OpenGL ES CTS rejects legal attribute interpolation for wide lines

[kusma]

The OpenGL ES 3.2 spec, section 13.6.2.1 ("Wide Lines") says the following:

The preferred method of attribute interpolation for a wide line is to generate the same attribute values for all fragments in the row or column described above, as if the adjusted line were used for interpolation and those values replicated to the other fragments, except for gl_FragCoord which is interpolated as usual. An implementation may instead interpolate each fragment according to the formula in "Basic Line Segment Rasterization", using the original line segment endpoints.

However, the code in verifySinglesampleWideLineGroupInterpolation() (tcuRasterizationVerifier.cpp) only checks for results matching the preferred method, causing valid implementations to fail these tests. This is currently a problem on LLVMpipe in Mesa, and while it's relatively easy to fix in a software rasterizer, it would be great if we could make the CTS not reject things that are allowed by the spec.

Here's a list of tests known to be affected:

• dEQP-GLES2.functional.rasterization.interpolation.basic.line_loop_wide
• dEQP-GLES2.functional.rasterization.interpolation.basic.line_strip_wide
• dEQP-GLES2.functional.rasterization.interpolation.basic.lines_wide
• dEQP-GLES2.functional.rasterization.interpolation.projected.line_loop_wide
• dEQP-GLES2.functional.rasterization.interpolation.projected.line_strip_wide
• dEQP-GLES2.functional.rasterization.interpolation.projected.lines_wide
• dEQP-GLES3.functional.rasterization.fbo.rbo_singlesample.interpolation.lines_wide
• dEQP-GLES3.functional.rasterization.fbo.texture_2d.interpolation.lines_wide
• dEQP-GLES3.functional.rasterization.interpolation.basic.line_loop_wide
• dEQP-GLES3.functional.rasterization.interpolation.basic.line_strip_wide
• dEQP-GLES3.functional.rasterization.interpolation.basic.lines_wide
• dEQP-GLES3.functional.rasterization.interpolation.projected.line_loop_wide
• dEQP-GLES3.functional.rasterization.interpolation.projected.line_strip_wide
• dEQP-GLES3.functional.rasterization.interpolation.projected.lines_wide

[mnetsch]

Hi, could you please try on the latest release 3.2.8.0 and propose a fix that works for Mesa if it still has issues? There's been some changes to relaxing this in CTS and the latest release might have some fixes.

[kusma]

I've modified Mesa to use the preferred interpolation instead. I suppose I could revert that change to give it a go, though. But I'm currently out on leave from work, so I won't be looking at this for at least another week or so.

[kusma]

Nope, still fails.

[kusma]

@mnetsch: Here you go: #309

[mnetsch]

I'll bring it in for review, thanks

[mnetsch]

Merged in a6cd9c7