attributes or
+ for comparison to other gradients or patterns.)
The serialization of a color for a color
+ value is a string, computed as follows: if it has alpha equal to 1.0, then the string is a
+ lowercase six-digit hex value, prefixed with a "#" character (U+0023 NUMBER SIGN), with the first
+ two digits representing the red component, the next two digits representing the green component,
+ and the last two digits representing the blue component, the digits being ASCII lower hex digits. Otherwise, the color value has alpha less than
+ 1.0, and the string is the color value in the CSS rgba()
+ functional-notation format: the literal string "rgba
" (U+0072 U+0067 U+0062
+ U+0061) followed by a U+0028 LEFT PARENTHESIS, a base-ten integer in the range 0-255 representing
+ the red component (using ASCII digits in the shortest form possible), a literal
+ U+002C COMMA and U+0020 SPACE, an integer for the green component, a comma and a space, an integer
+ for the blue component, another comma and space, a U+0030 DIGIT ZERO, if the alpha value is
+ greater than zero then a U+002E FULL STOP (representing the decimal point), if the alpha value is
+ greater than zero then one or more ASCII digits representing the fractional part of
+ the alpha, and finally a U+0029
+ RIGHT PARENTHESIS. User agents must express the fractional part of the alpha value, if any, with
+ the level of precision necessary for the alpha value, when reparsed, to be interpreted as the same
+ alpha value.
When the context is created, the fillStyle
and strokeStyle
attributes
+ must initially have the string value #000000
.
When the value is a color, it must not be affected by the transformation matrix when used to + draw on bitmaps.
+ + + +There are two types of gradients, linear gradients and radial gradients, both represented by
+ objects implementing the opaque CanvasGradient
interface.
Once a gradient has been created (see below), stops are placed along it to + define how the colors are distributed along the gradient. The color of the + gradient at each stop is the color specified for that stop. Between each such stop, the colors and + the alpha component must be linearly interpolated over the RGBA space without premultiplying the + alpha value to find the color to use at that offset. Before the first stop, the color must be the + color of the first stop. After the last stop, the color must be the color of the last stop. When + there are no stops, the gradient is transparent black.
+ +addColorStop
(offset, color)Adds a color stop with the given color to the gradient at the given offset. 0.0 is the offset + at one end of the gradient, 1.0 is the offset at the other end.
+ +Throws an "IndexSizeError
" DOMException
if the offset
+ is out of range. Throws a "SyntaxError
" DOMException
if
+ the color cannot be parsed.
createLinearGradient
(x0, y0, x1, y1)Returns a CanvasGradient
object that represents a
+ linear gradient that paints along the line given by the
+ coordinates represented by the arguments.
createRadialGradient
(x0, y0, r0, x1, y1, r1)Returns a CanvasGradient
object that represents a
+ radial gradient that paints along the cone given by the circles
+ represented by the arguments.
If either of the radii are negative, throws an
+ "IndexSizeError
" DOMException
exception.
The addColorStop(offset,
+ color)
method on the CanvasGradient
interface adds a new stop
+ to a gradient. If the offset is less than 0 or greater than 1 then an
+ "IndexSizeError
" DOMException
must be thrown. If the
+ color cannot be parsed as a CSS <color> value, then a
+ "SyntaxError
" DOMException
must be thrown. Otherwise, the
+ gradient must have a new stop placed, at offset offset relative to the whole gradient,
+ and with the color obtained by parsing
+ color as a CSS <color> value. If multiple stops are added at the same
+ offset on a gradient, then they must be placed in the order added, with the first one closest to
+ the start of the gradient, and each subsequent one infinitesimally further along towards the end
+ point (in effect causing all but the first and last stop added at each point to be ignored).
The createLinearGradient(x0,
+ y0, x1, y1)
method takes four arguments that
+ represent the start point (x0, y0) and end point (x1,
+ y1) of the gradient. The method, when invoked, must return a linear
+ CanvasGradient
initialized with the specified line.
Linear gradients must be rendered such that all points on a line perpendicular to the line that + crosses the start and end points have the color at the point where those two lines cross (with the + colors coming from the interpolation and extrapolation described + above). The points in the linear gradient must be transformed as described by the current transformation matrix when rendering.
+ +If x0 = x1 and y0 = y1, then the linear + gradient must paint nothing.
+ +The createRadialGradient(x0,
+ y0, r0, x1, y1, r1)
method
+ takes six arguments, the first three representing the start circle with origin (x0,
+ y0) and radius r0, and the last three representing the end circle with
+ origin (x1, y1) and radius r1. The values are in coordinate space
+ units. If either of r0 or r1 are negative, then an
+ "IndexSizeError
" DOMException
must be thrown. Otherwise,
+ the method, when invoked, must return a radial CanvasGradient
initialized with the
+ two specified circles.
Radial gradients must be rendered by following these steps:
+ +If x0 = x1 and y0 = y1 and r0 = r1, then the radial gradient must + paint nothing. Abort these steps.
Let x(ω) = (x1-x0)ω + x0
+ +Let y(ω) = (y1-y0)ω + y0
+ +Let r(ω) = (r1-r0)ω + r0
+ +Let the color at ω be the color at that position on the gradient + (with the colors coming from the interpolation and extrapolation + described above).
+ +For all values of ω where r(ω) > 0, starting with the value of ω nearest to positive infinity and ending with the value of ω nearest to negative infinity, draw the circumference of the circle with + radius r(ω) at position (x(ω), y(ω)), with the + color at ω, but only painting on the parts of the bitmap that have not + yet been painted on by earlier circles in this step for this rendering of the gradient.
This effectively creates a cone, touched by the two circles defined in the + creation of the gradient, with the part of the cone before the start circle (0.0) using the color + of the first offset, the part of the cone after the end circle (1.0) using the color of the last + offset, and areas outside the cone untouched by the gradient (transparent black).
+ +The resulting radial gradient must then be transformed as described by the current transformation matrix when rendering.
+ +Gradients must be painted only where the relevant stroking or filling effects requires that + they be drawn.
+ + + +Patterns are represented by objects implementing the opaque CanvasPattern
+ interface.
createPattern
(image, repetition)Returns a CanvasPattern
object that uses the given image and repeats in the
+ direction(s) given by the repetition argument.
The allowed values for repetition are repeat
(both
+ directions), repeat-x
(horizontal only), repeat-y
+ (vertical only), and no-repeat
(neither). If the repetition argument is empty, the value repeat
is used.
If the image isn't yet fully decoded, then nothing is drawn. If the image is a canvas with no
+ data, throws an "InvalidStateError
" DOMException
.
setTransform
(transform)Sets the transformation matrix that will be used when rendering the pattern during a fill or + stroke painting operation.
+ +The createPattern(image,
+ repetition)
method, when invoked, must run these steps:
Let image be the first argument and repetition be + the second argument.
Check the usability of the image argument. If this + returns aborted, then an exception has been thrown and the method doesn't return anything; + abort these steps. If it returns bad, then return null and abort these steps. Otherwise it + returns good; continue with these steps.
If repetition is the empty string, then set it to "repeat
".
If repetition is not a case-sensitive match for one of
+ "repeat
", "repeat-x
", "repeat-y
", or "no-repeat
", then throw a
+ "SyntaxError
" DOMException
and abort these steps.
Create a new CanvasPattern
object with the image image
+ and the repetition behavior given by repetition.
If the image argument is not origin-clean, then mark the
+ CanvasPattern
object as not
+ origin-clean.
Return the CanvasPattern
object.
Modifying the image used when creating a CanvasPattern
object
+ after calling the createPattern()
method must
+ not affect the pattern(s) rendered by the CanvasPattern
object.
Patterns have a transformation matrix, which controls how the pattern is used when it is + painted. Initially, a pattern's transformation matrix must be the identity transform.
+ +The setTransform(transform)
method,
+ when invoked, must reset the pattern's transformation matrix to the value represented by the
+ result of creating a DOMMatrix
+ from a dictionary transform, with ignore3D set to true.
When a pattern is to be rendered within an area, the user agent must run the following steps to + determine what is rendered:
+ +Create an infinite transparent black bitmap.
Place a copy of the image on the bitmap, anchored such that its top left corner is at the
+ origin of the coordinate space, with one coordinate space unit per CSS
+ pixel of the image, then place repeated copies of this image horizontally to the left and
+ right, if the repetition behavior is "repeat-x
", or vertically up and
+ down, if the repetition behavior is "repeat-y
", or in all four
+ directions all over the bitmap, if the repetition behavior is "repeat
".
If the original image data is a bitmap image, then the value painted at a point in the area
+ of the repetitions is computed by filtering the original image data. When scaling up, if the
+ imageSmoothingEnabled
attribute is
+ set to false, then the image must be rendered using nearest-neighbor interpolation. Otherwise,
+ the user agent may use any filtering algorithm (for example bilinear interpolation or
+ nearest-neighbor). User agents which support multiple filtering algorithms may use the value of
+ the imageSmoothingQuality
attribute
+ to guide the choice of filtering algorithm. When such a filtering algorithm requires a pixel
+ value from outside the original image data, it must instead use the value from wrapping the
+ pixel's coordinates to the original image's dimensions. (That is, the filter uses 'repeat'
+ behavior, regardless of the value of the pattern's repetition behavior.)
Transform the resulting bitmap according to the pattern's transformation matrix.
Transform the resulting bitmap again, this time according to the current transformation matrix.
Replace any part of the image outside the area in which the pattern is to be rendered with + transparent black.
The resulting bitmap is what is to be rendered, with the same origin and same + scale.
If a radial gradient or repeated pattern is used when the transformation matrix is singular, + then the resulting style must be transparent black (otherwise the gradient or pattern would be + collapsed to a point or line, leaving the other pixels undefined). Linear gradients and solid + colors always define all points even with singular transformation matrices.
+ + + + + + +Objects that implement the CanvasRect
interface provide the following methods for
+ immediately drawing rectangles to the bitmap. The methods each take four arguments; the first two
+ give the x and y coordinates of the top left of the rectangle, and the
+ second two give the width w and height h of the rectangle, respectively.
The current transformation matrix must be + applied to the following four coordinates, which form the path that must then be closed to get the + specified rectangle: (x, y), (x+w, y), (x+w, y+h), (x, y+h).
+ +Shapes are painted without affecting the current default path, and are subject to
+ the clipping region, and, with the exception of clearRect()
, also shadow
+ effects, global alpha, and global composition operators.
clearRect
(x, y, w, h)Clears all pixels on the bitmap in the given rectangle to transparent black.
+ +fillRect
(x, y, w, h)Paints the given rectangle onto the bitmap, using the current fill style.
+ +strokeRect
(x, y, w, h)Paints the box that outlines the given rectangle onto the bitmap, using the current stroke + style.
+ +The clearRect(x, y,
+ w, h)
method, when invoked, must run these steps:
If any of the arguments are infinite or NaN, then abort these steps.
Let pixels be the set of pixels in the specified rectangle that also + intersect the current clipping region.
Clear the pixels in pixels to a fully transparent black, erasing any + previous image.
If either height or width are zero, this method has no effect, since the set of + pixels would be empty.
+ +The fillRect(x, y,
+ w, h)
method, when invoked, must must run these steps:
If any of the arguments are infinite or NaN, then abort these steps.
If either w or h are zero, then abort these steps.
Paint the specified rectangular area using the fillStyle
.
The strokeRect(x, y,
+ w, h)
method, when invoked, must run these steps:
If any of the arguments are infinite or NaN, then abort these steps.
Take the result of tracing the path described below,
+ using the CanvasPathDrawingStyles
interface's line styles, and fill it with the
+ strokeStyle
.
If both w and h are zero, the path has a single subpath + with just one point (x, y), and no lines, and this method + thus has no effect (the trace a path algorithm returns an empty path in that + case).
+ +If just one of either w or h is zero, then the path has + a single subpath consisting of two points, with coordinates (x, y) and (x+w, y+h), in that order, connected by a single straight line.
+ +Otherwise, the path has a single subpath consisting of four points, with coordinates (x, y), (x+w, y), (x+w, + y+h), and (x, y+h), + connected to each other in that order by straight lines.
+ + + + + +Support: canvas-textChrome for Android 59+Chrome 4+UC Browser for Android 11.4+iOS Safari 3.2+Firefox 3.5+IE 9+Samsung Internet 4+Opera Mini NoneSafari 4+Android Browser 2.1+Opera 10.5+Edge 12+
Source: caniuse.com
fillText
(text, x, y [, maxWidth ] )strokeText
(text, x, y [, maxWidth ] )Fills or strokes (respectively) the given text at the given position. If a maximum width is + provided, the text will be scaled to fit that width if necessary.
+ +measureText
(text)Returns a TextMetrics
object with the metrics of the given text in the current
+ font.
width
actualBoundingBoxLeft
actualBoundingBoxRight
fontBoundingBoxAscent
fontBoundingBoxDescent
actualBoundingBoxAscent
actualBoundingBoxDescent
emHeightAscent
emHeightDescent
hangingBaseline
alphabeticBaseline
ideographicBaseline
Returns the measurement described below.
+ +Objects that implement the CanvasText
interface provide the following methods for
+ rendering text.
The fillText()
and strokeText()
methods take three or four arguments,
+ text, x, y, and optionally maxWidth, and render the
+ given text at the given (x, y) coordinates ensuring that the text
+ isn't wider than maxWidth if specified, using the current font
, textAlign
, and textBaseline
values. Specifically, when the methods
+ are invoked, the user agent must run these steps:
If any of the arguments are infinite or NaN, then abort these steps.
Run the text preparation algorithm, passing it text, the object
+ implementing the CanvasText
interface, and, if the maxWidth argument was
+ provided, that argument. Let glyphs be the result.
