[Mesa-dev] Looking for advice on how transform feedback should work with varying structs

[Paul Berry]

On 13 January 2013 15:25, Kenneth Graunke <kenneth at whitecape.org> wrote:

> On 01/13/2013 10:15 AM, Paul Berry wrote:
>
>> I'm working on a patch series to implement varying structs in Mesa, and
>> I'm trying to figure out how to adjust transform feedback accordingly.
>> Varying structs are a requirement of GLES 3.0 and GL 3.2.  The specs
>> aren't very helpful, and I'm considering filing a bug with Khronos, but
>> I wanted to check here first and see if anyone sees anything I missed.
>>
>> Here are the questions I'm trying to answer, and what I've discovered by
>> reading the specs and experimenting with an existing implementation (the
>> nVidia proprietary driver for Linux):
>>
>>
>> 1. Should it be possible to capture a full varying struct using
>> transform feedback, by simply passing the name of that variable to
>> glTransformFeedbackVaryings()?  If so, how should the structure be
>> packed as it is captured in the transform feedback buffer, and what data
>> should be returned from glGetTransformFeedbackVarying(**)?
>>
>> In other words, if the vertex shader contains:
>>
>> struct S {
>>    float f;
>>    vec4 v;
>> };
>> out S var;
>>
>> and the client code passes the string "var" to
>> glTransformFeedbackVaryings(), what should happen?
>>
>> The documentation of glTransformFeedbackVaryings() (section 2.11.8 of
>> GLES 3.0, section 2.14.6 of GL 3.2) simply says "varyings is an array of
>> count zero-terminated strings specifying the names of outputs to use for
>> transform feedback."  Which seems to indicate that this should work.
>> But there is no mention of how the data should be packed.
>>
>
> Since a packing hasn't been defined, it doesn't seem like any applications
> could safely use the feature.
>
>
>  Furthermore,
>> the documentation of glGetTransformFeedbackVarying(**) says that "The
>> type
>> returned can be any of the scalar, vector, or matrix attribute types
>> returned by GetActiveAttrib."  There is no mention of how structs should
>> be dealt with, and the documentation of GetActiveAttrib makes no
>> provision for structs either.
>>
>
> Both of these considerations suggest that transform feedback on whole
> structs should not be allowed, at least without further specification by
> Khronos.
>
>
>  The nVidia proprietary driver for Linux does not permit a full varying
>> struct to be captured using transform feedback.  If you try to do so,
>> you get a link error claiming "error: Varying (named var) specified but
>> not present in the program object."
>>
>
> This is further evidence that no application will use it.
>
>
>  2. Should it be possible to capture an element of a varying struct using
>> transform feedback, by using "." in a string passed to
>> glTransformFeedbackVaryings()?
>>
>> In other words, if the vertex shader contains:
>>
>> struct S {
>>    float f;
>>    vec4 v;
>> };
>> out S var;
>>
>> and the client code passes the strings "var.f" and "var.v" to
>> glTransformFeedbackVaryings(), what should happen?
>>
>
> This seems much more reasonable - at least there's a clear expectation of
> what data the application should get back.
>
>
>  I can't find text in any spec that specifically allows this for
>> glTransformFeedbackVaryings().  However, there are several reasons to
>> suspect that it might work:
>>
>> - This sort of thing is allowed for glGetUniformLocation(), and it seems
>> reasonable to expect that glGetUniformLocation() and
>> glTransformFeedbackVaryings() should behave similarly.
>> - "." is allowed in the strings passed to glTransformFeedbackVaryings()
>> for selecting elements of interface blocks.  Permitting it for selecting
>> the elements of structs is not a very big leap.
>> - Both Mesa and the nVidia proprietary driver already permit individual
>> array elements to be selected for transform feedback by passing a string
>> like "foo[3]" to glTransformFeedbackVaryings().  (However, I can't find
>> any spec text to justify this either, other than the similarity to
>> glGetUniformLocation()).  Again, it's not a very big leap from here to
>> permitting "." for selecting struct elements.
>>
>> The nVidia proprietary driver for Linux *does* not permit an element of
>>
>
> "does", you mean?


Argh, yes.  Thanks for guessing what I meant :)


>
>
>  a varying struct to be captured using transform feedback using ".", but
>> only if the varying occurs in an interface block.  So the example above
>> fails, but if the vertex shader instead contains:
>>
>> struct S {
>>    float f;
>>    vec4 v;
>> };
>> out Foo {
>>    S var;
>> };
>>
>> Then the client code may capture the two elements of the varying struct
>> by passing the strings "Foo.var.f" and "Foo.var.v" to
>> glTransformFeedbackVaryings().
>>
>> Unfortunately, it's hard to use the behaviour of the nVidia driver as a
>> guide for what to do in GLES 3.0, since GLES 3.0 does not support
>> interface blocks for shader inputs or outputs.
>>
>
> I'd probably take this up with Khronos but propose disallowing
> whole-struct transform feedback, but allowing individual members.
>

That seems like a reasonable plan.  Thanks.
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