CG Modeling 2: NURBS - An Introduction to Computer Graphics for Artists - page 259

Fig. 14.21 A polygonal object has one normal per face ( left ), but a NURBS surface has potentially
an infi nite number, depending on its tessellation settings

until you have done all you can with existing elements of your model. Usually, this
means that an artist will start by moving CVs to match a border of the target object.
The next step is to add a centerline, adjust its shape, then another, adjust it, and so on.

14.4.2.1 Surface Effi ciency

If you have more than ten isoparms in U and V, your model is starting to get heavy.
Some models have, and need, hundreds of isoparms in both directions, but this is
rare. More often, if you have ten divisions along both edges of your patch, you are
at your limit. At this point, you would normally use another patch to model addi-
tional detail.

14.4.3 NURBS Normals

NURBS objects have more normals than polygonal objects. This results in smoother
renders. The only limit to the number of normals used for lighting calculations is the
surface subdivision level, a parameter set by the artist. This can be set so low that the
NURBS object has a very small number of used normals. Practically speaking, a
NURBS object will always render more smoothly than a polygonal one (Fig. 14.21 ).

14.4.4 Appropriate NURBS Subjects

The following object types make good subjects for NURBS modeling, either as a
fi nished object, or an intermediate starting point for a polygonal fi nish object:

• Characters; Characters are built out of the same homotopic parts. This allows one

generic NURBS model to be a starting point for any character, and then through
manipulation become that character. Characters are most often defi ned by skin