Move all the shapes in glyphs to the right by x + CSS pixels and down by y CSS + pixels.
Paint the shapes given in glyphs, as transformed by the current transformation matrix, with each CSS pixel in the coordinate space of glyphs mapped to one + coordinate space unit.
+ +For fillText()
, fillStyle
must be applied to the shapes and strokeStyle
must be ignored. For strokeText()
, the reverse holds: strokeStyle
must be applied to the result of tracing the shapes using the object implementing the
+ CanvasText
interface for the line styles, and fillStyle
must be ignored.
These shapes are painted without affecting the current path, and are subject to shadow effects, global + alpha, the clipping region, and global composition operators.
+ +The measureText()
method takes one
+ argument, text. When the method is invoked, the user agent must run the
+ text preparation algorithm, passing it text and the object implementing
+ the CanvasText
interface, and then using the returned inline box must
+ create a new TextMetrics
object with its attributes set as described in the following
+ list.
+
+ If doing these measurements requires using a font that has an origin that is not the
+ same as that of the Document
object that owns the
+ canvas
element (even if "using a font" means just checking if that font has a
+ particular glyph in it before falling back to another font), then the method, when invoked, must
+ throw a "SecurityError
" DOMException
.
+
+ Otherwise, it must return the new TextMetrics
object.
+ [CSS]
+
+
width
attributeThe width of that inline box, in CSS pixels. (The + text's advance width.)
actualBoundingBoxLeft
attributeThe distance parallel to the baseline from the alignment point given by the textAlign
attribute to the left side of the bounding
+ rectangle of the given text, in CSS pixels; positive numbers
+ indicating a distance going left from the given alignment point.
+
+
The sum of this value and the next (actualBoundingBoxRight
) can be wider than
+ the width of the inline box (width
), in
+ particular with slanted fonts where characters overhang their advance width.
actualBoundingBoxRight
attributeThe distance parallel to the baseline from the alignment point given by the textAlign
attribute to the right side of the bounding
+ rectangle of the given text, in CSS pixels; positive numbers
+ indicating a distance going right from the given alignment point.
fontBoundingBoxAscent
attributeThe distance from the horizontal line indicated by the textBaseline
attribute to the top of the highest
+ bounding rectangle of all the fonts used to render the text, in CSS
+ pixels; positive numbers indicating a distance going up from the given baseline.
This value and the next are useful when rendering a background that have to have
+ a consistent height even if the exact text being rendered changes. The actualBoundingBoxAscent
attribute (and
+ its corresponding attribute for the descent) are useful when drawing a bounding box around
+ specific text.
fontBoundingBoxDescent
attributeThe distance from the horizontal line indicated by the textBaseline
attribute to the bottom of the lowest
+ bounding rectangle of all the fonts used to render the text, in CSS
+ pixels; positive numbers indicating a distance going down from the given
+ baseline.
actualBoundingBoxAscent
attributeThe distance from the horizontal line indicated by the textBaseline
attribute to the top of the bounding
+ rectangle of the given text, in CSS pixels; positive numbers
+ indicating a distance going up from the given baseline.
+
+
This number can vary greatly based on the input text, even if the first font
+ specified covers all the characters in the input. For example, the actualBoundingBoxAscent
of a lowercase
+ "o" from an alphabetic baseline would be less than that of an uppercase "F". The value can
+ easily be negative; for example, the distance from the top of the em box (textBaseline
value "top
") to the top of the bounding rectangle when
+ the given text is just a single comma ",
" would likely (unless the font is
+ quite unusual) be negative.
actualBoundingBoxDescent
attributeThe distance from the horizontal line indicated by the textBaseline
attribute to the bottom of the bounding
+ rectangle of the given text, in CSS pixels; positive numbers
+ indicating a distance going down from the given baseline.
emHeightAscent
attributeThe distance from the horizontal line indicated by the textBaseline
attribute to the highest top of the em
+ squares in the line box, in CSS pixels; positive numbers
+ indicating that the given baseline is below the top of that em square (so this value will usually
+ be positive). Zero if the given baseline is the top of that em square; half the font size if the
+ given baseline is the middle of that em square.
emHeightDescent
attributeThe distance from the horizontal line indicated by the textBaseline
attribute to the lowest bottom of the em
+ squares in the line box, in CSS pixels; positive numbers
+ indicating that the given baseline is below the bottom of that em square (so this value will
+ usually be negative). (Zero if the given baseline is the bottom of that em square.)
hangingBaseline
attributeThe distance from the horizontal line indicated by the textBaseline
attribute to the hanging baseline of the
+ line box, in CSS pixels; positive numbers indicating that
+ the given baseline is below the hanging baseline. (Zero if the given baseline is the hanging
+ baseline.)
alphabeticBaseline
attributeThe distance from the horizontal line indicated by the textBaseline
attribute to the alphabetic baseline of
+ the line box, in CSS pixels; positive numbers indicating
+ that the given baseline is below the alphabetic baseline. (Zero if the given baseline is the
+ alphabetic baseline.)
ideographicBaseline
attributeThe distance from the horizontal line indicated by the textBaseline
attribute to the ideographic baseline of
+ the line box, in CSS pixels; positive numbers indicating
+ that the given baseline is below the ideographic baseline. (Zero if the given baseline is the
+ ideographic baseline.)
Glyphs rendered using fillText()
and
+ strokeText()
can spill out of the box given by the
+ font size (the em square size) and the width returned by measureText()
(the text width). Authors are encouraged
+ to use the bounding box values described above if this is an issue.
A future version of the 2D context API might provide a way to render fragments of + documents, rendered using CSS, straight to the canvas. This would be provided in preference to a + dedicated way of doing multiline layout.
+ + + +Objects that implement the CanvasDrawPath
interface have a current default
+ path. There is only one current default path, it is not part of the
+ drawing state. The current default path is a path, as described above.
beginPath
()Resets the current default path.
+ +fill
( [ fillRule ] )fill
(path [, fillRule ] )Fills the subpaths of the current default path or the given path with the + current fill style, obeying the given fill rule.
+ +stroke
()stroke
(path)Strokes the subpaths of the current default path or the given path with the + current stroke style.
+ +clip
( [ fillRule ] )clip
(path [, fillRule ] )Further constrains the clipping region to the current default path or the given + path, using the given fill rule to determine what points are in the path.
+ +resetClip
()Unconstrains the clipping region.
+ +isPointInPath
(x, y [, fillRule ] )isPointInPath
(path, x, y [, fillRule ] )Returns true if the given point is in the current default path or the given + path, using the given fill rule to determine what points are in the path.
+ +isPointInStroke
(x, y)isPointInStroke
(path, x, y)Returns true if the given point would be in the region covered by the stroke of the + current default path or the given path, given the current stroke style.
+ +Spec bugs: 28217
The beginPath()
method, when invoked,
+ must empty the list of subpaths in the context's current default path so that the it
+ once again has zero subpaths.
Where the following method definitions use the term intended path, it means the
+ Path2D
argument, if one was provided, or the current default path
+ otherwise.
When the intended path is a Path2D
object, the coordinates and lines of its
+ subpaths must be transformed according to the
+ current transformation matrix on the object implementing the
+ CanvasTransform
interface when used by these methods (without affecting the
+ Path2D
object itself). When the intended path is the current default
+ path, it is not affected by the transform. (This is because transformations already affect
+ the current default path when it is constructed, so applying it when it is painted as
+ well would result in a double transformation.)
The fill()
method, when invoked, must fill
+ all the subpaths of the intended path, using fillStyle
, and using the fill rule indicated
+ by the fillRule argument. Open subpaths must be implicitly closed when being filled
+ (without affecting the actual subpaths).
The stroke()
method, when invoked, must
+ trace the intended path, using this
+ CanvasPathDrawingStyles
object for the line styles, and then fill the resulting path
+ using the strokeStyle
attribute, using the non-zero winding rule.
As a result of how the algorithm to trace a path is defined, + overlapping parts of the paths in one stroke operation are treated as if their union was what was + painted.
+ +The stroke style is affected by the transformation during painting, even + if the intended path is the current default path.
+ +Paths, when filled or stroked, must be painted without affecting the current default
+ path or any Path2D
objects, and must be subject to shadow effects, global
+ alpha, the clipping region, and global composition operators. (The effect
+ of transformations is described above and varies based on which path is being used.)
The clip()
method, when invoked, must
+ create a new clipping region by calculating the intersection of the current clipping
+ region and the area described by the intended path, using the fill rule indicated by
+ the fillRule argument. Open subpaths must be implicitly closed when computing the
+ clipping region, without affecting the actual subpaths. The new clipping region replaces the
+ current clipping region.
When the context is initialized, the clipping region must be set to the largest infinite + surface (i.e. by default, no clipping occurs).
+ + + +The resetClip()
method, when invoked,
+ must create a new clipping region that is the largest infinite surface. The new
clipping region replaces the current clipping region.
When the context is initialized, the clipping region must be set - to the rectangle with the top left corner at (0,0) and the width and - height of the coordinate space.
+The isPointInPath()
method, when
+ invoked, must return true if the point given by the x and y coordinates
+ passed to the method, when treated as coordinates in the canvas coordinate space unaffected by the
+ current transformation, is inside the intended path as determined by the fill rule
+ indicated by the fillRule argument; and must return false otherwise. Open subpaths must
+ be implicitly closed when computing the area inside the path, without affecting the actual
+ subpaths. Points on the path itself must be considered to be inside the path. If either of the
+ arguments are infinite or NaN, then the method must return false.
The isPointInPath(x, y)
method must
- return true if the point given by the x and y coordinates passed to the method, when treated as
- coordinates in the canvas coordinate space unaffected by the current
- transformation, is inside the current path as determined by the
- non-zero winding number rule; and must return false
- otherwise. Points on the path itself are considered to be inside the
- path. If either of the arguments is infinite or NaN, then the method
- must return false.
The isPointInStroke()
method,
+ when invoked, must return true if the point given by the x and y coordinates
+ passed to the method, when treated as coordinates in the canvas coordinate space unaffected by the
+ current transformation, is inside the path that results from tracing the intended path, using the non-zero winding rule, and using the
+ CanvasPathDrawingStyles
interface for the line styles; and must return false
+ otherwise. Points on the resulting path must be considered to be inside the path. If either of the
+ arguments are infinite or NaN, then the method must return false.
This canvas
element has a couple of checkboxes. The
- path-related commands are highlighted:
This canvas
element has a couple of checkboxes. The path-related commands are
+ highlighted:
<canvas height=400 width=750> <label><input type=checkbox id=showA> Show As</label> @@ -2356,11 +2796,9 @@ hairline width with transform. ack Shaun Morris. } context.beginPath(); context.rect(x-7, y-7, 12 + metrics.width+2, 14); - if (paint && context.drawCustomFocusRing(element)) { - context.strokeStyle = 'silver'; - context.stroke(); - } - context.restore(); + + context.drawFocusIfNeeded(element); + context.restore(); } function drawBase() { /* ... */ } function drawAs() { /* ... */ } @@ -2382,6 +2820,7 @@ hairline width with transform. ack Shaun Morris. var context = canvas.getContext('2d'); var x = event.clientX; var y = event.clientY; + var node = event.target; while (node) { x -= node.offsetLeft - node.scrollLeft; y -= node.offsetTop - node.scrollTop; @@ -2401,929 +2840,460 @@ hairline width with transform. ack Shaun Morris. document.getElementsByTagName('canvas')[0].addEventListener('click', processClick, false); redraw(); </script>- +
drawFocusIfNeeded
(element)drawFocusIfNeeded
(path, element)If the given element is focused, draws a focus ring around the current + default path or the given path, following the platform conventions for focus rings.
-scrollPathIntoView
()scrollPathIntoView
(path)font
[ = value ]Scrolls the current default path or the given path into view. This is especially + useful on devices with small screens, where the whole canvas might not be visible at once.
-Returns the current font settings.
+ -Can be set, to change the font. The syntax is the same as for - the CSS 'font' property; values that cannot be parsed as CSS font - values are ignored.
+ Objects that implement theCanvasUserInterface
interface provide the following
+ methods to control drawing focus rings and scrolling paths into view.
- Relative keywords and lengths are computed relative to the font
- of the canvas
element.
The drawFocusIfNeeded(element)
+ method, when invoked, must run these steps:
textAlign
[ = value ]If element is not focused or is not a descendant of the element with + whose context the method is associated, then abort these steps.
Draw a focus ring of the appropriate style along the intended path, following platform + conventions.
-Returns the current text alignment settings.
+Some platforms only draw focus rings around elements that have been focused from
+ the keyboard, and not those focused from the mouse. Other platforms simply don't draw focus
+ rings around some elements at all unless relevant accessibility features are enabled. This API
+ is intended to follow these conventions. User agents that implement distinctions based on the
+ manner in which the element was focused are encouraged to classify focus driven by the focus()
method based on the kind of user interaction event from which
+ the call was triggered (if any).
Can be set, to change the alignment. The possible values are
- start
, end
, left
, right
, and center
. Other values are ignored. The default is
- start
.
The focus ring should not be subject to the shadow effects, the
+ global alpha, the global composition operators, or any of
+ the members in the CanvasFillStrokeStyles
, CanvasPathDrawingStyles
,
+ CanvasTextDrawingStyles
interfaces, but should be subject to the
+ clipping region. (The effect of transformations is described above and varies based
+ on which path is being used.)
Inform the user that the focus is at the location given by the + intended path. User agents may wait until the next time the event loop reaches its + update the rendering step to optionally inform the user.
textBaseline
[ = value ]User agents should not implicitly close open subpaths in the intended path when drawing the + focus ring.
