2010/9/14 Kristian Høgsberg <k...@bitplanet.net>:
> 2010/9/14 Chia-I Wu <olva...@gmail.com>:
>> 2010/9/14 Kristian Høgsberg <k...@bitplanet.net>:
>>> 2010/9/13 keith whitwell <keith.whitw...@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)
vbo_exec_BeginVertices(ctx);
/* ... */
}
>From what I can tell, and I am be wrong, exec vtxfmt and PRE_LOOPBACK are to
avoid any unnecessary operation in vtxfmt functions, even as cheap as setting
FLUSH_UPDATE_CURRENT repeatedly. If that cannot be avoided, there seems to be
no need to have exec vtxfmt.
--
o...@lunarg.com
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