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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 back-
    ground image is computed as follows
      -  First  the  image  is thresholded between THRESHOLD$[1] and THRESH-
        OLD$[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 con-
    vergence especially in "blanked" areas (NITER$ = 20 is typical).


Gildas manager 2020-07-04