-This might be a moot point, however. For example, if the focus ring is drawn as an + axis-aligned bounding rectangle around the points in the intended path, then whether the subpaths + are closed or not has no effect. This specification intentionally does not specify precisely how + focus rings are to be drawn: user agents are expected to honor their platform's native + conventions.
-Returns the current baseline alignment settings.
+Can be set, to change the baseline alignment. The possible
- values and their meanings are given below. Other values are
- ignored. The default is alphabetic
.
The scrollPathIntoView()
+ method, when invoked, must run these steps:
Let the specified rectangle be the rectangle of the bounding box of + the intended path.
Let notional child be a hypothetical element that is a rendered child
+ of the canvas
element whose dimensions are those of the specified
+ rectangle.
Scroll notional child into + view with the align to top flag set.
-fillText
(text, x, y [, maxWidth ] )strokeText
(text, x, y [, maxWidth ] )Optionally, inform the user that the caret or selection (or both) + cover the specified rectangle of the canvas. The user agent may wait until the next + time the event loop reaches its update the rendering step to + optionally inform the user.
"Inform the user", as used in this section, does not imply any persistent state + change. It could mean, for instance, calling a system accessibility API to notify assistive + technologies such as magnification tools so that the user's magnifier moves to the given area of + the canvas. However, it does not associate the path with the element, or provide a region for + tactile feedback, etc.
-Fills or strokes (respectively) the given text at the given - position. If a maximum width is provided, the text will be scaled - to fit that width if necessary.
+ -measureText
(text)Objects that implement the CanvasDrawImage
interface have the drawImage
method to draw images.
Returns a TextMetrics
object with the metrics of the given text in the current font.
width
Returns the advance width of the text that was passed to the
- measureText()
- method.
The font
IDL
- attribute, on setting, must be parsed the same way as the 'font'
- property of CSS (but without supporting property-independent style
- sheet syntax like 'inherit'), and the resulting font must be
- assigned to the context, with the 'line-height' component forced to
- 'normal', with the 'font-size' component converted to CSS pixels,
- and with system fonts being computed to explicit values. If the new
- value is syntactically incorrect (including using
- property-independent style sheet syntax like 'inherit' or
- 'initial'), then it must be ignored, without assigning a new font
- value. [CSS]
Font names must be interpreted in the context of the
- canvas
element's stylesheets; any fonts embedded using
- @font-face
must therefore be available once
- they are loaded. (If a font is referenced before it is fully loaded,
- then it must be treated as if it was an unknown font, falling back
- to another as described by the relevant CSS specifications.) [CSSFONTS]
Only vector fonts should be used by the user agent; if a user - agent were to use bitmap fonts then transformations would likely - make the font look very ugly.
- -On getting, the font
- attribute must return the serialized form of the current font of the context
- (with no 'line-height' component). [CSSOM]
For example, after the following statement:
- -context.font = 'italic 400 12px/2 Unknown Font, sans-serif';- -
...the expression context.font
would
- evaluate to the string "italic 12px "Unknown Font", sans-serif
". The
- "400" font-weight doesn't appear because that is the default
- value. The line-height doesn't appear because it is forced to
- "normal", the default value.
When the context is created, the font of the context must be set
- to 10px sans-serif. When the 'font-size' component is set to lengths
- using percentages, 'em' or 'ex' units, or the 'larger' or 'smaller'
- keywords, these must be interpreted relative to the computed value
- of the 'font-size' property of the corresponding canvas
- element at the time that the attribute is set. When the
- 'font-weight' component is set to the relative values 'bolder' and
- 'lighter', these must be interpreted relative to the computed value
- of the 'font-weight' property of the corresponding
- canvas
element at the time that the attribute is
- set. If the computed values are undefined for a particular case
- (e.g. because the canvas
element is not in a
- Document
), then the relative keywords must be
- interpreted relative to the normal-weight 10px sans-serif
- default.
The textAlign
IDL
- attribute, on getting, must return the current value. On setting, if
- the value is one of start
, end
, left
, right
, or center
, then the
- value must be changed to the new value. Otherwise, the new value
- must be ignored. When the context is created, the textAlign
attribute must
- initially have the value start
.
The textBaseline
- IDL attribute, on getting, must return the current value. On
- setting, if the value is one of top
, hanging
, middle
, alphabetic
,
- ideographic
,
- or bottom
,
- then the value must be changed to the new value. Otherwise, the new
- value must be ignored. When the context is created, the textBaseline
attribute
- must initially have the value alphabetic
.
The textBaseline
- attribute's allowed keywords correspond to alignment points in the
- font:
The keywords map to these alignment points as follows:
- -top
- hanging
- middle
- alphabetic
- ideographic
- bottom
- The fillText()
and
- strokeText()
- methods take three or four arguments, text, x, y, and optionally maxWidth, and render the given text at the given (x, y) coordinates ensuring that the text isn't wider
- than maxWidth if specified, using the current
- font
, textAlign
, and textBaseline
- values. Specifically, when the methods are called, the user agent
- must run the following steps:
If maxWidth is present but less than or - equal to zero, return without doing anything; abort these - steps.
Let font be the current font of the
- context, as given by the font
attribute.
Replace all the space - characters in text with U+0020 SPACE - characters.
Form a hypothetical infinitely wide CSS line box containing
- a single inline box containing the text text,
- with all the properties at their initial values except the 'font'
- property of the inline box set to font, the
- 'direction' property of the inline box set to the
- directionality of the canvas
element, and the
- 'white-space' property set to 'pre'. [CSS]
If the maxWidth argument was specified - and the hypothetical width of the inline box in the hypothetical - line box is greater than maxWidth CSS pixels, - then change font to have a more condensed font - (if one is available or if a reasonably readable one can be - synthesized by applying a horizontal scale factor to the font) or a - smaller font, and return to the previous step.
Let the anchor point be a point on the
- inline box, determined by the textAlign
and textBaseline
values, as
- follows:
Horizontal position:
- -textAlign
is left
textAlign
is start
and the directionality of the
- canvas
element is 'ltr'textAlign
is end
and the directionality of the
- canvas
element is 'rtl'textAlign
is right
textAlign
is end
and the directionality of the
- canvas
element is 'ltr'textAlign
is start
and the directionality of the
- canvas
element is 'rtl'textAlign
is center
Vertical position:
- -textBaseline
is top
textBaseline
is hanging
textBaseline
is middle
textBaseline
is alphabetic
textBaseline
is ideographic
textBaseline
is bottom
Paint the hypothetical inline box as the shape given by the - text's glyphs, as transformed by the current transformation - matrix, and anchored and sized so that before applying the - current transformation - matrix, the anchor point is at (x, y) and each CSS pixel is - mapped to one coordinate space unit.
- -For fillText()
- fillStyle
must be
- applied to the glyphs and strokeStyle
must be
- ignored. For strokeText()
the reverse
- holds and strokeStyle
must be
- applied to the glyph outlines and fillStyle
must be
- ignored.
Text is painted without affecting the current path, and is - subject to shadow effects, global alpha, the clipping region, and global composition - operators.
- -The measureText()
- method takes one argument, text. When the method
- is invoked, the user agent must replace all the space characters in text with
- U+0020 SPACE characters, and then must form a hypothetical
- infinitely wide CSS line box containing a single inline box
- containing the text text, with all the
- properties at their initial values except the 'white-space' property
- of the inline element set to 'pre' and the 'font' property of the
- inline element set to the current font of the context as given by
- the font
attribute, and
- must then return a new TextMetrics
object with its
- width
attribute set to
- the width of that inline box, in CSS pixels. [CSS]
The TextMetrics
interface is used for the objects
- returned from measureText()
. It has one
- attribute, width
, which is set
- by the measureText()
- method.
Glyphs rendered using fillText()
and strokeText()
can spill out
- of the box given by the font size (the em square size) and the width
- returned by measureText()
(the text
- width). This version of the specification does not provide a way to
- obtain the bounding box dimensions of the text. If the text is to be
- rendered and removed, care needs to be taken to replace the entire
- area of the canvas that the clipping region covers, not just the box
- given by the em square height and measured text width.
A future version of the 2D context API may provide a - way to render fragments of documents, rendered using CSS, straight - to the canvas. This would be provided in preference to a dedicated - way of doing multiline layout.
- - - -To draw images onto the canvas, the drawImage
method
- can be used.
This method can be invoked with three different sets of arguments:
-drawImage(image, dx, dy)
- drawImage(image, dx, dy, dw, dh)
- drawImage(image, sx, sy, sw, sh, dx, dy, dw, dh)
- Each of those three can take either an
- HTMLImageElement
, an HTMLCanvasElement
, or
- an HTMLVideoElement
for the image
- argument.
drawImage(image, dx, dy)
+ drawImage(image, dx, dy, dw, dh)
+ drawImage(image, sx, sy, sw, sh, dx, dy, dw, dh)
+ drawImage
(image, dx, dy)drawImage
(image, dx, dy, dw, dh)drawImage
(image, sx, sy, sw, sh, dx, dy, dw, dh)drawImage
(image, dx, dy)drawImage
(image, dx, dy, dw, dh)drawImage
(image, sx, sy, sw, sh, dx, dy, dw, dh)Draws the given image onto the canvas. The arguments are interpreted as follows:
- + -If the first argument isn't an img
,
- canvas
, or video
element, throws a
- TYPE_MISMATCH_ERR
exception. If the image has no
- image data, throws an INVALID_STATE_ERR
exception. If
- the one of the source rectangle dimensions is zero, throws an
- INDEX_SIZE_ERR
exception. If the image isn't yet
- fully decoded, then nothing is drawn.
If the image isn't yet fully decoded, then nothing is drawn. If the image is a canvas with no
+ data, throws an "InvalidStateError
" DOMException
.
If not specified, the dw and dh arguments must default to the values of sw and sh, interpreted such that
- one CSS pixel in the image is treated as one unit in the canvas
- coordinate space. If the sx, sy, sw, and sh arguments are omitted, they must default to 0, 0,
- the image's intrinsic width in image pixels, and the image's
- intrinsic height in image pixels, respectively. If the image has no
- intrinsic dimensions, the concrete object size must be used
- instead, as determined using the CSS "Concrete
- Object Size Resolution" algorithm, with the specified
- size having neither a definite width nor height, nor any
- additional contraints, the object's intrinsic properties being those
- of the image argument, and the default object
- size being the size of the canvas
element. [CSSIMAGES]
When the drawImage()
method is invoked, the user
+ agent must run these steps:
The image argument is an instance of either
- HTMLImageElement
, HTMLCanvasElement
, or
- HTMLVideoElement
.
If any of the arguments are infinite or NaN, then abort these steps.
Check the usability of the image argument. If this + returns aborted, then an exception has been thrown and the method doesn't return anything; + abort these steps. If it returns bad, then abort these steps without drawing anything. + Otherwise it returns good; continue with these steps.
If the image argument is an
- HTMLImageElement
object that is not fully decodable, or if the image argument is an HTMLVideoElement
- object whose readyState
- attribute is either HAVE_NOTHING
or HAVE_METADATA
, then the
- implementation must return without drawing anything.
Establish the source and destination rectangles as follows:
-If the image argument is an
- HTMLCanvasElement
object with either a horizontal
- dimension or a vertical dimension equal to zero, then the
- implementation must raise an INVALID_STATE_ERR
- exception.
If not specified, the dw and dh arguments must default to the values of + sw and sh, interpreted such that one CSS pixel + in the image is treated as one unit in the output bitmap's coordinate space. If the + sx, sy, sw, and sh arguments are omitted, then they + must default to 0, 0, the image's intrinsic width in image pixels, and the image's + intrinsic height in image pixels, respectively. If the image has no intrinsic + dimensions, then the concrete object size must be used instead, as determined + using the CSS "Concrete Object + Size Resolution" algorithm, with the specified size having neither a definite width + nor height, nor any additional constraints, the object's intrinsic properties being those of the + image argument, and the default object size being the size of the + output bitmap. [CSSIMAGES]
-The source rectangle is the rectangle whose corners are the four - points (sx, sy), (sx+sw, sy), (sx+sw, sy+sh), (sx, sy+sh).
+The source rectangle is the rectangle whose corners are the four points (sx, sy), (sx+sw, sy), (sx+sw, sy+sh), + (sx, sy+sh).
-If one of the sw or sh
- arguments is zero, the implementation must raise an
- INDEX_SIZE_ERR
exception.
The destination rectangle is the rectangle whose corners are the four points (dx, dy), (dx+dw, dy), (dx+dw, dy+dh), + (dx, dy+dh).
-The destination rectangle is the rectangle whose corners are the - four points (dx, dy), - (dx+dw, dy), (dx+dw, dy+dh), (dx, dy+dh).
+When the source rectangle is outside the source image, the source rectangle must be clipped + to the source image and the destination rectangle must be clipped in the same proportion.
-When drawImage()
is
- invoked, the region of the image specified by the source rectangle
- must be painted on the region of the canvas specified by the
- destination rectangle, after applying the current transformation
- matrix to the points of the destination rectangle.
When the destination rectangle is outside the destination image (the + output bitmap), the pixels that land outside the output bitmap are + discarded, as if the destination was an infinite canvas whose rendering was clipped to the + dimensions of the output bitmap.
-The original image data of the source image must be used, not the
- image as it is rendered (e.g. width
and height
attributes on the source
- element have no effect). The image data must be processed in the
- original direction, even if the dimensions given are negative.
If one of the sw or sh arguments is zero, then abort + these steps. Nothing is painted.
This specification does not define the algorithm to - use when scaling the image, if necessary.
