GLUBEGINTRIM(3G) OpenGL Manual GLUBEGINTRIM(3G)
gluBeginTrim - delimit a NURBS trimming loop definition
void gluBeginTrim(GLUnurbs* nurb);
void gluEndTrim(GLUnurbs* nurb);
Specifies the NURBS object (created with gluNewNurbsRenderer()).
Use gluBeginTrim to mark the beginning of a trimming loop and gluEndTrim() to mark the end
of a trimming loop. A trimming loop is a set of oriented curve segments (forming a closed
curve) that define boundaries of a NURBS surface. You include these trimming loops in the
definition of a NURBS surface, between calls to gluBeginSurface() and gluEndSurface().
The definition for a NURBS surface can contain many trimming loops. For example, if you
wrote a definition for a NURBS surface that resembled a rectangle with a hole punched out,
the definition would contain two trimming loops. One loop would define the outer edge of
the rectangle; the other would define the hole punched out of the rectangle. The
definitions of each of these trimming loops would be bracketed by a
The definition of a single closed trimming loop can consist of multiple curve segments,
each described as a piecewise linear curve (see gluPwlCurve()) or as a single NURBS curve
(see gluNurbsCurve()), or as a combination of both in any order. The only library calls
that can appear in a trimming loop definition (between the calls to gluBeginTrim and
gluEndTrim()) are gluPwlCurve() and gluNurbsCurve().
The area of the NURBS surface that is displayed is the region in the domain to the left of
the trimming curve as the curve parameter increases. Thus, the retained region of the
NURBS surface is inside a counterclockwise trimming loop and outside a clockwise trimming
loop. For the rectangle mentioned earlier, the trimming loop for the outer edge of the
rectangle runs counterclockwise, while the trimming loop for the punched-out hole runs
If you use more than one curve to define a single trimming loop, the curve segments must
form a closed loop (that is, the endpoint of each curve must be the starting point of the
next curve, and the endpoint of the final curve must be the starting point of the first
curve). If the endpoints of the curve are sufficiently close together but not exactly
coincident, they will be coerced to match. If the endpoints are not sufficiently close, an
error results (see gluNurbsCallback()).
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.
This code fragment defines a trimming loop that consists of one piecewise linear curve,
and two NURBS curves:
gluBeginSurface(), gluNewNurbsRenderer(), gluNurbsCallback(), gluNurbsCurve(),
Copyright (C) 1991-2006 Silicon Graphics, Inc. This document is licensed under the SGI
Free Software B License. For details, see http://oss.sgi.com/projects/FreeB/.
opengl.org 06/10/2014 GLUBEGINTRIM(3G)