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

[Tapani Pälli]

Patches 5..10

Reviewed-by: Tapani Pälli <tapani.palli at intel.com>

Also tested that these fix the new Piglit tests and esconform3 test. I
made a little proposal for patch 10 but not sure if this change would be
worth the trouble.


On 06/21/2014 04:00 AM, Ian Romanick wrote:
> 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+.
>
> 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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