[Mesa-dev] Gallium depth clamp support

[Roland Scheidegger]

On 22.07.2010 01:27, Brian Paul wrote:
> On 07/21/2010 05:19 PM, Marek Olšák wrote:
>> On Thu, Jul 22, 2010 at 12:17 AM, Roland Scheidegger <sroland at vmware.com
>> <mailto:sroland at vmware.com>> wrote:
>>
>>     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
>>
>>
>>     One thing I was wondering does it actually make sense to call it
>>     depth clamp? I think d3d10 name for once (which calls this
>>     functionality depth clip though of course this means enable/disable
>>     is reversed) makes more sense - this disables near/far plane
>>     clipping, hence the necessity to clamp to 0/1.
>>     I guess having this in clip state is ok - d3d10 puts depth clip into
>>     rasterizer state, but then again d3d10 doesn't have any clip state...
>>
>>
>> Depth clip does sound better to me, but honestly I don't care how we
>> call it. If you like the D3D naming, so be it.
> 
> There's two aspects to the GL extension:
>   - near/far Z clipping
>   - fragment Z clamping during interpolation
> 
> I think pipe_clip_state::depth_clamp should become 
> pipe_clip_state::depth_clip.
> 
> Then, I'd probably add the second part of this as 
> pipe_rasterizer_state::depth_clamp to enable/disable fragment Z clamping.
> 
> GL would set both pieces of state in tandem.  I think we should have 
> two pieces of state to avoid interdependencies between clip state and 
> rasterization state in the drivers.
> 
> I'm not sure how this is expressed in Direct3D.

d3d10 always clamps z to viewport min/max depth, regardless if depth
clipping is enabled or not (hmm does it really make a difference when
depth clipping is enabled? Maybe only for OGL rasterpos and similar
legacy stuff?).

Roland