Name ARB_post_depth_coverage Name Strings GL_ARB_post_depth_coverage Contact Piers Daniell, NVIDIA Corporation (pdaniell 'at' nvidia.com) Contributors Jeff Bolz, NVIDIA Pat Brown, NVIDIA James Helferty, NVIDIA Eric Werness, NVIDIA Notice Copyright (c) 2015 The Khronos Group Inc. Copyright terms at http://www.khronos.org/registry/speccopyright.html Specification Update Policy Khronos-approved extension specifications are updated in response to issues and bugs prioritized by the Khronos OpenGL Working Group. For extensions which have been promoted to a core Specification, fixes will first appear in the latest version of that core Specification, and will eventually be backported to the extension document. This policy is described in more detail at https://www.khronos.org/registry/OpenGL/docs/update_policy.php Status Complete. Approved by the ARB on June 26, 2015. Ratified by the Khronos Board of Promoters on August 7, 2015. Version Last Modified Date: June 25, 2015 Revision: 3 Number ARB Extension #180 Dependencies This extension is written against the OpenGL 4.5 specification (Compatibility Profile). This extension is written against version 4.50 of the OpenGL Shading Language Specification. This extension interacts with NV_fragment_program4. This extension interacts with EXT_post_depth_coverage. 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; Use of this feature implicitly enables early fragment tests. New Procedures and Functions None. New Tokens None. Additions to Chapter 15 of the OpenGL 4.5 (Compatibility Profile) Specification (Rasterization) Modify Section 15.1 Fragment Shader Variables, p. 558 (modify the third paragraph on p. 559) ...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. 560 (modify the fourth paragraph on p. 562) ...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 4.50 Including the following line in a shader can be used to control the language features described in this extension: #extension GL_ARB_post_depth_coverage : where is as specified in section 3.3. New preprocessor #defines are added to the OpenGL Shading Language: #define GL_ARB_post_depth_coverage 1 Modify Section 4.4.1.3 Fragment Shader Inputs (p. 63) (replace the discussion of early_fragment_tests on p. 64) 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 neither this nor post_depth_coverage are 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. Dependencies on NV_fragment_program4 Modify Section 2.X.6.Y of the NV_fragment_program4 specification (add new option section) + Post-depth Coverage (ARB_post_depth_coverage) If a fragment program specifies the "ARB_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 ARB_post_depth_coverage option and not the NV_early_fragment_tests option, it will fail to compile. Dependencies on EXT_post_depth_coverage If a shader enables both EXT_post_depth_coverage and ARB_post_depth_coverage the behavior of layout(post_depth_coverage) will match ARB_post_depth_coverage. 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. (2) What happens if you write gl_FragDepth? What about side-effects? RESOLVED: Because the post_depth_coverage option implies early_fragment_tests, the behavior for these cases is inhereted from the basic early_fragment_tests behavior, so writes to gl_FragDepth are ignored and side effects are only executed on shaders that are executed which have already passed the early fragment operations. Revision History Revision 3, June 25, 2015 Add interaction between ARB_post_depth_coverage and EXT_post_depth_coverage behaviors Revision 2, April 29, 2015 Implicitly enable early_fragment_tests when post_depth_coverage is enabled. Add a clarification issue. Revision 1, April 20, 2015 Initial ARB spec branched from EXT_post_depth_coverage