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kc3-lang/angle/extensions/EXT_draw_buffers.txt
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- extensions/EXT_draw_buffers.txt
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Name EXT_draw_buffers Name Strings GL_EXT_draw_buffers Contributors Contributors to GL_NV_draw_buffers Contributors to GL_NV_fbo_color_attachments Contributors to the OpenGL ES 2.0 specification Contributors to the OpenGLSL ES 1.0.17 specification Contributors to the OpenGL ES 3.0 specification Nicolas Capens, TransGaming Inc. Daniel Koch, TransGaming Inc. Alastair Patrick, Google Inc. Kenneth Russell, Google Inc. Greg Roth, NVIDIA Corporation Ben Bowman, Imagination Technologies Members of the WebGL and OpenGL ES working groups Contact Daniel Koch (daniel 'at' transgaming.com) Status Draft Complete Version Last Modified Date: January 30, 2013 Revision: #7 Number TBD Dependencies OpenGL ES 2.0 is required. The extension is written against the OpenGL ES 2.0 specification. ANGLE_framebuffer_blit affects the definition of this extension. APPLE_framebuffer_multisample affects the definitin of this extension. Overview This extension increases the number of available framebuffer object color attachment points, extends OpenGL ES 2.0 to allow multiple output colors, and provides a mechanism for directing those outputs to multiple color buffers. This extension is similar to the combination of the GL_NV_draw_buffers and GL_NV_fbo_color_attachments extensions, but imposes certain restrictions informed by the OpenGL ES 3.0 API. New Procedures and Functions void DrawBuffersEXT(sizei n, const enum *bufs); New Tokens Accepted by the <pname> parameter of GetIntegerv: MAX_COLOR_ATTACHMENTS_EXT 0x8CDF Accepted by the <pname> parameters of GetIntegerv and GetFloatv: MAX_DRAW_BUFFERS_EXT 0x8824 DRAW_BUFFER0_EXT 0x8825 DRAW_BUFFER1_EXT 0x8826 DRAW_BUFFER2_EXT 0x8827 DRAW_BUFFER3_EXT 0x8828 DRAW_BUFFER4_EXT 0x8829 DRAW_BUFFER5_EXT 0x882A DRAW_BUFFER6_EXT 0x882B DRAW_BUFFER7_EXT 0x882C DRAW_BUFFER8_EXT 0x882D DRAW_BUFFER9_EXT 0x882E DRAW_BUFFER10_EXT 0x882F DRAW_BUFFER11_EXT 0x8830 DRAW_BUFFER12_EXT 0x8831 DRAW_BUFFER13_EXT 0x8832 DRAW_BUFFER14_EXT 0x8833 DRAW_BUFFER15_EXT 0x8834 Accepted by the <attachment> parameter of FramebufferRenderbuffer, FramebufferTexture2D and GetFramebufferAttachmentParameteriv, and by the <bufs> parameter of DrawBuffersEXT: COLOR_ATTACHMENT0_EXT 0x8CE0 COLOR_ATTACHMENT1_EXT 0x8CE1 COLOR_ATTACHMENT2_EXT 0x8CE2 COLOR_ATTACHMENT3_EXT 0x8CE3 COLOR_ATTACHMENT4_EXT 0x8CE4 COLOR_ATTACHMENT5_EXT 0x8CE5 COLOR_ATTACHMENT6_EXT 0x8CE6 COLOR_ATTACHMENT7_EXT 0x8CE7 COLOR_ATTACHMENT8_EXT 0x8CE8 COLOR_ATTACHMENT9_EXT 0x8CE9 COLOR_ATTACHMENT10_EXT 0x8CEA COLOR_ATTACHMENT11_EXT 0x8CEB COLOR_ATTACHMENT12_EXT 0x8CEC COLOR_ATTACHMENT13_EXT 0x8CED COLOR_ATTACHMENT14_EXT 0x8CEE COLOR_ATTACHMENT15_EXT 0x8CEF The COLOR_ATTACHMENT0_EXT constant is equal to the COLOR_ATTACHMENT0 constant. Each COLOR_ATTACHMENT<i>_EXT adheres to COLOR_ATTACHMENT<i>_EXT = COLOR_ATTACHMENT0_EXT + <i>. Changes to Chapter 3 of the OpenGL ES 2.0 Specification (Rasterization) Section 3.2, (Multisampling). Replace the second paragraph: An additional buffer, called the multisample buffer, is added to the window system-provided framebuffer. Pixel sample values, including color, depth, and stencil values, are stored in this buffer. Samples contain separate color values for each fragment color. When the window system-provided framebuffer includes a multisample buffer, it does not include depth or stencil buffers, even if the multisample buffer does not store depth or stencil values. Color buffers do coexist with the multisample buffer, however. Section 3.8.2, (Shader Execution) Replace subsection "Shader Outputs": The OpenGL ES Shading Language specification describes the values that may be output by a fragment shader. These are gl_FragColor and gl_FragData[n]. The final fragment color values or the final fragment data values written by a fragment shader are clamped to the range [0, 1] and then converted to fixed-point as described in section 2.1.2 for framebuffer color components. Writing