CG Modeling 2: NURBS - An Introduction to Computer Graphics for Artists - page 241

Chapter 14
CG Modeling 2: NURBS

14.1 Introduction

NURBS geometry is used to create mathematically accurate curved surfaces, such
as those in automobiles and airplanes. In fi lm, the advantage of NURBS surfaces is
that they render well and are more easily controlled for animation than dense polyg-
onal meshes. In video games, NURBS are rarely used, but when they are, it is as
intermediate objects made during the polygonal modeling process or as high resolu-
tion geometry rendered for cinematic sequences.
NURBS modeling techniques are meaningfully different from polygonal tech-
niques. The difference between the two concepts can make it diffi cult to transition
from one to the other, but when both are understood, they can work well together.
On a simplifi ed level, polygonal modeling is additive in the sense that a base object
has points added to it, then moved, and more points added to it, and so on. With
curve based modeling, it is more reductive .
A polygonal model can be made out of one piece, with many insets and extru-
sions. This is not true of a NURBS object. If you are accustomed to modeling in
polygons, you will naturally want to take a NURBS base object and modify it by
adding and moving control vertices until you have what you want. This can work
with simple shapes, but for complex objects you will fi nd that you very quickly have
a model with so many control points that it is unwieldy to work with. This is the
central reason polygonal modelers fi nd NURBS objects diffi cult to use. By treating
a NURBS patch as if it was a polyset, it is made too complex. Instead, a modeler
who is using NURBS, especially if he is working with curves, must instead think of
an object as a collection of connected surfaces.
To create a good likeness of an object in NURBS, the artist must learn to visualize
the shape from which the fi nal object is cut, and then carve it out by successive
reduction of the initial objects. When designing an object from scratch, this work-
fl ow is very effi cient, but when trying to reverse engineer something that already
exists, it can take some effort to come up with the original shapes from which the
fi nal object was carved (Fig. 14.1 ).