[Mesa-dev] [PATCH] vbo: Set FLUSH_UPDATE_CURRENT when setting vertex attibutes

Chia-I Wu olvaffe at gmail.com
Tue Sep 14 08:18:46 PDT 2010

2010/9/14 Kristian Høgsberg <krh at bitplanet.net>:
> 2010/9/14 Chia-I Wu <olvaffe at gmail.com>:
>> 2010/9/14 Kristian Høgsberg <krh at bitplanet.net>:
>>> 2010/9/13 keith whitwell <keith.whitwell at gmail.com>:
>>>> Hey Kristian,
>>>> The first question is whether this is necessary - from vague memory I
>>>> have an idea that current attributes need not be updated by vertex
>>>> buffer rendering - ie. it's optional/implementation-dependent.
>>>> I assume you're concerned with the case where you have something like
>>>>   // ctx->Current.Color is xyz
>>>>   glDrawArrays();
>>>>   // has ctx->Current.Color been updated??
>>>> But assuming I'm wrong about that & we really do want to make
>>>> DrawArrays set the current values, the patch looks good...
>>> No, what I'm seeing is that the code in question sets three generic
>>> vertex attributes and then calls glDrawArrays().  The value of the
>>> last attribute is not propagates into the shader.
>>> The problem is that the vertex array code keeps the values in
>>> exec->vtx.vertex, but the implementation of glDrawArrays looks in
>>> ctx->Current (that's what I assume, I didn't track that down).  When
>>> the code hits a case where the size of an attribute is smaller that
>>> what we're trying to set, it recomputes the layout of the
>>> exec->vtx.vertex values and as a side effect copies the
>>> exec->vtx.vertex values to ctx->Current.  Since we start out with
>>> attrsz == 0 for all attributes, each new attribute will trigger this
>>> recomputation and thus effectively flushes all previous values to
>>> ctx->Current.  Which is why all but the last attribute make it to the
>>> shader.
>>> Note that the ATTR macro is defined differently, depending on
>>> FEATURE_beginend - the !FEATURE_beginend case sets the
>>> FLUSH_UPDATE_CURRENT flag too.  I don't know why we wouldn't also set
>>> it in the FEATURE_beginend case, not using begin/end in that case is
>>> still an option.
>> The way glColor4f is dispatched depends on whether it is GL or ES:
>>  GL (with FEATURE_beginend): glColor4f -> neutral_Color4f -> vbo_Color4f
>>  ES (w/o  FEATURE_beginend): glColor4f -> _es_Color4f -> vbo_Color4f
>> In the former case, FLUSH_UPDATE_CURRENT should have been set by
>> vbo_exec_BeginVertices which is called by neutral_Color4f.  In the latter case,
>> the flag must be set in vbo_Color4f.  Could it be a bug some where in vtxfmt?
>> One issue I noticed just last week is that the current scheme does not take
>> into account "ES with FEATURE_beginend".  This happens with --enable-gles2
>> build.  Since FEATURE_beginend is enabled in such build, vbo_Color4f does not
>> set FLUSH_UPDATE_CURRENT.  Yet, an ES context does not use neutral_Color4f.
> And that's exactly the problem I have.  Setting FLUSH_UPDATE_CURRENT
> in the ATTR macro makes sure that the FLUSH_CURRENT in
> vbo_exec_DrawArrays (and other array draw funcs) ends up calling
> vbo_exec_copy_to_current(), which then pushes the values into
> ctx->Current before the draw call.  I don't see a problem with this
> approach; in a begin/end, this flag is already set, so there's no
> overhead, outside begin/end (ES2) it's required for correct behaviour.
If this patch is to be applied, it makes sense to also stop setting
FLUSH_UPDATE_CURRENT in vbo_exec_BeginVertices, and remove the
!FEATURE_beginend case.

But then, does it make sense to remove exec vtxfmt in main/vtxfmt.c and the
related code (a big chunk!), and define ATTR to

  #define ATTR(...) do {
     struct vbo_exec_context *exec = &vbo_context(ctx)->exec;

     if (!exec->ctx->Driver.NeedFlush)

     /* ... */

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