[Mesa-dev] [Bug 97231] GL_DEPTH_CLAMP doesn't clamp to the far plane

[bugzilla-daemon at freedesktop.org]

https://bugs.freedesktop.org/show_bug.cgi?id=97231

--- Comment #20 from Roland Scheidegger <sroland at vmware.com> ---
(In reply to Ilia Mirkin from comment #18)
> FWIW this also fails on nouveau right now... this is a part of the patch I'm
> going to have to apply (still working out the full details, but this should
> have the general idea):
> 
> @@ -329,8 +329,21 @@ nvc0_validate_viewport(struct nvc0_context *nvc0)
>        PUSH_DATA (push, (w << 16) | x);
>        PUSH_DATA (push, (h << 16) | y);
>  
> -      zmin = vp->translate[2] - fabsf(vp->scale[2]);
> -      zmax = vp->translate[2] + fabsf(vp->scale[2]);
> +      if (nvc0->rast->pipe.clip_halfz) {
> +         zmin = vp->translate[2];
> +         zmax = vp->translate[2] + vp->scale[2];
> +         if (zmax < zmin) {
> +            float t = zmax;
> +            zmax = zmin;
> +            zmin = t;
> +         }
> +      } else {
> +         zmin = vp->translate[2] - fabsf(vp->scale[2]);
> +         zmax = vp->translate[2] + fabsf(vp->scale[2]);
> +      }
> 
> Note that the near/far planes have to be calculated differently based on
> whether clip_halfz is enabled.

Yes, that's pretty much what I had in mind for fixing llvmpipe. Albeit llvmpipe
already does different calculation depending on halfz.
It does:
      if (lp->rasterizer->clip_halfz == 0) {
         float half_depth = viewports[i].scale[2];
         min_depth = viewports[i].translate[2] - half_depth;
         max_depth = min_depth + half_depth * 2.0f;
      } else {
         min_depth = viewports[i].translate[2];
         max_depth = min_depth + viewports[i].scale[2];
      }
That said, it doesn't do the abs part. Is that really required?
And don't you need the zmin/zmax swap part too without clip_halfz?
(And as said, llvmpipe has problems with depth clamp as it does only actually
clamp the values if they are output by fs.)

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