Graphics Programs Reference
In-Depth Information
NURBS!
NURBS modeling depends on surfaces that are created using curves. Just as control vertices
(CVs) are connected to form curved lines, NURBS surfaces are created by connecting (or
spanning ) curves. Therefore, typical NURBS modeling pipelines first involve the creation
of curves that define the edges, outline, paths, and/or boundaries of surfaces.
When a surface is created, its shape is defined and governed by its isoparms. These sur-
face curves, or curves that reside solely on a surface, show the outline of a surface's shape
much as the chicken wire in a wire mesh sculpture does. CVs on the isoparms define and
govern the shape of these isoparms just as they would regular curves. Adjusting a NURBS
surface involves manipulating the CVs of the object, somewhat like sculpting.
You can create a NURBS surface in several ways. The easiest way is to create a NURBS
primitive. You can sculpt the primitive surface by moving its CVs, but you can also cut it
apart to create different surface swatches or patches to use as needed, which you'll see in
a steam pump model for the locomotive later in this chapter. A primitive need not retain
its original shape, and it frequently can be shaped to it the artist's needs. Using the sur-
facing tools available under the Surfaces menu set, you can detach, cut, and attach pieces
into and out of a primitive to get the exact shapes you need.
You can also make surfaces in several ways without using a primitive. All these meth-
ods involve first creating or using existing NURBS curves, or curves on another surface,
to define a part or parts of the surface, and then using one of the methods described in
the following sections to create the surfaces.
Lofting
The most common surfacing method is lofting , which takes at least two curves and cre-
ates a surface span between each selected curve in the order in which they're selected.
Figure 5.1 shows the result of lofting two curves together.
Figure 5.1
A simple loft
created between
two curves
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