[Mesa-dev] [PATCH 2/2] draw: inject frontface info into wireframe outputs

[Jose Fonseca]

----- Original Message -----
> > > +   if (draw_will_inject_frontface(lp_context->draw) &&
> > I think it's annoying you have to do these calls to determine if there's
> > a valid frontface here for each line instead of just per draw call but
> > it doesn't seem easy to avoid it.
> 
> Yea, there's no trivial way of avoiding it.
> 
> > Also, no love for llvmpipe point face? I realize d3d10 doesn't require
> > it but OpenGL (and IIRC d3d9) do.
> 
> I didn't know of any tests for the points and we care only about lines right
> now. It's just four extra lines of code or so, so I can trivially add it but
> I don't have anything to test it with.
> 
> > Looks like quite a heavy interface (and sort of silly to allocate 128
> > bits in the vertex data (so actually twice that for one line) for 1 bit
> > of information but given all our data passed on to the line/point funcs
> > are float4 I don't really see any other easy way neither), but seems all
> > necessary unfortunately. I guess another option would be to pass the
> > face info always along the vertex data no matter what (which would mean
> > all those additional calls for setting up outputs, determining if
> > there's a valid frontface etc. could go along with the storage needed)
> > for all primitives to the point/line/tri funcs but I'm not really
> > thrilled about that idea neither (passing it for tris so it doesn't have
> > to be recalculated may or may not be a good idea neither).
> 
> Yes, plus then we'd need a brand new pipeline stage that is always run and
> that is largely useless for vast majority of rendering. It's sort of a lose
> lose scenario. The only thing that is clear is that we have to pass the data
> along the shader outputs, everything else is a messy glue to make it
> possible.

The only other thing I can think of would be to modify draw module unfilled stage so that it further decomposed wireframe's lines into face-preserving triangles, when it notices that the fragment shader reads face register. Draw module already has stages to decompose line into triangles, so could be a matter of patching things up. Performance wouldn't be as good, but it would avoid adding complexity to draw->driver interface.

Just a thought. The patch looks good too.

Jose