On 4 October 2011 13:15, Ian Romanick <i...@freedesktop.org> wrote: > On 10/03/2011 02:17 PM, Paul Berry wrote: > >> Before this patch, clip planes didn't work properly in Mesa when using >> vertex shaders, because Mesa assigned both gl_ClipVertex and >> gl_Position to the same gl_vert_result (VERT_RESULT_HPOS). As a >> result, backends couldn't distinguish between the two variables, so >> any shader that wrote different values to them would fail to work >> properly. >> >> This patch paves the way for proper support of gl_ClipVertex by >> creating a new enumerated value in gl_vert_result for it >> (VERT_RESULT_CLIP_VERTEX). After this patch, a back-end may add >> > > What happens in drivers that aren't expecting / don't know about > VERT_RESULT_CLIP_VERTEX? In other words, does this break (more) i915 and > all Gallium drivers? I understand that gl_ClipVertex already doesn't work > anywhere, but it would be a shame to replace incorrect rendering with a > crash. > > I can test out (and patch up) i915 today, but I'd like some feedback from > people that rely on TGSI. >
Dang, I should have thought of this. BTW, an easy way to assess the effect of this patch on a driver would be to run Piglit with "-t clip" after applying this patch. The patch should only regress "vs-clip-vertex-const-reject" (which only used to pass by dumb luck), and nothing should crash or abort. Also, if "clip-plane-transformation fixed" passes, we can have pretty high confidence that old fashioned fixed function clipping still works. I just did this test on Gallium's LLVM pipe and on vanilla swrast, and everything looked ok--no regressions other than vs-clip-vertex-const-reject. BTW, both gallium LLVM pipe and vanilla swrast have a crash (in "vs-clip-vertex-enables" and "clip-plane-transformation arb", respectively), but in both cases the crash seems to be unrelated to this patch. Unfortunately I don't have the hardware to test anything else so I'd appreciate feedback from others. If worse comes to worst and we do wind up regressing a driver, I suppose a workaround would be to add a flag to gl_shader_compiler_options so that the driver can tell the GLSL compiler either "I understand VERT_RESULT_CLIP_VERTEX" or "I don't, just do the old buggy behavior". But it seems ugly so I would rather avoid that sort of thing if we can.
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