gluBeginTrim, gluEndTrim - delimit a NURBS trimming loop definition
GLUnurbs* nurb ); void gluEndTrim(
GLUnurbs* nurb );
Specifies the NURBS object (created with gluNewNurbsRenderer()).
Use gluBeginTrim() to mark the beginning of a trimming loop, and glu‐
EndTrim() 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 glu‐
The definition for a NURBS surface can contain many trimming loops.
For example, if you wrote a definition for a NURBS surface that resem‐
bled a rectangle with a hole punched out, the definition would contain
two trimming loops. One loop would define the outer edge of the rectan‐
gle; the other would define the hole punched out of the rectangle. The
definitions of each of these trimming loops would be bracketed by a
The definition of a single closed trimming loop can consist of multiple
curve segments, each described as a piecewise linear curve (see gluPwl‐
Curve()) or as a single NURBS curve (see gluNurbsCurve()), or as a com‐
bination 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.
This code fragment defines a trimming loop that consists of one piece‐
wise linear curve, and two NURBS curves: gluBeginTrim(nobj);
gluNurbsCurve(..., GLU_MAP1_TRIM_3); gluEndTrim(nobj);
SEE ALSOgluBeginSurface(3), gluNewNurbsRenderer(3), gluNurbsCallback(3),