B
gluBeginTrim
Chapter 2
26
gluBeginTrim
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 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 piece wise 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).
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