Hi Stefan, thanks for testing that. So it looks like Wings3D is using the "s" statements for that?

Officially the s statements are used for setting "smoothing groups" but it's not referring to sub-d smoothing, it's for handling vertex normal generation where only polygons from the same smoothing group will be used for averaging vertex normals. That's for "smooth shading" control, not sub-d smoothing control.

The .obj file format predates the use of sub-d smoothing and so doesn't really have sub-d specific functions in it.

- Michael