CG Modeling 2: NURBS - An Introduction to Computer Graphics for Artists - page 267

14.4.11.5 Cracked

This only happens between two adjacent surfaces. There is more than one reason for
this error:

• The surfaces have a different number of control points along the common edge
• The subdivision level for either patch is different
• Although the surfaces may be partly tangent, they are not tangent at the crack,
causing the surfaces to move away from each other at this point.

14.4.12 NURBS and Grouping

NURBS modeling requires more parts than a polygonal model of the same subject.
This is because NURBS patches are the equivalent of a polyset, but they rarely rep-
resent as much information as a typical polygonal polyset. This differs depending on
the type of target in question.
A character, for instance, can be very effi ciently modeled in NURBS. It would
probably have a separate patch for each limb, the head, ears, and the torso, plus
fi ngers, for a total of 18 parts. A polygonal model of the same thing would probably
be one polyset. If the subject is architectural, a NURBS model could easily have six
patches for every cube-shaped object, but a polygonal model might be broken down
into different polysets only where certain parts need to be animated separately from
the main polyset, like for windows and doors. In such a case, the NURBS scene will
have a much more complicated grouping of parts compared to a polygonal model.

14.4.13 Curve Detail

Even if your fi nal object will be polygonal, it is a good idea to build curved shapes
in NURBS fi rst, and then convert them to polygons. This is because the NURBS
defi nition of the curve will give you a better distribution of vertices around the curve
than you would get if working in polygons.

14.4.14 Poly Conversion

When a NURBS object of any curved surface is converted to polygons, it will always
lose volume, because NURBS surfaces contain an infi nite degree of surface informa-
tion and polygons do not (Fig. 14.31 ). Converting a NURBS object into polygons
can be accomplished with the push of a button. It is helpful, however, to know a little
about how it is done, in order to get the most out of the conversion operation.