+Paint the region of the image argument specified by the source rectangle + on the region of the rendering context's output bitmap specified by the + destination rectangle, after applying the current + transformation matrix to the destination rectangle.
-When a canvas is drawn onto itself, the drawing - model requires the source to be copied before the image is drawn - back onto the canvas, so it is possible to copy parts of a canvas - onto overlapping parts of itself.
+The image data must be processed in the original direction, even if the dimensions given are + negative.
-If the original image data is a bitmap image, the value painted - at a point in the destination rectangle is computed by filtering the - original image data. The user agent may use any filtering algorithm - (for example bilinear interpolation or nearest-neighbor). When the - filtering algorithm requires a pixel value from outside the original - image data, it must instead use the value from the nearest edge - pixel. (That is, the filter uses 'clamp-to-edge' behavior.)
- - +When scaling up, if the imageSmoothingEnabled
attribute is set to
+ true, the user agent should attempt to apply a smoothing algorithm to the image data when it is
+ scaled. User agents which support multiple filtering algorithms may use the value of the imageSmoothingQuality
attribute to guide
+ the choice of filtering algorithm when the imageSmoothingEnabled
attribute is set to
+ true. Otherwise, the image must be rendered using nearest-neighbor interpolation.
When the drawImage()
method
- is passed an animated image as its image
- argument, the user agent must use the poster frame of the animation,
- or, if there is no poster frame, the first frame of the
- animation.
This specification does not define the precise algorithm to use when scaling an
+ image down, or when scaling an image up when the imageSmoothingEnabled
attribute is set to
+ true.
When the image argument is an
- HTMLVideoElement
, then the frame at the current
- playback position must be used as the source image, and the
- source image's dimensions must be the intrinsic width and
- intrinsic height
- of the media resource (i.e. after any aspect-ratio
- correction has been applied).
When a canvas
element is drawn onto itself, the drawing
+ model requires the source to be copied before the image is drawn, so it is possible to
+ copy parts of a canvas
element onto overlapping parts of itself.
Images are painted without affecting the current path, and are - subject to shadow effects, global alpha, the clipping region, and global composition - operators.
+If the original image data is a bitmap image, then the value painted at a point in the + destination rectangle is computed by filtering the original image data. The user agent may use + any filtering algorithm (for example bilinear interpolation or nearest-neighbor). When the + filtering algorithm requires a pixel value from outside the original image data, it must instead + use the value from the nearest edge pixel. (That is, the filter uses 'clamp-to-edge' behavior.) + When the filtering algorithm requires a pixel value from outside the source rectangle but inside + the original image data, then the value from the original image data must be used.
+ + -Thus, scaling an image in parts or in whole will have the same effect. This does
+ mean that when sprites coming from a single sprite sheet are to be scaled, adjacent images in
+ the sprite sheet can interfere. This can be avoided by ensuring each sprite in the sheet is
+ surrounded by a border of transparent black, or by copying sprites to be scaled into temporary
+ canvas
elements and drawing the scaled sprites from there.
Images are painted without affecting the current path, and are subject to shadow effects, global + alpha, the clipping region, and global composition operators.
+ +If the image argument is not origin-clean, then set the
+ CanvasRenderingContext2D
's origin-clean flag to false.
ImageData
(sw, sh)createImageData
(sw, sh)createImageData
(sw, sh)Returns an ImageData
object with the given dimensions. All the pixels in the
+ returned object are transparent black.
Throws an "IndexSizeError
" DOMException
if either of
+ the width or height arguments are zero.
Returns an ImageData
object with the given
- dimensions in CSS pixels (which might map to a different number of
- actual device pixels exposed by the object itself). All the pixels
- in the returned object are transparent black.
createImageData
(imagedata)Returns an ImageData
object with the same dimensions as the argument. All the
+ pixels in the returned object are transparent black.
createImageData
(imagedata)ImageData
(data, sw [, sh ] )Returns an ImageData
object using the data provided in the Uint8ClampedArray
argument, interpreted using the given
+ dimensions.
Returns an ImageData
object with the same
- dimensions as the argument. All the pixels in the returned object
- are transparent black.
As each pixel in the data is represented by four numbers, the length of the data needs to be + a multiple of four times the given width. If the height is provided as well, then the length + needs to be exactly the width times the height times 4.
-Throws an "IndexSizeError
" DOMException
if the given
+ data and dimensions can't be interpreted consistently, or if either dimension is zero.
getImageData
(sx, sy, sw, sh)getImageData
(sx, sy, sw, sh)Returns an ImageData
object containing the image data for the given rectangle of
+ the bitmap.
Returns an ImageData
object containing the image
- data for the given rectangle of the canvas.
Throws an "IndexSizeError
" DOMException
if the either
+ of the width or height arguments are zero.
Throws a NOT_SUPPORTED_ERR
exception if any of the
- arguments are not finite. Throws an INDEX_SIZE_ERR
- exception if the either of the width or height arguments are
- zero.
width
height
Returns the actual dimensions of the data in the ImageData
object, in
+ pixels.
width
height
data
Returns the one-dimensional array containing the data in RGBA order, as integers in the range + 0 to 255.
-Returns the actual dimensions of the data in the ImageData
object, in device pixels.
putImageData
(imagedata, dx, dy [, dirtyX, dirtyY, dirtyWidth, dirtyHeight ] )Paints the data from the given ImageData
object onto the bitmap. If a dirty
+ rectangle is provided, only the pixels from that rectangle are painted.
data
The globalAlpha
and globalCompositeOperation
attributes, as
+ well as the shadow attributes, are ignored for the purposes of this method call; pixels in the
+ canvas are replaced wholesale, with no composition, alpha blending, no shadows, etc.
Throws an "InvalidStateError
" DOMException
if the
+ imagedata object's data's [[Detached]] internal slot value is true.
Returns the one-dimensional array containing the data in RGBA order, as integers in the range 0 to 255.
+putImageData
(imagedata, dx, dy [, dirtyX, dirtyY, dirtyWidth, dirtyHeight ])Objects that implement the CanvasImageData
interface provide the following methods
+ for reading and writing pixel data to the bitmap.
The ImageData()
constructors and the
+ createImageData()
methods are
+ used to instantiate new ImageData
objects.
Paints the data from the given ImageData
object
- onto the canvas. If a dirty rectangle is provided, only the pixels
- from that rectangle are painted.
When the ImageData()
constructor is invoked with two
+ numeric arguments sw and sh, it must create an ImageData
+ object with parameter pixelsPerRow set to sw, and rows set
+ to sh. The image data of the newly created ImageData
object must be
+ initialized to transparent black. If both sw and sh are non-zero, then
+ return the new ImageData
object. If one or both of sw and sh
+ are zero, then the constructor must throw an "IndexSizeError
"
+ DOMException
instead.
The globalAlpha
- and globalCompositeOperation
- attributes, as well as the shadow attributes, are ignored for the
- purposes of this method call; pixels in the canvas are replaced
- wholesale, with no composition, alpha blending, no shadows,
- etc.
When the ImageData()
constructor is invoked with its first
+ argument being an Uint8ClampedArray
source
+ and its second and optional third arguments being numeric arguments sw and
+ sh, it must run these steps:
If the first argument is null, throws a
- TYPE_MISMATCH_ERR
exception. Throws a
- NOT_SUPPORTED_ERR
exception if any of the other
- arguments are not finite.
Let length be the number of bytes in source.
If length is not a non-zero integral multiple of four, then throw an
+ "InvalidStateError
" DOMException
and abort these
+ steps.
Let length be length divided by four.
-If length is not an integral multiple of sw, then throw an
+ "IndexSizeError
" DOMException
and abort these steps.
The createImageData()
- method is used to instantiate new blank ImageData
- objects. When the method is invoked with two arguments sw and sh, it must return an
- ImageData
object representing a rectangle with a width
- in CSS pixels equal to the absolute magnitude of sw and a height in CSS pixels equal to the absolute
- magnitude of sh. When invoked with a single imagedata argument, it must return an
- ImageData
object representing a rectangle with the same
- dimensions as the ImageData
object passed as the
- argument. The ImageData
object returned must be filled
- with transparent black.
At this step, the length is guaranteed to be greater than zero (otherwise the + second step above would have aborted the steps), so if sw is zero, this + step will throw the exception and abort these steps.
-The getImageData(sx, sy, sw,
- sh)
method must return an
- ImageData
object representing the underlying pixel data
- for the area of the canvas denoted by the rectangle whose corners are
- the four points (sx, sy),
- (sx+sw, sy), (sx+sw, sy+sh), (sx, sy+sh), in canvas
- coordinate space units. Pixels outside the canvas must be returned
- as transparent black. Pixels must be returned as non-premultiplied
- alpha values.
Let height be length divided by sw.
If the sh argument was not omitted, and its value is not equal to
+ height, then throw an "IndexSizeError
"
+ DOMException
and abort these steps.
If any of the arguments to createImageData()
or
- getImageData()
- are infinite or NaN, the method must instead raise a
- NOT_SUPPORTED_ERR
exception. If either the sw or sh arguments are zero,
- the method must instead raise an INDEX_SIZE_ERR
- exception.
Create an ImageData
object, with parameter pixelsPerRow
+ set to sw, rows set to sh, and using source.
+ Return the newly created ImageData
object.
ImageData
objects must be initialized so that their
- width
attribute
- is set to w, the number of physical device
- pixels per row in the image data, their height
attribute is
- set to h, the number of rows in the image data,
- and their data
- attribute is initialized to a CanvasPixelArray
object
- holding the image data. At least one pixel's worth of image data
- must be returned.
The resulting object's data is not a copy of source, it's
+ the actual Uint8ClampedArray
object passed as the
+ first argument to the constructor.
The CanvasPixelArray
object provides ordered,
- indexed access to the color components of each pixel of the image
- data. The data must be represented in left-to-right order, row by
- row top to bottom, starting with the top left, with each pixel's
- red, green, blue, and alpha components being given in that order for
- each pixel. Each component of each device pixel represented in this
- array must be in the range 0..255, representing the 8 bit value for
- that component. The components must be assigned consecutive indices
- starting with 0 for the top left pixel's red component.
The CanvasPixelArray
object thus represents h×w×4 integers. The
- length
- attribute of a CanvasPixelArray
object must return this
- number.
When the createImageData()
method is
+ invoked with two numeric arguments sw and sh, it must create an
+ ImageData
object, with parameter pixelsPerRow set to the
+ absolute magnitude of sw, and parameter rows set to the absolute magnitude
+ of sh. Initialize the image data of the new ImageData
object to
+ transparent black. If both sw and sh are non-zero, then return
+ the new ImageData
object. If one or both of sw and sh are
+ zero, then throw an "IndexSizeError
" DOMException
+ instead.
The object's supported property indices are the - numbers in the range 0 .. h×w×4-1.
+When the createImageData()
method is
+ invoked with a single imagedata argument, it must create an
+ ImageData
object, with parameter pixelsPerRow set to the value of
+ the width
attribute of the ImageData
+ object passed as the argument, and the rows parameter set to the value of the
+ height
attribute.
+ Initialize the image data of the new ImageData
object to transparent black. Return
+ the newly created ImageData
object.
To determine the value of - an indexed property index, the user agent - must return the value of the indexth component - in the array.
+The getImageData(sx,
+ sy, sw, sh)
method, when invoked, must,
+ if either the sw or sh arguments are zero, throw an
+ "IndexSizeError
" DOMException
; otherwise,
+
+ if the CanvasRenderingContext2D
's origin-clean flag is set to false, it must throw a
+ "SecurityError
" DOMException
;
+
+ otherwise, it must create an ImageData
object, with parameter
+ pixelsPerRow set to sw, and parameter rows set to sh.
+ Set the pixel values of the image data of the newly created ImageData
object to
+ represent the output bitmap for the area of that bitmap denoted by the rectangle
+ whose corners are the four points (sx, sy), (sx+sw, sy), (sx+sw, sy+sh), (sx,
+ sy+sh), in the bitmap's coordinate space units.
+ Pixels outside the output bitmap must be set to transparent black. Pixel values
+ must not be premultiplied by alpha.
To set the value of an - existing indexed property index to value - value, the value of the indexth component in the array must be set to value.
+When the user agent is required to create an ImageData
object, given a
+ positive integer number of rows rows, a positive integer number of pixels per row
+ pixelsPerRow, and an optional Uint8ClampedArray
source, it must run these
+ steps:
The width and height (w and h) might be different from the sw - and sh arguments to the above methods, e.g. if - the canvas is backed by a high-resolution bitmap, or if the sw and sh arguments are - negative.
+Let imageData be a new uninitialized ImageData
object.
If source is specified, then assign the data
attribute of imageData to
+ source.
If source is not specified, then initialize the data
attribute of imageData to a new Uint8ClampedArray
object. The Uint8ClampedArray
object must use a new Canvas
+ Pixel ArrayBuffer
for its storage, and must have a
+ zero start offset and a length equal to the length of its storage, in bytes. The Canvas
+ Pixel ArrayBuffer
must have the correct size to
+ store rows × pixelsPerRow pixels.
The putImageData(imagedata, dx, dy, dirtyX, dirtyY, dirtyWidth, dirtyHeight)
method writes data from
- ImageData
structures back to the canvas.
If the Canvas Pixel ArrayBuffer
cannot be
+ allocated, then rethrow the RangeError
thrown by JavaScript,
+ and abort these steps.
Initialize the width
attribute of
+ imageData to pixelsPerRow.
Initialize the height
attribute of
+ imageData to rows.
Return imageData.
If any of the arguments to the method are infinite or NaN, the
- method must raise a NOT_SUPPORTED_ERR
exception.
