[Bug 103047] lighting render issue with i965 on SKL+

[bugzilla-daemon at freedesktop.org]

https://bugs.freedesktop.org/show_bug.cgi?id=103047

Danylo <danylo.piliaiev at gmail.com> changed:

           What    |Removed                     |Added
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                 CC|                            |danylo.piliaiev at gmail.com

--- Comment #14 from Danylo <danylo.piliaiev at gmail.com> ---
I found that the underlying cause of the issue lies in the combination of flat
shading, provoking vertex and clipped polygons.

Reproducing the issue is easy:

- Have glShadeModel set to GL_FLAT, smooth doesn't have such issue
- Have glProvokingVertexEXT set to GL_LAST_VERTEX_CONVENTION_EXT, first vertex
mode seems not to exhibit the issue
- Create triangle strip with 3 vertices out of screen with blue color, two
vertices on screen where the last one has red color (colors themself don’t 
- Expected color of the triangle is red because color should be taken from last
vertex but on Intel GPU triangle is blue

I asked Jason to help me, I'll quote his answer:
> I think what you've found is a hardware bug. :-(  
> It appears that the clipper is messing up provoking vertex whenever entire 
> polygons are clipped.

> What do we do about it?  That's an interesting question.  
> The most obvious thing that jumps out to me would be to do a shader 
> workaround where we tell the hardware that flat inputs are not flat 
> (so it passes in all three vertices) and then manually grab the correct 
> one of the three interpolants based on which provoking vertex is set.  
> I don't think provoking vertex is something that applications change 
> frequently so having a bit in the FS shader key probably isn't too 
> bad for this.

I've sent a patch to Piglit which reproduces the bug -
https://patchwork.freedesktop.org/patch/229260/

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