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

[Keith Whitwell]

On Mon, 2010-11-15 at 01:28 -0800, Keith Whitwell wrote:
> 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

Another question is how to tell the pipe driver which of these to use --
probably we want an explicit flag in one of the state atoms
(rasterizer?) to select between the two possibilities?

Keith