[Mesa-dev] [Bug 78914] New: Front/Backfaces do not cover the same pixels when rasterized

[bugzilla-daemon at freedesktop.org]

https://bugs.freedesktop.org/show_bug.cgi?id=78914

          Priority: medium
            Bug ID: 78914
          Assignee: mesa-dev at lists.freedesktop.org
           Summary: Front/Backfaces do not cover the same pixels when
                    rasterized
          Severity: normal
    Classification: Unclassified
                OS: All
          Reporter: florianlink at gmail.com
          Hardware: Other
            Status: NEW
           Version: 10.1
         Component: Mesa core
           Product: Mesa

When trying to run my GLSL raycaster with Mesa/llvmpipe, I recognized artifacts
due to the wrong ray start/end positions. I render the start/end rays as boxes
where the front face is the start position and the back face is the back
position. The problem seems to be that the front/back faces do not cover the
exact same pixels in the framebuffer, even when they share the same edges.
On NVidia/ATI cards with native driver, the back faces cover the same pixels as
the front faces.

This can be reproduced by rendering a triangle with culling turned off and
blending turned on. When rotating the triangle one can see the artifacts on the
borders of the triangle.

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