Am 21.06.2014 03:00, schrieb Ian Romanick: > This patch series fixes bugs in the i965 w.r.t. several uses of > gl_VertexID. OpenGL (desktop and ES) have the following expectations of > gl_VertexID: > > 1. When used with BaseVertex drawing commands, gl_VertexID will include > the value of basevertex. This differens from "the other API," but the > change in OpenGL was based on feedback from application developers. > This only affects OpenGL 3.2+.
I don't question this, but note some binary drivers may not agree. There are unforunately lots of man pages around of glDrawElementsInstancedBaseVertexBaseInstance too which state: "The basevertex has no effect on the shader-visible value of gl_VertexID." There's a bug filed about this (https://www.khronos.org/bugzilla/show_bug.cgi?id=932) which is still open though maybe it has all been resolved... (I'll just mention that llvmpipe right now will do d3d10-style vertexID though I guess indeed at some point we need to make it either switchable or implement GL_ARB_shader_draw_parameters so it can be worked around easily in the state tracker.) Roland > > 2. When used with DrawArrays drawing commands, gl_VertexID will count > from the 'start' value instead of zero. This affects OpenGL 3.0+ and > OpenGL ES 3.0+. > > The i965 driver botched both of these for slightly different reasons. > For #1, our hardware was designed with the "other API's" semantic in > mind, so gl_VertexID doesn't include the basevertex value. > > For #2, we implement DrawArrays with a non-zero start index by > reprogramming the array base offset. As a result, gl_VertexID always > counts from zero. > > I suspect other hardware may suffer from one or both of these issues. I > have sent tests to the piglit list to reproduce them. > > To fix both of these issues, the shader needs to know the basevertex > value. A later GL extension, GL_ARB_shader_draw_parameters, adds > gl_BaseVertex and gl_BaseInstance as built-in variables. I believe some > hardware implements these as system values much like gl_VertexID. > > I started this series with the assumption that we could have a > SYSTEM_VALUE_BASE_VERTEX that might come from a uniform. In the end, I > couldn't make that work. I left patch 7 in the series, but we may want > to remove it. > > I added a STATE_BASE_VERTEX uniform instead. > > There is now a lowering pass that converts gl_VertexID to > gl_VertexIDMESA + gl_BaseVertex (backed by a uniform). Some of these > names should probably be changed. > > I have also contemplated adding an extension that exposes the other > semantic for gl_VertexID for applications that actually want that. This > is primarily things that are porting content (or "bridging" content) > from the other API. That, however, can happen later. > > _______________________________________________ > mesa-dev mailing list > mesa-dev@lists.freedesktop.org > https://urldefense.proofpoint.com/v1/url?u=http://lists.freedesktop.org/mailman/listinfo/mesa-dev&k=oIvRg1%2BdGAgOoM1BIlLLqw%3D%3D%0A&r=F4msKE2WxRzA%2BwN%2B25muztFm5TSPwE8HKJfWfR2NgfY%3D%0A&m=E56BlL24I31lr1fFtEsKBJjAprQ%2BQdhUczD1EsQ3dIY%3D%0A&s=2634834afd3400c841ab62f11a9b5350d604592529cfea97f2297c9398c0fd93 > _______________________________________________ mesa-dev mailing list mesa-dev@lists.freedesktop.org http://lists.freedesktop.org/mailman/listinfo/mesa-dev