EXT_post_depth_coverage

GL_EXT_post_depth_coverage

Jeff Bolz, NVIDIA Corporation (jbolz 'at' nvidia.com)
Pat Brown, NVIDIA Corporation (pbrown 'at' nvidia.com)

Jeff Bolz, NVIDIA
Pat Brown, NVIDIA
James Helferty, NVIDIA

Shipping

Last Modified Date:         March 27, 2015
Revision:                   2

OpenGL Extension #461
OpenGL ES Extension #225

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.

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;

None.

None.

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 <n> of element <w> 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. ...

None.

None.

None.

None.

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 : <behavior>

where <behavior> 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.

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.

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.

None.

(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 2, 2015/03/27
  - Add ES interactions

Revision 1, September 12, 2014 (jbolz, pbrown, jhelferty)

  Internal spec development.