ImageData
objects are serializable objects. Their serialization
+ steps, given value and serialized, are:
When the last four arguments are omitted, they must be assumed to
- have the values 0, 0, the width
member of the imagedata structure, and the height
member of the imagedata structure, respectively.
Set serialized.[[Data]] to the sub-serialization of the value of
+ value's data
attribute.
Set serialized.[[Width]] to the value of value's width
attribute.
Set serialized.[[Height]] to the value of value's height
attribute.
When invoked with arguments that do not, per the last few
- paragraphs, cause an exception to be raised, the putImageData()
method
- must act as follows:
Their deserialization steps, given serialized and value, + are:
+ +Initialize value's data
attribute
+ to the sub-deserialization of serialized.[[Data]].
Initialize value's width
attribute
+ to serialized.[[Width]].
Initialize value's height
attribute
+ to serialized.[[Height]].
A Canvas Pixel ArrayBuffer
is an ArrayBuffer
whose data is represented in left-to-right order, row
+ by row top to bottom, starting with the top left, with each pixel's red, green, blue, and alpha
+ components being given in that order for each pixel. Each component of each pixel represented in
+ this array must be in the range 0..255, representing the 8 bit value for that component. The
+ components must be assigned consecutive indices starting with 0 for the top left pixel's red
+ component.
The putImageData()
method writes
+ data from ImageData
structures back to the rendering context's output
+ bitmap. Its arguments are: imagedata, dx, dy,
+ dirtyX, dirtyY, dirtyWidth, and dirtyHeight.
When the last four arguments to this method are omitted, they must be assumed to have the
+ values 0, 0, the width
member of the imagedata structure, and the height
+ member of the imagedata structure, respectively.
The method, when invoked, must act as follows:
Let dxdevice be the x-coordinate - of the device pixel in the underlying pixel data of the canvas - corresponding to the dx coordinate in the - canvas coordinate space.
+If imagedata's data
attribute value's
+ [[Detached]] internal slot value is true, then throw an
+ "InvalidStateError
" DOMException
and abort these
+ steps.
Let dydevice be the y-coordinate - of the device pixel in the underlying pixel data of the canvas - corresponding to the dy coordinate in the - canvas coordinate space.
+If dirtyWidth is negative, then let dirtyX be dirtyX+dirtyWidth, and let dirtyWidth be equal + to the absolute magnitude of dirtyWidth.
-If dirtyHeight is negative, then let dirtyY be dirtyY+dirtyHeight, and let dirtyHeight be + equal to the absolute magnitude of dirtyHeight.
-If dirtyWidth is negative, let dirtyX be dirtyX+dirtyWidth, and let dirtyWidth be equal to the absolute magnitude of - dirtyWidth.
+If dirtyHeight is negative, let dirtyY be dirtyY+dirtyHeight, and let dirtyHeight be equal to the absolute magnitude of - dirtyHeight.
+If dirtyX is negative, then let dirtyWidth be dirtyWidth+dirtyX, and let dirtyX be zero.
-If dirtyY is negative, then let dirtyHeight be dirtyHeight+dirtyY, and let dirtyY be zero.
-If dirtyX is negative, let dirtyWidth be dirtyWidth+dirtyX, and - let dirtyX be zero.
+If dirtyX+dirtyWidth is greater than the width
attribute of the imagedata argument, then
+ let dirtyWidth be the value of that width
+ attribute, minus the value of dirtyX.
If dirtyY is negative, let dirtyHeight be dirtyHeight+dirtyY, and - let dirtyY be zero.
+If dirtyY+dirtyHeight is greater than the height
attribute of the imagedata argument, then
+ let dirtyHeight be the value of that height
attribute, minus the value of dirtyY.
If, after those changes, either dirtyWidth or dirtyHeight are negative + or zero, then abort these steps without affecting any bitmaps.
-If dirtyX+dirtyWidth is greater than the width
attribute of the imagedata argument, let dirtyWidth be the value of that width
attribute, minus the
- value of dirtyX.
For all integer values of x and y where dirtyX ≤ x < dirtyX+dirtyWidth and dirtyY ≤ y < dirtyY+dirtyHeight, copy the
+ four channels of the pixel with coordinate (x, y) in
+ the imagedata data structure's Canvas Pixel
+ ArrayBuffer
to the pixel with coordinate (dx+x, dy+y)
+ in the rendering context's output bitmap.
If dirtyY+dirtyHeight is greater than the height
attribute of the imagedata argument, let dirtyHeight be the value of that height
attribute, minus the
- value of dirtyY.
Due to the lossy nature of converting to and from premultiplied alpha color
+ values, pixels that have just been set using putImageData()
might be returned to an equivalent
+ getImageData()
as different values.
The current path, transformation matrix, + shadow attributes, global + alpha, the clipping region, and global composition operator must not + affect the methods described in this section.
-If, after those changes, either dirtyWidth - or dirtyHeight is negative or zero, stop these - steps without affecting the canvas.
- -Otherwise, for all integer values of x - and y where dirtyX ≤ x < dirtyX+dirtyWidth - and dirtyY ≤ y < dirtyY+dirtyHeight, copy the four channels of - the pixel with coordinate (x, y) in the imagedata data - structure to the pixel with coordinate (dxdevice+x, - dydevice+y) in the underlying pixel data of the - canvas.
The handling of pixel rounding when the specified coordinates do - not exactly map to the device coordinate space is not defined by - this specification, except that the following must result in no - visible changes to the rendering:
- -context.putImageData(context.getImageData(x, y, w, h), p, q);- -
...for any value of x, y, - w, and h and where p is the smaller of x and the sum - of x and w, and q is the smaller of y and the sum - of y and h; and except that - the following two calls:
- -context.createImageData(w, h); -context.getImageData(0, 0, w, h);- -
...must return ImageData
objects with the same
- dimensions, for any value of w and h. In other words, while user agents may round the
- arguments of these methods so that they map to device pixel
- boundaries, any rounding performed must be performed consistently
- for all of the createImageData()
, getImageData()
and putImageData()
- operations.
Due to the lossy nature of converting to and from
- premultiplied alpha color values, pixels that have just been set
- using putImageData()
might be
- returned to an equivalent getImageData()
as
- different values.
The current path, transformation matrix,
- shadow attributes, global alpha, the clipping region, and global composition
- operator must not affect the getImageData()
and putImageData()
- methods.
The data returned by getImageData()
is at the
- resolution of the canvas backing store, which is likely to not be
- one device pixel to each CSS pixel if the display used is a high
- resolution display.
In the following example, the script generates an
- ImageData
object so that it can draw onto it.
In the following example, the script generates an ImageData
object so that it can
+ draw onto it.
// canvas is a reference to a <canvas> element var context = canvas.getContext('2d'); @@ -3348,11 +3318,11 @@ function AddCloud(data, x, y) { ... }
Here is an example of using getImageData()
and putImageData()
to
- implement an edge detection filter.
Here is an example of using getImageData()
and putImageData()
to implement an edge detection
+ filter.
<!DOCTYPE HTML> -<html> +<html lang="en"> <head> <title>Edge detection demo</title> <script> @@ -3375,9 +3345,9 @@ function AddCloud(data, x, y) { ... }var output = context.createImageData(canvas.width, canvas.height); // alias some variables for convenience - // notice that we are using input.width and input.height here - // as they might not be the same as canvas.width and canvas.height - // (in particular, they might be different on high-res displays) + // In this case input.width and input.height + // match canvas.width and canvas.height + // but we'll use the former to keep the code generic. var w = input.width, h = input.height; var inputData = input.data; var outputData = output.data; @@ -3408,102 +3378,322 @@ function AddCloud(data, x, y) { ... }
When a shape or image is painted, user agents must follow these - steps, in the order given (or act as if they do):
- -Render the shape or image onto an infinite transparent black - bitmap, creating image A, as described in the - previous sections. For shapes, the current fill, stroke, and line - styles must be honored, and the stroke must itself also be - subjected to the current transformation matrix.
When shadows are drawn, render the shadow from - image A, using the current shadow styles, - creating image B.
When shadows are drawn, multiply the alpha
- component of every pixel in B by globalAlpha
.
When shadows are drawn, composite B within the clipping region over the - current canvas bitmap using the current composition - operator.
Multiply the alpha component of every pixel in A by globalAlpha
.
Composite A within the clipping - region over the current canvas bitmap using the current - composition operator.
globalAlpha
[ = value ]Returns the current alpha value applied to rendering operations.
+ +Can be set, to change the alpha value. Values outside of the range 0.0 .. 1.0 are + ignored.
+ +globalCompositeOperation
[ = value ]Returns the current composition operation, from the values defined in the Compositing and + Blending specification. [COMPOSITE].
+ +Can be set, to change the composition operation. Unknown values are ignored.
+ +Spec bugs: 27313
All drawing operations on an object which implements the CanvasCompositing
+ interface are affected by the global compositing attributes, globalAlpha
and globalCompositeOperation
.
The globalAlpha
attribute gives an
+ alpha value that is applied to shapes and images before they are composited onto the output
+ bitmap. The value must be in the range from 0.0 (fully transparent) to 1.0 (no additional
+ transparency). If an attempt is made to set the attribute to a value outside this range, including
+ Infinity and Not-a-Number (NaN) values, then the attribute must retain its previous value. When
+ the context is created, the globalAlpha
attribute
+ must initially have the value 1.0.
The globalCompositeOperation
attribute
+ sets the current composition operator, which controls how shapes and images are drawn onto the
+ output bitmap, once they have had globalAlpha
and the current transformation matrix
+ applied. The possible values are those defined in the Compositing and Blending specification, and
+ include the values source-over
+ and copy
.
+ [COMPOSITE]
These values are all case-sensitive — they must be used exactly as defined. User agents + must not recognize values that are not a case-sensitive match for one of the values + given in the Compositing and Blending specification. [COMPOSITE]
+ +On setting, if the user agent does not recognize the specified value, it must be ignored,
+ leaving the value of globalCompositeOperation
unaffected.
+ Otherwise, the attribute must be set to the given new value.
When the context is created, the globalCompositeOperation
attribute must
+ initially have the value source-over
.
imageSmoothingEnabled
[ = value ]Returns whether pattern fills and the drawImage()
method will attempt to smooth images if
+ their pixels don't line up exactly with the display, when scaling images up.
Can be set, to change whether images are smoothed (true) or not (false).
+ +imageSmoothingQuality
[ = value ]Returns the current image-smoothing-quality preference.
+ +Can be set, to change the preferred quality of image smoothing. The possible values are
+ "low
", "medium
" and "high
". Unknown values are ignored.
Objects that implement the CanvasImageSmoothing
interface have attributes that
+ control how image smoothing is performed.
The imageSmoothingEnabled
+ attribute, on getting, must return the last value it was set to. On setting, it must be set to the
+ new value. When the object implementing the CanvasImageSmoothing
interface is
+ created, the attribute must be set to true.
The imageSmoothingQuality
+ attribute, on getting, must return the last value it was set to. On setting, it must be set to the
+ new value. When the object implementing the CanvasImageSmoothing
interface is
+ created, the attribute must be set to "low
".
All drawing operations on an object which implements the CanvasShadowStyles
+ interface are affected by the four global shadow attributes.
shadowColor
[ = value ]Returns the current shadow color.
+ +Can be set, to change the shadow color. Values that cannot be parsed as CSS colors are ignored.
+ +shadowOffsetX
[ = value ]shadowOffsetY
[ = value ]Returns the current shadow offset.
+ +Can be set, to change the shadow offset. Values that are not finite numbers are ignored.
+ +shadowBlur
[ = value ]Returns the current level of blur applied to shadows.
+ +Can be set, to change the blur level. Values that are not finite numbers greater than or + equal to zero are ignored.
+ +The shadowColor
attribute sets the
+ color of the shadow.
When the context is created, the shadowColor
+ attribute initially must be fully-transparent black.
On getting, the serialization of the color + must be returned.
+ +On setting, the new value must be parsed as a CSS <color> value and the + color assigned. If the value cannot be parsed as a CSS <color> value then it + must be ignored, and the attribute must retain its previous value. [CSSCOLOR]
+ +The shadowOffsetX
and shadowOffsetY
attributes specify the distance
+ that the shadow will be offset in the positive horizontal and positive vertical distance
+ respectively. Their values are in coordinate space units. They are not affected by the current
+ transformation matrix.
When the context is created, the shadow offset attributes must initially have the value + 0.
+ +On getting, they must return their current value. On setting, the attribute being set must be + set to the new value, except if the value is infinite or NaN, in which case the new value must be + ignored.
+ +The shadowBlur
attribute specifies
+ the level of the blurring effect. (The units do not map to coordinate space units, and are not
+ affected by the current transformation matrix.)
When the context is created, the shadowBlur
+ attribute must initially have the value 0.
On getting, the attribute must return its current value. On setting the attribute must be set + to the new value, except if the value is negative, infinite or NaN, in which case the new value + must be ignored.
+ +Shadows are only drawn if the opacity component of
+ the alpha component of the color of shadowColor
is
+ non-zero and either the shadowBlur
is non-zero, or
+ the shadowOffsetX
is non-zero, or the shadowOffsetY
is non-zero.
When shadows are drawn, they must be rendered as follows:
+ +Let A be an infinite transparent black bitmap on which the source + image for which a shadow is being created has been rendered.
Let B be an infinite transparent black bitmap, with a coordinate + space and an origin identical to A.
Copy the alpha channel of A to B, offset by shadowOffsetX
in the positive x
+ direction, and shadowOffsetY
in the positive
+ y direction.
