[Mesa-dev] [PATCH v2 12/23] glsl: Validate vertex emission in geometry shaders.

[Kenneth Graunke]

On Friday, June 20, 2014 08:51:14 AM Iago Toral wrote:
> On Wed, 2014-06-18 at 13:38 -0700, Ian Romanick wrote:
> > On 06/18/2014 02:51 AM, Iago Toral Quiroga wrote:
> (...)
> > > +      /* From the ARB_gpu_shader5 spec:
> > > +       *
> > > +       *   "Multiple vertex streams are supported only if the output 
primitive
> > > +       *    type is declared to be "points".  A program will fail to 
link if it
> > > +       *    contains a geometry shader calling EmitStreamVertex() or
> > > +       *    EndStreamPrimitive() if its output primitive type is not 
"points".
> > > +       *
> > > +       * However, in the same spec:
> > > +       *
> > > +       *   "The function EmitVertex() is equivalent to calling 
EmitStreamVertex()
> > > +       *    with <stream> set to zero."
> > > +       *
> > > +       * And:
> > > +       *
> > > +       *   "The function EndPrimitive() is equivalent to calling
> > > +       *    EndStreamPrimitive() with <stream> set to zero."
> > > +       *
> > > +       * Since we can call EmitVertex() and EndPrimitive() when we 
output
> > > +       * primitives other than points, calling EmitStreamVertex(0) or
> > > +       * EmitEndPrimitive(0) should not produce errors. This it also 
what Nvidia
> > > +       * does. Currently we only set prog->Geom.UsesStreams to TRUE 
when
> > > +       * EmitStreamVertex() or EmitEndPrimitive() are called with a 
non-zero
> > > +       * stream.
> > 
> > Does AMD also behave this way?  If so, I can submit a spec bug to make
> > these explicitly legal.  Otherwise, I think we should not allow it.  We
> > probably ought to check Apple and the Intel Windows driver too...
> 
> I could test this on the proprietary AMD Linux driver:
> 
> A geometry shader with output triangle_strip using EmitStreamVertex(0)
> and EmitEndPrimitive(0) works fine: no link errors, correct output.
> 
> A geometry shader with output triangle_strip using EmitStreamVertex(1)
> does not produce a link error (which is wrong by the spec) and produces
> incorrect output (blank screen).
> 
> A geometry shader with output triangle_strip using EmitStreamVertex(0)
> that also calls EmitStreamPrimitive(1) does not produce a link error
> (which is wrong by the spec) and produces correct output (the end stream
> primitive on stream 1 is really a no-op since there are no vertices
> emitted on that stream).
> 
> So I'd say that AMD is not respecting the spec too much here, but it
> looks like using EmitStreamVertex(0) or EndStreamPrimitive(0) with
> output types other than points works on both Nvidia and AMD proprietary
> drivers in the end.
> 
> So Ian, what do you think we should do?
> 
> Iago

For what it's worth, AMD also supports writing lines and triangles to multiple 
streams, not just points:

http://www.opengl.org/registry/specs/AMD/transform_feedback3_lines_triangles.txt

--Ken
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