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543 строки
24 KiB
Plaintext
Name
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EXT_draw_buffers
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Name Strings
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GL_EXT_draw_buffers
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Contributors
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Contributors to GL_NV_draw_buffers
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Contributors to GL_NV_fbo_color_attachments
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Contributors to the OpenGL ES 2.0 specification
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Contributors to the OpenGLSL ES 1.0.17 specification
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Contributors to the OpenGL ES 3.0 specification
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Nicolas Capens, TransGaming Inc.
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Daniel Koch, TransGaming Inc.
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Alastair Patrick, Google Inc.
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Kenneth Russell, Google Inc.
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Greg Roth, NVIDIA Corporation
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Ben Bowman, Imagination Technologies
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Members of the WebGL and OpenGL ES working groups
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Contact
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Daniel Koch (daniel 'at' transgaming.com)
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Status
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Draft Complete
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Version
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Last Modified Date: January 30, 2013
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Revision: #7
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Number
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TBD
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Dependencies
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OpenGL ES 2.0 is required.
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The extension is written against the OpenGL ES 2.0 specification.
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ANGLE_framebuffer_blit affects the definition of this extension.
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APPLE_framebuffer_multisample affects the definitin of this extension.
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Overview
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This extension increases the number of available framebuffer object
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color attachment points, extends OpenGL ES 2.0 to allow multiple output
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colors, and provides a mechanism for directing those outputs to
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multiple color buffers.
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This extension is similar to the combination of the GL_NV_draw_buffers
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and GL_NV_fbo_color_attachments extensions, but imposes certain
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restrictions informed by the OpenGL ES 3.0 API.
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New Procedures and Functions
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void DrawBuffersEXT(sizei n, const enum *bufs);
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New Tokens
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Accepted by the <pname> parameter of GetIntegerv:
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MAX_COLOR_ATTACHMENTS_EXT 0x8CDF
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Accepted by the <pname> parameters of GetIntegerv and GetFloatv:
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MAX_DRAW_BUFFERS_EXT 0x8824
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DRAW_BUFFER0_EXT 0x8825
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DRAW_BUFFER1_EXT 0x8826
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DRAW_BUFFER2_EXT 0x8827
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DRAW_BUFFER3_EXT 0x8828
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DRAW_BUFFER4_EXT 0x8829
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DRAW_BUFFER5_EXT 0x882A
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DRAW_BUFFER6_EXT 0x882B
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DRAW_BUFFER7_EXT 0x882C
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DRAW_BUFFER8_EXT 0x882D
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DRAW_BUFFER9_EXT 0x882E
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DRAW_BUFFER10_EXT 0x882F
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DRAW_BUFFER11_EXT 0x8830
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DRAW_BUFFER12_EXT 0x8831
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DRAW_BUFFER13_EXT 0x8832
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DRAW_BUFFER14_EXT 0x8833
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DRAW_BUFFER15_EXT 0x8834
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Accepted by the <attachment> parameter of FramebufferRenderbuffer,
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FramebufferTexture2D and GetFramebufferAttachmentParameteriv, and by
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the <bufs> parameter of DrawBuffersEXT:
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COLOR_ATTACHMENT0_EXT 0x8CE0
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COLOR_ATTACHMENT1_EXT 0x8CE1
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COLOR_ATTACHMENT2_EXT 0x8CE2
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COLOR_ATTACHMENT3_EXT 0x8CE3
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COLOR_ATTACHMENT4_EXT 0x8CE4
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COLOR_ATTACHMENT5_EXT 0x8CE5
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COLOR_ATTACHMENT6_EXT 0x8CE6
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COLOR_ATTACHMENT7_EXT 0x8CE7
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COLOR_ATTACHMENT8_EXT 0x8CE8
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COLOR_ATTACHMENT9_EXT 0x8CE9
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COLOR_ATTACHMENT10_EXT 0x8CEA
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COLOR_ATTACHMENT11_EXT 0x8CEB
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COLOR_ATTACHMENT12_EXT 0x8CEC
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COLOR_ATTACHMENT13_EXT 0x8CED
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COLOR_ATTACHMENT14_EXT 0x8CEE
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COLOR_ATTACHMENT15_EXT 0x8CEF
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The COLOR_ATTACHMENT0_EXT constant is equal to the
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COLOR_ATTACHMENT0 constant.
