8

1082 Chapter 9: Sur face Modeling
Going immediately to the Modify panel avoids the
problem of creating additional top-level NURBS
objects, which you can’t use to build relational,
dependent sub-objects. (The exception is using
cur ves for loft and sweep surfaces. See
ULoft
Surface (p age 1–1197)
, UV Loft Surface (page
1–1202)
, 1-Rail Sweep Surface (page 1–1205) ,or
2-Rail Sweep Surface (page 1–1210) .)
Two general references for modeling with NURBS
are
Curves and Surfaces for Computer-Aided
Geometric Design: A Pract ical Guide
by Gerald
Farin (Academic Press, fourth edition 1996) and
Interact ive Cur ves and Surfaces: A Multimedia
Tutor i a l on C omputer Aided Gr aphic D esi g n
by
Alyn Rockwood and Peter Chambers (Morgan
Kaufman Publishers, 1996).
Surface Trimming
To trim a surface is to use a curve on the surface
to cut away part of the surface, or to cut a hole in
the surface.
Before you trim a surface, you must create a curve
on that surface. These are the kinds of curves that
can trim surfaces:
•
U iso and V iso curves (page 1–1170)
• Surface-surface intersection curve (page
1–1167)
• Normal projected curve (page 1–1171)
• Vector projected curve (page 1–1172)
• CV curve on surface (page 1–1174)
• Point curve on surface (p age 1–1176)
Once you’ve created the curve, you t rim the surface
by turning on Trim in the curve sub-object’s
parameters. A Flip Trim control inverts the trim
direction.
The direct ion of the curve determines the
initial direction of the trim. For example, a
closed curve on surface created in a clockwise
direction trims inward, creating a hole in the
surface; w hile a closed curve on sur face created
in a counterclockwise direction trims outward,
creating a curve-shaped portion of the surface.
When a surface is trimmed, its untrimmed version
is still present in the 3ds Max scene. You can select
itforthepurposesofeditingit,orreplacingitasa
parent to a
dependent sub-object (p age 1–1088) .
To d o s o, us e th e se l e c t - b y - n a m e d i a l o g . S e e
Sub-Object Select ion (page 1–1086) .
Procedures
Exa mple: To cut a hole in a CV sur f ace:
1.
Create a CV surface in the Top viewpor t.
2. Create a closed C V curve sub-object that lies on
top of (or above) the surface.
3. Inthetoolbox,turnonNormalProjected
Curve, then in the Top v iewport select first the
CV curve, then the surface.
This creates a projection of the CV cur ve that
lies on the sur face, and can tr im it.
4. In the normal projected curve’s par ameters,
click to turn on Trim.
Aholeappearsinthesurface.Dependingon
the orientation of the Normal Projected curve,
you might see e verything but the hole.
5. UsetheFlipTrimtoggletoinvertthetrim.
Note: Tr ims aren’t displayed in viewports if the
NURBS surface’s Surface Trims toggle is turned
off on the General rollout’s Display group bo x.