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+'\" e
+'\"! eqn | mmdoc
+'\"macro stdmacro
+.ds Vn Version 1.2
+.ds Dt 6 March 1997
+.ds Re Release 1.2.0
+.ds Dp May 02 11:53
+.ds Dm 37 begintrim
+.ds Xs 15822 6 begintrim.gl
+.TH GLUBEGINTRIM 3G
+.SH NAME
+.B "gluBeginTrim, gluEndTrim
+\- delimit a NURBS trimming loop definition
+
+.SH C SPECIFICATION
+void \f3gluBeginTrim\fP(
+GLUnurbs* \fInurb\fP )
+.nf
+.fi
+
+void \f3gluEndTrim\fP(
+GLUnurbs* \fInurb\fP )
+.nf
+.fi
+
+.EQ
+delim $$
+.EN
+.SH PARAMETERS
+.TP \w'\f2nurb\fP\ \ 'u 
+\f2nurb\fP
+Specifies the NURBS object (created with \%\f3gluNewNurbsRenderer\fP).
+.SH DESCRIPTION
+Use \%\f3gluBeginTrim\fP to mark the beginning of a
+trimming loop, and \%\f3gluEndTrim\fP 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 \%\f3gluBeginSurface\fP and \%\f3gluEndSurface\fP.
+.P
+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
+\%\f3gluBeginTrim\fP/\%\f3gluEndTrim\fP pair.
+.P
+The definition of a single closed trimming loop can consist
+of multiple curve segments, each described as a piecewise
+linear curve (see \%\f3gluPwlCurve\fP) or as a single NURBS
+curve (see \%\f3gluNurbsCurve\fP), 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 \%\f3gluBeginTrim\fP and \%\f3gluEndTrim\fP) are 
+\%\f3gluPwlCurve\fP and \%\f3gluNurbsCurve\fP.
+.P
+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.
+.P
+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 \%\f3gluNurbsCallback\fP).
+.P
+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.
+.P
+If no trimming information is given
+for a NURBS surface, the entire surface is drawn.
+.SH EXAMPLE
+This code fragment defines a trimming loop that consists of one
+piecewise linear curve, and two NURBS curves:
+.sp
+.Ex
+gluBeginTrim(nobj);
+   gluPwlCurve(..., GLU_MAP1_TRIM_2);
+   gluNurbsCurve(..., GLU_MAP1_TRIM_2);
+   gluNurbsCurve(..., GLU_MAP1_TRIM_3);
+gluEndTrim(nobj);
+.Ee
+.SH SEE ALSO
+\%\f3gluBeginSurface(3G)\fP, \%\f3gluNewNurbsRenderer(3G)\fP, \%\f3gluNurbsCallback(3G)\fP, 
+\%\f3gluNurbsCurve(3G)\fP, \%\f3gluPwlCurve(3G)\fP
+
+