Friday, July 20, 2007

gl_FragCoord + ATI = World of Hurt

Well we just had a bit of a firedrill when we learned that gl_FragCoord causes some ATI cards on windows to revert to software shading. I don't know if this is fixed in a driver upgrade - we used a simple workaround suggested by a user on
  • We store the clip-space coordinate in a varying variable. (We have to compute that anyway, it's just the result of ftransform()).
  • We then perform a perspective divide and change the coordinate range in the fragment shader.
Fortunately we wanted the screen coordinate as 0..1 so having clip coordinates is almost what we want anyway. The perspective divide must be per-pixel - varying variables only interpolate in a perspective-correct manner in homogenous clip coordinates, not window coordinates. At least, I think. :-)

This is a case where "the Google" comes in really handy -- having confirmation from other users of a known bug gives us a lot more confidence in the workaround.