SubD Limitations and Disadvantages

Rhino
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SubD modeling has some limitations and disadvantages compared to NURBS modeling which are rarely mentioned. These limitation and disadvantages can be significant, restricting both what geometry can be created and how that geometry can be used, depending on the desired shapes and objectives. They may outweight the advantages of SubD modeling in many situations.

  1. SubD surfaces have zero curvature along naked edges and creases. The curvature in one direction along naked edges and creases is inherently zero in SubD. That is how the math works and it can fundamentally limit the possible shapes. Closed surfaces without openings are not affected. In contrast the curvature of NURBS surfaces is not restricted along naked edges and creases.

  2. SubD surfaces cannot be matched (exactly or within an arbitrary tolerance) to other surfaces or curves. This may change in Rhino V9. NURBS surfaces can be matched within any desired tolerance to other surfaces or curves.

  3. SubD surfaces cannot be trimmed or split while maintaining shape. NURBS surfaces can be trimmed and split arbitrarily without affecting the shape of the resulting surfaces.

  4. Internal continuity of SubD surfaces is always curvature (G2) continuity except at creases. Internal continuity of NURBS surfaces can be anything with use of appropriate degree surfaces.

  5. Converting a SubD surface to NURBS usually results in a more complex NURBS surface/polysurface than is needed to model the same geometry using NURBS. A SubD converted to NURBS surface may have a factor of two, three or more control points than is needed for an equivalent surface created directly using NURBS.