Nurbs Surfaces

Nurbs surfaces are geometric primitives that are the equivalent of an infinitely flexible thin square sheet of rubber. One of the trickiest issues to deal with for these objects is that they must have four sides, as in a square, but that they can be manipulated into the shape of a sphere, perfect circle, or almost anything else. An easy way to visualize this is to literally draw four-sided boundary edges (in two colors for horizontal or vertical sides) directly onto various objects, with a simple grid in between. Any area you can describe in this manner can be built as a nurbs surface (Fig. 13.1).