[Mesa-dev] [Bug 98846] triangle clipping causes wrong vertex's attribute to be broadcasted for flat interpolation

bugzilla-daemon at freedesktop.org bugzilla-daemon at freedesktop.org
Mon Apr 15 16:58:57 UTC 2019


https://bugs.freedesktop.org/show_bug.cgi?id=98846

--- Comment #3 from Roland Scheidegger <sroland at vmware.com> ---
(In reply to Zebediah Figura from comment #2)
> This also affects softpipe and llvmpipe. I'm not familiar with mesa, but I
> think there are multiple places that do the wrong thing. For example,
> do_clip_tri() in src/gallium/auxiliary/draw/draw_pipe_clip.c gets passed the
> vertices in winding order, but it doesn't take that into account when
> determining provoking vertex.
> 
> If this should be split off to a separate bug, then I'll do so.
> 
> This causes failures in Wine's conformance test suite, since we use
> GL_FIRST_VERTEX_CONVENTION universally, and test flat shading with
> GL_TRIANGLESTRIP and some vertices slightly clipped outside of the viewport.
> Since our test suite is run in a virtual machine, it uses llvmpipe, and so
> is affected by this bug.

I suspect it's a different issue. For llvmpipe the piglit test you pointed out
works just fine.
(I have some suspicion for llvmpipe/softpipe it would make a difference if you
use interpolation qualifier flat vs. shade model flat. See the FIXME in
draw_pt_decompose.h - the order of decomposing should only be determined by
flatshade_first, and not depend on if flatshade is actually active. But we need
this to compensate for some bug elsewhere, apparently...)

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