[Mesa-dev] per-vertex array max_index

[Brian Paul]

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