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GLUBEGINTRIM(3G)					      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 bound‐
       aries of a NURBS surface. You include these trimming loops in the defi‐
       nition  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 defi‐
       nitions	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	piece‐
       wise 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(3G),   gluNewNurbsRenderer(3G),	 gluNurbsCallback(3G),
       gluNurbsCurve(3G), gluPwlCurve(3G)

							      GLUBEGINTRIM(3G)
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