[Mesa-dev] [PATCH 2/2] gallium: Replaced gl_rasterization_rules with lower_left_origin and half_pixel_center.

[Jose Fonseca]

----- Original Message -----
> Do we really need the lower_left_origin state? I think I can't
> implement it for radeon and it's the kind of stuff that should be
> taken care of by the state tracker anyway. 

My understanding is that hardware had switches for this sort of thing. It's really hard to provide fully-conforming rasterization for opengl, dx9 & dx10 without it.

If your hardware allows to put a negative pitch on rendertargets, then that should also do it.

If you know what is the hardware's sub-pixel rasterization resolution, then adding a vertical bias equal to that amount, depending on this state, would give a very close approximation. (This would get the top/bottom edges right, at expense of small inaccuracies on non-horizontal edges)

> Isn't it sufficient to just
> set a viewport which is upside down, like we do now?

I'm not aware of rasterization top-left rule being affected by the viewport flipping.

Do both

 ./bin/triangle-rasterization -auto
 ./bin/triangle-rasterization -use_fbo -auto

currently work for you?


If drivers don't provide this state, the only way to workaround it I know would be to store textures (or drawables?) up-side down, and flip them on gl(Get)TexImage & friends.  This would be like using a cannon to shoot a fly (a lot of work and a lot of overheads for a small correctness detail).  I think the drivers are better equipped to handle this.

And you always have the option of merely ignoring this state.  Top-left rule correct rasterization has, after all, been ignored till date, and nobody cared.


For the record, my motivation here is simple: llvmpipe gets the right behavior on GL drawables, and fails on GL FBOs & D3D 9/10. I want to get the right behavior on D3D 9/10 without causing regressions on GL drawables.

BTW, I'd imagine that if hardware rasterizer behavior is hardcoded to anything, it would be to D3D 9/10 behavior. That is, they would get GL FBO right, but drawables wrong.


Jose