CG Modeling 3: Advanced - An Introduction to Computer Graphics for Artists - page 274

Fig. 15.2 A subdivision surface on left with its control mesh hidden. On right , the control mesh
has been edited and the surface changed

15.2.4 Conversion to Polys or NURBS

Subdivision surfaces are primitives that obey the topological constraints of NURBS
patches by subdividing the quad-based topology of the polygonal control mesh. To
adjust smoothness, or what might be described as curvature, edges are given smoothing
values on an edge-by-edge basis. The defi nition of the surface allows different edge
smoothness values to be interpolated correctly for perfect continuity across all boundar-
ies. Because subdivision surfaces obey NURBS topological rules, they can be converted
into NURBS patches or polygons. Conversely, NURBS and polygonal geometry can be
converted to subdivision surfaces, though polygon conversions are prone to create poor
patch layouts unless carefully controlled when the object was originally modeled.

15.2.5 Subdivision Surface Limitation

Controlling curvature around edges in subdivision surfaces utilizes an indexed set
of pre-defi ned values. If the curve you want is different from the options offered by
your application, you will be forced to make your model far more complex at the
edges to force the shape you want by moving points manually. The only other alter-
native is to not use the shape you have in mind. This is not a small limitation, and it
may be why NURBS surfaces are still used as the standard geometry type in indus-
trial and automotive design. Subdivision surfaces make certain types of objects very
well, but if an exact replica of a shape that is not easily made with subdivision sur-
faces is needed, polygons or NURBS may be a better option.