[Mesa-dev] Gallium depth clamp support

Keith Whitwell keithw at vmware.com
Wed Jul 21 06:54:14 PDT 2010

On Wed, 2010-07-21 at 01:49 -0700, Marek Olšák wrote:
> Hi,
> there is a new branch gallium-depth-clamp in the main repository which
> implements ARB_depth_clamp in Gallium. Wine uses this extension to
> disable clipping when it's requested from a D3D9 app, so it's an
> important one.
> There is a new state "depth_clamp" in pipe_clip_state, and a new cap
> PIPE_CAP_DEPTH_CLAMP. If you think this feature should be mandatory in
> Gallium instead, it's ok with me. There are several reasons I put the
> enable bit in pipe_clip_state, but the most important one is that Z
> clipping must be disabled in hardware first for it to work. It also
> implements depth_clamp handling in cso_cache and wires up
> ARB_depth_clamp in st/mesa.
> The support in Draw has also been implemented, and both softpipe and
> llvmpipe pass piglit/depth_clamp and piglit/depth-clamp-range.
> http://cgit.freedesktop.org/mesa/mesa/log/?h=gallium-depth-clamp
> Please review.
> -Marek


This looks like a useful change. 

I'm not sure it's actually going go give the expected results for
softpipe/llvmpipe or other users of the path you've put into the draw

In particular it looks like the spec calls for clamping depth on a
per-pixel basis, where as this is doing it per-vertex.   

The difference is that the spec would allow a triangle with one distant
vertex to hit the far plane and then be clamped to it, whereas your
change means that the triangle as a whole will be altered to be in front
of the depth plane and touch it at one vertex.

Basically, I think the draw change needs to reduced to just disabling
depth clipping, and (if you like) a clamp(depth, 0, 1) introduced into
softpipe/llvmpipe.  Or just don't advertise the cap for


 I would 

 It adds an extra branch to the clipmask computation, but it's not like
that code is tuned at this point anyway...  I'

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