Hi Pilou & adamio - I think there is some further misunderstanding with the paper referenced.
The technical report https://www.cl.cam.ac.uk/techreports/UCAM-CL-TR-773.pdf is titled "NURBS-compatible subdivision surfaces". However, it's not about exporting polygon models to NURBS, it's basically the reverse of that.
It's about a different type of subdivision surface (not the typical "Catmull-Clark" method) that uses the NURBS concepts of degree and non-uniform knot vectors as part of the polygon structure and subdivision rules, making it a superset of NURBS. It basically means the subdivision model can load an untrimmed NURBS surface into it exactly. But once you introduce an extraordinary vertex you'll be outside the subset of NURBS and the model isn't exactly convertible back to NURBS anymore, it would require some kind of refitting process briefly suggested in just one paragraph.
- Michael
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