If shadowBlur
is greater than 0:
Let σ be half the value of shadowBlur
.
Perform a 2D Gaussian Blur on B, using σ + as the standard deviation.
User agents may limit values of σ to an implementation-specific + maximum value to avoid exceeding hardware limitations during the Gaussian blur operation.
+ +Set the red, green, and blue components of every pixel in B to the
+ red, green, and blue components (respectively) of the color of shadowColor
.
Multiply the alpha component of every pixel in B by the alpha
+ component of the color of shadowColor
.
The shadow is in the bitmap B, and is rendered as part of the + drawing model described below.
If the current composition operation is copy
, then shadows
+ effectively won't render (since the shape will overwrite the shadow).
All drawing operations on an object which implements the CanvasFilters
+ interface are affected by the global filter
+ attribute.
filter
[ = value ]Returns the current filter.
+ +Can be set, to change the filter. Values that cannot be parsed as a + <filter-function-list> value are ignored.
+ +The filter
attribute, on getting, must
+ return the last value it was successfully set to. The value must not be re-serialized. On setting,
+ if the new value is 'none' (not the empty string, null, or undefined), filters must be disabled
+ for the context. Otherwise, the value must be parsed as a
+ <filter-function-list> value. If the value cannot be parsed as a
+ <filter-function-list> value, where using property-independent style sheet
+ syntax like 'inherit' or 'initial' is considered an invalid value, then it must be ignored, and
+ the attribute must retain its previous value. When creating the object implementing the
+ CanvasFilters
interface, the attribute must be set to 'none'.
A <filter-function-list> value consists of a sequence of one or more + filter functions or references to SVG filters. The input to the filter is used as the input + to the first item in the list. Subsequent items take the output of the previous item as + their input. [FILTERS]
+ +Coordinates used in the value of the filter
attribute are interpreted such that one pixel is
+ equivalent to one SVG user space unit and to one canvas coordinate space unit. Filter coordinates
+ are not affected by the current transformation
+ matrix. The current transformation matrix affects only the input to the filter. Filters
+ are applied in the output bitmap's coordinate space.
When the value of the filter
attribute defines
+ lengths using percentages or using 'em' or 'ex' units, these must be
+ interpreted relative to the computed value of the 'font-size' property
+ of the font style source object at the time that the attribute is set, if it is an
+ element. If the computed values are undefined for a
+ particular case (e.g. because the font style source object is not an element or is
+ not being rendered), then the relative keywords must be interpreted relative to the
+ default value of the font
attribute. The 'larger' and
+ 'smaller' keywords are not supported.
If the value of the filter
attribute refers to an
+ SVG filter in the same document, and this SVG filter changes, then the changed filter is used for
+ the next draw operation.
If the value of the filter
attribute refers to an
+ SVG filter in an external resource document and that document is not loaded when a drawing
+ operation is invoked, then the drawing operation must proceed with no filtering.
This section is non-normative.
-When a canvas is interactive, authors should include focusable - elements in the element's fallback content corresponding to each - focusable part of the canvas, as in the example above.
+Since drawing is performed using filter value 'none' until an externally-defined
+ filter has finished loading, authors might wish to determine whether such a filter
+ has finished loading before proceeding with a drawing operation. One way to accomplish
+ this is to load the externally-defined filter elsewhere within the same page in some
+ element that sends a load
event (for example, an SVG
+ use
element), and wait for the load
event to be
+ dispatched.
To indicate which focusable part of the canvas is currently
- focused, authors should use the drawSystemFocusRing()
- method, passing it the element for which a ring is being drawn. This
- method only draws the focus ring if the element is focused, so that
- it can simply be called whenever drawing the element, without
+
+
+
When a shape or image is painted, user agents must follow these steps, in the order given (or + act as if they do):
+ +Render the shape or image onto an infinite transparent black bitmap, creating image A, as described in the previous sections. For shapes, the current fill, stroke, + and line styles must be honored, and the stroke must itself also be subjected to the current + transformation matrix.
When the filter attribute is set to a value other than 'none' and all the
+ externally-defined filters it references, if any, are in documents that are currently loaded,
+ then use image A as the input to the filter
, creating image B. Otherwise, let
+ B be an alias for A.
When shadows are drawn, render the shadow from image B, + using the current shadow styles, creating image C.
When shadows are drawn, multiply the alpha component of every pixel in C by globalAlpha
.
When shadows are drawn, composite C within the + clipping region over the current output bitmap using the current + composition operator.
Multiply the alpha component of every pixel in B by globalAlpha
.
Composite B within the clipping region over the current + output bitmap using the current composition operator.
When compositing onto the output bitmap, pixels that would fall outside of the + output bitmap must be discarded.
+ + + + + + +When a canvas is interactive, authors should include focusable elements in the element's + fallback content corresponding to each focusable part of the canvas, as in the example above.
+ +When rendering focus rings, to ensure that focus rings have the appearance of native focus
+ rings, authors should use the drawFocusIfNeeded()
method, passing it the
+ element for which a ring is being drawn. This method only draws the focus ring if the element is
+ focused, so that it can simply be called whenever drawing the element, without
checking whether the element is focused or not first.
Authors should avoid implementing text editing controls using the
- canvas
element. Doing so has a large number of
- disadvantages:
In addition to drawing focus rings, authors should use the scrollPathIntoView()
method when an element in
+ the canvas is focused, to make sure it is visible on the screen (if applicable).
Authors should avoid implementing text editing controls
+ using the canvas
element. Doing so has a large number of disadvantages:
This is a huge amount of work, and authors are most strongly
- encouraged to avoid doing any of it by instead using the
- input
element, the textarea
element, or
- the contenteditable
- attribute.
This is a huge amount of work, and authors are most strongly encouraged to avoid doing any of
+ it by instead using the input
element, the textarea
element, or the
+ contenteditable
attribute.
This section is non-normative.
@@ -3554,140 +3744,835 @@ function AddCloud(data, x, y) { ... }The 2D rendering context for canvas
is often used for sprite-based games. The
+ following example demonstrates this:
Here is the source for this example:
-The canvas
APIs must perform color correction at
- only two points: when rendering images with their own gamma
- correction and color space information onto the canvas, to convert
- the image to the color space used by the canvas (e.g. using the 2D
- Context's drawImage()
- method with an HTMLImageElement
object), and when
- rendering the actual canvas bitmap to the output device.
Thus, in the 2D context, colors used to draw shapes
- onto the canvas will exactly match colors obtained through the getImageData()
- method.
The toDataURL()
method
- must not include color space information in the resource
- returned. Where the output format allows it, the color of pixels in
- resources created by toDataURL()
must match those
- returned by the getImageData()
- method.
In user agents that support CSS, the color space used by a
- canvas
element must match the color space used for
- processing any colors for that element in CSS.
The gamma correction and color space information of images must
- be handled in such a way that an image rendered directly using an
- img
element would use the same colors as one painted on
- a canvas
element that is then itself
- rendered. Furthermore, the rendering of images that have no color
- correction information (such as those returned by the toDataURL()
method) must be
- rendered with no color correction.
Thus, in the 2D context, calling the drawImage()
method to render
- the output of the toDataURL()
method to the
- canvas, given the appropriate dimensions, has no visible effect.
canvas
elementsInformation leakage can occur if scripts from - one origin can access information (e.g. read pixels) - from images from another origin (one that isn't the same).
- -To mitigate this, canvas
elements are defined to
- have a flag indicating whether they are origin-clean. All
- canvas
elements must start with their
- origin-clean set to true. The flag must be set to false if
- any of the following actions occur:
The element's 2D context's drawImage()
method is
- called with an HTMLImageElement
or an
- HTMLVideoElement
whose origin is not the
- same as that of the
- Document
object that owns the canvas
- element.
The element's 2D context's drawImage()
method is
- called with an HTMLCanvasElement
whose
- origin-clean flag is false.
The element's 2D context's fillStyle
attribute is set
- to a CanvasPattern
object that was created from an
- HTMLImageElement
or an HTMLVideoElement
- whose origin was not the same as that of the Document
object
- that owns the canvas
element when the pattern was
- created.
The element's 2D context's fillStyle
attribute is set
- to a CanvasPattern
object that was created from an
- HTMLCanvasElement
whose origin-clean flag was
- false when the pattern was created.
The element's 2D context's strokeStyle
attribute is
- set to a CanvasPattern
object that was created from an
- HTMLImageElement
or an HTMLVideoElement
- whose origin was not the same as that of the Document
object
- that owns the canvas
element when the pattern was
- created.
The element's 2D context's strokeStyle
attribute is
- set to a CanvasPattern
object that was created from an
- HTMLCanvasElement
whose origin-clean flag was
- false when the pattern was created.
The element's 2D context's fillText()
or strokeText()
methods are
- invoked and consider using a font that has an origin
- that is not the same as that of
- the Document
object that owns the canvas
- element. (The font doesn't even have to be used; all that matters
- is whether the font was considered for any of the glyphs
- drawn.)
Whenever the toDataURL()
method of a
- canvas
element whose origin-clean flag is set to
- false is called, the method must raise a SECURITY_ERR
- exception.
Whenever the getImageData()
method of
- the 2D context of a canvas
element whose
- origin-clean flag is set to false is called with otherwise
- correct arguments, the method must raise a SECURITY_ERR
- exception.
Whenever the measureText()
method of
- the 2D context of a canvas
element ends up using a font
- that has an origin that is not the same as that of the Document
object that
- owns the canvas
element, the method must raise a
- SECURITY_ERR
exception.
Even resetting the canvas state by changing its
- width
or height
attributes doesn't reset
- the origin-clean flag.
<!DOCTYPE HTML> +<meta charset="utf-8"> +<title>Blue Robot Demo</title> +<style> + html { overflow: hidden; min-height: 200px; min-width: 380px; } + body { height: 200px; position: relative; margin: 8px; } + .buttons { position: absolute; bottom: 0px; left: 0px; margin: 4px; } +</style> +<canvas width="380" height="200"></canvas> +<script> + var Landscape = function (context, width, height) { + this.offset = 0; + this.width = width; + this.advance = function (dx) { + this.offset += dx; + }; + this.horizon = height * 0.7; + // This creates the sky gradient (from a darker blue to white at the bottom) + this.sky = context.createLinearGradient(0, 0, 0, this.horizon); + this.sky.addColorStop(0.0, 'rgb(55,121,179)'); + this.sky.addColorStop(0.7, 'rgb(121,194,245)'); + this.sky.addColorStop(1.0, 'rgb(164,200,214)'); + // this creates the grass gradient (from a darker green to a lighter green) + this.earth = context.createLinearGradient(0, this.horizon, 0, height); + this.earth.addColorStop(0.0, 'rgb(81,140,20)'); + this.earth.addColorStop(1.0, 'rgb(123,177,57)'); + this.paintBackground = function (context, width, height) { + // first, paint the sky and grass rectangles + context.fillStyle = this.sky; + context.fillRect(0, 0, width, this.horizon); + context.fillStyle = this.earth; + context.fillRect(0, this.horizon, width, height-this.horizon); + // then, draw the cloudy banner + // we make it cloudy by having the draw text off the top of the + // canvas, and just having the blurred shadow shown on the canvas + context.save(); + context.translate(width-((this.offset+(this.width*3.2)) % (this.width*4.0))+0, 0); + context.shadowColor = 'white'; + context.shadowOffsetY = 30+this.horizon/3; // offset down on canvas + context.shadowBlur = '5'; + context.fillStyle = 'white'; + context.textAlign = 'left'; + context.textBaseline = 'top'; + context.font = '20px sans-serif'; + context.fillText('WHATWG ROCKS', 10, -30); // text up above canvas + context.restore(); + // then, draw the background tree + context.save(); + context.translate(width-((this.offset+(this.width*0.2)) % (this.width*1.5))+30, 0); + context.beginPath(); + context.fillStyle = 'rgb(143,89,2)'; + context.lineStyle = 'rgb(10,10,10)'; + context.lineWidth = 2; + context.rect(0, this.horizon+5, 10, -50); // trunk + context.fill(); + context.stroke(); + context.beginPath(); + context.fillStyle = 'rgb(78,154,6)'; + context.arc(5, this.horizon-60, 30, 0, Math.PI*2); // leaves + context.fill(); + context.stroke(); + context.restore(); + }; + this.paintForeground = function (context, width, height) { + // draw the box that goes in front + context.save(); + context.translate(width-((this.offset+(this.width*0.7)) % (this.width*1.1))+0, 0); + context.beginPath(); + context.rect(0, this.horizon - 5, 25, 25); + context.fillStyle = 'rgb(220,154,94)'; + context.lineStyle = 'rgb(10,10,10)'; + context.lineWidth = 2; + context.fill(); + context.stroke(); + context.restore(); + }; + }; +</script> +<script> + var BlueRobot = function () { + this.sprites = new Image(); + this.sprites.src = 'blue-robot.png'; // this sprite sheet has 8 cells + this.targetMode = 'idle'; + this.walk = function () { + this.targetMode = 'walk'; + }; + this.stop = function () { + this.targetMode = 'idle'; + }; + this.frameIndex = { + 'idle': [0], // first cell is the idle frame + 'walk': [1,2,3,4,5,6], // the walking animation is cells 1-6 + 'stop': [7], // last cell is the stopping animation + }; + this.mode = 'idle'; + this.frame = 0; // index into frameIndex + this.tick = function () { + // this advances the frame and the robot + // the return value is how many pixels the robot has moved + this.frame += 1; + if (this.frame >= this.frameIndex[this.mode].length) { + // we've reached the end of this animation cycle + this.frame = 0; + if (this.mode != this.targetMode) { + // switch to next cycle + if (this.mode == 'walk') { + // we need to stop walking before we decide what to do next + this.mode = 'stop'; + } else if (this.mode == 'stop') { + if (this.targetMode == 'walk') + this.mode = 'walk'; + else + this.mode = 'idle'; + } else if (this.mode == 'idle') { + if (this.targetMode == 'walk') + this.mode = 'walk'; + } + } + } + if (this.mode == 'walk') + return 8; + return 0; + }, + this.paint = function (context, x, y) { + if (!this.sprites.complete) return; + // draw the right frame out of the sprite sheet onto the canvas + // we assume each frame is as high as the sprite sheet + // the x,y coordinates give the position of the bottom center of the sprite + context.drawImage(this.sprites, + this.frameIndex[this.mode][this.frame] * this.sprites.height, 0, this.sprites.height, this.sprites.height, + x-this.sprites.height/2, y-this.sprites.height, this.sprites.height, this.sprites.height); + }; + }; +</script> +<script> + var canvas = document.getElementsByTagName('canvas')[0]; + var context = canvas.getContext('2d'); + var landscape = new Landscape(context, canvas.width, canvas.height); + var blueRobot = new BlueRobot(); + // paint when the browser wants us to, using requestAnimationFrame() + function paint() { + context.clearRect(0, 0, canvas.width, canvas.height); + landscape.paintBackground(context, canvas.width, canvas.height); + blueRobot.paint(context, canvas.width/2, landscape.horizon*1.1); + landscape.paintForeground(context, canvas.width, canvas.height); + requestAnimationFrame(paint); + } + paint(); + // but tick every 150ms, so that we don't slow down when we don't paint + setInterval(function () { + var dx = blueRobot.tick(); + landscape.advance(dx); + }, 100); +</script> +<p class="buttons"> + <input type=button value="Walk" onclick="blueRobot.walk()"> + <input type=button value="Stop" onclick="blueRobot.stop()"> +<footer> + <small> Blue Robot Player Sprite by <a href="https://johncolburn.deviantart.com/">JohnColburn</a>. + Licensed under the terms of the Creative Commons Attribution Share-Alike 3.0 Unported license.</small> + <small> This work is itself licensed under a <a rel="license" href="https://creativecommons.org/licenses/by-sa/3.0/">Creative + Commons Attribution-ShareAlike 3.0 Unported License</a>.</small> +</footer> +
ImageBitmap
rendering contextImageBitmapRenderingContext
is a performance-oriented interface that provides a
+ low overhead method for displaying the contents of ImageBitmap
objects. It uses
+ transfer semantics to reduce overall memory consumption. It also streamlines performance by
+ avoiding intermediate compositing, unlike the drawImage()
method of
+ CanvasRenderingContext2D
.
