[Mesa-dev] first attempt at shader stencil export

José Fonseca jfonseca at vmware.com
Thu Sep 30 10:43:22 PDT 2010

On Thu, 2010-09-30 at 10:19 -0700, Roland Scheidegger wrote:
> On 30.09.2010 16:44, José Fonseca wrote:
> > On Thu, 2010-09-30 at 07:32 -0700, Brian Paul wrote:
> >> On 09/30/2010 12:41 AM, Dave Airlie wrote:
> >>> some background:
> >>>
> >>> so on r600g, the only way to directly write to the stencil is via the
> >>> shader, using a transfer would require an extra step to flush the DS
> >>> buffer out via the pipe again to make it useable by the hw as a DS
> >>> buffer. So using the current gallium stencil draw would be a major
> >>> overhead, compared to just doing it properly.
> >>>
> >>> So to do it properly I decided to expose the
> >>> GL_ARB_shader_stencil_export type functionality.
> >>>
> >>> http://cgit.freedesktop.org/~airlied/mesa/log/?h=gallium-stencil-export
> >>>
> >>> 7 commits in here:
> >>>
> >>> two simple one liners, add the cap to gallium headers, and add
> >>> FRAG_RESULT_STENCIL to mesa.
> >>> then a "fix" to the mesa texstore so it will store S8 in an 8/24 so we
> >>> can put the stencil values in a texture.
> >>> mesa state tracker support to use the new capability to hw accel
> >>> drawpixels on the stencil (outputs to Y of FRAG_RESULT_STENCIL).
> >>> r600g support for the capability take the second
> >>> TGSI_SEMANTIC_POSITION output and use its Y value as stencil (looking
> >>> at this now, I should probably be taking the X value really).
> >>> very initial mesa/GLSL support for the extension (completely untested).
> >>> initial softpipe support for the capability a demonstration.
> >>>
> >>> TODO:
> >>> probably stop using Y (this was just because the hw uses Y), the GLSL
> >>> extension just defines an integer output for the fragprog.
> >>> fix the 24/8 texstore variant.
> >>> write some test code for piglit and test the GL extension/GLSL bits.
> >>>
> >>> I'm a lot more interested in the non-GL extension bits as it allows
> >>> stencil writes to work properly on r600g.
> >> A few comments.
> >>
> >> Instead of using semantic name=TGSI_SEMANTIC_POSITION, index=1 to 
> >> indicate the stencil output, how about just defining a new 
> >> TGSI_SEMANTIC_STENCIL label instead?  I think that'd be clearer.
> >>
> >> I think the fragment stencil result is supposed to be an integer.  But 
> >> in the softpipe changes, it looks like you're doing a [0,1]->[0,255] 
> >> scaling at one point.
> >>
> >> If we want to use this feature for implementing glDraw/CopyPixels 
> >> functionality, the stencil values will be coming from a texture.  How 
> >> do we sample that texture?  That is, which components of the texture 
> >> image return the Z and/or stencil components?  Basically, we need to 
> >> add a new row to the table in the tgsi.rst file for Z/Stencil 
> >> textures.  Also, we need to clarify that fetching a stencil value 
> >> returns an integer value, not a float normalized to [0,1].
> > 
> > DirectX 10 documentation concerning depth textures is quite peculiar,
> > they suggest using typecasts:
> > 
> > http://msdn.microsoft.com/en-us/library/bb205074(v=VS.85).aspx#reading_the_depth-stencil_buffer_as_a_texture
> > 
> > But they specifically mention reading z24s8 stencil values this way,
> > although it could be certainly possible, by reading the texture as i32. 
> No this won't work as you can't read a z24s8 format a i32 (not the same
> typeless group, number of components needs to be the same as well as all
> components need to have same number of bits).
> But the format list suggests this is possible. For z24s8 you could
> presumably use DXGI_FORMAT_R24G8_TYPELESS. For depthstencil view it
> should be possible to then use DXGI_FORMAT_D24_UNORM_S8_UINT. For
> reading depth use DXGI_FORMAT_R24_UNORM_X8_TYPELESS - for reading
> stencil similarly use DXGI_FORMAT_X24_TYPELESS_G8_UINT. That's at least
> my interpretation. Note you can't read both components at once (there is
> no DXGI_FORMAT_R24_UNORM_G8_UINT format), you need different resource
> views for that (presumably because the components don't have the same
> type, so this is disallowed). Despite that reading depth will return
> first (red) component, stencil second (green).


Gallium should have similar restraints: I recall Dave Airlie saying that
AMD's recent ships have separate storage for depth and stencil, and
DirectX10 rules accommodate that quite well.

This means that Z24_S8 should be disallowed in sampler views: either it
is Z24_X8 or X24_S8. And only one component is returned.

It also means we need to add X24_S8 formats to gallium.


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