C                  INTERNATIONAL AVS CENTER
C        (This disclaimer must remain at the top of all files)
C
C WARRANTY DISCLAIMER
C
C This module and the files associated with it are distributed free of charge.
C It is placed in the public domain and permission is granted for anyone to use,
C duplicate, modify, and redistribute it unless otherwise noted.  Some modules
C may be copyrighted.  You agree to abide by the conditions also included in
C the AVS Licensing Agreement, version 1.0, located in the main module
C directory located at the International AVS Center ftp site and to include
C the AVS Licensing Agreement when you distribute any files downloaded from
C that site.
C
C The International AVS Center, MCNC, the AVS Consortium and the individual
C submitting the module and files associated with said module provide absolutely
C NO WARRANTY OF ANY KIND with respect to this software.  The entire risk as to
C the quality and performance of this software is with the user.  IN NO EVENT
C WILL The International AVS Center, MCNC, the AVS Consortium and the individual
C submitting the module and files associated with said module BE LIABLE TO
C ANYONE FOR ANY DAMAGES ARISING FROM THE USE OF THIS SOFTWARE, INCLUDING,
C WITHOUT LIMITATION, DAMAGES RESULTING FROM LOST DATA OR LOST PROFITS, OR ANY
C SPECIAL, INCIDENTAL, OR CONSEQUENTIAL DAMAGES.
C
C This AVS module and associated files are public domain software unless
C otherwise noted.  Permission is hereby granted to do whatever you like with
C it, subject to the conditions that may exist in copyrighted materials. Should
C you wish to make a contribution toward the improvement, modification, or
C general performance of this module, please send us your comments:  why you
C liked or disliked it, how you use it, and most important, how it helps your
C work. We will receive your comments at avs@ncsc.org.
C
C Please send AVS module bug reports to avs@ncsc.org.
C
C  NOTE:  THIS MODULE AND SOURCE CODE IS FOR USE 
C  WITH THE AVS SOFTWARE ENVIRONMENT ONLY 
      SUBROUTINE FACET_NORMAL( X, Y, Z, FNX, FNY, FNZ )
*
      REAL X(*), Y(*), Z(*), FNX, FNY, FNZ
*
      REAL X12, Y12, Z12, X23, Y23, Z23, LEN
*
      X12 = X(2) - X(1)
      Y12 = Y(2) - Y(1)
      Z12 = Z(2) - Z(1)
*
      X23 = X(3) - X(2)
      Y23 = Y(3) - Y(2)
      Z23 = Z(3) - Z(2)
* 
      FNX = Y12*Z23 - Z12*Y23
      FNY = Z12*X23 - X12*Z23
      FNZ = X12*Y23 - Y12*X23
*
      LEN = SQRT( FNX*FNX + FNY*FNY + FNZ*FNZ )
      IF (LEN.GT.1E-9) THEN
        FNX = FNX / LEN
        FNY = FNY / LEN
        FNZ = FNZ / LEN
      ELSE
        PRINT *, 'ERROR : facet normal has zero length'
        FNX = 0.0
        FNY = 0.0
        FNZ = 1.0
      END IF
*
      RETURN
      END