--bg-subsample 12 blows up previously invisible spots to become annoying.

[rmast]

When I try to lower the footprint of the irrelevant background picture with --bg-subsample 12 the previously invisible spots are blown up in a way they become annoying.

Reproduction steps just the same as in #16, except for the --bg-subsample 12 option:

When I encode the included Scantailored image with this statement in the standard didjvu (master from jwilk) with Python2.7 and
http://nl.archive.ubuntu.com/ubuntu/pool/universe/g/gamera/python-gamera_3.4.2+git20160808.1725654-2_amd64.deb in Mint 20.2

./didjvu encode ~/scantailorin/out/outputbase2-000-raar-effect-onderste-regel-didjvu\ zonder\ tekst.tif -o jaarverslagraar.djvu --bg-subsample 12
outputbase2-000-raar-effect-onderste-regel-didjvu zonder tekst.zip

Result

jaarverslagvlekken.zip

[rmast]

As this issue shows the background picture is quite clear at the default of --bg-subsample 3, via c44, I've replayed the scenario's for --bg-subsample 1..12. It appears all values except 3, 4 and 5 leave annoying spots on different places:
--bg-subsample 1:

--bg-subsample 2:

--bg-subsample 3:

--bg-subsample 4:

--bg-subsample 5:

--bg-subsample 6:

--bg-subsample 7:

--bg-subsample 8:

--bg-subsample 9:

--bg-subsample 10:

--bg-subsample 11:

--bg-subsample 12: