On Sun, Jan 11, 2015 at 7:43 PM, Tobias Klausmann
<tobias.johannes.klausm...@mni.thm.de> wrote:
>
>
> On 11.01.2015 06:05, Ilia Mirkin wrote:
>>
>> Can you elaborate a bit as to why that's the right thing to do?
>>
>> On Wed, Jan 7, 2015 at 1:52 PM, Tobias Klausmann
>> <tobias.johannes.klausm...@mni.thm.de> wrote:
>>>
>>> If we capture transform feedback from n stream in (n-1) buffers we face a
>>> NULL buffer, use the buffer (n-1) to capture the output of stream n.
>>>
>>> This fixes one piglit test with nvc0:
>>> arb_gpu_shader5-xfb-streams-without-invocations
>>>
>>> Signed-off-by: Tobias Klausmann <tobias.johannes.klausm...@mni.thm.de>
>>> ---
>>> src/mesa/state_tracker/st_cb_xformfb.c | 5 +++++
>>> 1 file changed, 5 insertions(+)
>>>
>>> diff --git a/src/mesa/state_tracker/st_cb_xformfb.c
>>> b/src/mesa/state_tracker/st_cb_xformfb.c
>>> index 8f75eda..5a12da4 100644
>>> --- a/src/mesa/state_tracker/st_cb_xformfb.c
>>> +++ b/src/mesa/state_tracker/st_cb_xformfb.c
>>> @@ -123,6 +123,11 @@ st_begin_transform_feedback(struct gl_context *ctx,
>>> GLenum mode,
>>> struct st_buffer_object *bo =
>>> st_buffer_object(sobj->base.Buffers[i]);
>>>
>>> if (bo) {
>>> + if (!bo->buffer)
>>> + /* If we capture transform feedback from n streams into
>>> (n-1)
>>> + * buffers we have to write to buffer (n-1) for stream n.
>>> + */
>>> + bo = st_buffer_object(sobj->base.Buffers[i-1]);
>>> /* Check whether we need to recreate the target. */
>>> if (!sobj->targets[i] ||
>>> sobj->targets[i] == sobj->draw_count ||
>>> --
>>> 2.2.1
>
> Quoted from Ilia Mirkin, to specify what shall be elaborated:
> "Can you explain (on-list) why using buffer n - 1 is the right thing to
> do to capture output of stream n? I would have thought that the output
> for that stream should be discarded or something.
>
> Like with a spec quotation or some other justification. i.e. why is
> the code you wrote correct? Why is it better than, say, bo =
> buffers[0], or some other thing entirely?"
>
> Yeah thats the most concerning point i see as well. The problem is that
> there is a interaction between arb_gpu_shader5 and arb_transform_feedback3,
> but after a bit of reading i think the patch is actually what we should do:
>
> From the arb_transfrom_feedback3 spec:
> "
> (3) How might you use transform feedback with geometry shaders and
> multiple vertex streams?
>
> RESOLVED: As a simple example, let's say you are processing triangles
> and capture both processed triangle vertices and some values that are
> computed per-primitive (e.g., facet normal). The geometry shader
> might declare its outputs like the following:
>
> layout(stream = 0) out vec4 position;
> layout(stream = 0) out vec4 texcoord;
> layout(stream = 1) out vec4 normal;
>
> "position" and "texcoord" would be per-vertex attributes written to
> vertex stream 0; "normal" would be a per-triangle facet normal. The
> geometry shader would emit three vertices to stream zero (the
> processed
> input vertices) and a single vertex to stream one (the per-triangle
> data). The transform feedback API usage for this case would be
> something like:
>
> // Set up buffer objects 21 and 22 to capture data for per-vertex
> and
> // per primitive values.
> glBindBufferBase(GL_TRANSFORM_FEEDBACK_BUFFER, 0, 21);
> glBindBufferBase(GL_TRANSFORM_FEEDBACK_BUFFER, 1, 22);
>
> // Set up XFB to capture position and texcoord to buffer binding
> // point 0 (buffer 21 bound), and normal to binding point 1 (buffer
> // 22 bound).
> char *strings[] = { "position", "texcoord", "gl_NextBuffer",
> "normal" };
> "
>
> -> Especially the comments are enlightening as to where the outputs should
> go. Thats what happens with the
> "arb_gpu_shader5-xfb-streams-without-invocations" test, where two
> stream(outputs) are captured into one buffer.
>
> One might argue now if we have to count .Buffers[i-1] for all buffers after
> this...
>
> Comments and additional feedback is always appreciated!
The thing you're quoting is talking about the case where everything's
supposed to work. I haven't investigated, but I'm guessing that the
test has a layout(stream=1) but no buffer is bound at index 1. Is that
right? In that case, I would imagine that the TF output should
actually just get dropped on the floor. I would assume that this is in
the ARB_tf3 spec, but I don't have time to go digging right now.
-ilia
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