[Mesa-dev] [PATCH] vbo: count min/max_index before vbo->draw_prims

[Eric Anholt]

On Sun, 25 Dec 2011 12:26:25 +0800, Liu Aleaxander <aleaxander at gmail.com> wrote:
> On Sun, Dec 25, 2011 at 8:03 AM, Eric Anholt <eric at anholt.net> wrote:
> > On Thu, 22 Dec 2011 18:55:50 +0800, Yuanhan Liu <yuanhan.liu at linux.intel.com> wrote:
> >> For the case that index data is stored in element array buffer object,
> >> and user called glMultiDrawElements, count the min/max_index before
> >> calling vbo->draw_prims. vbo_get_minmax_index() isn't friendly to this
> >> case. So do it while building the prim info.
> >>
> >> Signed-off-by: Yuanhan Liu <yuanhan.liu at linux.intel.com>
> >
> > I'm really failing to understand what this commit is for.  Does it fix
> > some testcase?
> 
> Sorry, my bad. I should note something more about this issue. And
> yes, it does fixes something. Say for following case:
> 
> GLubyte indices_data[] = {0, 1, 2, 3, 4, 5, 6,    1, 7, 8, 9, 10, 11};
> GLsizei count[] = {7, 6}
> GLvoid * indices_off[2] = {0, 7};
> 
> /* here copy the indices data into Element Array Buffer Object */
> .....glBind..glBufferData....
> 
> /* Here call glMulitDrawElements */
> glMultiDrawElements(GL_LINE_STRIP, count,
>                                 GL_UNSIGNED_BYTE,
>                                 indices_off, 2);
> 
> The current code in brw_draw.c would just count the min_index
> and max_index of the first prim, thus got a min_index of 0, and
> max_index of 6. This is not right, thus the second prim(and any
>  later, if you have more than 2 prim to render) rendered incorrectly.
> 
> I found it's a little ugly to add those code similar in this patch at
> brw_draw_prims(), then I figured out a way to add it at the caller,
> vbo_validated_multidrawelements(). I guess it's reasonable, since
> the prim info is built at there.

What about when the next caller that does multiple primitives comes
along.  Would you add workaround code to it too?  Please fix the actual
broken code in brw_draw.c instead.

This would be a good time to check for the same breakage in other
callers of vbo_get_minmax_index(), and see if we want a function that
does this multi-primitive vbo_get_minmax_index() logic.
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