CG Modeling 2: NURBS - An Introduction to Computer Graphics for Artists - page 250

Fig. 14.9 A curve is fi rst projected to create a curve on surface. Then, the curve on surface is used
to defi ne a section of the surface to be trimmed away

14.2.6 Curve Boundaries

These are the four boundaries of a patch, either an actual patch boundary or the
curves that will be used to make it.

14.2.7 Internal Control Points

These are points along internal surface isoparms and between curves at surface
boundaries. They may be moved or re-weighted to alter the shape of a curve or
surface.

14.2.8 Curve Direction

The control point designated as the fi rst point in the curve and the next control point
defi ne a curve's direction. If the directions of curve pairs do not match, any surfaces,
blends, or attempts to join across mismatched curves will result in failure or a seri-
ously distorted curve or surface.

14.2.9 Curve Intersection

Curve intersection sometimes looks like the best way to build a shape, because planar
curves in perpendicular planes, such as front view and side views of an automobile,
resemble orthographic reference. However, it is often easier to build surfaces differ-
ently, without attempting to draw the curves as you would in a 2D drawing.