module henon

  implicit none
  contains

!-----------------------------------------------------

subroutine compute_pixel_henon(a, b, maxpasse, passe, limit, rx, ry)
  implicit none
  double precision, intent(in)         :: a, b, limit
  integer, intent(in)                  :: maxpasse
  integer, intent(out)                 :: passe
  double precision, intent(out)        :: rx, ry

  double precision                     :: x, y, x2, y2

  write(0, fmt="('compute pixel:', (2F8.3, I6, F8.3))")       &
                              a, b, maxpasse, limit

  x = 0.0
  y = 0.0

  do passe=1, maxpasse

    x2 = 1d0 + y - (a * x * x)
    y2 = b * x
    x = x2
    y = y2
    write(0, fmt="(i4, 2F8.3)") passe, x, y
    if (x .lt. -limit) exit
    if (x .gt.  limit) exit
    if (y .lt. -limit) exit
    if (y .gt.  limit) exit

  enddo

  rx = x
  ry = y

end subroutine

!-----------------------------------------------------

end module