Using an img
element as an intermediate for getting an image resource into a
+ canvas, for example, would result in two copies of the decoded image existing in memory at the
+ same time: the img
element's copy, and the one in the canvas's backing store. This
+ memory cost can be prohibitive when dealing with extremely large images. This can be avoided by
+ using ImageBitmapRenderingContext
.
Using ImageBitmapRenderingContext
, here is how to transcode an image to the JPEG
+ format in a memory- and CPU-efficient way:
createImageBitmap(inputImageBlob).then(image => { + const canvas = document.createElement('canvas'); + const context = canvas.getContext('bitmaprenderer'); + context.transferFromImageBitmap(image); + + canvas.toBlob(outputJPEGBlob => { + // Do something with outputJPEGBlob. + }, 'image/jpeg'); +});+
ImageBitmapRenderingContext
interface[Exposed=Window] +interface ImageBitmapRenderingContext { + readonly attribute HTMLCanvasElement canvas; + void transferFromImageBitmap(ImageBitmap? bitmap); +}; + +dictionary ImageBitmapRenderingContextSettings { + boolean alpha = true; +};+ +
getContext
('bitmaprenderer' [, { [ alpha
: false ] } ] )Returns an ImageBitmapRenderingContext
object that is permanently bound to a
+ particular canvas
element.
If the alpha
setting is
+ provided and set to false, then the canvas is forced to always be opaque.
canvas
Returns the canvas
element that the context is bound to.
transferFromImageBitmap
(imageBitmap)Transfers the underlying bitmap data
+ from imageBitmap to context, and the bitmap becomes the contents of the
+ canvas
element to which context is bound.
transferFromImageBitmap
(
+ null)Replaces contents of the canvas
element to which context
+ is bound with a transparent black bitmap whose size corresponds to the width
and height
+ content attributes of the canvas
element.
The canvas
attribute
+ must return the value it was initialized to when the object was created.
An ImageBitmapRenderingContext
object has an output bitmap, which is a
+ reference to bitmap data.
An ImageBitmapRenderingContext
object has a bitmap mode, which can be set to
+ valid or blank. A value of valid indicates that the context's
+ output bitmap refers to
+ bitmap data that was acquired via transferFromImageBitmap()
.
+ A value blank indicates that the
+ context's output
+ bitmap is a default transparent bitmap.
An ImageBitmapRenderingContext
object also has an alpha flag, which can be set to true or
+ false. When an ImageBitmapRenderingContext
object has its alpha flag set to false, the contents
+ of the canvas
element to which the context is bound are obtained by
+ compositing the context's output bitmap onto an opaque
+ black bitmap of the same size using the source-over composite operation. If the alpha flag is set to true, then the
+ output bitmap is used
+ as the contents of the canvas
element to which the context is bound. [COMPOSITE]
The step of compositing over an opaque black bitmap ought to be elided whenever + equivalent results can be obtained more efficiently by other means.
+ +When a user agent is required to set an ImageBitmapRenderingContext
's output
+ bitmap, with a context argument that is an
+ ImageBitmapRenderingContext
object and an optional argument bitmap that
+ refers to bitmap data, it must run these
+ steps:
If a bitmap argument was not provided, then:
+ +Set context's bitmap mode to blank.
Let canvas be the canvas
element to which context
+ is bound.
Set context's output bitmap to be fully
+ transparent black with an intrinsic width equal to the numeric value of
+ canvas's width
attribute and an
+ intrinsic height equal to the numeric value of canvas's height
attribute, those values being interpreted in CSS pixels.
Set the output bitmap's origin-clean flag to true.
If a bitmap argument was provided, then:
+ +Set context's bitmap mode to valid.
Set context's output bitmap to refer + to the same underlying bitmap data as bitmap, without making a copy.
+ +The origin-clean flag of + bitmap is included in the bitmap data to be referenced by context's + output bitmap.
+The ImageBitmapRenderingContext
creation algorithm, which is passed a
+ target (a canvas
element) and optionally some arguments, consists of
+ running the following steps:
If the algorithm was passed some arguments, then let arg be the first such + argument. Otherwise, let arg be undefined.
Let settings be the result of coercing the arg context arguments for
+ ImageBitmapRenderingContext
.
Create a new ImageBitmapRenderingContext
object.
Initialize its canvas
attribute to point to
+ target.
Let the new ImageBitmapRenderingContext
object's output bitmap and
+ target's bitmap be a shared reference to the same bitmap.
Run the steps to set an ImageBitmapRenderingContext
's output
+ bitmap, using the new ImageBitmapRenderingContext
object as the
+ context argument, and with no bitmap argument.
Initialize context's alpha flag to true. + +
Process each of the members of settings as follows:
+ +alpha
Return the new ImageBitmapRenderingContext
object.
When a user agent is required to coerce context arguments for
+ ImageBitmapRenderingContext
, it must run these steps:
Let input be the argument to coerce.
Let jsval be the result of converting input to a JavaScript value. (This can + throw an exception.)
Let dict be the result of converting
+ jsval to the dictionary type ImageBitmapRenderingContextSettings
. (This
+ can throw an exception.)
Return dict.
The transferFromImageBitmap(imageBitmap)
+ method, when invoked, must run these steps:
Let bitmapContext be the ImageBitmapRenderingContext
object on
+ which the transferFromImageBitmap()
+ method was called.
If imageBitmap is null, then run the steps to set an + ImageBitmapRenderingContext's output bitmap, with bitmapContext as the + context argument and no bitmap argument, then abort these steps.
If the value of imageBitmap's [[Detached]] internal slot is set to
+ true, then throw an "InvalidStateError
" DOMException
and
+ abort these steps.
Run the steps to set an ImageBitmapRenderingContext
's output
+ bitmap, with the context argument equal to bitmapContext, and the
+ bitmap argument referring to imageBitmap's underlying bitmap data.
+
+
Set the value of imageBitmap's [[Detached]] internal slot to + true.
Unset imageBitmap's bitmap + data.
OffscreenCanvas
interfaceSupport: offscreencanvasChrome for Android NoneChrome NoneUC Browser for Android NoneiOS Safari NoneFirefox NoneIE NoneSamsung Internet NoneOpera Mini NoneSafari NoneAndroid Browser NoneOpera NoneEdge None
Source: caniuse.com
typedef (OffscreenCanvasRenderingContext2D or + WebGLRenderingContext) OffscreenRenderingContext; + +dictionary ImageEncodeOptions { + DOMString type = "image/png"; + unrestricted double quality = 1.0; +}; + +enum OffscreenRenderingContextType { "2d", "webgl" }; + +[Constructor([EnforceRange] unsigned long long width, [EnforceRange] unsigned long long height), Exposed=(Window,Worker), Transferable] +interface OffscreenCanvas : EventTarget { + attribute unsigned long long width; + attribute unsigned long long height; + + OffscreenRenderingContext? getContext(OffscreenRenderingContextType contextType, any... arguments); + ImageBitmap transferToImageBitmap(); + Promise<Blob> convertToBlob(optional ImageEncodeOptions options); +}; ++ +
OffscreenCanvas
objects are used to create rendering contexts, much like an
+ HTMLCanvasElement
, but with no connection to the DOM. This makes it possible to
+ use canvas rendering contexts in workers.
An OffscreenCanvas
object may hold a weak reference to a placeholder canvas
element, which is
+ typically in the DOM, whose embedded content is provided by the OffscreenCanvas
+ object. The bitmap of the OffscreenCanvas
object is pushed to the placeholder canvas
element by
+ calling the commit()
method of the
+ OffscreenCanvas
object's rendering context. All rendering context types that
+ can be created by an OffscreenCanvas
object must implement a commit()
method. The exact behavior of the commit
+ method (e.g. whether it copies or transfers bitmaps) may vary, as defined by the rendering
+ contexts' respective specifications. Only the 2D context for offscreen canvases is defined
+ in this specification.
OffscreenCanvas
(width,
+ height)Returns a new OffscreenCanvas
object that is not linked to a placeholder canvas
element, and
+ whose bitmap's size is determined by the width and height arguments.
getContext
(contextType [, ... ] )Returns an object that exposes an API for drawing on the OffscreenCanvas
+ object. The first argument specifies the desired API, either "2d
", or "webgl
". Subsequent arguments are handled by that
+ API.
This specification defines the "2d
" context below,
+ which is similar but distinct from the "2d
"
+ context that is created from a canvas
element. There is also a specification that
+ defines a "webgl
" context.
+ [WEBGL]
Returns null if the canvas has already been initialized with another context type (e.g.
+ trying to get a "2d
" context after getting a
+ "webgl
" context).
An OffscreenCanvas
object has an internal bitmap that is initialized when the object
+ is created. The width and height of the bitmap are
+ equal to the values of the width
and height
attributes of the OffscreenCanvas
+ object. Initially, all the bitmap's pixels are transparent black.
An OffscreenCanvas
object can have a rendering context bound to it. Initially,
+ it does not have a bound rendering context. To keep track of whether it has a rendering context
+ or not, and what kind of rendering context it is, an OffscreenCanvas
object also
+ has a context mode, which is initially none but can be changed to either 2d, webgl or detached by algorithms defined in this
+ specification.
The constructor OffscreenCanvas(width,
+ height)
, when invoked, must create a new OffscreenCanvas
+ object with its bitmap initialized to a rectangular
+ array of transparent black pixels of the dimensions specified by width and
+ height; and its width
and height
attributes initialized to width and
+ height respectively.
OffscreenCanvas
objects are transferable. Their transfer steps, given value and
+ dataHolder, are as follows:
If value's context mode is
+ not equal to none, then throw an
+ "InvalidStateError
" DOMException
.
Set value's context mode to + detached.
Let width and height be the dimensions of value's bitmap.
Unset value's bitmap.
Set dataHolder.[[With]] to width, and + dataHolder.[[Height]] to height.
Set dataHolder.[[PlaceholderCanvas]] to be a weak reference to
+ value's placeholder canvas
+ element, if value has one, or null if it does not.
Their transfer-receiving steps, given dataHolder and value, + are:
+ +Initialize value's bitmap to a + rectangular array of transparent black pixels with width given by dataHolder.[[Width]] + and height given by dataHolder.[[Height]].
If dataHolder.[[PlaceholderCanvas]] is not null, set value's placeholder canvas
element to
+ dataHolder.[[PlaceholderCanvas]] (while maintaining the weak reference
+ semantics).
The getContext(contextId,
+ arguments...)
method of an OffscreenCanvas
object, when
+ invoked, must run the steps in the cell of the following table whose column header describes the
+ OffscreenCanvas
object's context
+ mode and whose row header describes the method's first argument.
+ | none + | 2d + | webgl + | detached + |
---|---|---|---|---|
"2d "
+ |
+ Follow the steps to create an
+ offscreen 2D context defined in the section below, passing it the
+ OffscreenCanvas object and the method's arguments..., to obtain
+ an OffscreenCanvasRenderingContext2D object; if this does not throw an
+ exception, then set the OffscreenCanvas object's context mode to 2d, and return the new
+ OffscreenCanvasRenderingContext2D object.
