c
c  . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
c  .                                                             .
c  .                  copyright (c) 1998 by UCAR                 .
c  .                                                             .
c  .       University Corporation for Atmospheric Research       .
c  .                                                             .
c  .                      all rights reserved                    .
c  .                                                             .
c  .                                                             .
c  .                         SPHEREPACK                          .
c  .                                                             .
c  . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
c
c ... file visequ.f
c
c     contains documentation and code for subroutine visgau
c
c     subroutine visequ (nlat,nlon,h,len,eyer,eyelat,eyelon,
c    +                                    wk,lwk,iwk,liwk,ierror)
c
c     subroutine visequ will display a function on the sphere
c     as a solid. ie. as a "lumpy" sphere. visequ calls subroutine
c     vsurf to produce the visible surface rendering.
c
c     requires routines visequ1 interp sptc diag stride triang vsurf
c                       vsurf1 prjct box icvmg projct
c
c     visgeo uses the ncar graphics package.
c     compile with: ncargf77 (all programs above)
c
c     execute with:  a.out
c
c     on screen display with:  ctrans -d x11 gmeta
c                          
c     print with:  ctrans -d ps.color gmeta > gmeta.ps
c                  lpr -p(your printer) gmeta.ps 
c
c     input parameters
c
c     nlat   the number of colatitudes on the full sphere including the
c            poles. for example, nlat = 37 for a five degree grid.
c            nlat determines the grid increment in colatitude as
c            pi/(nlat-1).  if nlat is odd the equator is located at
c            grid point i=(nlat+1)/2. if nlat is even the equator is
c            located half way between points i=nlat/2 and i=nlat/2+1.
c            nlat must be at least 3. note: on the half sphere, the
c            number of grid points in the colatitudinal direction is
c            nlat/2 if nlat is even or (nlat+1)/2 if nlat is odd.
c
c     nlon   the number of distinct londitude points.  nlon determines
c            the grid increment in longitude as 2*pi/nlon. for example
c            nlon = 72 for a five degree grid. nlon must be greater
c            than or equal to 4. the efficiency of the computation is
c            improved when nlon is a product of small prime numbers.
c
c
c     h      a two dimensional array that contains the discrete
c            function to be displayed. h(i,j) is the distance from the
c            center of the sphere to the surface at the colatitude 
c            point theta(i) = (i-1)*pi/(nlat-1) and longitude point 
c            phi(j) = (j-1)*2*pi/nlon.
c
c     len    the first dimension of the array h as it appears in the
c            program that calls visequ.
c
c     eyer   the distance from the center of the sphere to the eye.
c
c     eyelat the colatitudinal coordinate of the eye (in degrees).
c
c     eyelon the longitudinal  coordinate of the eye (in degrees).
c
c     wk     a real work array 
c
c     lwk    the dimension of the array wk as it appears in the
c            program that calls visequ. lwk must be at least 
c                       46*nlat*(nlon+1).
c
c     iwk    an integer work array
c
c     liwk   the dimension of the array iwk as it appears in the
c            program that calls visequ. liwk must be at least 
c                       14*nlat*(nlon+1).
c
c     ierror = 0    no error
c            = 1    the eye is positioned inside the sphere
c            = 2    lwk  is less than 46*nlat*(nlon+1)
c            = 3    liwk is less than 14*nlat*(nlon+1)
c
c