8

1080 Chapter 9: Sur face Modeling
NURBS surfaces, on the other hand, are
analy tically generated. They are more efficient to
calculate, and you can render a NURBS surface
that appears to be seamless. (A rendered NURBS
surface is actually approximated by polygons,
but the NURBS approximation can be very fine
grained.)
NURBS Models: Objects and Sub-Objects (page
1–1080)
NURBS and Modifiers (page 1–1091)
NURBS and Animation (page 1–1092)
NURBS Concepts (page 1–1093)
Wo rking with NURBS
Models
NUR B S M odels: Obj ects a nd
Sub-Objects
Like Shape (page 1–257) objects, a NURBS
model can be an assemblage of multiple NURBS
sub-objects. For example, a NU RBS object might
con tain two surfaces that are separate in space.
NURBS curves and NURBS surfaces are controlled
by either point or control ver tex (CV) sub-objects.
Points and CVs behave somewhat like the vertices
of spline objects, but there are differences.
The parent object in a NURBS model is either a
NURBS surface or a NURBS curve. Sub-objects
canbeanyoftheobjectslistedhere.ANURBS
cur ve remains a Shape object unless you add a
surfacesub-objecttoitwhenyouconvertittoa
NURBS surface (without changing its name).
Surf aces—There are two k inds of NURBS surfaces.
A
point surface (page 1–1103) is controlled by
points, which always lie on the sur face. A
CV
surface (page 1–1105)
is controlled by control
vertices (CVs). Instead of lying on the surface,
CVs form a
control lattice (page 3–1017) that
surroundsthesurface. (Thisissimilartothe
lattice used by the FFD [free-form deformation]
modifiers.)
See
Creating Surface Sub-Objects (page 1–1179)
and Editing Surface Sub-Objects (page 1–1142) .
Curves—There are also two kinds of NURBS
curves. These correspond exactly to the two
kinds of surfaces. A
point cur ve (page 1–1108) is
controlled by points, which always lie on the curve.
A
CV cur ve (page 1–1112) is controlled by CVs,
which don’t necessarily lie on the curve.
See
Creating Surface Sub-Objects (page 1–1179)
and Editing Curve Sub-Objects (page 1–1136) .
Point s—Pointsurfacesandpointcurveshave point
(page 1–1221)
sub-objects. You can also create
separate point sub-objects that are not part of a
surface or a curve.
See
Creating and Editing Point Sub-Objects (page
1–1220)
.
CVs—CV surfaces and CV curves have CV
sub-objects. Un like points, CVs are always par t
of a surface or a curve.
See
Editing Curve CV Sub-Objects (page 1–1128)
and Editing Surface CV Sub-Objects (page
1–1132)
.
Impor ts—Importsare3dsMaxobjects,including
other NURBS objects. Within the NURBS model,
they render as NURBS; but they retain their
original parameters and modifiers.
See
Attaching and Importing 3ds Max Objects
(page 1–1121)
.
Sub-objects can be
dependent (page 1–1088)
sub-objects whose geometry is related to the
geometry of other sub-objects.