GLUBEGINTRIM(3G)						   OpenGL Manual						  GLUBEGINTRIM(3G)

NAME

       gluBeginTrim - delimit a NURBS trimming loop definition

C SPECIFICATION

       void gluBeginTrim(GLUnurbs* nurb);

C SPECIFICATION

       void gluEndTrim(GLUnurbs* nurb);

PARAMETERS

       nurb
	   Specifies the NURBS object (created with gluNewNurbsRenderer()).

DESCRIPTION

       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 gluBeginTrim/gluEndTrim() pair.

       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 clockwise.

       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.

EXAMPLE

       This code fragment defines a trimming loop that consists of one piecewise linear curve, and two NURBS curves:

	   gluBeginTrim(nobj);
	      gluPwlCurve(..., GLU_MAP1_TRIM_2);
	      gluNurbsCurve(..., GLU_MAP1_TRIM_2);
	      gluNurbsCurve(..., GLU_MAP1_TRIM_3);
	   gluEndTrim(nobj);

SEE ALSO

       gluBeginSurface(), gluNewNurbsRenderer(), gluNurbsCallback(), gluNurbsCurve(), gluPwlCurve()

COPYRIGHT

       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/.

AUTHORS

       opengl.org

opengl.org							    06/10/2014							  GLUBEGINTRIM(3G)