[Mesa-dev] [Bug 78914] [llvmpipe] Front/Backfaces do not get the same depth values when interpolated

bugzilla-daemon at freedesktop.org bugzilla-daemon at freedesktop.org
Mon Nov 24 12:30:08 PST 2014


José Fonseca <jfonseca at vmware.com> changed:

           What    |Removed                     |Added
            Summary|[llvmpipe] Front/Backfaces  |[llvmpipe] Front/Backfaces
                   |do not cover the same       |do not get the same depth
                   |pixels when rasterized      |values when interpolated

--- Comment #20 from José Fonseca <jfonseca at vmware.com> ---
> depth test as such is as accurate as it could be. Doing interpolation with as much precision as possible is not all that easy due to properties of floating point arithmetic. In particular for the math the order of vertices matter. Reordering would be possible, though still does not guarantee the same results for fragments along a shared edge (unless the tri shares all vertices, that is it's really the same tri with reordered edges).

I agree.  It might be possible to do better (barycentric interpolation might
help), but it is complicated to ensure we get exactly the same depth values in
all circumstances, particularly because llvmpipe wants to strike a good balance
of performance/accuracy.

OpenGL application developers should make use of glPolygonOffset to prevent
depth-fighting in a portable way.

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