[Mesa-dev] [PATCH 1/6] mesa: Add field gl_renderbuffer.Unwrapped

[Chad Versace]

On 06/16/2011 08:01 AM, Alex Deucher wrote:
> On Wed, Jun 15, 2011 at 9:09 PM, Chad Versace <chad at chad-versace.us> wrote:
>> If a renderbuffer wraps multiple renderbuffers, then Unwrapped points to
>> the them.
>>
>> For example, if hardware requires separate depth and stencil buffers
>> (X8_Z24 and S8), then glRenderbufferStorage(GL_DEPTH24_STENCIL8) may
>> create a fake S8_Z24 renderbuffer for which Unwrapped[BUFFER_DEPTH] and
>> Unwrapped[BUFFER_STENCIL] point to the real X8_Z24 and S8 renderbuffers.
>>
> 
> FWIW, evergreen and newer chips only support separate stencil and
> depth, so we had a similar issue.  We ended up using
> GL_ARB_shader_stencil_export to implement writes to the stencil.
> Start of the thread:
> http://lists.freedesktop.org/archives/mesa-dev/2010-September/003318.html
> 
> Alex

The thread mentions some strange behaviour on r600:
> 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

I've observed mysterious behaviour when writing to intel's stencil buffer through a memory mapped pointer. It's seems that the
hardware fails to recognize that the writes ever occurred. (Writes via rendering work fine).  I still need to investigate the
problem further, but perhaps GL_ARB_shader_stencil_export is what the intel driver needs to work around this problem.

Thanks for pointing this out.

-- 
Chad Versace
chad at chad-versace.us