The normal wont update until you call updatenormals and that can be a slow command on a large mesh. If the retexturing is being done on the fly it might be a bit slow.
I think the suggestion I made works though doesn't it? The two range calculations work out the length of the two funnily named edges of the triangle and atan2 returns the angle based on those edge-lengths.
I think the suggestion I made works though doesn't it? The two range calculations work out the length of the two funnily named edges of the triangle and atan2 returns the angle based on those edge-lengths.