Procedure

   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

 
 
   1) Compute an upward normal of an equilateral triangle lying 
      in the X-Y plane and centered at the origin. 
 
      Example code begins here. 
 
         #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 = %18.11e %18.11e %18.11e\n", 
                     normal[0], normal[1], normal[2]      );

            return ( 0 );
         }

  
      When run on a PC/Linux/gcc/64-bit platform, the output 
      from this program was: 
 
 
   NORMAL =  0.000000000000E+00  0.000000000000E+00  0.259807621135E+01 
 
 

Restrictions

 
   None. 
 

Literature_References

 
   None. 
 

Author_and_Institution

 
   N.J. Bachman    (JPL) 
 

Version

 
   -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