Index Page saelgv_c
A B C D E F G H I J K L M N O P Q R S T U V W X
Procedure Abstract Required_Reading Keywords Brief_I/O Detailed_Input | Detailed_Output Parameters Exceptions Files Particulars Examples | Restrictions Literature_References Author_and_Institution Version Index_Entries
#### Procedure
       void saelgv_c ( ConstSpiceDouble   vec1  [3],
                       ConstSpiceDouble   vec2  [3],
                       SpiceDouble        smajor[3],
                       SpiceDouble        sminor[3]  )
#### Abstract
       Find semi-axis vectors of an ellipse generated by two arbitrary
       three-dimensional vectors.
#### Required_Reading
       ELLIPSES
#### Keywords
       ELLIPSE
       GEOMETRY
       MATH
#### Brief_I/O
       Variable  I/O  Description
       --------  ---  --------------------------------------------------
       vec1,
       vec2       I   Two vectors used to generate an ellipse.
       smajor     O   Semi-major axis of ellipse.
       sminor     O   Semi-minor axis of ellipse.
#### Detailed_Input
       vec1,
       vec2           are two vectors that define an ellipse.
                      The ellipse is the set of points in 3-space

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

                      where theta is in the interval ( -pi, pi ] and
                      center is an arbitrary point at which the ellipse
                      is centered.  An ellipse's semi-axes are
                      independent of its center, so the vector center
                      shown above is not an input to this routine.

                      vec2 and vec1 need not be linearly independent;
                      degenerate input ellipses are allowed.
#### Detailed_Output
       smajor
       sminor         are semi-major and semi-minor axes of the ellipse,
                      respectively.  smajor and sminor may overwrite
                      either of vec1 or vec2.
#### Parameters
       None.
#### Exceptions
       1)  If one or more semi-axes of the ellipse is found to be the
           zero vector, the input ellipse is degenerate.  This case is
           not treated as an error; the calling program must determine
           whether the semi-axes are suitable for the program's intended
           use.
#### Files
       None.
#### Particulars
       We note here that two linearly independent but not necessarily
       orthogonal vectors vec1 and vec2 can define an ellipse
       centered at the origin:  the ellipse is the set of points in
       3-space

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

       where theta is in the interval (-pi, pi] and center is an
       arbitrary point at which the ellipse is centered.

       This routine finds vectors that constitute semi-axes of an
       ellipse that is defined, except for the location of its center,
       by vec1 and vec2.  The semi-major axis is a vector of largest
       possible magnitude in the set

          cos(theta) vec1  +  sin(theta) vec2

       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 is in the interval (-pi, pi].

       The capability of finding the axes of an ellipse is useful in
       finding the image of an ellipse under a linear transformation.
       Finding this image is useful for determining the orthogonal and
       gnomonic projections of an ellipse, and also for finding the limb
       and terminator of an ellipsoidal body.
#### Examples
       1)  An example using inputs that can be readily checked by
           hand calculation.

              Let

                 vec1 = ( 1.,  1.,  1. )
                 vec2 = ( 1., -1.,  1. )

             The function call

                saelgv_c ( vec1, vec2, smajor, sminor );

             returns

                smajor = ( -1.414213562373095,
                            0.0,
                           -1.414213562373095 )
             and

                sminor = ( -2.4037033579794549D-17
                            1.414213562373095,
                           -2.4037033579794549D-17 )

       2)   This example is taken from the code of the CSPICE routine
            pjelpl_c, which finds the orthogonal projection of an ellipse
            onto a plane.  The code listed below is the portion used to
            find the semi-axes of the projected ellipse.

               #include "SpiceUsr.h"
                     .
                     .
                     .

               /.
               Project vectors defining axes of ellipse onto plane.
               ./
               vperp_c ( vec1,   normal,  proj1 );
               vperp_c ( vec2,   normal,  proj2 );

                  .
                  .
                  .

               saelgv_c ( proj1,  proj2,  smajor,  sminor );

            The call to saelgv_c determines the required semi-axes.
#### Restrictions
       None.
#### Literature_References
       [1]  Calculus, Vol. II.  Tom Apostol.  John Wiley & Sons, 1969.
            See Chapter 5, `Eigenvalues of Operators Acting on Euclidean
            Spaces'.
#### Author_and_Institution
       N.J. Bachman   (JPL)
       W.L. Taber     (JPL)
#### Version
       -CSPICE Version 1.0.0, 12-JUN-1999 (NJB)
#### Index_Entries
       semi-axes of ellipse from generating vectors
#### Link to routine saelgv_c source file saelgv_c.c

    Wed Jun  9 13:05:29 2010