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Each COLOR_ATTACHMENT<i>_EXT adheres to COLOR_ATTACHMENT<i>_EXT
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= COLOR_ATTACHMENT0_EXT + <i>.
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Changes to Chapter 3 of the OpenGL ES 2.0 Specification (Rasterization)
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Section 3.2, (Multisampling). Replace the second paragraph:
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An additional buffer, called the multisample buffer, is added to the
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window system-provided framebuffer. Pixel sample values, including
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color, depth, and stencil values, are stored in this buffer. Samples
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contain separate color values for each fragment color. When the
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window system-provided framebuffer includes a multisample buffer, it
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does not include depth or stencil buffers, even if the multisample
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buffer does not store depth or stencil values. Color buffers do
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coexist with the multisample buffer, however.
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Section 3.8.2, (Shader Execution) Replace subsection "Shader
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Outputs":
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The OpenGL ES Shading Language specification describes the values
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that may be output by a fragment shader. These are gl_FragColor and
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gl_FragData[n]. The final fragment color values or the final
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fragment data values written by a fragment shader are clamped to the
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range [0, 1] and then converted to fixed-point as described in
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section 2.1.2 for framebuffer color components.
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Writing to gl_FragColor specifies the fragment color (color number
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zero) that will be used by subsequent stages of the pipeline.
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Writing to gl_FragData[n] specifies the value of fragment color
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number n. Any colors, or color components, associated with a
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fragment that are not written by the fragment shader are undefined.
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A fragment shader may not statically assign values to both
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gl_FragColor and gl_FragData. In this case, a compile or link error
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will result. A shader statically assigns a value to a variable if,
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after preprocessing, it contains a statement that would write to the
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variable, whether or not run-time flow of control will cause that
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statement to be executed.
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Changes to Chapter 4 of the OpenGL ES 2.0 Specification (Per-Fragment
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Operations and the Frame Buffer)
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Modify the overview of Chapter 4 and replace the sentences
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of the fifth paragraph which read:
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"The name of the color buffer of an application-created framebuffer
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object is COLOR_ATTACHMENT0. The names of the depth and stencil buffers
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are DEPTH_ATTACHMENT and STENCIL_ATTACHMENT."
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With the following:
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"A framebuffer object has an array of color buffer attachment points,
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numbered zero through <n>, a depth buffer attachment point, and a
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stencil buffer attachment point."
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Insert Table 4.3 to Section 4.2.1 (and renumber subsequent tables):
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Symbolic Constant Meaning
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----------------- ---------------------
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NONE No buffer
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COLOR_ATTACHMENT<i>_EXT (see caption) Output fragment color to image
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attached at color attachment
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point i
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Table 4.3: Arguments to DrawBuffersEXT when the context is bound to a
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framebuffer object, and the buffers they indicate. <i> in
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COLOR_ATTACHMENT<i>_EXT may range from zero to the value of
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MAX_COLOR_ATTACHMENTS_EXT minus one.
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Replace Section 4.2.1, "Selecting a Buffer for Writing" with the following:
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"By default, color values are written into the front buffer for
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single buffered surfaces or into the back buffer for back buffered
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surfaces as determined when making the context current. To control
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the color buffer into which each of the fragment color values is
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written, DrawBuffersEXT is used.
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The command
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void DrawBuffersEXT(sizei n, const enum *bufs);
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defines the draw buffers to which all fragment colors are written.
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<n> specifies the number of buffers in <bufs>. <bufs> is a pointer
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to an array of symbolic constants specifying the buffer to which
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each fragment color is written.