to gl_FragColor specifies the fragment color (color number zero) that will be used by subsequent stages of the pipeline. Writing to gl_FragData[n] specifies the value of fragment color number n. Any colors, or color components, associated with a fragment that are not written by the fragment shader are undefined. A fragment shader may not statically assign values to both gl_FragColor and gl_FragData. In this case, a compile or link error will result. A shader statically assigns a value to a variable if, after preprocessing, it contains a statement that would write to the variable, whether or not run-time flow of control will cause that statement to be executed. Changes to Chapter 4 of the OpenGL ES 2.0 Specification (Per-Fragment Operations and the Frame Buffer) Modify the overview of Chapter 4 and replace the sentences of the fifth paragraph which read: "The name of the color buffer of an application-created framebuffer object is COLOR_ATTACHMENT0. The names of the depth and stencil buffers are DEPTH_ATTACHMENT and STENCIL_ATTACHMENT." With the following: "A framebuffer object has an array of color buffer attachment points, numbered zero through <n>, a depth buffer attachment point, and a stencil buffer attachment point." Insert Table 4.3 to Section 4.2.1 (and renumber subsequent tables): Symbolic Constant Meaning ----------------- --------------------- NONE No buffer COLOR_ATTACHMENT<i>_EXT (see caption) Output fragment color to image attached at color attachment point i Table 4.3: Arguments to DrawBuffersEXT when the context is bound to a framebuffer object, and the buffers they indicate. <i> in COLOR_ATTACHMENT<i>_EXT may range from zero to the value of MAX_COLOR_ATTACHMENTS_EXT minus one. Replace Section 4.2.1, "Selecting a Buffer for Writing" with the following: "By default, color values are written into the front buffer for single buffered surfaces or into the back buffer for back buffered surfaces as determined when making the context current. To control the color buffer into which each of the fragment color values is written, DrawBuffersEXT is used. The command void DrawBuffersEXT(sizei n, const enum *bufs); defines the draw buffers to which all fragment colors are written. <n> specifies the number of buffers in <bufs>. <bufs> is a pointer to an array of symbolic constants specifying the buffer to which each fragment color is written. Each buffer listed in <bufs> must be BACK, NONE, or one of the values from table 4.3. Further, acceptable values for the constants in <bufs> depend on whether the GL is using the default framebuffer (i.e., DRAW_FRAMEBUFFER_BINDING is zero), or a framebuffer object (i.e., DRAW_FRAMEBUFFER_BINDING is non-zero). For more information about framebuffer objects, see section 4.4. If the GL is bound to the default framebuffer, then <n> must be 1 and the constant must be BACK or NONE. When draw buffer zero is BACK, color values are written into the sole buffer for single- buffered contexts, or into the back buffer for double-buffered contexts. If DrawBuffersEXT is supplied with a constant other than BACK and NONE, the error INVALID_OPERATION is generated. If the GL is bound to a draw framebuffer object, then each of the constants must be one of the values listed in table 4.3. In both cases, the draw buffers being defined correspond in order to the respective fragment colors. The draw buffer for fragment colors beyond <n> is set to NONE. The maximum number of draw buffers is implementation-dependent. The number of draw buffers supported can be queried by calling GetIntegerv with the symbolic constant MAX_DRAW_BUFFERS_EXT. An INVALID_VALUE error is generated if <n> is greater than MAX_DRAW_BUFFERS_EXT. If the GL is bound to a draw framebuffer object, the <i>th buffer listed in <bufs> must be COLOR_ATTACHMENT<i>_EXT or NONE. Specifying a buffer out of order, BACK, or COLOR_ATTACHMENT<m>_EXT where <m> is greater than or equal to the value of MAX_COLOR_ATTACHMENTS_EXT, will generate the error INVALID_OPERATION. If a fragment shader writes to "gl_FragColor", DrawBuffersEXT specifies the set of draw buffers into which the color written to "gl_FragColor" is written. If a fragment shader writes to "gl_FragData", DrawBuffersEXT specifies a set of draw buffers into which each of the multiple output colors defined by these variables are separately written. If a fragment shader writes to neither "gl_FragColor" nor "gl_FragData" the values of the fragment colors following shader execution are undefined, and may differ for each fragment color. Indicating a buffer or buffers using DrawBuffersEXT causes subsequent pixel color value writes to affect the indicated buffers. If the GL is bound to a draw framebuffer object and a draw buffer selects an attachment that has no image attached, then that fragment color is not written. Specifying NONE as the draw buffer for a fragment color will inhibit that fragment color from being written. The state required to handle color buffer selection for each framebuffer is an integer for each supported fragment color. For the default framebuffer, in the initial state the draw buffer for fragment color zero is BACK if there is a default framebuffer associated with the context, otherwise NONE. For framebuffer objects, in the initial state the draw buffer for fragment color zero is COLOR_ATTACHMENT0_EXT. For both the default framebuffer and framebuffer objects, the initial state of draw buffers for fragment colors other than zero is NONE. The value of the draw buffer selected for fragment color <i> can be queried by calling GetIntegerv with the symbolic constant DRAW_BUFFER<i>_EXT." Modify Section 4.2.3 (Clearing the Buffers) and replace the first two paragraphs with the following: "The GL provides a means for setting portions of every pixel in a particular buffer to the same value. The argument to void Clear(bitfield buf); is the bitwise OR of a number of values indicating which buffers are to be cleared. The values are COLOR_BUFFER_BIT, DEPTH_BUFFER_BIT, and STENCIL_BUFFER_BIT, indicating the buffers currently enabled for color writing, the depth buffer, and the stencil buffer (see below), respectively. The value to which each buffer is cleared depends on the setting of the clear value for that buffer. If the mask is not a bitwise OR of the specified values, then the error INVALID_VALUE is generated. void ClearColor(clampf r, clampf, g, clampf b, clampf a); sets the clear value for fixed-point color buffers. Each of the specified components is clamped to [0, 1] and converted to fixed-point as described in section 2.1.2 for framebuffer color components." Replace the second paragraph of Section 4.4.1 (Binding and Managing Framebuffer Objects) with the following: "The namespace for framebuffer objects is the unsigned integers, with zero reserved by OpenGL ES to refer to the default framebuffer. A framebuffer object is created by binding an unused name to the target FRAMEBUFFER, DRAW_FRAMEBUFFER, or READ_FRAMEBUFFER. The binding is effected by calling void BindFramebuffer(enum target, uint framebuffer); with <target> set the desired framebuffer target and <framebuffer> set to the unused name. The resulting framebuffer object is a new state vector. There is a number of color attachment points, plus one each for the depth and stencil attachment points. The number of color attachment points is equal to the value of MAX_COLOR_ATTACHMENTS_EXT." Replace the third item in the bulleted list in Section 4.4.1 (Binding and Managing Framebuffer Objects) with the following: " * The only color buffer bitplanes are the ones defined by the framebuffer attachments points named COLOR_ATTACHMENT0_EXT through COLOR_ATTACHMENT<n>_EXT." Modify Section 4.4.3 (Renderbuffer Objects) in the "Attaching Renderbuffer Images to a Framebuffer" subsection as follows: Insert the following table: Name of attachment --------------------------------------- COLOR_ATTACHMENT<i>_EXT (see caption) DEPTH_ATTACHMENT STENCIL_ATTACHMENT Table 4.x: Framebuffer attachment points. <i> in COLOR_ATTACHMENT<i>_EXT may range from zero to the value of MAX_COLOR_ATTACHMENTS_EXT minus 1. Modify the third sentence of the paragraph following the definition of FramebufferRenderbuffer to be as follows: "<attachment> should be set to one of the attachment