1GLUBEGINTRIM(3G)                                              GLUBEGINTRIM(3G)
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NAME

6       gluBeginTrim, gluEndTrim - delimit a NURBS trimming loop definition
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C SPECIFICATION

10       void gluBeginTrim( GLUnurbs* nurb )
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12       void gluEndTrim( GLUnurbs* nurb )
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PARAMETERS

16       nurb  Specifies the NURBS object (created with gluNewNurbsRenderer).
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DESCRIPTION

19       Use  gluBeginTrim  to  mark  the  beginning  of  a  trimming  loop, and
20       gluEndTrim to mark the end of a trimming loop. A trimming loop is a set
21       of  oriented curve segments (forming a closed curve) that define bound‐
22       aries of a NURBS surface. You include these trimming loops in the defi‐
23       nition  of  a  NURBS  surface,  between  calls  to  gluBeginSurface and
24       gluEndSurface.
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26       The definition for a NURBS surface can contain many trimming loops. For
27       example, if you wrote a definition for a NURBS surface that resembled a
28       rectangle with a hole punched out, the  definition  would  contain  two
29       trimming  loops. One loop would define the outer edge of the rectangle;
30       the other would define the hole punched out of the rectangle. The defi‐
31       nitions  of  each  of  these  trimming  loops  would  be bracketed by a
32       gluBeginTrim/gluEndTrim pair.
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34       The definition of a single closed trimming loop can consist of multiple
35       curve  segments,  each  described  as  a  piecewise  linear  curve (see
36       gluPwlCurve) or as a single NURBS curve (see gluNurbsCurve),  or  as  a
37       combination  of  both  in  any  order.  The only library calls that can
38       appear in a trimming loop definition (between the calls to gluBeginTrim
39       and gluEndTrim) are gluPwlCurve and gluNurbsCurve.
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41       The  area  of  the NURBS surface that is displayed is the region in the
42       domain to the left  of  the  trimming  curve  as  the  curve  parameter
43       increases.  Thus,  the retained region of the NURBS surface is inside a
44       counterclockwise trimming loop and outside a clockwise  trimming  loop.
45       For  the  rectangle  mentioned earlier, the trimming loop for the outer
46       edge of the rectangle runs counterclockwise, while  the  trimming  loop
47       for the punched-out hole runs clockwise.
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49       If  you  use  more than one curve to define a single trimming loop, the
50       curve segments must form a closed loop (that is, the endpoint  of  each
51       curve must be the starting point of the next curve, and the endpoint of
52       the final curve must be the starting point of the first curve). If  the
53       endpoints  of the curve are sufficiently close together but not exactly
54       coincident, they will be coerced to match.  If the  endpoints  are  not
55       sufficiently close, an error results (see gluNurbsCallback).
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57       If  a  trimming loop definition contains multiple curves, the direction
58       of the curves must be consistent (that is, the inside must  be  to  the
59       left  of all of the curves). Nested trimming loops are legal as long as
60       the curve orientations alternate correctly.   If  trimming  curves  are
61       self-intersecting, or intersect one another, an error results.
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63       If  no  trimming  information  is given for a NURBS surface, the entire
64       surface is drawn.
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EXAMPLE

67       This code fragment defines a trimming loop that consists of one  piece‐
68       wise linear curve, and two NURBS curves:
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70       gluBeginTrim(nobj);
71          gluPwlCurve(..., GLU_MAP1_TRIM_2);
72          gluNurbsCurve(..., GLU_MAP1_TRIM_2);
73          gluNurbsCurve(..., GLU_MAP1_TRIM_3); gluEndTrim(nobj);
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SEE ALSO

76       gluBeginSurface(3G),   gluNewNurbsRenderer(3G),   gluNurbsCallback(3G),
77       gluNurbsCurve(3G), gluPwlCurve(3G)
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83                                                              GLUBEGINTRIM(3G)