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

[bugzilla-daemon at freedesktop.org]

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

Roland Scheidegger <sroland at vmware.com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
            Summary|Front/Backfaces do not      |[llvmpipe] Front/Backfaces
                   |cover the same pixels when  |do not cover the same
                   |rasterized                  |pixels when rasterized
          Component|Mesa core                   |Other

--- Comment #1 from Roland Scheidegger <sroland at vmware.com> ---
So, in order to get front and backface tris, you draw essentially the same tri
twice, but once you draw index 0,1,2 and once you draw 0,2,1? I could see this
getting different results for interpolated attributes (in fact I know it will
happen...). I am not actually sure it's guaranteed to get the same results,
this is very tricky to get right (the reason is the interpolation /
interpolation setup is not quite symmetric wrt all triangle corners, the float
math can give different results). Though this should only affect interpolated
attribute values, not rasterization itself (which happens with fixed point
math). If it actually rasterizes different pixels this is a bug. Hence if you
could provide some minimal test case that would be great.
This only affects llvmpipe right?

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