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<STRONG>NAME</STRONG>
<STRONG>gluBeginTrim,</STRONG> <STRONG>gluEndTrim</STRONG> - delimit a NURBS trimming loop
definition
<STRONG>C</STRONG> <STRONG>SPECIFICATION</STRONG>
void <STRONG>gluBeginTrim</STRONG>( GLUnurbs* <EM>nurb</EM> )
void <STRONG>gluEndTrim</STRONG>( GLUnurbs* <EM>nurb</EM> )
<STRONG>PARAMETERS</STRONG>
<EM>nurb</EM> Specifies the NURBS object (created with
<STRONG>gluNewNurbsRenderer</STRONG>).
<STRONG>DESCRIPTION</STRONG>
Use <STRONG>gluBeginTrim</STRONG> to mark the beginning of a trimming loop,
and <STRONG>gluEndTrim</STRONG> 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 <STRONG>gluBeginSurface</STRONG> and <STRONG>gluEndSurface</STRONG>.
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 <STRONG>gluBeginTrim</STRONG>/<STRONG>gluEndTrim</STRONG> pair.
The definition of a single closed trimming loop can consist
of multiple curve segments, each described as a piecewise
linear curve (see <STRONG>gluPwlCurve</STRONG>) or as a single NURBS curve
(see <STRONG>gluNurbsCurve</STRONG>), 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 <STRONG>gluBeginTrim</STRONG> and
<STRONG>gluEndTrim</STRONG>) are <STRONG>gluPwlCurve</STRONG> and <STRONG>gluNurbsCurve</STRONG>.
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
<STRONG>gluNurbsCallback</STRONG>).
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.
<STRONG>EXAMPLE</STRONG>
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);
<STRONG>SEE</STRONG> <STRONG>ALSO</STRONG>
<STRONG>gluBeginSurface</STRONG>, <STRONG>gluNewNurbsRenderer</STRONG>, <STRONG>gluNurbsCallback</STRONG>,
<STRONG>gluNurbsCurve</STRONG>, <STRONG>gluPwlCurve</STRONG>
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