Blender Git Loki

Fix for #31581, UVs shrink on edge slide.

Issue is that all loops of a face adjacent to the sliding verts were
getting project-corrected. Introduced a test to only project the
affected loops.

The projection code introduces a small offset to the boundaries so that
any boundary tests can work as expected, but this leads to shrinking of
the barycentric coordinates of the projection, causing a shrink of the
uvs in turn. This even affects the uvs that -should- be affected though
the unfixed behavior works strangely in a correctish way (my guess is
because the projection uses the same face as the opposite sliding loop).

I fixed the behaviour by taking the mean value of the uvs. This won't
support seams but current code doesn't either. Also, all CustomData to
exhibit this unfixed behaviour. I only fixed the uv case, other data
(Vcolors, etc) will have discontinuities when edge sliding. I expect
that the CorrectUV code I am working on may address some of these
issues.

Also, added NULL checks for utility function (was intended for this bug
but wasn't needed after all)