Changing this threshholds changes how aggressively dbblast removes
dust bunnies when surrounded by a dark colored background. For
example, if you find that dbblast doesn't remove bunnies from dark
blue sky, try lowering this threshhold to 90 or even lower.
Image areas that have a low range of colors tend to have less fine
detail that areas that have higher ranges. Since dbblast tends to
smudge areas with lots of fine details, we can usually get by without
blasting bunnies in areas with fine details especially since the
details tend to hide the dust bunnies anyway.
Raising this threshhold value will make dbblast more aggressively remove
bunnies in areas with fine details.