8

1116 Chapter 9: Sur face Modeling
Cho rd L e n g t h —
Chooses the chord-length
algorithm for reparameterization.
Chord-length reparameterization spaces knots
(in
parameter space (page 3–1082) )basedonthe
square root of the length of each curve segment.
Chord-length reparameterization is usually the
best choice.
Unifor m—Spaces the k nots uniformly.
A uniform knot vector has the advantage t hat the
curve or surface changes only locally when you edit
it. With the other two forms of parameterization,
moving any CV can change the entire sub-object.
Creat ing NURBS Curve and
Surface Objects
Creati ng I ndependent S ur f aces
from NURBS Curve Objects
To create independent surfaces from top-level
NURBS c urve objects, use the
Extrude (page
1–671)
and Lathe (page 1–699) modifiers. Extrude
adds height to the cur ve, creating a shape by
sweepingitalongthecurve’slocalZaxis. Lathe
creates a surface of rotation, revolving the shape
along a specified axis. You c an also use the
Bevel
(page 1–542)
and Bevel Profile (p age 1–546)
modifiers on NURBS c u rves.
ThesemodifierstreatNURBScurvesthesameway
they treat
shapes (page 1–257) . The advantage of
usingNURBScurvesinsteadofshapesisinthe
different po ssible shapes that NURBS geometry
and editi ng provide.
Tip: When you create a complicated surface,
especial ly with the Lathe modifier, you often want
to render both sides of the surface. Turn on Force
2-Sided in the Render Scene dialog to see both
sides of the extruded or lathed surface. To see both
sidesinviewports,turnonForce2-Sidedinthe
View por t Configuration dialog.
By default, an object with Extrude or Lathe
collapses to an
editable mesh (page 1–984) object.
To have Ext rude or Lathe output collapse to a
NURBS object, change the setting to NURBS in t he
Output group box of the Extrude or Lathe rollout,
and then collapse the
modifier stack (page 3–802) .
Cre ating N URBS Curves fro m
Splines
Selec t Spline object. > Modify panel > Right-click the
spline name. > Convert To: NURBS
Splines become N URBS curves that then become NURBS
surfaces.
Yo u c a n t u r n a spline (page 1–261) into a NURBS
object formed of
CV curves (page 1–1107) .Once
converted, you can no longer edit the spline shape
parametrically, but yo u can edit it as a NURBS
object, moving CVs and so on.
When you convert a spline
circle (page 1–268)
to a NURBS curve, the direction of the curve is
reversed. This facilitates usi ng the curve to trim a
surface: if the direction weren’t reversed, usually
the circle would trim outward instead of inward.