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and fit points. The fit points and control vertices offer different editing options
such as knot parameterization and degree options.
Moving Fit Points vs. Moving Control Vertices
NURBS curves have both fit points and control vertices. The fit points lie on
the line, and the control vertices lay outside the line. Use fit points to make
a change to one small part of a curve; use control vertices to make changes
that will affect the shape of the curve as a whole.
Clamp Surfaces and Curves with Open and Closed Geometry
NURBS surfaces and curves can have a clamp, closed, or open form. The form
affects how the object deforms.
■ Open Curves and surfaces - have their start and end CVs in different
positions - it doesn’t form a loop. If you snap the start and end CVs of an
open curve to the same position, it’s still an open curve, because you can
still drag these points away from one another.
■ Closed Curves and Surfaces - a loop with coinciding start and end CVs.
Where they meet is called a seam. If you move one CV, the other moves
with it.
■ Clamp Curve - is a closed loop with a seam that creates extra, unseen CVs.
These unseen CVs can cause the shape to wrinkle and crease when it is
reshaped.
Quick Reference
Commands
SPLINE
Creates a smooth curve that passes through or near a set of fit points, or that
is defined by the vertices in a control frame.
SPLINEDIT
Modifies the parameters of a spline or converts a spline-fit polyline to a
spline.
1102 | Chapter 24 Create 3D Models