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CLPTR

       SUBROUTINE  CLPTR (U1,V1,MOVED1, U2,V2,MOVED2, VISSEG)
 C$    (2-Dimensional Triangular Coordinate Clip)
 C$    Clip  a   line  segment   defined  by   its  endpoints   in
 C$    2-dimensional triangular coordinates to the unit  triangle.
 C$    The arguments are:
 C$
 C$    (U1,V1)........Triangular coordinates of point 1.
 C$    MOVED1.........LOGICAL  value  set  .TRUE.  if  point  1 is
 C$                   moved.
 C$    (U2,V2)........Triangular coordinates of point 2.
 C$    MOVED2.........LOGICAL  value  set  .TRUE.  if  point  2 is
 C$                   moved.
 C$    VISSEG.........LOGICAL value set .TRUE. if  the  segment is
 C$                   visible.  If VISSEG=.FALSE., then the values
 C$                   of   all   preceding   parameters   may   be
 C$                   undefined.
 C$
 C$    This routine is  adapted from the  general polyhedral  clip
 C$    routine, CLPPH (written by  John Halleck of the  University
 C$    of Utah  Computing Center),  for the  special case  of  2-D
 C$    triangular coordinates, for which the inward-pointing  edge
 C$    normals are (1,0,0,0), (0,1,0,0), and (-1,-1,0,1).
 C$    (07-JAN-82)