CG Modeling 2: NURBS - An Introduction to Computer Graphics for Artists - page 257

Fig. 14.18 The soft vase on the left and the brittle vase on the right could be made of NURBS or
polygonal geometry. When treated as a primitive, NURBS makes soft transitions in comparison to
polygons, which tend to behave as if brittle

• Curve direction

• Surface direction/orientation

• Subdivision level in U

• Subdivision level in V

14.4.1.1 Flexibility

Flexibility , in the context of NURBS primitives, is one of the justifi cations for using
NURBS objects. A NURBS surface is not a simple collection of individual vertex
coordinates that can exist independently of each other, as in a polyset. Because
every component of a NURBS surface contains surface information that is shared
by or related to the surface information of neighboring components, changing the
position or other values of any part of a surface will automatically affect others
(Fig. 14.18 ). This does not happen with a polyset without attaching special controls.
As a result, a polyset is less fl exible than a NURBS surface.

14.4.1.2 Simplifi ed Curve Display

A very smooth NURBS object can be represented with only a few curves (Fig. 14.19 ).
This is one of the advantages of working with NURBS. To build a polygonal model
with the same level of curve detail would require thousands more edge loops, the
poly equivalent of curves, than needed for a NURBS surface of equivalent detail.
This simplifi cation of your model is not an insignifi cant advantage. It allows you to
work more quickly, and you will be less prone to mistakes. You will not, for instance,
have to sort through a confusing mess of thousands of edges to fi nd the one you need
to work on. Instead, because of the extremely small number of isoparms, you will
more easily fi nd the elements you need to work with, and all of your transform edits
will be accomplished more quickly.