gluBeginTrim() gluBeginTrim()
gluBeginTrim, gluEndTrim  delimit a NURBS trimming loop
definition
void gluBeginTrim(
GLUnurbs* nurb ); void gluEndTrim(
GLUnurbs* nurb );
Specifies the NURBS object (created with gluNewNurbsRenderer()).
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 punchedout 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
selfintersecting, or intersect one another, an error
results.
If no trimming information is given for a NURBS surface,
the entire surface is drawn.
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);
gluBeginSurface(3), gluNewNurbsRenderer(3), gluNurbsCallback(3), gluNurbsCurve(3), gluPwlCurve(3)
gluBeginTrim()
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