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Each buffer listed in <bufs> must be BACK, NONE, or one of the
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values from table 4.3. Further, acceptable values for the constants
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in <bufs> depend on whether the GL is using the default framebuffer
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(i.e., DRAW_FRAMEBUFFER_BINDING is zero), or a framebuffer object
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(i.e., DRAW_FRAMEBUFFER_BINDING is non-zero). For more information
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about framebuffer objects, see section 4.4.
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If the GL is bound to the default framebuffer, then <n> must be 1
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and the constant must be BACK or NONE. When draw buffer zero is
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BACK, color values are written into the sole buffer for single-
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buffered contexts, or into the back buffer for double-buffered
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contexts. If DrawBuffersEXT is supplied with a constant other than
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BACK and NONE, the error INVALID_OPERATION is generated.
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If the GL is bound to a draw framebuffer object, then each of the
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constants must be one of the values listed in table 4.3.
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In both cases, the draw buffers being defined correspond in order to
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the respective fragment colors. The draw buffer for fragment
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colors beyond <n> is set to NONE.
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The maximum number of draw buffers is implementation-dependent. The
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number of draw buffers supported can be queried by calling
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GetIntegerv with the symbolic constant MAX_DRAW_BUFFERS_EXT. An
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INVALID_VALUE error is generated if <n> is greater than
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MAX_DRAW_BUFFERS_EXT.
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If the GL is bound to a draw framebuffer object, the <i>th buffer listed
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in <bufs> must be COLOR_ATTACHMENT<i>_EXT or NONE. Specifying a
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buffer out of order, BACK, or COLOR_ATTACHMENT<m>_EXT where <m> is
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greater than or equal to the value of MAX_COLOR_ATTACHMENTS_EXT,
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will generate the error INVALID_OPERATION.
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If a fragment shader writes to "gl_FragColor", DrawBuffersEXT
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specifies the set of draw buffers into which the color
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written to "gl_FragColor" is written. If a fragment shader writes to
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"gl_FragData", DrawBuffersEXT specifies a set of draw buffers
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into which each of the multiple output colors defined by these
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variables are separately written. If a fragment shader writes to
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neither "gl_FragColor" nor "gl_FragData" the values of the
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fragment colors following shader execution are undefined, and may
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differ for each fragment color.
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Indicating a buffer or buffers using DrawBuffersEXT causes
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subsequent pixel color value writes to affect the indicated
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buffers. If the GL is bound to a draw framebuffer object and a draw
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buffer selects an attachment that has no image attached, then that
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fragment color is not written.
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Specifying NONE as the draw buffer for a fragment color will inhibit
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that fragment color from being written.
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The state required to handle color buffer selection for each
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framebuffer is an integer for each supported fragment color. For the
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default framebuffer, in the initial state the draw buffer for
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fragment color zero is BACK if there is a default framebuffer
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associated with the context, otherwise NONE. For framebuffer
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objects, in the initial state the draw buffer for fragment color
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zero is COLOR_ATTACHMENT0_EXT.
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For both the default framebuffer and framebuffer objects, the
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initial state of draw buffers for fragment colors other than zero is
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NONE.
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The value of the draw buffer selected for fragment color <i> can be
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queried by calling GetIntegerv with the symbolic constant
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DRAW_BUFFER<i>_EXT."
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Modify Section 4.2.3 (Clearing the Buffers) and replace the first
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two paragraphs with the following:
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"The GL provides a means for setting portions of every pixel in a
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particular buffer to the same value. The argument to
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void Clear(bitfield buf);
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is the bitwise OR of a number of values indicating which buffers are
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to be cleared. The values are COLOR_BUFFER_BIT, DEPTH_BUFFER_BIT, and
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STENCIL_BUFFER_BIT, indicating the buffers currently enabled for color
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writing, the depth buffer, and the stencil buffer (see below),
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respectively. The value to which each buffer is cleared depends on
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the setting of the clear value for that buffer. If the mask is not a
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bitwise OR of the specified values, then the error INVALID_VALUE is
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generated.
