CG Modeling 2: NURBS - An Introduction to Computer Graphics for Artists - page 260

stretched over hundreds of muscles in discrete limb segments. This type of topology
is easily represented and manipulated with NURBS, and can quickly be made
into a desired shape for this type of object.
• Vehicles; Many vehicle types are designed with NURBS. This means that a
NURBS modeling solution will more likely match the overall shape of a target
of this type more quickly than polygonal techniques. Simple models of these
subjects may be generated very quickly with NURBS primitives, and then be
converted to polygons for more detailed modeling.
• Industrial design; most modern examples of industrial design, like vacuum
cleaners and cappuccino machines, are built from curve-based models.
• Objects that will be seen close-up in pre-rendered graphics. Because NURBS
have a very large number of normals compared to polysets, renders made from
them have much smoother lighting than polygons.
• Organic objects; for reasons similar to those for building previs characters with
NURBS primitives, other organic objects are likewise quite easy to build and
yield excellent representations of the target objects.

14.4.5 Making NURBS Surfaces from Curves

To build a surface from curves, you need only select the appropriate curves and
then invoke a surface generation tool. Unlike polygons, which normally require
considerable editing before they resemble your target object, if you have built your
curves properly, the resulting surface will be either an exact match or will be very
close to one.
Planning your modeling process is important to success with NURBS objects.
Because they are reductive, it isn't always obvious what the original shape of an
object is before it is cut into a different shape. You will have to become sensitive to
the steps necessary to build certain objects so that when you build them in NURBS,
you are able to layout the steps properly (Fig. 14.22 ). This is not much of a problem
when designing an object from scratch, but if you are copying an existing product,
it can take some time to reverse engineer how it was built.
The trick to working with curves is to remember that the curves you build must
describe your target object completely. Every change of curve direction, every tan-
gent created by a stacked group of curves, every bit of data you create in your
curves, must belong to your object and be accurate. The good news is that working
with curves does not require you to use a high level of detail. You only need to
ensure that a few parameters are correct and the application will generate the surface
accurately. This is because NURBS modeling packages interpolate in-between
curve states between isoparms just as animation programs interpolate in-between
frames based on keyframe data. If the isoparms are built correctly, the software will
do the rest for you.
Ideally, you will plan your curves in advance so that any surface built from them
matches the shape you intend to build. Be careful when you do this to avoid using