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entry variable: $fslope -- overlay contrast
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default: 1.5

Controls steepness of contrast sigmoid (or double
sigmoids with two-ended/signed color scales).
Smaller values give a more linear relation
between statistic and color saturation.  At large
values (>10), a hard threshold is approached.  At
statistic values below fmid, overlay colors fade
smoothly into light and dark background grays.

If an additional masking statistic has been read
in ($overlaymaskflag is set), two fslope entries
will appear (re-labeled "sl").  The first is the
same as before (affects abruptness of
thresholding of amplitude), and the second
affects abruptness of thresholding using mask
statistic units.
