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

[Marek Olšák]

Same as Dave. The first one fails, the second one (-use_fbo) passes.

I just hope lower_left_origin doesn't affect the Y axis of the
viewport, scissor and gl_FragCoord. If it does, a PIPE_CAP would be
useful to keep the current behavior.

Marek

On Sun, Apr 21, 2013 at 9:36 AM, Jose Fonseca <jfonseca at vmware.com> wrote:
> ----- 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