hartley_image

  HARTLEY_IMAGE - A Hartley image function
   IM = HARTLEY_IMAGE(KX, KY, M)

   Returns the Hartley image defined by the equation

   H(kx,ky) = cas(2*pi*(kx*l+ky*m)/M)
    where l and m run from 0 to M-1.

   From Ringach et al., 1997