9

1080 Chapter 9: Surface Modeling
• Youcanturnapatch grid (page 1–993) object
(Bezier patch) into a NURBS object.
• Youcanturnaloft (page 1–352) object into a
NURBS object.
To tur n objects other than NURBS curves and
surfaces into NURBS objects, use the Modif y
panel. R i ght-click the object’s name in the s t ack
display (see Modifier Stack (page 3–760))and
choose Convert To: NURBS.
In viewports, the quad menu (page 3–694) also
lets you convert objects to NURBS. Select and
then right-click the object, and in the Tra nsform
(lower-right) quadrant, choose Convert To: >
Convert to NURBS.
• In addition, the modifiers Extrude (page 1–680)
and Lathe (page 1–707) let you choose NURBS
output, which creates a NURBS object.
Work ing with NURBS Models
When you work with NURBS models, usually you
follow these overall steps:
• Create one NURBS object as the "starter"
object. This c an b e a surface object, a curve
object, or a converted geometry primitive, as
described in Creating NURBS Models (page
1–1079).
Often modelers like to identify a single, master
surface as the main component of the model.
Converted geometry primitives are good if you
wantthestartersurfacetobecomethemaster
surface. See Creating NURBS Surfaces from
Geometric Primitives (page 1–1116).Pointand
CV surfaces are good as starters for rectangular
surfaces.
•OntheModifypanel,youcanedittheoriginal
object, or you can create additional sub-objects.
See Using the NURBS Toolbox to Create
Sub-Objects (page 1–1083).
You might even choose to delete the original,
starter object once you have built a model from
newer sub-objects.
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
curves for loft and sweep surfaces. See ULoft
Surface (page 1–1196), UV Loft Surface (page
1–1200), 1-Rail Sweep Surface (page 1–1204),or
2-Rail Sweep Surface (page 1–1209).)
Two general references for modeling with NURBS
are Curves and Surfaces for Computer-Aided
Geometric Design: A Practical Guide by Gerald
Farin (Academic Press, fourth edition 1996) and
Interactive Curves and Surfaces: A Multimedia
Tutori al on Computer Aid ed Graphic D es ig n by
Alyn Rockwood and Peter Chambers (Morgan
Kaufman Publishers, 1996).
S urfa ce Tri mmin g
To trim a surface is to use a curve on t he surface
tocutawaypartofthesurface,ortocutaholein
the surface.
Before you trim a surface, you must create a curve
onthatsurface.Thesearethekindsofcurvesthat
can t rim surfaces:
• U iso and V iso curves (page 1–1168)
• Surface-surface intersection curve (page 1–1166)
• Normal projected curve (page 1–1169)
• Vector projected curve (page 1–1171)
• CV curve on surface (page 1–1172)
• Point curve on surface (page 1–1175)
Once you’ve created the curve, you trim the surface
by turning on Trim in the curve sub-object’s
parameters. A Flip Tr im cont rol inver ts the trim
direction.