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void ClearColor(clampf r, clampf, g, clampf b, clampf a);
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sets the clear value for fixed-point color buffers. Each of the
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specified components is clamped to [0, 1] and converted to fixed-point
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as described in section 2.1.2 for framebuffer color components."
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Replace the second paragraph of Section 4.4.1 (Binding and Managing
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Framebuffer Objects) with the following:
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"The namespace for framebuffer objects is the unsigned integers, with
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zero reserved by OpenGL ES to refer to the default framebuffer. A
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framebuffer object is created by binding an unused name to the
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target FRAMEBUFFER, DRAW_FRAMEBUFFER, or READ_FRAMEBUFFER. The binding
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is effected by calling
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void BindFramebuffer(enum target, uint framebuffer);
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with <target> set the desired framebuffer target and <framebuffer> set
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to the unused name. The resulting framebuffer object is a new state
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vector. There is a number of color attachment points, plus one each
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for the depth and stencil attachment points. The number of color attachment
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points is equal to the value of MAX_COLOR_ATTACHMENTS_EXT."
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Replace the third item in the bulleted list in Section 4.4.1 (Binding
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and Managing Framebuffer Objects) with the following:
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" * The only color buffer bitplanes are the ones defined by the
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framebuffer attachments points named COLOR_ATTACHMENT0_EXT through
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COLOR_ATTACHMENT<n>_EXT."
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Modify Section 4.4.3 (Renderbuffer Objects) in the
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"Attaching Renderbuffer Images to a Framebuffer" subsection as follows:
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Insert the following table:
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Name of attachment
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---------------------------------------
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COLOR_ATTACHMENT<i>_EXT (see caption)
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DEPTH_ATTACHMENT
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STENCIL_ATTACHMENT
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Table 4.x: Framebuffer attachment points. <i> in COLOR_ATTACHMENT<i>_EXT
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may range from zero to the value of MAX_COLOR_ATTACHMENTS_EXT minus 1.
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Modify the third sentence of the paragraph following the definition of
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FramebufferRenderbuffer to be as follows:
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"<attachment> should be set to one of the attachment points of the
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framebuffer listed in Table 4.x."
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Modify Section 4.4.3 (Renderbuffer Objects) in the "Attaching Texture
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Images to a Framebuffer" subsection as follows:
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Modify the last sentence of the paragraph following the definition of
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FramebufferTexture2D to be as follows:
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"<attachment> must be one of the attachment points of the framebuffer
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listed in Table 4.x."
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Modify Section 4.4.5 (Framebuffer Completeness) and replace the 3rd
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item in the bulleted list in the "Framebuffer Attachment Completeness"
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subsection with the following:
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" * If <attachment> is COLOR_ATTACHMENT<i>_EXT, then <image> must
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have a color-renderable internal format."
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Changes to Chapter 6 of the OpenGL ES 2.0 Specification (State and
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State Requests)
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In section 6.1.3 (Enumerated Queries) modify the third sentence in
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the definition of GetFramebufferAttachmentParameteriv to be as follows:
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"<attachment> must be one of the attachment points of the framebuffer
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listed in Table 4.x."
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Changes to Chapter 3 of the OpenGL ES Shading Language 1.0.17 Specification (Basics)
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Add a new section:
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3.4.1 GL_EXT_draw_buffers Extension
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To use the GL_EXT_draw_buffers extension in a shader it
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must be enabled using the #extension directive.
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The shading language preprocessor #define
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GL_EXT_draw_buffers will be defined to 1, if the
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GL_EXT_draw_buffers extension is supported.
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Dependencies on ANGLE_framebuffer_blit and APPLE_framebuffer_multisample:
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If neither ANGLE_framebuffer_blit nor APPLE_framebuffer_multisample are
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supported, then all references to "draw framebuffers" should be replaced
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with references to "framebuffers". References to DRAW_FRAMEBUFFER_BINDING
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should be replaced with references to FRAMEBUFFER_BINDING. References to
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DRAW_FRAMEBUFFER and READ_FRAMEBUFFER should be removed.
