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SUBROUTINE PL2SP (PH,TH,VISBLE)
C$ (2-D Spherical Polar Pen Movement)
C$ Change the angular variables (PH,TH) to the Cartesian
C$ coordinates X,Z so as to define directly in spherical polar
C$ coordinates points which lie upon the surface of a constant
C$ sphere and graph their projection on the X-Z plane, where
C$ the Z axis is vertical, and the X axis is positive to the
C$ right. Points with positive Y values lie behind the plane
C$ and are marked as invisible. (PH,TH) are all assumed to be
C$ scaled to the unit interval. The Cartesian coordinates
C$ (X,Y,Z) are adjusted to the unit interval and passed to
C$ LINA2/MOVA2 as (X,Z), so that the default view plane (the
C$ X-Y plane) will receive the image. Spherical coordinate
C$ ranges are
C$
C$ 0 .LE. RHO .LE. infinity
C$ 0 .LE. PHI .LE. 2*pi
C$ 0 .LE. THETA .LE. pi
C$
C$ See H. Margenau and G.M. Murphy, "Mathematics of Physics
C$ and Chemistry", 2nd Ed., Van Nostrand (1956), Vol 1, p.
C$ 177. These are related to the Cartesian coordinates by
C$
C$ X = RHO*SIN(THETA)*COS(PHI)
C$ Y = RHO*SIN(THETA)*SIN(PHI)
C$ Z = RHO*COS(THETA)
C$
C$ The coordinate surfaces are
C$ (1) Concentric spheres about the origin (RHO = constant)
C$ (2) Right circular cones with apex at the origin and axis
C$ along Z (THETA = constant)
C$ (3) Half planes from the Z axis (PHI = constant).
C$
C$ To obtain coordinates (PH,TH) expressed on the unit
C$ interval, (PHI,THETA) are transformed as follows:
C$
C$ PH = PHI/TWOPI
C$ TH = THETA/PI
C$ (09-APR-82)