[Bug 89649] dEQP-GLES3: line color interpolation does not have enough precision

[bugzilla-daemon at freedesktop.org]

https://bugs.freedesktop.org/show_bug.cgi?id=89649

--- Comment #11 from Jason Ekstrand <jason at jlekstrand.net> ---
Unfortunately, the HW doesn't give us a perpendicular barycentric coordinate so
we have no idea where we are in the line other than gl_FragCoord.xy.  The best
solution I've been able to come up with so far is to add an extra vec4 varying
out of the last geometry stage into the fragment shader which duplicates
gl_Position.  Then we can use that to find the slope of the line to determine
major/minor axis and, using gl_FragCoord.xy, manually calculate the point
vertical or horizontal and compute our own barycentrics.  Of course, we'll have
to have different math for perspective correct vs. not etc.  Oh, fun!

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