Procedure

   void el2cgv_c ( ConstSpiceEllipse   * ellipse,
                   SpiceDouble           center[3],
                   SpiceDouble           smajor[3],
                   SpiceDouble           sminor[3]  ) 

Abstract

 
   Convert a CSPICE ellipse to a center vector and two generating 
   vectors.  The selected generating vectors are semi-axes of the 
   ellipse. 
 

Required_Reading

 
   ELLIPSES 
 

Keywords

 
   ELLIPSE 
   GEOMETRY 
 

Brief_I/O

 
   Variable  I/O  Description 
   --------  ---  -------------------------------------------------- 
   ellipse    I   A CSPICE ellipse. 
   center, 
   smajor, 
   sminor     O   Center and semi-axes of ellipse. 
 

Detailed_Input

 
   ellipse        is a CSPICE ellipse. 
 

Detailed_Output

 
   center, 
   smajor, 
   sminor         are, respectively, a center vector, a semi-major 
                  axis vector, and a semi-minor axis vector that 
                  generate the input ellipse.  This ellipse is the 
                  set of points 
 
                     center + cos(theta) smajor + sin(theta) sminor 
 
                  where theta ranges over the interval (-pi, pi]. 
 

Parameters

 
   None. 
 

Exceptions

 
   Error free. 
 

Files

 
   None. 
 

Particulars

 
   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 ) 
 
   A word about the output of this routine:   the semi-major axis of 
   an ellipse is a vector of largest possible magnitude in the set 
 
      cos(theta) vec1  +  sin(theta) vec2, 
 
   where theta is in the interval (-pi, pi].  There are two such 
   vectors; they are additive inverses of each other. The semi-minor 
   axis is an analogous vector of smallest possible magnitude.  The 
   semi-major and semi-minor axes are orthogonal to each other.  If 
   smajor and sminor are choices of semi-major and semi-minor axes, 
   then the input ellipse can also be represented as the set of 
   points 
 
 
      center + cos(theta) smajor + sin(theta) sminor 
 
   where theta ranges over the interval (-pi, pi]. 
 
 

Examples

 
   1)  Find the semi-axes of the limb of an ellipsoid. 
 
          #include "SpiceUsr.h"
                  .
                  .
                  .
          /.
          Our viewing location is viewpt.  The radii of the 
          ellipsoid are a, b, and c. 
          ./
          edlimb_c ( a, b, c, viewpt, &limb );

          el2cgv_c ( &limb, center, smajor, sminor ); 
 

Restrictions

 
   None. 
 

Literature_References

 
   None. 
 

Author_and_Institution

 
   N.J. Bachman   (JPL) 
 

Version

 
   -CSPICE Version 1.0.0, 12-JUN-1999 (NJB)

Index_Entries

 
   ellipse to center and generating vectors 
 

Link to routine el2cgv_c source file el2cgv_c.c