nurbscurve(3G)

NAME
     nurbscurve - controls the shape of a NURBS curve

C SPECIFICATION
     void nurbscurve(long knot_count, double knot_list, long
     double *ctlarray, long order, long type)

PARAMETERS
     knot_count   number of knots in knot_list. knot_count equals the number
                  of control points plus the order.

     knot_list    array of knot_count non-decreasing knot values.

     offset       offset (in bytes) between successive curve control points.

     ctlarray     pointer to an array of control points.  The coordinates must
                  agree with the type specified below.

     order        order of the NURBS curve.  order equals degree + 1, hence, a
                  cubic curve has an order of 4.  The maximum order is given
                  by getgdesc (GD_NURBS_ORDER).

     type         type of curve:

                  N_P2D          for a polynomial (u,v) trimming curve

                  N_P2DR         for a rational (wu,wv,w) trimming curve

                  N_V3D          for a polynomial (x,y,z) curve

                  N_V3DR         for a rational (x,y,z,w) curve

DESCRIPTION
     Use nurbscurve to describe a NURBS curve.

     The curve is either a curve in space, or a curve used to trim (define the
     visible region of) a NURBS surface.  Curve types N_V3D and N_V3DR specify
     curves in space; types N_P2D and N_P2DR specify curves used as trimming
     curves.

     Use bgncurve and endcurve to delimit the NURBS curve, unless it is a
     trimming curve, in which case it will appear within a bgntrim/endtrim
     block.  Specify the knot sequence, the control points and the order of
     the NURBS curve.

     You specify trimming regions within a NURBS surface definition (see
     bgnsurface) to define the visible regions of the NURBS surface. Trimming
     loops are closed, oriented curves that define the trimming regions.  You
     can describe a trimming loop by using a series of NURBS curves, piecewise
     linear curves (see pwlcurve), or both.
     Use bgntrim to mark the beginning of a trimming loop definition; use
     endtrim to mark the end of a trimming loop definition.

     The system displays the region of the NURBS surface that is to the left
     of the trimming curve as the parameter increases. Thus, for a counter-
     clockwise oriented trimming curve, the region inside the curve is
     displayed.  For a clockwise oriented trimming curve, the region outside
     the curve is displayed.

     The offset parameter is used in case the control points are part of an
     array of larger structure elements.  The nurbscurve routine searches for
     the n-th control point pair or triple beginning at byte address
     ctlarray + n * offset.

     See the Graphics Library Programming Guide for a mathematical description
     of a NURBS curve.

EXAMPLE
          double ctlpts [4] [3] ={
          {-.75, -.75, 0.0}, {-.5, .75, 0.0}, {.5, .75, 0.0} {.75, -.75, 0.0}
          };
          double knots[8] = {0.0, 0.0, 0.0, 0.0, 1.0, 1.0, 1.0, 1.0};
          bgncurve()
             nurbscurve(8, knots, 3*sizeof(double), &ctlpts[0] [0], 4, N_V3D);
          endcurve();

SEE ALSO
     bgncurve, bgnsurface, bgntrim, endcurve, endsurface, endtrim,
     getnurbsproperty, nurbssurface, pwlcurve, setnurbsproperty