MAKE_BACK

        MAKE_BACK

    Computes  the  background  image  of   an   input    image    using    a
    thresholding  and  a smoothing algorithm based on the conjugate gradient
    method.  The background image is computed as follows
      -  First  the   image    is    thresholded    between    THRESHOLD$[1]
        and THRESHOLD$[2].  All pixels outside this range are blanked.
      - Then the image is smoothed using  the  conjugate  gradient   method,
        with NITER$ iterations and a smoothing parameter P$.  Blanked pixels
        are thus interpolated in the process.

    The output image is the  background   image.    P$   should   be   large
    enough  to  yield  some smoothing (1 to 100, try...), and NITER$ also to
    assure convergence  especially  in  "blanked"  areas  (NITER$  =  20  is
    typical).