Geometry Shaders For Mesa Not Yet Ready
While there's been early code available for several months, Mesa support for OpenGL Geometry Shaders still isn't ready for merging into mainline Mesa.
While Mesa 9.1 has picked up many features over the past six months, OpenGL Geometry Shaders won't be found in the release due out by late February.
Geometry Shaders are GLSL programs that deal with primitives and can yield more primitives. Geometry Shaders have been part of the OpenGL Core specification since version 3.2 with the ARB_geometry_shader4 extension.
There's been early OpenGL GS Mesa code going back to last summer thanks to a Mesa branch that was worked on by Bryan Cain. While it ended up progressing and back in September it looked like Geometry Shaders would merge soon for Mesa, that didn't end up happening. Geometry Shaders are one of the last holdouts for Mesa being able to advertise itself as OpenGL 3.2 friendly.
Earlier this month when writing about R600 Gallium3D Getting Close On OpenGL 3.3 Support, David Airlie wrote about the state of geometry shaders in the Phoronix Forums. "There is a tree on github with unproven geometry shader code for r600g, intel would need to add 965 support, it just needs a lot of piglit tests and a lot of review...but writing the piglit tests for geometry shaders is probably the biggest problem." (Also see The State Of Open-Source Radeon Driver Features.)
Brian Paul, Mesa founder and current VMware employee, asked today on the Mesa developers' mailing list about the state of Geometry Shaders for this open-source graphics stack. Intel's Paul Berry responded with the current open-source state of affairs for OpenGL Geometry Shaders:
It looks like Bryan made a lot of headway before he set the branch aside in October, so I'm planning to use that as a starting point. My impression from talking to Bryan is that he doesn't have too much time/interest to put into geometry shaders these days, so for the moment I'm planning to take over responsibility for the branch myself.So getting the GS support cleaned-up and merged isn't at the top of Intel's TODO list, but at least they are working on it. This won't happen for Mesa 9.1 obviously, but hopefully this OpenGL 3.2 feature will be baked and ready for tasting by the next Mesa release six months down the road. The follow-up release will be Mesa 9.2 or more than likely it will be dubbed Mesa 10.0 assuming a major new OpenGL revision is reached (likely OpenGL 3.2/3.3 compliance in one go).
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