nurbsc(3G)

NAME
     nurbsc - controls the shape of a NURBS curve

FORTRAN 77 SPECIFICATION
      subroutine nurbsc(knotcount, knotlist, offset, ctlarray,
     +                  order, type)
      integer*4 knotcount, offset
      double precision knotlist(knotcount), ctlarray(*)
      integer*4 order, type

PARAMETERS
     knotcount    number of knots in knotlist.  knotcount equals the number of
                  control points plus the order.

     knotlist     array of knotcount 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 nurbsc 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 NV3D and NV3DR specify
     curves in space; types NP2D and NP2DR specify curves used as trimming
     curves.

     Use bgncur and endcur to delimit the NURBS curve, unless it is a trimming
     curve, in which case it will appear within a bgntri/endtri 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
     bgnsur). 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 pwlcur), or both.

     Use bgntri to mark the beginning of a trimming loop definition; use
     endtri 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
     bgncur, bgnsur, bgntri, endcur, endsur, endtri, getnur, nurbss, pwlcur,
     setnur