[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

--- Comment #6 from Roland Scheidegger <sroland at vmware.com> ---
Actually I think I know what's happening here. Our rasterizer can only handle
counterclockwise triangles (so the edge function sign indicates if a fragment
is inside or outside a plane and ultimately the tri). To make it work we rotate
the triangle if the triangle is clockwise to make it counterclockwise (if
backface culling is disabled). So far so good. But for correct fill rules (to
get cases right where fragment centers are on edges) we also fix up the plane
coefficients - and we do this based on the sign of the steepness of the plane
function, but because we exchanged two vertices in case when we have front face
culling instead of back face culling (so actual winding of the tri did not
really change, we just treat it as if it would have changed) we get it wrong.
Though this requires some further analysis, it is quite possible I made some
mistake here, this is tricky to get right!

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