Hi Micha, just one more detail to try and explain why you see those shading artifacts in Rhino when you do a "full weld" while inside of Rhino.

As I explained before, Rhino's mesh structure does not allow for only partial sharing between faces, a face can only either have all of its information (3d point, uv, and normal) shared, or none shared with neighboring faces.

If you tell Rhino to force a welding to make all faces share the same vertices, that means that they will also share the same vertex normal for shading as well, so that smooth shading will then try to be calculated across all those faces. In the case of a box you don't want shading to flow across all faces, but that is what you get anyway with welding in Rhino.

The way that Rhino defines its meshes just does not make it possible to have welded points but different smoothing per face.

Some programs handle these issues in different ways - there is the possibility to have "per face" polygon attributes like I described earlier which is the most flexible, but also some programs that have only "per vertex" type data like Rhino use an additional mechanism called "smoothing groups" where they can assign faces to different groups and then will not let smoothing intermingle outside of one of those groups.

When MoI exports a welded OBJ file, it will actually put out both kinds of data - it will have per-face attributes and also emit smoothing groups as well so I'm not quite sure which method your unwrap program actually uses.

- Michael