nurbssurface(3G)

NAME
     nurbssurface - controls the shape of a NURBS surface

C SPECIFICATION
     void nurbssurface(s_knot_count, s_knot, t_knot_count, t_knot,
                       s_offset, t_offset, ctlarray,
                       s_order, t_order, type)
     long s_knot_count, t_knot_count;
     double s_knot[], t_knot[];
     long s_offset, t_offset;
     double *ctlarray;
     long s_order, t_order, type;

PARAMETERS
     s_knot_count   expects the number of knots in the parametric s direction.

     s_knot         expects an array of s_knot_count non-decreasing knot
                    values in the parametric s direction.

     t_knot_count   expects the number of knots in the parametric t direction.

     t_knot         expects an array of t_knot_count non-decreasing knot
                    values in the parametric t direction.

     s_offset       expects the offset (in bytes) between successive control
                    points in the parametric s direction in ctlarray.

     t_offset       expects the offset (in bytes) between successive control
                    points in the parametric t direction in ctlarray.

     ctlarray       expects an array containing control points for the NURBS
                    surface.  The coordinates must appear as either (x, y, z)
                    triples or as (wx, wy, wz, w) quadruples.  The offsets
                    between successive control points in the parametric s and
                    t directions are given by s_offset and t_offset.

     s_order        expects the order of the NURBS surface in the parametric s
                    direction.  The order is one more than the degree, hence,
                    a cubic surface has an order of 4.

     t_order        expects the order of the NURBS surface in the parametric t
                    direction.  The order is one more than the degree, hence,
                    a cubic surface has an order of 4.

     type           expects a value indicating the control point type.
                    Current options are N_V3D, N_V3DR, N_C4D, N_C4DR, and
                    N_T2D, N_T2DR.  Types N_V3D and N_V3DR denote double-
                    precision positional coordinates in a three-dimensional
                    model space.  N_V3D denotes non-rational (3) coordinates
                    and N_V3DR denotes rational (4) coordinates.  Types N_C4D
                    and N_C4DR denote double-precision color coordinates in a
                    four-dimensional RGBA color space.  N_C4D denotes non-
                    rational coordinates and N_C4DR denotes rational
                    coordinates.  Types N_T2D and N_T2DR denote double-
                    precision texture coordinates in a two-dimensional texture
                    space.  N_T2D denotes non-rational coordinates and N_T2DR
                    denotes rational coordinates.

DESCRIPTION
     Use the nurbssurface command within a NURBS (Non-Uniform Rational B-
     Spline) surface definition to describe the shape of a NURBS surface
     before any trimming takes place.  To mark the beginning of a NURBS
     surface definition, use the bgnsurface command.  To mark the end of a
     NURBS surface definition, use the endsurface command.  Call nurbssurface
     within a NURBS surface definition only.

     Positional, texture, and color coordinates are associated by presenting
     each as a separate nurbssurface between a bgnsurface/endsurface pair.  No
     more than one call to nurbssurface for each of color and texture data may
     be made within a single bgnsurface/endsurface pair.  Exactly one call
     must be made to describe position data and it must be the last call to
     nurbssurface between the bracketing bgnsurface/endsurface.

EXAMPLE
          bgnsurface();
              nurbssurface( ..., N_C4D );   /* color data */
              nurbssurface( ..., N_T2D );   /* texture data */
              nurbssurface( ..., N_V3D );   /* position data */
          endsurface();


     You can trim a NURBS surface by using the commands nurbscurve and
     pwlcurve between calls to bgntrim and endtrim.

     Observe that a nurbssurface with s_knot_count knots in the s direction
     and t_knot_count knots in the t direction with orders s_order and t_order
     must have (s_knot_count - s_order) x (t_knot_count - t_order) control
     points.

     The system renders a NURBS surface as a polygonal mesh and analytically
     calculates normal vectors at the corners of the polygons within the mesh.
     Therefore, your code should specify a lighting model when it uses NURBS
     surfaces. Otherwise, you loose all the interesting surface information.
     Use lmdef and lmbind to define or modify materials and their properties.

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

SEE ALSO
     bgnsurface, nurbscurve, bgntrim, pwlcurve, getnurbsproperty,
     setnurbsproperty, texbind

NOTE
     nurbssurface commands specifying color or texture coordinates currently
     have no effect on IRIS-4D G, GT, GTX, and Personal IRIS.