points of the framebuffer listed in Table 4.x." Modify Section 4.4.3 (Renderbuffer Objects) in the "Attaching Texture Images to a Framebuffer" subsection as follows: Modify the last sentence of the paragraph following the definition of FramebufferTexture2D to be as follows: "<attachment> must be one of the attachment points of the framebuffer listed in Table 4.x." Modify Section 4.4.5 (Framebuffer Completeness) and replace the 3rd item in the bulleted list in the "Framebuffer Attachment Completeness" subsection with the following: " * If <attachment> is COLOR_ATTACHMENT<i>_EXT, then <image> must have a color-renderable internal format." Changes to Chapter 6 of the OpenGL ES 2.0 Specification (State and State Requests) In section 6.1.3 (Enumerated Queries) modify the third sentence in the definition of GetFramebufferAttachmentParameteriv to be as follows: "<attachment> must be one of the attachment points of the framebuffer listed in Table 4.x." Changes to Chapter 3 of the OpenGL ES Shading Language 1.0.17 Specification (Basics) Add a new section: 3.4.1 GL_EXT_draw_buffers Extension To use the GL_EXT_draw_buffers extension in a shader it must be enabled using the #extension directive. The shading language preprocessor #define GL_EXT_draw_buffers will be defined to 1, if the GL_EXT_draw_buffers extension is supported. Dependencies on ANGLE_framebuffer_blit and APPLE_framebuffer_multisample: If neither ANGLE_framebuffer_blit nor APPLE_framebuffer_multisample are supported, then all references to "draw framebuffers" should be replaced with references to "framebuffers". References to DRAW_FRAMEBUFFER_BINDING should be replaced with references to FRAMEBUFFER_BINDING. References to DRAW_FRAMEBUFFER and READ_FRAMEBUFFER should be removed. If ANGLE_framebuffer_blit is supported, DRAW_FRAMEBUFFER_BINDING, DRAW_FRAMEBUFFER and READ_FRAMEBUFFER all refer to corresponding _ANGLE suffixed names (they have the same token values). If APPLE_framebuffer_multisample is supported, DRAW_FRAMEBUFFER_BINDING, DRAW_FRAMEBUFFER and READ_FRAMEBUFFER all refer to the corresponding _APPLE suffixed names (they have the same token values). Errors The INVALID_OPERATION error is generated if DrawBuffersEXT is called when the default framebuffer is bound and any of the following conditions hold: - <n> is greater than 1 and less than MAX_DRAW_BUFFERS_EXT, - <bufs> contains a value other than BACK or NONE. The INVALID_OPERATION error is generated if DrawBuffersEXT is called when bound to a draw framebuffer object and any of the following conditions hold: - the <i>th value in <bufs> is not COLOR_ATTACHMENT<i>_EXT or NONE. The INVALID_VALUE error is generated if DrawBuffersEXT is called with a value of <n> which is greater than MAX_DRAW_BUFFERS_EXT. The INVALID_ENUM error is generated by FramebufferRenderbuffer if the <attachment> parameter is not one of the values listed in Table 4.x. The INVALID_ENUM error is generated by FramebufferTexture2D if the <attachment> parameter is not one of the values listed in Table 4.x. The INVALID_ENUM error is generated by GetFramebufferAttachmentParameteriv if the <attachment> parameter is not one of the values listed in Table 4.x. New State Add Table 6.X Framebuffer (State per framebuffer object): State Type Get Command Initial Value Description ------------------ ---- ------------ ------------- ----------- DRAW_BUFFER<i>_EXT Z10* GetIntegerv see 4.2.1 Draw buffer selected for fragment color i Add to Table 6.18 (Implementation Dependent Values) Get value Type Get Cmnd Minimum Value Description Sec. -------------------- ---- ----------- ------------- ----------- ----- MAX_DRAW_BUFFERS_EXT Z+ GetIntegerv 1 Maximum number of 4.2.1 active draw buffers MAX_COLOR_ATTACHMENTS_EXT Z+ GetIntegerv 1 Number of framebuffer 4.4.1 color attachment points Issues See ARB_draw_buffers for relevant issues. 1) What are the differences between this extension and NV_draw_buffers + NV_fbo_color_attachments? RESOLVED. This extension: - adds interactions with blit_framebuffer and the separate draw/read binding points - The draw buffer and color attachment limits are global instead of per-fbo (see Issue 2) - can be used to with default framebuffer to set NONE/BACK (see Issue 4) - retains the ordering restrictions on color attachments that are imposed by ES 3.0. 