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PL3EL

       SUBROUTINE  PL3EL (XI,ETA,ZETA,VISBLE)
 C$    (3-D Elliptic Cylinder)
 C$    Change  the  variables   (XI,ETA,ZETA)  to  the   Cartesian
 C$    coordinates X,Y,Z  so  as  to  define  points  directly  in
 C$    elliptic cylindrical coordinates and graph their projection
 C$    on the X-Z plane,  where the Z axis  is vertical and the  X
 C$    axis is  positive to  the right.   Points with  positive  Y
 C$    values lie behind  the plane and  are marked as  invisible.
 C$    (XI,ETA,ZETA) are  all assumed  to be  scaled to  the  unit
 C$    interval.  The Cartesian  coordinates (X,Y,Z) are  adjusted
 C$    to the unit interval and passed to MOVA3/LINA3 as  (X,Z,Y),
 C$    so that the default view plane (the X-Y plane) will receive
 C$    the image.  Elliptical cylindrical coordinate ranges are:
 C$
 C$    0.0 .LE. U .LE. infinity
 C$    0.0 .LE. V .LE. 2*pi
 C$    -infinity .LE. W .LE. +infinity
 C$
 C$    See H. Margenau  and G.M. Murphy,  "Mathematics of  Physics
 C$    and Chemistry", 2nd  Ed., Van  Nostrand (1956),  Vol 1,  p.
 C$    182.  These are related to the Cartesian coordinates by
 C$
 C$    X = A COSH(U) COS(V)
 C$    Y = A SINH(U) SIN(V)
 C$    Z = W
 C$
 C$    Where the semi-major axis is "A".  The coordinate  surfaces
 C$    are
 C$    (1) Elliptic cylinders (U = constant)
 C$    (2) Hyperbolic cylinders (V = constant)
 C$    (3) Planes parallel to the X-Y plane (W = constant).
 C$
 C$    To obtain coordinates (XI,ETA,ZETA), expressed on the  unit
 C$    interval, (U,V,W) are transformed as follows:
 C$
 C$    XI = U/UMAX
 C$    ETA = V/TWOPI
 C$    ZETA = W   (simply clipped to 0..1)
 C$
 C$    To obtain  a reasonable  scaling,  the semi-major  axis  is
 C$    chosen to be  A = 1/COSH(UMAX)  = 0.5, from  which UMAX  is
 C$    found to be UMAX = ARCCOSH(2.0) = 1.3169578.
 C$    (09-APR-82)