Procedure

   vprojg_c ( Vector projection, general dimension ) 

   void vprojg_c ( ConstSpiceDouble    a      [],
                   ConstSpiceDouble    b      [],
                   SpiceInt            ndim,
                   SpiceDouble         p      [] )

Abstract

   Compute the projection of one vector onto another vector. All
   vectors are of arbitrary dimension.

Required_Reading

   None.

Keywords

   VECTOR

Brief_I/O

   VARIABLE  I/O  DESCRIPTION
   --------  ---  --------------------------------------------------
   a          I   The vector to be projected.
   b          I   The vector onto which `a' is to be projected.
   ndim       I   Dimension of `a', `b', and `p'.
   p          O   The projection of `a' onto `b'.

Detailed_Input

   a           is a double precision vector of arbitrary dimension.
               This vector is to be projected onto the vector `b'.

   b           is a double precision vector of arbitrary dimension.
               This vector is the vector which receives the
               projection.

   ndim        is the dimension of `a', `b' and `p'.

Detailed_Output

   p           is a double precision vector of arbitrary dimension
               containing the projection of `a' onto `b'. (`p' is
               necessarily parallel to `b'.)

Parameters

   None.

Exceptions

   Error free.

Files

   None.

Particulars

   The projection of a vector `a' onto a vector `b' is, by definition,
   that component of `a' which is parallel to `b'. To find this
   component it is enough to find the scalar ratio of the length of
   `b' to the projection of `a' onto `b', and then use this number to
   scale the length of `b'. This ratio is given by

       ratio = < a, b > / < b, b >

   where <,> denotes the general vector dot product. This routine
   does not attempt to divide by zero in the event that `b' is the
   zero vector.

Examples

   The numerical results shown for this example may differ across
   platforms. The results depend on the SPICE kernels used as
   input, the compiler and supporting libraries, and the machine
   specific arithmetic implementation.

   1) Define two sets of vectors and compute the projection of
      each vector of the first set on the corresponding vector of
      the second set.

      Example code begins here.


      /.
         Program vprojg_ex1
      ./
      #include <stdio.h>
      #include "SpiceUsr.h"

      int main( )
      {

         /.
         Local parameters.
         ./
         #define NDIM         4
         #define SETSIZ       4

         /.
         Local variables.
         ./
         SpiceDouble          pvec   [NDIM];

         SpiceInt             i;

         /.
         Define the two vector sets.
         ./
         SpiceDouble          seta   [SETSIZ][NDIM] = {
                                   {6.0,  6.0,  6.0,  0.0},
                                   {6.0,  6.0,  6.0,  0.0},
                                   {6.0,  6.0,  0.0,  0.0},
                                   {6.0,  0.0,  0.0,  0.0} };

         SpiceDouble          setb   [SETSIZ][NDIM] = {
                                   {2.0,  0.0,  0.0,  0.0},
                                   {-3.0,  0.0,  0.0,  0.0},
                                   {0.0,  7.0,  0.0,  0.0},
                                   {0.0,  0.0,  9.0,  0.0} };

         /.
         Calculate the projection
         ./
         for ( i = 0; i < SETSIZ; i++ )
         {

            vprojg_c ( seta[i], setb[i], NDIM, pvec );
            printf( "Vector A  :  %4.1f %4.1f %4.1f %4.1f\n",
                    seta[i][0], seta[i][1], seta[i][2], seta[i][3] );
            printf( "Vector B  :  %4.1f %4.1f %4.1f %4.1f\n",
                    setb[i][0], setb[i][1], setb[i][2], setb[i][3] );
            printf( "Projection:  %4.1f %4.1f %4.1f %4.1f\n",
                           pvec[0], pvec[1], pvec[2], pvec[3] );
            printf( " \n" );

         }

         return ( 0 );
      }


      When this program was executed on a Mac/Intel/cc/64-bit
      platform, the output was:


      Vector A  :   6.0  6.0  6.0  0.0
      Vector B  :   2.0  0.0  0.0  0.0
      Projection:   6.0  0.0  0.0  0.0

      Vector A  :   6.0  6.0  6.0  0.0
      Vector B  :  -3.0  0.0  0.0  0.0
      Projection:   6.0 -0.0 -0.0 -0.0

      Vector A  :   6.0  6.0  0.0  0.0
      Vector B  :   0.0  7.0  0.0  0.0
      Projection:   0.0  6.0  0.0  0.0

      Vector A  :   6.0  0.0  0.0  0.0
      Vector B  :   0.0  0.0  9.0  0.0
      Projection:   0.0  0.0  0.0  0.0

Restrictions

   1)  No error detection or recovery schemes are incorporated into
       this routine except to insure that no attempt is made to
       divide by zero. Thus, the user is required to make sure that
       the vectors `a' and `b' are such that no floating point overflow
       will occur when the dot products are calculated.

Literature_References

   [1]  G. Thomas and R. Finney, "Calculus and Analytic Geometry,"
        7th Edition, Addison Wesley, 1988.

Author_and_Institution

   J. Diaz del Rio     (ODC Space)

Version

   -CSPICE Version 1.0.0, 01-NOV-2021 (JDR)

Index_Entries

   n-dimensional vector projection