[Mesa-dev] [PATCH 00/13] Fix gl_VertexID on i965

Roland Scheidegger sroland at vmware.com
Sat Jun 21 08:36:56 PDT 2014

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.)


> 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.
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