gluBeginTrim(3G) OpenGL Reference gluBeginTrim(3G)
gluBeginTrim, gluEndTrim  delimit a NURBS trimming loop definition
void gluBeginTrim( GLUnurbs* nurb )
void gluEndTrim( GLUnurbs* nurb )
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).
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gluBeginTrim(3G) OpenGL Reference gluBeginTrim(3G)
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, gluNewNurbsRenderer, gluNurbsCallback, gluNurbsCurve,
gluPwlCurve
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