[Mesa-dev] gl_FragCoord / FBOs vs mesa/st

[Keith Whitwell]

On Sun, 2010-11-14 at 20:18 -0800, Dave Airlie wrote:
> Eric just checked in a test into piglit that tests that the
> gl_FragCoord works the right way up for FBOs,
> 
> Now all the gallium drivers fail this currently and fixing it creates
> an ugly linkage between the currently bound buffer and the fragment
> shader, since if you swap from an FBO to rendering to the front
> buffer, you need recompile the fragment shader to emit a proper wpos
> manipulation. Just wondering if anyone sees a nicer way to do this,
> than caching frag shaders with some sort of key in the state tracker,
> (which is pretty much what 965 has done.).

I guess the other possibility would be to have a couple of constants in
the constant buffer which get factored into the fragcood calculation in
such a way as to effect a flip based on their value, eg:

   fc' = fc * const[0].x + const[0].y

where const[0] is either
-> {1, 0} for non-flipped
-> {-1, fb_height} for flipped

Keith