program grabuge
  use genplotting
  implicit none

  call genp_init (0)
  call do_surface (51)
  call genp_end (0)

contains
! ---------------------------------------------------------
real function displacement(x, y, a)
  real, intent(in)        :: x, y
  integer, intent(in)     :: a

  displacement = cos(((5.8+x+y)*(y+x))/real(a))

end function
! ---------------------------------------------------------
subroutine do_surface (amp)
  integer, intent(in)     :: amp
  integer                 :: ix, iy, color
  real                    :: rx, ry, dy, disp
  logical                 :: startline

  do iy= -amp, amp
    ry = real(iy)
    startline = .true.
    do ix = -(amp-1), amp
      rx = real(ix)
      disp = displacement (rx, ry, amp)
      dy = ry + disp
      if (disp .lt. 0.0) then
        color = 6
      else
        color = 3
      endif
      if (startline) then
        call genp_move(real(-amp), dy)
        startline = .false.
      else
        call genp_draw(rx, dy, color)
      endif

    enddo
  enddo

end subroutine
! ---------------------------------------------------------
end program