MirBSD manpage: gluBeginTrim(3), gluEndTrim(3)

GLUBEGINTRIM(3G)    UNIX Programmer's Manual     GLUBEGINTRIM(3G)


     gluBeginTrim, gluEndTrim - delimit a NURBS trimming loop


     void gluBeginTrim( GLUnurbs* nurb )

     void gluEndTrim( GLUnurbs* nurb )


     nurb  Specifies the NURBS object (created with


     Use gluBeginTrim to mark the beginning of a trimming loop,
     and gluEndTrim to mark the end of a trimming loop. A trim-
     ming 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 brack-
     eted 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 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

MirBSD #10-current     Printed 2021-12-07                       1

GLUBEGINTRIM(3G)    UNIX Programmer's Manual     GLUBEGINTRIM(3G)

     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 coin-
     cident, they will be coerced to match. If the endpoints are
     not sufficiently close, an error results (see

     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 piecewise linear curve, and two NURBS curves:

        gluPwlCurve(..., GLU_MAP1_TRIM_2);
        gluNurbsCurve(..., GLU_MAP1_TRIM_2);
        gluNurbsCurve(..., GLU_MAP1_TRIM_3); gluEndTrim(nobj);


     gluBeginSurface(3G), gluNewNurbsRenderer(3G),
     gluNurbsCallback(3G), gluNurbsCurve(3G), gluPwlCurve(3G)

MirBSD #10-current     Printed 2021-12-07                       2

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