+ | + Return the same object as was returned the last time the method was invoked with this same + first argument. + | + Return null. + |
+ Throw an "InvalidStateError " DOMException .
+
+ |
"webgl "
+ |
+ Follow the instructions given in the WebGL specification's Context Creation section to
+ obtain either a WebGLRenderingContext or null; if the returned value is null,
+ then return null and abort these steps, otherwise, set the OffscreenCanvas
+ object's context mode to webgl, and return the
+ WebGLRenderingContext object. [WEBGL]
+ | + Return null. + | + Return the same value as was returned the last time the method was invoked with this same + first argument. + |
+ Throw an "InvalidStateError " DOMException .
+
+ |
width
[
+ = value ]height
[
+ = value ]These attributes return the dimensions of the OffscreenCanvas
object's bitmap.
They can be set, to replace the bitmap with a + new, transparent black bitmap of the specified dimensions (effectively resizing it).
+If either the width
or height
attributes of an
+ OffscreenCanvas
object are set (to a new value or to the same value as before) and
+ the OffscreenCanvas
object's context
+ mode is 2d, then replace the
+ OffscreenCanvas
object's bitmap with a
+ new transparent black bitmap and reset the rendering context to its default state.
+ The new bitmap's dimensions are equal to the new values of the width
and height
attributes.
The resizing behavior for "webgl
"
+ contexts is defined in the WebGL specification. [WEBGL]
If an OffscreenCanvas
object whose dimensions were changed has
+ a placeholder canvas
element, then
+ the placeholder canvas
element's
+ intrinsic size will only be updated via the commit()
method of the OffscreenCanvas
+ object's rendering context.
convertToBlob
( [options] )Returns a promise that will fulfill with a new Blob
object representing a file
+ containing the image in the OffscreenCanvas
object.
The argument, if provided, is a dictionary that controls the encoding options of the image
+ file to be created. The type
+ field specifies the file format and has a default value of "image/png
"; that type
+ is also used if the requested type isn't supported. If the image format supports variable
+ quality (such as "image/jpeg
"), then the quality
field is a number in the range 0.0
+ to 1.0 inclusive indicating the desired quality level for the resulting image.
transferToImageBitmap
()Returns a newly created ImageBitmap
object with the image in the
+ OffscreenCanvas
object. The image in the OffscreenCanvas
object is
+ replaced with a new blank image.
The convertToBlob(options)
method,
+ when invoked, must run the following steps:
If the value of this OffscreenCanvas
object's [[Detached]]
+ internal slot is set to true, then return a promise rejected with an
+ "InvalidStateError
" DOMException
and abort these
+ steps.
If this OffscreenCanvas
object's context mode is 2d and the rendering context's bitmap's origin-clean flag is set to false, then return a
+ promise rejected with a "SecurityError
" DOMException
and
+ abort these steps.
If this OffscreenCanvas
object's bitmap has no pixels (i.e. either its
+ horizontal dimension or its vertical dimension is zero) then return a promise rejected with an
+ "IndexSizeError
" DOMException
and abort these
+ steps.
Let result be a new promise object.
Return result, and queue a task to run the remaining steps + in parallel.
Let blob be a Blob
object, created in the relevant Realm of this OffscreenCanvas
+ object, representing a serialization of
+ this OffscreenCanvas
object's bitmap as
+ a file, passing the values of the type
and quality
fields of options, if
+ options was specified. [FILEAPI]
If blob is null, then reject result with an
+ "EncodingError
" DOMException
.
Otherwise, resolve result with blob.
The transferToImageBitmap()
method,
+ when invoked, must run the following steps:
If the value of this OffscreenCanvas
object's [[Detached]]
+ internal slot is set to true, then throw an "InvalidStateError
"
+ DOMException
and abort these steps.
If this OffscreenCanvas
object's context mode is set to none, then throw an
+ "InvalidStateError
" DOMException
and abort these
+ steps.
Let image be a newly created ImageBitmap
object that references
+ the same underlying bitmap data as this OffscreenCanvas
object's bitmap.
Set this OffscreenCanvas
object's bitmap to reference a newly created bitmap of the
+ same dimensions as the previous bitmap, and with its pixels initialized to transparent black,
+ or opaque black if the rendering context's alpha flag is set to false.
Return image.
[Exposed=(Window,Worker)] +interface OffscreenCanvasRenderingContext2D { + void commit(); + readonly attribute OffscreenCanvas canvas; +}; + +OffscreenCanvasRenderingContext2D implements CanvasState; +OffscreenCanvasRenderingContext2D implements CanvasTransform; +OffscreenCanvasRenderingContext2D implements CanvasCompositing; +OffscreenCanvasRenderingContext2D implements CanvasImageSmoothing; +OffscreenCanvasRenderingContext2D implements CanvasFillStrokeStyles; +OffscreenCanvasRenderingContext2D implements CanvasShadowStyles; +OffscreenCanvasRenderingContext2D implements CanvasFilters; +OffscreenCanvasRenderingContext2D implements CanvasRect; +OffscreenCanvasRenderingContext2D implements CanvasDrawPath; +OffscreenCanvasRenderingContext2D implements CanvasDrawImage; +OffscreenCanvasRenderingContext2D implements CanvasImageData; +OffscreenCanvasRenderingContext2D implements CanvasPathDrawingStyles; +OffscreenCanvasRenderingContext2D implements CanvasPath; ++ +
The OffscreenCanvasRenderingContext2D
is a rendering context
+ interface for drawing to the bitmap of an
+ OffscreenCanvas
object. It is similar to CanvasRenderingContext2D
,
+ with the following differences:
text rendering is not supported;
there is no support for user interface + features;
its canvas
attribute refers to an
+ OffscreenCanvas
object rather than a canvas
element;
it has a commit()
method for pushing the
+ rendered image to the context's OffscreenCanvas
object's placeholder canvas
element.
An OffscreenCanvasRenderingContext2D
object has a bitmap that is initialized when the object is
+ created.
The bitmap has an origin-clean flag, which can be set to true or + false. Initially, when one of these bitmaps is created, its origin-clean flag must be set to true.
+ +An OffscreenCanvasRenderingContext2D
object also has an alpha flag, which can be set to true or false. Initially,
+ when the context is created, its alpha flag must be set to true. When an
+ OffscreenCanvasRenderingContext2D
object has its alpha flag set to false, then its alpha channel must be
+ fixed to 1.0 (fully opaque) for all pixels, and attempts to change the alpha component of any pixel
+ must be silently ignored.
An OffscreenCanvasRenderingContext2D
object has an associated
+ OffscreenCanvas
object, which is the OffscreenCanvas
object
+ from which the OffscreenCanvasRenderingContext2D
object was created.
+
+
commit
()Copies the rendering context's bitmap to
+ the bitmap of the placeholder canvas
+ element of the associated OffscreenCanvas
object. The copy
+ operation is asynchronous.
canvas
Returns the associated OffscreenCanvas
object.
The offscreen 2D context creation algorithm, which is passed a
+ target (an OffscreenCanvas
object) and optionally some arguments,
+ consists of running the following steps:
If the algorithm was passed some arguments, let arg be the first such + argument. Otherwise, let arg be undefined.
Let settings be the result of coercing the arg context arguments + for 2D.
Let context be a new OffscreenCanvasRenderingContext2D
+ object.
Set context's associated OffscreenCanvas
object to
+ target.
Process each of the members of settings as follows:
+ +Set context's bitmap to a newly
+ created bitmap with the dimensions specified by the width
and height
attributes of target, and set
+ target's bitmap to the same bitmap (so that they are shared).
If context's alpha flag is set + to true, initialize all the pixels of context's bitmap to transparent black. Otherwise, initialize + the pixels to opaque black.
Return context.
The commit()
method, when invoked,
+ must run the following steps:
If this OffscreenCanvasRenderingContext2D
's associated
+ OffscreenCanvas
object does not have a placeholder canvas
element, abort
+ these steps.
Let image be a copy of this OffscreenCanvasRenderingContext2D
's
+ bitmap, including the value of its origin-clean flag.
Queue a task in the placeholder canvas
element's
+ relevant settings object's responsible event loop (which will be a
+ browsing context event loop) to set the placeholder canvas
element's
+ output bitmap to be a reference to image.
If image has different dimensions than the bitmap previously
+ referenced as the placeholder canvas
+ element's output bitmap, then this task will result in a change in
+ the placeholder canvas
+ element's intrinsic size, which can affect
+ document layout.
Implementations are encouraged to short-circuit the graphics update steps of
+ the browsing context event loop for the purposes of updating the
+ contents of a placeholder canvas
+ element to the display. This could mean, for example, that the commit()
method can copy the bitmap contents directly
+ to a graphics buffer that is mapped to the physical display location of the placeholder canvas
element. This or
+ similar short-circuiting approaches can significantly reduce display latency, especially in cases
+ where the commit()
method is invoked from a worker
+ and the event loop of the placeholder canvas
element's
+ browsing context is busy. However, such shortcuts can not have any
+ script-observable side-effects. This means that the committed bitmap still needs to be sent to
+ the placeholder canvas
element, in
+ case the element is used as a CanvasImageSource
, as an
+ ImageBitmapSource
, or in case toDataURL()
+ or toBlob()
are called on it.
The canvas
attribute, on getting,
+ must return this OffscreenCanvasRenderingContext2D
's associated
+ OffscreenCanvas
object.
+
+
The canvas
APIs must perform color correction at only two points: when rendering
+ images with their own gamma correction and color space information onto a bitmap, to convert the
+ image to the color space used by the bitmaps (e.g. using the 2D Context's drawImage()
method with an
+ HTMLOrSVGImageElement
object), and when rendering the actual canvas bitmap to the
+ output device.
Thus, in the 2D context, colors used to draw shapes onto the canvas will exactly
+ match colors obtained through the getImageData()
method.
The toDataURL()
method, when invoked, must not
+ include color space information in the resources they return. Where the output format allows it,
+ the color of pixels in resources created by toDataURL()
+ must match those returned by the getImageData()
+ method.
In user agents that support CSS, the color space used by a canvas
element must
+ match the color space used for processing any colors for that element in CSS.
The gamma correction and color space information of images must be handled in such a way that
+ an image rendered directly using an img
element would use the same colors as one
+ painted on a canvas
element that is then itself rendered. Furthermore, the rendering
+ of images that have no color correction information (such as those returned by the toDataURL()
method) must be rendered with no color
+ correction.
Thus, in the 2D context, calling the drawImage()
method to render the output of the toDataURL()
method to the canvas, given the appropriate
+ dimensions, has no visible effect.
When a user agent is to create a + serialization of the bitmap as a file, given an optional type and + quality, it must create an image file in the format given by type, or if + type was not supplied, in the PNG format. If an error occurs during the creation of + the image file (e.g. an internal encoder error), then the result of the serialization is null. + [PNG]
+ +The image file's pixel data must be the bitmap's pixel data scaled to one image pixel per + coordinate space unit, and if the file format used supports encoding resolution metadata, the + resolution must be given as 96dpi (one image pixel per CSS pixel).
+ +If type is supplied, then it must be interpreted as a MIME + type giving the format to use. If the type has any parameters, then it must be treated as + not supported.
+ +For example, the value "image/png
" would mean to generate a PNG
+ image, the value "image/jpeg
" would mean to generate a JPEG image, and the value
+ "image/svg+xml
" would mean to generate an SVG image (which would require that the
+ user agent track how the bitmap was generated, an unlikely, though potentially awesome,
+ feature).
User agents must support PNG ("image/png
"). User agents may support other types.
+ If the user agent does not support the requested type, then it must create the file using the PNG
+ format. [PNG]
User agents must convert the provided type to ASCII + lowercase before establishing if they support that type.
+ +For image types that do not support an alpha channel, the serialized image must be the bitmap + image composited onto a solid black background using the source-over operator.
+ +If type is an image format that supports variable quality (such as
+ "image/jpeg
") and quality is given, then, if Type(quality) is Number, and quality is in the range
+ 0.0 to 1.0 inclusive, the user agent must treat quality as the desired quality level.
+ If Type(quality) is not Number, or if quality
+ is outside that range, the user agent must use its default quality value, as if the
+ quality argument had not been given.
The use of type-testing here, instead of simply declaring quality as
+ a Web IDL double
, is a historical artifact.
canvas
elementsThis section is non-normative.
+ +Information leakage can occur if scripts from one origin can + access information (e.g. read pixels) from images from another origin (one that isn't the same).
+ +To mitigate this, bitmaps used with canvas
elements and ImageBitmap
+ objects are defined to have a flag indicating whether they are origin-clean. All bitmaps start with their origin-clean set to true. The flag is set to false
+ when cross-origin images are used.
The toDataURL()
, toBlob()
, and getImageData()
methods check the flag and will
+ throw a "SecurityError
" DOMException
rather than leak
+ cross-origin data.
The value of the origin-clean flag is
+ propagated from a source canvas
element's bitmap to a new ImageBitmap
+ object by createImageBitmap()
. Conversely, a
+ destination canvas
element's bitmap will have its origin-clean flags set to false by drawImage
if the source image is an
+ ImageBitmap
object whose bitmap has its origin-clean flag set to false.
The flag can be reset in certain situations; for example, when changing the value of the
+ width
or the height
content attribute of the canvas
element
+ to which a CanvasRenderingContext2D
is bound, the bitmap is
+ cleared and its origin-clean flag is reset.
When using an ImageBitmapRenderingContext
, the value of the origin-clean flag is propagated from
+ ImageBitmap
objects when they are transferred to the canvas
+ via transferFromImageBitmap().