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

[bugzilla-daemon at freedesktop.org]

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

            Bug ID: 98846
           Summary: triangle clipping causes wrong vertex's attribute to
                    be broadcasted for flat interpolation
           Product: Mesa
           Version: git
          Hardware: Other
                OS: All
            Status: NEW
          Severity: normal
          Priority: medium
         Component: Drivers/Gallium/swr
          Assignee: mesa-dev at lists.freedesktop.org
          Reporter: imirkin at alum.mit.edu
        QA Contact: mesa-dev at lists.freedesktop.org

See example at
generated_tests/spec/glsl-1.10/execution/interpolation/interpolation-none-gl_FrontColor-flat-fixed.shader_test

This sets a position s.t. the clipping at +/-1 takes effect. And it works just
fine - the clipped result is correct. However the color output is green instead
of blue - the first vertex's color is broadcast to the new triangles rather
than the last vertex's color.

I traced through the code a bit, and it seems to *want* to do the right thing,
but for some reason doesn't. Smooth shading works fine in this clipping
scenario.

With user-provided clip planes (or distances), i.e. in where the clipping stage
does nothing, the color is picked correctly.

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