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If ANGLE_framebuffer_blit is supported, DRAW_FRAMEBUFFER_BINDING, DRAW_FRAMEBUFFER
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and READ_FRAMEBUFFER all refer to corresponding _ANGLE suffixed names
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(they have the same token values).
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If APPLE_framebuffer_multisample is supported, DRAW_FRAMEBUFFER_BINDING,
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DRAW_FRAMEBUFFER and READ_FRAMEBUFFER all refer to the corresponding _APPLE
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suffixed names (they have the same token values).
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Errors
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The INVALID_OPERATION error is generated if DrawBuffersEXT is called
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when the default framebuffer is bound and any of the following conditions
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hold:
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- <n> is greater than 1 and less than MAX_DRAW_BUFFERS_EXT,
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- <bufs> contains a value other than BACK or NONE.
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The INVALID_OPERATION error is generated if DrawBuffersEXT is called
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when bound to a draw framebuffer object and any of the following
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conditions hold:
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- the <i>th value in <bufs> is not COLOR_ATTACHMENT<i>_EXT or NONE.
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The INVALID_VALUE error is generated if DrawBuffersEXT is called
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with a value of <n> which is greater than MAX_DRAW_BUFFERS_EXT.
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The INVALID_ENUM error is generated by FramebufferRenderbuffer if
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the <attachment> parameter is not one of the values listed in Table 4.x.
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The INVALID_ENUM error is generated by FramebufferTexture2D if
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the <attachment> parameter is not one of the values listed in Table 4.x.
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The INVALID_ENUM error is generated by GetFramebufferAttachmentParameteriv
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if the <attachment> parameter is not one of the values listed in Table 4.x.
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New State
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Add Table 6.X Framebuffer (State per framebuffer object):
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State Type Get Command Initial Value Description
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------------------ ---- ------------ ------------- -----------
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DRAW_BUFFER<i>_EXT Z10* GetIntegerv see 4.2.1 Draw buffer selected
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for fragment color i
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Add to Table 6.18 (Implementation Dependent Values)
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Get value Type Get Cmnd Minimum Value Description Sec.
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-------------------- ---- ----------- ------------- ----------- -----
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MAX_DRAW_BUFFERS_EXT Z+ GetIntegerv 1 Maximum number of 4.2.1
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active draw buffers
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MAX_COLOR_ATTACHMENTS_EXT Z+ GetIntegerv 1 Number of framebuffer 4.4.1
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color attachment points
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Issues
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See ARB_draw_buffers for relevant issues.
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1) What are the differences between this extension and NV_draw_buffers
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+ NV_fbo_color_attachments?
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RESOLVED. This extension:
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- adds interactions with blit_framebuffer and the separate
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draw/read binding points
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- The draw buffer and color attachment limits are global instead
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of per-fbo (see Issue 2)
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- can be used to with default framebuffer to set NONE/BACK (see Issue 4)
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- retains the ordering restrictions on color attachments that are
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imposed by ES 3.0.
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2) Should the MAX_DRAW_BUFFERS_EXT and MAX_COLOR_ATTACHMENTS_EXT limits
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be per-framebuffer values or implementation dependent constants?
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DISCUSSION: In ARB_draw_buffers this was per-context (see Issue 2).
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EXT_framebuffer_object (and subsequently ARB_framebuffer_object, and GL 3.0
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through GL 4.2) made these queries framebuffer-dependent.
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However in GL 4.3 and GLES 3.0, these limits were changed from
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framebuffer-dependent state to implementation-dependent state after
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much discussion (Bug 7990).
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NV_draw_buffers has MAX_DRAW_BUFFERS listed as per-framebuffer state,
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but NV_fbo_color_attachments has MAX_COLOR_ATTACHMENTS as an
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implementation-dependent constant.
