Form a CSPICE ellipse from a center vector and two generating 
   vectors. 

   ELLIPSES 

   Variable  I/O  Description 
   --------  ---  -------------------------------------------------- 
   center, 
   vec1, 
   vec2       I   Center and two generating vectors for an ellipse. 
   ellipse    O   The CSPICE ellipse defined by the input vectors. 

   center, 
   vec1, 
   vec2           are a center and two generating vectors defining 
                  an ellipse in three-dimensional space.  The 
                  ellipse is the set of points 

                     center  +  cos(theta) vec1  +  sin(theta) vec2 

                  where theta ranges over the interval (-pi, pi]. 
                  vec1 and vec2 need not be linearly independent. 

   ellipse        is the CSPICE ellipse defined by the input 
                  vectors. 

   None. 

   CSPICE ellipses serve to simplify calling sequences and reduce 
   the chance for error in declaring and describing argument lists 
   involving ellipses. 

   The set of ellipse conversion routines is 

      cgv2el_c ( Center and generating vectors to ellipse ) 
      el2cgv_c ( Ellipse to center and generating vectors ) 

   1)  Find the intersecton of an ellipse with a plane.  The ellipse 
       is defined by the vectors center, vec1, and vec2.  The plane 
       is defined by the normal vector n and the constant c. 

          #include "SpiceUsr.h"
                    .
                    .
                    .
          /.
          Make a CSPICE ellipse.  Make a plane while we're at it. 
          ./
          cgv2el_c ( center, vec1, vec2,  &ellipse ); 
          nvc2pl_c ( n,      c,           &plane   ); 

          /.
          Find the intersection of the ellipse and plane. 
          nxpts is the number of intersection points; xpt1 
          and xpt2 are the points themselves. 
          ./
          inelpl_c ( &ellipse, &plane, &nxpts, xpt1, xpt2 );

   None. 

   1)  If vec1 and vec2 are linearly dependent, ellips will be 
       degenerate.  CSPICE ellipses are allowed to represent 
       degenerate geometric ellipses. 

   None. 

   N.J. Bachman   (JPL) 

   None. 

   -CSPICE Version 1.0.0, 05-MAR-1999 (NJB)

   center and generating vectors to ellipse