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    Computes the background image of a given map.  The basic idea is to make
    a  crude  mesh  model of the background by finding the most likely value
    of  the  original map (within some intensity range) for each cell of the
    crude  mesh.  The most likely value is found by making an  histogram  of
    the  intensity  distribution  in the  specified  range  for  all  points
    lying  in  a circle around the cell center.  The circle may (and should)
    be greater than the cell size.  If  the  number  of  pixels  within  the
    intensity range is too small, no value is attributed to the intermediate

    The  intermediate  model  is then resampled to the original map  using a
    general   triangulation   technique.   This  procedure  has  several ad-
    vantages  over  a simpler method which would compute a "smoothed"  image
    as  the background, because it is not biased by any emission outside the
    selected range, and it is able to interpolate  over  large  non  sampled
    area of the intermediate mesh.  Other tasks with similar names use other
    interpolation algorithms.  They will hopefully be merged into  a  single
    task one (with a switch for the interpolation algorithm).

    CAUTION:  In the present version, blanking in the  input  image  is  not
    correctly  handled.  This can usually be worked around by specifiying an
    apropriate range.

Gildas manager 2020-12-02