HP b2600 Reference Manual page 27

B
gluBeginTrim
If a trimming loop definition contains multiple curves, the direction of the curves must
be consistent (that is, the inside must be to the left of all of the curves). Nested trimming
loops are legal as long as the curve orientations alternate correctly. If trimming curves
are self-intersecting, or intersect one another, an error results.
If no trimming information is given for a NURBS surface, the entire surface is drawn.
See Also
gluBeginSurface,
gluNewNurbsRenderer,
gluNurbsCallback,
gluNurbsCurve,
gluPwlCurve
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Chapter 2