gluBeginTrim(3G)	       OpenGL Reference		      gluBeginTrim(3G)



NAME
     gluBeginTrim, gluEndTrim -	delimit	a NURBS	trimming loop definition


C SPECIFICATION
     void gluBeginTrim(	GLUnurbs* nurb )

     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).




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gluBeginTrim(3G)	       OpenGL Reference		      gluBeginTrim(3G)



     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.	Trimming curves	cannot be self-
     intersecting, nor can they	intersect one another (or 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





























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