2) Should the MAX_DRAW_BUFFERS_EXT and MAX_COLOR_ATTACHMENTS_EXT limits be per-framebuffer values or implementation dependent constants? DISCUSSION: In ARB_draw_buffers this was per-context (see Issue 2). EXT_framebuffer_object (and subsequently ARB_framebuffer_object, and GL 3.0 through GL 4.2) made these queries framebuffer-dependent. However in GL 4.3 and GLES 3.0, these limits were changed from framebuffer-dependent state to implementation-dependent state after much discussion (Bug 7990). NV_draw_buffers has MAX_DRAW_BUFFERS listed as per-framebuffer state, but NV_fbo_color_attachments has MAX_COLOR_ATTACHMENTS as an implementation-dependent constant. This is relevant because some implementations are not able to support multisampling in conjuction with multiple color attachments. If the query is per-framebuffer, they can report a maximum of one attachment when there are multisampled attachments, but a higher limit when only single-sampled attachments are present. RESOLVED. Make this global context state as this is most consistent with GLES 3.0 and updated GL drivers. In an implementation cannot support multisampling in conjunction with multiple color attachments, perhaps such an implementation could report FBO incomplete in this situation, but this is less than desirable. 3) Should we support broadcast from gl_FragColor to all gl_FragData[x] or should it be synonymous with gl_FragData[0]? DISCUSSION: With NV_draw_buffers, writing to gl_FragColor writes to all the enabled draw buffers (ie broadcast). In OpenGL ES 3.0 when using ESSL 1.0, gl_FragColor is equivalent to writing a single output to gl_FragData[0] and multiple outputs are not possible. When using ESSL 3.0, only user-defined out variables may be used. If broadcast is supported, some implementations may have to replace writes to gl_FragColor with replicated writes to all possible gl_FragData locations when this extension is enabled. RESOLVED: Writes to gl_FragColor are broadcast to all enabled color buffers. ES 3.0 using ESSL 1.0 doesn't support broadcast because ESSL 1.0 was not extended to have multiple color outputs (but that is what this extension adds). ESSL 3.0 doesn't support the broadcast because it doesn't have the gl_FragColor variable at all, and only has user- defined out variables. This extension extends ESSL 1.0 to have multiple color outputs. Broadcasting from gl_FragColor to all enabled color buffers is the most consistent with existing draw buffer extensions to date (both NV_draw_buffers and desktop GL). 4) Should we allow DrawBuffersEXT to be called when the default FBO is bound? DISCUSSION: NV_draw_buffers specifies that DrawBuffersNV errors with INVALID_OPERATION when the default FBO is bound. OpenGL ES 3.0 allows DrawBuffers to toggle between BACK and NONE on the default FBO. An implementation that does not natively support disabling the drawbuffer on the default FBO could emulate this by disabling color writes. RESOLVED: Allow DrawBuffersEXT to be called for the default FBO. This is more forward looking and is compatible with ES 3.0. 5) What are the requirements on the color attachment sizes and formats? RESOLVED: ES 2.0 requires that all color buffers attached to application- created framebuffer objects must have the same number of bitplanes (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). This extension does not lift those requirements, and failing to meet them will result in an incomplete FBO. 6) Does this have any interactions with glClear? 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. Revision History 01/30/2013 dgkoch add issue 6 and clear interactions renamed to EXT_draw_buffers based on feedback changed resolution of issue 3. 01/23/2013 dgkoch add resolutions to issues 2-4. add issue 5. Add Table 4.x and update various explicit references to COLOR_ATTACHMENT0. Add errors. 11/13/2012 dgkoch add revision history add text from updated ES 3.0 spec 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