A forum post was made on the FBX forum a couple years back which was in reference to importing meshes in FBX files that had been exported out of Maya. I participated in the discussion as I had encountered the problem while coding an FBX import. It was determined that meshes that are exported from Maya as FBX have their normals deformed as they are at time of export, an odd behavior which leads to improper FBX output and bad artifacts on import. The problem being reported turned out to be a bug in Maya's export specifically, but that bug was never reported in the Maya forum. Now I'm reporting the bug. The post in question is here:
https://forums.autodesk.com/t5/fbx-forum/fbx-sdk-normals-incorrect-with-skinned-meshes-with-animatio...
Maya should export the normals of any mesh in the same space as its geometry, whether deformed or not, but if the mesh happens to be deformed at the time of export, its normals will be deformed, while its geometry is not. Strange behavior indeed. It's as though a programmer at the time saved out the normals from a cache for display rather than the original geometry.
It's hard to imagine a reason how this would be useful in any way, and the behavior contradicts the FBX documentation. So it must be a bug. While a lot of users will keep their animations in separate files and always export in bind pose, and a lot will be fine with recomputing normals in other software, some will not, and may not notice the issue.
It seems the issue has likely existed for many years, with the general suggestion that normals should be recomputed by whatever software imports an FBX. That, however, is a potentially destructive operation (lost custom normals), so it is not actually a solution. An obvious workaround may be to put all meshes in bind pose before exporting, but a user may want a deformation that's not animated, and that would be lost.
My hope in reporting the bug is that Maya might make a correction to its FBX export, and if there really is some sort of reason for the behavior, explain it, and make it a configurable option. This should definitely not be default behavior. I have found no other 3D software that does this.
I can deal with the issue because I'm aware of it, but it's an extra step to forget, and it's frustrating when I do.
I can produce a visual example, but there are some examples on that post. It's reproduced by simply having a mesh deformed at time of export and importing it into Blender without recomputing normals. Its normals will be wrong for any deformed vertex.
Thanks.
