[Mesa-dev] per-vertex array max_index

Christoph Bumiller e0425955 at student.tuwien.ac.at
Fri Apr 15 11:20:58 PDT 2011

On 15.04.2011 18:04, Brian Paul wrote:
> Hi Marek,
> Back on Jan 10 you removed the per-vertex array max_index field
> (commit cdca3c58aa2d9549f5188910e2a77b438516714f).  I believe this was
> a mistake.
> I noticed today that the piglit draw-instanced-divisor test is
> failing.  A bisection points to Jose's commit
> 17bbc1f0425b3768e26473eccea5f2570dcb26d3.  But that's a red herring.
> If I disable the SSE code paths, the regression goes back to the Jan
> 10 change.
> With the GL_ARB_instanced_arrays extension vertex array indexing
> changes quite a bit.  Specifically, some arrays (those with divisors
> != 0) are now indexed by instance ID, not the primitive's vertex
> index.  The draw_info::max_index field doesn't let us take this into
> account.  I believe that we really need a per-array max_index value.
> As an example, suppose we're drawing a star field with 1 million
> instanced stars, each star drawn as a 4-vertex quad.  We might use a
> vertex array indexed by the instance ID to color the stars.
> The draw call would look like:
> glDrawArraysInstanced(GL_QUADS, 0, 4, 1000*1000);
> In this case we'd have two vertex arrays.  The first array is the quad
> vertex positions with four elements.  The second array is the star
> colors with 1 million elements.  As it is now, we're setting
> draw_info::max_index = 4 and we errantly clamp the index into the
> second array to 4 instead of 1 million.
> As a temporary work around we can disable clamping of array indexes
> for instance arrays.  But I think we need to revert the Jan 10 commit
> and then rework some of Jose's related changes.
> -Brian
You know which vertex elements are per-instance, you know the divisor,
and you know the max instance index - that should be all the information
you need.

You just have to clamp it to startInstance + instanceCount for them instead.

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