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PERDST

 PERDST.......Distance  of  perspective  projective  point  (eye  point)
              measured from view  reference point in  direction of  view
              plane  normal.    If   specified,   this   option   forces
              PARLEL=.FALSE. and  sets PPXYZ(*)  = RPXYZ(*)  + PERDST  *
              (unit view  plane normal).   Since the  view plane  normal
              points into the window, the projective point should be  in
              front of  the  window,  so PERDST  should  in  general  be
              negative.  For very large  negative values of PERDST,  the
              perspective projection  becomes indistinguishable  from  a
              parallel projection.   When non-skewed  perspective  views
              are required, PERDST is  a more convenient parameter  than
              setting PARLEL and PPXYZ(*) together.