How to properly export/import meshes and SubDs with texture coordinates?

I’ll continue here with a look at importing meshes and SubDs.

1. Import meshes
This is a similar mesh (as above in 1.) created in blender, plane with 4 polygons, 9 vertices and split UVs.
image image
Exported as OBJ, imported in Rhino, result in a mesh with 12 vertices, UVs are imported as expected.

Now lets test this with FBX, same mesh as above: A wild vertex appeared! Cool, there are now 13 vertices.

This means, importing split UVs will also split the mesh, which in my opinion is not ideal as it changes the mesh itself. Editing the mesh in Rhino behaves as if the mesh is not split (/unwelded), which I don’t understand. Why would I unweld a mesh in the first place if I can’t move the faces seperately afterwards?

2. Import SubDs
Slightly modified mesh from 1. with SubD:
image

Exported as FBX, import into Rhino:
image

It’s there, but not imported as a SubD, simple to fix, but annoying for proper production cases where there there can be hundreds of meshes and SubD in a scene, mixed together.

The FBX importer provides an option to import meshes as SubD, cool…:
image
…but using this will remove transforms and convert all meshes and UVs are not imported correctly:
image

To show, that other software can handle this, here is a screenshot after importing into Modo:
image image

Boundary rules are different in Modo by default, using creased edges by default, rather easy fix, can also be scripted.

Overall, this is a little frustrating for me, as some of the UV tools in Rhino are quite nice (e.g. curve constraints). Currently the exporting meshes and SubDs is more important for me, though there will likely be cases where we will import them into Rhino as well.
Currently these points limit us from properly using Rhino and all its tools. I’m happy to provide more use cases and examples. For this topic I kept it simple to show the actual problem we are facing without any distracting stuff and to easily reproduce it.

I’ve also looked into other ways to get it working, but ran into more issues, see here: How to get UVs for SubD with RhinoCommon or rhino3dm.py? - Rhino Developer / rhino3dm - McNeel Forum

For now this means I will have to jump to other tools for UVs as I can’t get them out of Rhino.