nurbss(3G)

NAME
     nurbss - controls the shape of a NURBS surface

FORTRAN 77 SPECIFICATION
      subroutine nurbss(sknotcount, s_knot, tknotcount, t_knot,
     +                  soffset, toffset, ctlarray, sorder, torder, type)
      integer*4 sknotcount, tknotcount
      double precision sknot(sknotcount), tknot(tknotcount)
      integer*4 soffset, toffset
      double precision ctlarray(*)
      integer*4 sorder, torder, type

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

     sknot          expects an array of sknotcount non-decreasing knot values
                    in the parametric s direction.

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

     tknot          expects an array of tknotcount non-decreasing knot values
                    in the parametric t direction.

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

     toffset        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 soffset and toffset.

     sorder         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.

     torder         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 NV3D, NV3DR, NC4D, NC4DR, and NT2D,
                    NT2DR.  Types NV3D and NV3DR denote double-precision
                    positional coordinates in a three-dimensional model space.
                    NV3D denotes non-rational (3) coordinates and NV3DR
                    denotes rational (4) coordinates.  Types NC4D and NC4DR
                    denote double-precision color coordinates in a four-
                    dimensional RGBA color space.  NC4D denotes non-rational
                    coordinates and NC4DR denotes rational coordinates.  Types
                    NT2D and NT2DR denote double-precision texture coordinates
                    in a two-dimensional texture space.  NT2D denotes non-
                    rational coordinates and NT2DR denotes rational
                    coordinates.

DESCRIPTION
     Use the nurbss 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 bgnsur command.  To mark the end of a NURBS surface
     definition, use the endsur command.  Call nurbss within a NURBS surface
     definition only.

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

EXAMPLE
          call bgnsur
          call nurbss( ..., N_C4D )
          call nurbss( ..., N_T2D )
          call nurbss( ..., N_V3D )
          call endsur


     You can trim a NURBS surface by using the commands nurbsc and pwlcur
     between calls to bgntri and endtri.

     Observe that a nurbss with sknotcount knots in the s direction and
     tknotcount knots in the t direction with orders sorder and torder must
     have (sknotcount - sorder) x (tknotcount - torder) 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
     bgnsur, nurbsc, bgntri, pwlcur, setnur, getnur, texbin

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