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)

1 files changed, 6 insertions, 1 deletions

diff --git a/source/blender/bmesh/intern/bmesh_queries.c b/source/blender/bmesh/intern/bmesh_queries.c
index ce4ce87b31f..b6a56e64dcf 100644
--- a/source/blender/bmesh/intern/bmesh_queries.c
+++ b/source/blender/bmesh/intern/bmesh_queries.c
@@ -196,7 +196,12 @@ BMLoop *BM_loop_other_vert_loop(BMLoop *l, BMVert *v)
 
 BMLoop *BM_vert_find_first_loop(BMVert *v)
 {
-	BMEdge *e = bmesh_disk_faceedge_find_first(v->e, v);
+	BMEdge *e;
+
+	if(!v || !v->e)
+		return NULL;
+
+	e = bmesh_disk_faceedge_find_first(v->e, v);
 	return bmesh_radial_faceloop_find_first(e->l, v);
 }