Bug 572680 - Add nsLayoutUtils::DrawPixelSnapped. r=roc

layout/base/nsLayoutUtils.cpp

@@ -95,6 +95,8 @@
 #include "ImageLayers.h"
 #include "mozilla/dom/Element.h"
 #include "nsCanvasFrame.h"
+#include "gfxDrawable.h"
+#include "gfxUtils.h"
 
 #ifdef MOZ_SVG
 #include "nsSVGUtils.h"
@@ -3034,6 +3036,49 @@ DrawImageInternal(nsIRenderingContext* aRenderingContext,
   return NS_OK;
 }
 
+/* static */ void
+nsLayoutUtils::DrawPixelSnapped(nsIRenderingContext* aRenderingContext,
+                                gfxDrawable*         aDrawable,
+                                gfxPattern::GraphicsFilter aFilter,
+                                const nsRect&        aDest,
+                                const nsRect&        aFill,
+                                const nsPoint&       aAnchor,
+                                const nsRect&        aDirty)
+{
+  nsCOMPtr<nsIDeviceContext> dc;
+  aRenderingContext->GetDeviceContext(*getter_AddRefs(dc));
+  PRInt32 appUnitsPerDevPixel = dc->AppUnitsPerDevPixel();
+  gfxContext* ctx = aRenderingContext->ThebesContext();
+  gfxIntSize drawableSize = aDrawable->Size();
+  nsIntSize imageSize(drawableSize.width, drawableSize.height);
+
+  SnappedImageDrawingParameters drawingParams =
+    ComputeSnappedImageDrawingParameters(ctx, appUnitsPerDevPixel, aDest, aFill,
+                                         aAnchor, aDirty, imageSize);
+
+  if (!drawingParams.mShouldDraw)
+    return;
+
+  gfxContextMatrixAutoSaveRestore saveMatrix(ctx);
+  if (drawingParams.mResetCTM) {
+    ctx->IdentityMatrix();
+  }
+
+  gfxRect sourceRect =
+    drawingParams.mUserSpaceToImageSpace.Transform(drawingParams.mFillRect);
+  gfxRect imageRect(0, 0, imageSize.width, imageSize.height);
+  gfxRect subimage(drawingParams.mSubimage.x, drawingParams.mSubimage.y,
+                   drawingParams.mSubimage.width, drawingParams.mSubimage.height);
+
+  NS_ASSERTION(!sourceRect.Intersect(subimage).IsEmpty(),
+               "We must be allowed to sample *some* source pixels!");
+
+  gfxUtils::DrawPixelSnapped(ctx, aDrawable,
+                             drawingParams.mUserSpaceToImageSpace, subimage,
+                             sourceRect, imageRect, drawingParams.mFillRect,
+                             gfxASurface::ImageFormatARGB32, aFilter);
+}
+
 /* static */ nsresult
 nsLayoutUtils::DrawSingleUnscaledImage(nsIRenderingContext* aRenderingContext,
                                        imgIContainer*       aImage,

layout/base/nsLayoutUtils.h

@@ -66,6 +66,7 @@ class nsClientRectList;
 #include "nsCSSPseudoElements.h"
 
 class nsBlockFrame;
+class gfxDrawable;
 
 /**
  * nsLayoutUtils is a namespace class used for various helper
@@ -907,6 +908,29 @@ public:
                             const nsRect&        aDirty,
                             PRUint32             aImageFlags);
 
+  /**
+   * Draw a drawable using the pixel snapping algorithm.
+   * See https://wiki.mozilla.org/Gecko:Image_Snapping_and_Rendering
+   *   @param aRenderingContext Where to draw the image, set up with an
+   *                            appropriate scale and transform for drawing in
+   *                            app units.
+   *   @param aDrawable         The drawable we want to draw.
+   *   @param aFilter           The graphics filter we should draw with.
+   *   @param aDest             Where one copy of the image should mapped to.
+   *   @param aFill             The area to be filled with copies of the
+   *                            image.
+   *   @param aAnchor           A point in aFill which we will ensure is
+   *                            pixel-aligned in the output.
+   *   @param aDirty            Pixels outside this area may be skipped.
+   */
+  static void DrawPixelSnapped(nsIRenderingContext* aRenderingContext,
+                               gfxDrawable*         aDrawable,
+                               gfxPattern::GraphicsFilter aFilter,
+                               const nsRect&        aDest,
+                               const nsRect&        aFill,
+                               const nsPoint&       aAnchor,
+                               const nsRect&        aDirty);
+
   /**
    * Draw a whole image without scaling or tiling.
    *