[Mesa-dev] [Bug 110451] triangle strip clipping with GL_FIRST_VERTEX_CONVENTION causes wrong vertex's attribute to be broadcasted for flat interpolation

[bugzilla-daemon at freedesktop.org]

https://bugs.freedesktop.org/show_bug.cgi?id=110451

            Bug ID: 110451
           Summary: triangle strip clipping with
                    GL_FIRST_VERTEX_CONVENTION causes wrong vertex's
                    attribute to be broadcasted for flat interpolation
           Product: Mesa
           Version: git
          Hardware: All
                OS: All
            Status: NEW
          Severity: normal
          Priority: medium
         Component: Drivers/Gallium/llvmpipe
          Assignee: mesa-dev at lists.freedesktop.org
          Reporter: z.figura12 at gmail.com
        QA Contact: mesa-dev at lists.freedesktop.org

This is superficially similar to bug 98846, but not identical.

It affects softpipe and llvmpipe at least. When drawing clipped triangle strips
(and, I believe, only triangle strips), and using GL_FIRST_VERTEX_CONVENTION,
the wrong vertex of every other clipped triangle is broadcasted for flat
interpolation. This seems to be because do_clip_tri() in
src/gallium/auxiliary/draw/draw_pipe_clip.c gets passed the vertices in winding
order, but assumes the first vertex will be the provoking vertex (when in
reality it may be the second).

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'm not familiar with Mesa, but I'd like to at least take a shot at fixing
this. I guess the first step is to add a new test to piglit.

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