1 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).
2 P$
        TASK\REAL "Smoothing parameter" P$

    Smoothing parameter for the background.  Use 1 to 100
2 NITER$
        TASK\INTEGER "Number of smoothing iterations" NITER$

    NITER$ must be large  enough   to   interpolate   values   properly   in
    blanked areas.  Use 10 to 30.
2 IN$
        TASK\CHARACTER "Input file name" IN$

    Input image (not modified)
2 OUT$
        TASK\CHARACTER "Output file name" OUT$

    Output background image name (created)
2 THRESHOLD$
        TASK\REAL "Low, high threshold, and guess" THRESHOLD$[3]

    Three values indicating the lowest  significant  background  value,  the
    highest,   and   an   initial  guess  for blanked pixels.  THRESHOLD$[3]
    should usually be either equal to  THRESHOLD$[2]  or  to  THRESHOLD$[1],
    depending whether the signal is positive or negative...
1 END