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tickbox variable: $overlaymaskflag -- toggle mask
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keyboard equiv: <none>

The unmarked tickbox next to "mskn:" (or
"mskup:") toggles application of statistical mask
to a real or complex-valued amplitude overlay.

If ticked ($overlaymaskflag is 1), the fthresh,
fmid, and fslope entries are each doubled (and
their labels are abbreviated to "thr", "md", and
"sl").

The first entry in each pair controls the
threshold and contrast of the values themselves.
For example, if a value is below current
$fthresh, it will not be visible in the overlay.

The second set of entries in each pair controls
whether the values are masked based on what is
found in the currently read-in stat mask.  For
example, if a the value of of the .stat field for
this pixel is below the current $sfthresh, the
*value* will be masked.

If unticked, the interface toggles back to one
set of "fthresh", "fmid", and "fslope" entries
for controlling the overlay amplitude threshold
and contrast directly.

Loading Stat Mask

The stat mask field can be loaded with either
native (EPI) resolution stats or already
upsampled stats:

  (1) a native (EPI) resolution stat mask is
  read, registered (w/register.dat), upsampled
  to structural res with:

    setfile statpatt <statvolume>
    read_native_stat 2
    do_regupsamp_stat

  where statfile is either a C printf-style
  bfloat filename pattern, or a stat BRIK.

  (2) An already registered and upsampled BRIK is
  read with:

    read_regupsamp_stat <statbrik>