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This is relevant because some implementations are not able to support
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multisampling in conjuction with multiple color attachments. If the
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query is per-framebuffer, they can report a maximum of one attachment
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when there are multisampled attachments, but a higher limit when only
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single-sampled attachments are present.
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RESOLVED. Make this global context state as this is most consistent
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with GLES 3.0 and updated GL drivers. In an implementation cannot
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support multisampling in conjunction with multiple color attachments,
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perhaps such an implementation could report FBO incomplete in this
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situation, but this is less than desirable.
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3) Should we support broadcast from gl_FragColor to all gl_FragData[x]
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or should it be synonymous with gl_FragData[0]?
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DISCUSSION: With NV_draw_buffers, writing to gl_FragColor writes to all
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the enabled draw buffers (ie broadcast). In OpenGL ES 3.0 when using
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ESSL 1.0, gl_FragColor is equivalent to writing a single output to
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gl_FragData[0] and multiple outputs are not possible. When using ESSL 3.0,
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only user-defined out variables may be used.
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If broadcast is supported, some implementations may have to replace
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writes to gl_FragColor with replicated writes to all possible gl_FragData
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locations when this extension is enabled.
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RESOLVED: Writes to gl_FragColor are broadcast to all enabled color
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buffers. ES 3.0 using ESSL 1.0 doesn't support broadcast because
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ESSL 1.0 was not extended to have multiple color outputs (but that is
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|
what this extension adds). ESSL 3.0 doesn't support the broadcast because
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it doesn't have the gl_FragColor variable at all, and only has user-
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|
defined out variables. This extension extends ESSL 1.0 to have multiple
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|
color outputs. Broadcasting from gl_FragColor to all enabled color
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|
buffers is the most consistent with existing draw buffer extensions to
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|
date (both NV_draw_buffers and desktop GL).
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|
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4) Should we allow DrawBuffersEXT to be called when the default FBO is
|
|
bound?
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|
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DISCUSSION: NV_draw_buffers specifies that DrawBuffersNV errors with
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|
INVALID_OPERATION when the default FBO is bound. OpenGL ES 3.0 allows
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|
DrawBuffers to toggle between BACK and NONE on the default FBO.
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|
|
|
An implementation that does not natively support disabling the drawbuffer
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|
on the default FBO could emulate this by disabling color writes.
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|
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RESOLVED: Allow DrawBuffersEXT to be called for the default FBO. This
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|
is more forward looking and is compatible with ES 3.0.
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|
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|
5) What are the requirements on the color attachment sizes and formats?
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|
|
|
RESOLVED: ES 2.0 requires that all color buffers attached to application-
|
|
created framebuffer objects must have the same number of bitplanes
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|
(Chapter 4 overview p91). ES 2.0 also requires that all attached images
|
|
have the same width and height (Section 4.4.5 Framebuffer Completeness).
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|
This extension does not lift those requirements, and failing to meet
|
|
them will result in an incomplete FBO.
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|
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|
6) Does this have any interactions with glClear?
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|
|
|
RESOLVED: Yes. When multiple color buffers are enabled for writing,
|
|
glClear clears all of the color buffers. Added language clarifying
|
|
that glClear and glClearColor may affect multiple color buffers.
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Revision History
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|
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01/30/2013 dgkoch add issue 6 and clear interactions
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|
renamed to EXT_draw_buffers based on feedback
|
|
changed resolution of issue 3.
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|
01/23/2013 dgkoch add resolutions to issues 2-4.
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|
add issue 5.
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|
Add Table 4.x and update various explicit
|
|
references to COLOR_ATTACHMENT0.
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|
Add errors.
|
|
11/13/2012 dgkoch add revision history
|
|
add text from updated ES 3.0 spec
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|
add issues for discussion
|
|
10/16/2012 kbr update name string
|
|
10/16/2012 kbr remove restrition requiring draw buffer 0 to be non-NULL
|
|
10/12/2012 kbr remove references to GetDoublev and ReadBuffer
|
|
10/11/2012 kbr initial draft extension
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