[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 #9 from Jason Ekstrand <jason at jlekstrand.net> ---
If the commentary on the Khronos and Google issues (which aren't public) is to
be believed, Intel HW is providing barycentrics to the shader which are
interpolated diagonally as if the line is drawn as a rectangle rather than
simply replicating the barycentrics from the original line vertically or
horizontally.  If this is really the case, then ouch; we're going to have to do
some "fun" in the shader.

One option would be to use derivatives of barycentrics to figure out the slope
of the line and, from there, figure out if we're supposed to be horizontal or
vertical.  However, I'm a bit concerned about the stability of those
calculations for 45-degree lines and I'm not sure how we would figure out from
there how to correct to get back to the pixel we're vertically or horizontally
associated with.

Another (rather horrible) option would be to somehow push both ends of the line
into the fragment shader (probably via a geometry shader?  Yuck!) and use
gl_FragCoord to figure out our own barycentrics.

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