Name EXT_post_depth_coverage Name Strings GL_EXT_post_depth_coverage Contact Jeff Bolz, NVIDIA Corporation (jbolz 'at' nvidia.com) Pat Brown, NVIDIA Corporation (pbrown 'at' nvidia.com) Contributors Jeff Bolz, NVIDIA Pat Brown, NVIDIA James Helferty, NVIDIA Status Shipping Version Last Modified Date: March 27, 2015 Revision: 2 Number OpenGL Extension #461 OpenGL ES Extension #225 Dependencies This extension is written against the OpenGL 4.3 specification (Compatibility Profile). This extension is written against version 4.30 of the OpenGL Shading Language Specification. This extension interacts with NV_fragment_program4. This extension interacts with OpenGL ES 3.1 If implemented in OpenGL ES 3.1, OES_sample_variables (providing gl_SampleMaskIn) is required. Overview This extension allows the fragment shader to control whether values in gl_SampleMaskIn[] reflect the coverage after application of the early depth and stencil tests. This feature can be enabled with the following layout qualifier in the fragment shader: layout(post_depth_coverage) in; To use this feature, early fragment tests must also be enabled in the fragment shader via: layout(early_fragment_tests) in; New Procedures and Functions None. New Tokens None. Additions to Chapter 15 of the OpenGL 4.3 (Compatibility Profile) Specification (Rasterization) Modify Section 15.1 Fragment Shader Variables, p. 508 (modify the third paragraph on p. 509) ...When interpolating variables declared using "centroid in", the variable is sampled at a location within the pixel covered by the primitive generating the fragment. The fragment shader layout qualifier "post_depth_coverage" (Section 15.2.2) does not affect the determination of the centroid location. Modify Section 15.2.2 Shader Inputs, p. 511 (modify the first paragraph on p. 513) ...Bit of element in the array is set if and only if the sample numbered <32w + n> is considered covered for this fragment shader invocation. If the fragment shader specifies the "early_fragment_tests" and "post_depth_coverage" layout qualifiers, then the sample is considered covered if and only if the sample is covered by the primitive and the sample passes the early fragment tests (as described in Section 15.2.4). If these layout qualifiers are not specified, then the sample is considered covered if the sample is covered by the primitive, regardless of the result of the fragment tests. ... New Implementation Dependent State None. New State None. Additions to the AGL/GLX/WGL Specifications None. GLX Protocol None. Modifications to the OpenGL Shading Language Specification, Version 4.30 Including the following line in a shader can be used to control the language features described in this extension: #extension GL_EXT_post_depth_coverage : where is as specified in section 3.3. New preprocessor #defines are added to the OpenGL Shading Language: #define GL_EXT_post_depth_coverage 1 Modify Section 4.4.1.3 Fragment Shader Inputs (p. 58) (replace the discussion of early_fragment_tests on p. 59) Fragment shaders also allow the following layout qualifiers on "in" only (not with variable declarations) layout-qualifier-id early_fragment_tests post_depth_coverage For example, layout(early_fragment_tests) in; layout(post_depth_coverage) in; "early_fragment_tests" requests that fragment tests be performed before fragment shader execution, as described in section 15.2.4 "Early Fragment Tests" of the OpenGL Specification. If this is not declared, per-fragment tests will be performed after fragment shader execution. "post_depth_coverage" requests that the built-in "gl_SampleMaskIn[]" will reflect the result of the early fragment tests, as described in section 15.2.2 "Shader Inputs" of the OpenGL Specification. Only one fragment shader (compilation unit) need declare these, though more than one can. If at least one fragment shader declares one of these, then it is enabled. If any fragment shader declares "post_depth_coverage" and none declare "early_fragment_tests", a link-time error will occur. Dependencies on NV_fragment_program4 Modify Section 2.X.6.Y of the NV_fragment_program4 specification (add new option section) + Post-depth Coverage (EXT_post_depth_coverage) If a fragment program specifies the "EXT_post_depth_coverage" option, the sample mask will reflect the result of the early fragment tests, as described in Section 15.2.2 "Shader Inputs". If a fragment program specifies the EXT_post_depth_coverage option and not the NV_early_fragment_tests option, it will fail to compile. Interactions with OpenGL ES 3.1 If OpenGL ES 3.1 is supported, edits similar to those above are applied to OpenGL ES 3.1 and GLSL ES 3.10 specifications. Modify the edits under "Fragment Shader Inputs" as follows: Add the following to the paragraph discussing behavior when "early_fragment_tests" is enabled to reflect a pre-existing error not shared with GLSL 4.30: It is an error to statically write to gl_FragDepth in the fragment shader. Change the paragraph beginning "Only one fragment shader (compilation unit) need declare..." to the following, since GLSL ES 3.10 does not support linking multiple shaders of the same type: If a fragment shader declares "post_depth_coverage" and doesn't declare "early_fragment_tests", a link-time error will occur. Errors None. Issues (1) Should the determination of a fragment's centroid use the pre-depth or post-depth coverage? RESOLVED: In this extension, it uses the pre-depth coverage. This way the centroid location (and hence the result of shading) does not depend on the rendering order, which is almost certainly the desired result for 3D rendering. For path rendering, it would be desirable to use post-depth centroid since the stencil test really determines whether samples are inside the primitive rather than whether samples are "occluded," and guaranteeing attributes are sampled inside the path would be nice. Revision History Revision 2, 2015/03/27 - Add ES interactions Revision 1, September 12, 2014 (jbolz, pbrown, jhelferty) Internal spec development.