[Mesa-dev] [PATCH 0/2 v2] Add support for clip distances in Gallium

[Alex Deucher]

On Tue, Dec 13, 2011 at 4:18 PM, Christoph Bumiller
<e0425955 at student.tuwien.ac.at> wrote:
> On 12/13/2011 09:58 PM, Jose Fonseca wrote:
>>
>>
>> ----- Original Message -----
>>> On 12/13/2011 09:11 PM, Jose Fonseca wrote:
>>>>
>>>>
>>>> ----- Original Message -----
>>>>> This is an updated version of the patch set I sent to the list a
>>>>> few
>>>>> hours
>>>>> ago.
>>>>
>>>>
>>>>> There is now a TGSI property called
>>>>> TGSI_PROPERTY_NUM_CLIP_DISTANCES
>>>>> that drivers can use to determine how many of the 8 available clip
>>>>> distances
>>>>> are actually used by a shader.
>>>>
>>>> Can't the info in TGSI_PROPERTY_NUM_CLIP_DISTANCES be easily
>>>> derived from the shader, and queried through
>>>> src/gallium/auxiliary/tgsi/tgsi_scan.h ?
>>>>
>>>>
>>>> Could you also elaborate on why TGSI_SEMANTIC_CLIPDIST is useful
>>>> for the drivers? I personally don't have nothing against it, but
>>>> just like to understand why it makes a difference.
>>>>
>>>
>>> Why does TGSI_SEMANTIC_*POSITION* make a difference ?
>>>
>>> Right, because the position values are consumed by the fixed function
>>> rasterizer. So are the clip distances.
>>>
>>> This is not about pipe_clip_state.ucp but about what this legacy
>>> cruft
>>> has to be turned into if GL_CLIP_PLANEi is used instead of GLSL 1.3's
>>> gl_ClipDistance[i].
>>
>> I'm just surprised because I thought there was no more fixed function clipping.
>>
>
> nvfx and r300, and I think even r600, are probably happy to have UCPs in
> the interface and can make direct use of them, see nvfx_ucp_validate in
> nvfx_state_emit.c or r300_set_clip_state in r300_state.c

R3xx-cayman all support legacy user clip planes (see
r600_set_clip_state() and evergreen_set_clip_state()).  r6xx-cayman
also support shader exported clip/cull distance.  Clip/cull distance
is exported from the vertex shader (as 1 or 2 vec4s) via special
export locations (similar to position, point size, edge flag, rt
index, viewport index, and kill flag).  See the comment starting with
"Special export handling in shaders" in r600_shader.c.

Alex

>
> nv50 and nvc0 have to use the same shader outputs for both legacy
> glClipPlanes and gl_ClipDistance[].
>
> On nv50, there is shader-external state that designates which outputs
> are clip distances, on nvc0 they have to be written to special output
> slots/addresses so the semantic information is important to be able to
> assign slots at compile time (the GPU was specially designed to make
> this possible and to support ARB_separate_shader_objects nicely).
>
> In D3D10/11, there is a system value semantic for clip distances and
> cull distances (cull distance is a different clipping mode and consumes
> the same slots as clip distances).
> See
> http://msdn.microsoft.com/en-us/library/windows/desktop/bb509647%28v=vs.85%29.aspx.
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