Procedure

   nplnpt_c ( Nearest point on line to point ) 

   void nplnpt_c ( ConstSpiceDouble    linpt  [3],
                   ConstSpiceDouble    lindir [3],
                   ConstSpiceDouble    point  [3],
                   SpiceDouble         pnear  [3],
                   SpiceDouble       * dist       )

Abstract

   Find the nearest point on a line to a specified point, and find
   the distance between the two points.

Required_Reading

   None.

Keywords

   GEOMETRY
   MATH
   VECTOR

Brief_I/O

   VARIABLE  I/O  DESCRIPTION
   --------  ---  --------------------------------------------------
   linpt,
   lindir     I   Point on a line and the line's direction vector.
   point      I   A second point.
   pnear      O   Nearest point on the line to point.
   dist       O   Distance between point and pnear.

Detailed_Input

   linpt,
   lindir      are, respectively, a point and a direction vector
               that define a line in 3-dimensional space. The
               line is the set of points

                  linpt   +   t * lindir

               where t is any real number.

   point       is a point in 3-dimensional space.

Detailed_Output

   pnear       is the nearest point on the input line to the input
               point.

   dist        is the distance between the input line and input
               point.

Parameters

   None.

Exceptions

   1)  If the line direction vector `lindir' is the zero vector, the
       error SPICE(ZEROVECTOR) is signaled.

Files

   None.

Particulars

   For every line L and point P, there is a unique closest point
   on L to P. Call this closest point C. It is always true that
   P - C  is perpendicular to L, and the length of P - C is called
   the "distance" between P and L.

Examples

   1)  Suppose a line passes through the point ( 1, 2, 3 ) and
       has direction vector ( 0, 1, 1 ).  We wish to find the
       closest point on the line to the point ( -6, 9, 10 ).  We
       can use the code fragment

          #include "SpiceUsr.h"
               .
               .
               .
          LINPT[0]   =  1.0;
          LINPT[1]   =  2.0;
          LINPT[2]   =  3.0;

          LINDIR[0]  =  0.0;
          LINDIR[1]  =  1.0;
          LINDIR[2]  =  1.0;

          POINT[0]   = -6.0;
          POINT[1]   =  9.0;
          POINT[2]   = 10.0;

          nplnpt_c ( linpt, lindir, point, pnear, &dist );


       After the call, pnear will take the value

          ( 1., 9., 10. );

       dist will be 7.0.

Restrictions

   None.

Literature_References

   None.

Author_and_Institution

   N.J. Bachman        (JPL)
   J. Diaz del Rio     (ODC Space)

Version

   -CSPICE Version 1.0.1, 04-AUG-2021 (JDR)

       Edited the header to comply with NAIF standard.

   -CSPICE Version 1.0.0, 16-AUG-1999 (NJB)

Index_Entries

   distance between point and line
   nearest point on line to point

Link to routine nplnpt_c source file nplnpt_c.c