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entry variable: $fthresh -- overlay hard threshold
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default: 0.3

(1) threshold simple amplitude statistic

An overlay amplitude statistic below this value
is thresholded -- i.e., the background gray
surface color is displayed.

Use "fmid" to achieve a smoother roll-off into
the background color.  The units are the same as
those of fmid.  The rapidity of the fmid roll-off
is controlled by "fslope".  Setting a large
fslope makes fmid behave like fthresh.

(2) amplitude masked by second statistic

If, in addition to an amplitude, a statistical
mask has also been read in with the "msk{n,s}:"
field (or csurf VOLUME-STATS with Data Type set
to "real+Fstat" or "real+tstat"), then a more
complex masking takes place.  The masking can be
toggled on and off with the unmarked checkbutton
next to "msk{n,s}:" ($overlaymaskflag).

When masking is toggled on, 3 extra entries
appear -- one extra beside fthresh, fmid, and
fslope (truncating the labels to "th", "md", and
"sl").

In this case, the amplitude is subject to *two*
sigmoidal masks.   The first entry in each pair
behaves identically to "fthresh", "fmid" and
"fslope", rolling off the amplitude using the
amplitude itself.

The second entry in each pair masks the amplitude
by an independent sigmoid that uses the value of
the mask statistic to roll off the amplitude.

Bugs

The BRy color scale misbehaves if you set "fmid"
(soft thresh) lower than "fthresh" (hard thresh).
