Procedure

   pltnrm_c ( DSK, compute outward normal of plate ) 

   void pltnrm_c ( ConstSpiceDouble    v1[3],
                   ConstSpiceDouble    v2[3],
                   ConstSpiceDouble    v3[3],
                   SpiceDouble         normal[3] )

Abstract

   Compute an outward normal vector of a triangular plate.
   The vector does not necessarily have unit length.

Required_Reading

   DSK

Keywords

   DSK
   FILES
   TOPOGRAPHY

Brief_I/O

   VARIABLE  I/O  DESCRIPTION
   --------  ---  --------------------------------------------------
   v1,
   v2,
   v3         I   Vertices of a plate.
   normal     O   Plate's outward normal vector.

Detailed_Input

   v1,
   v2,
   v3          are vertices of a triangular plate.

Detailed_Output

   normal      is an outward normal vector of the plate defined by
               the input vertices. The order of the vertices is
               used to determine the choice of normal direction:
               the normal vector is

                  ( v2 - v1 ) x ( v3 - v2 )

Parameters

   None.

Exceptions

   1)  The input plate may be degenerate: it may be a line segment
       or a point. These are not considered to be erroneous inputs.

Files

   None.

Particulars

   This routine saves computation time by not scaling the output
   vector to unit length. The caller can scale the vector using
   the routine vhat_c.

Examples

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

   1) Compute an upward normal of an equilateral triangle lying
      in the X-Y plane and centered at the origin.

      Example code begins here.


      /.
         Program pltnrm_ex1
      ./

      #include <stdio.h>
      #include <math.h>
      #include "SpiceUsr.h"

      int main()
      {
         /.
         Local variables
         ./
         SpiceDouble             normal[3];
         SpiceDouble             s;
         SpiceDouble             v1[3];
         SpiceDouble             v2[3];
         SpiceDouble             v3[3];

         s = sqrt(3.0)/2;

         vpack_c (   s,  -0.5,  0.0, v1 );
         vpack_c ( 0.0,   1.0,  0.0, v2 );
         vpack_c (  -s,  -0.5,  0.0, v3 );

         pltnrm_c ( v1, v2, v3, normal );

         printf ( "NORMAL = %18.14f %18.14f %18.14f\n",
                  normal[0], normal[1], normal[2]      );

         return ( 0 );
      }


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


      NORMAL =   0.00000000000000   0.00000000000000   2.59807621135332

Restrictions

   None.

Literature_References

   None.

Author_and_Institution

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

Version

   -CSPICE Version 1.0.1, 10-AUG-2021 (JDR)

       Edited header to comply with NAIF standard.

       Changed code example output format for the solution to fit
       within the -Examples section without modifications.

   -CSPICE Version 1.0.0, 26-JAN-2016 (NJB)

Index_Entries

   compute normal vector of triangular plate from vertices

Link to routine pltnrm_c source file pltnrm_c.c