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

The "fthresh:" entry sets the vertex-wise overlay
value (or complex amplitude value) below which
the background gray surface color is displayed.
Use an equivalent fmid to achieve a smoother
roll-off into the background color.

For most color scales, the units of fthresh are
the same as those of fmid.

When "mask" is checked, the data is subject to
two different thresholds -- the amplitude data
(in .val[.val2]) compared to $fthresh, but also
the stat data (in .stat) compared to $sfthresh.

For complex-valued data, the stat field and the
real and imaginary data fields can be manually
loaded from the tksurfer tools interface as follows
(this is usu. done by csurf):

  1) type name stat wfile in "val:" field
  2) "R" button (on "val:" line)
  3) optional SMOOTH (w/val ticked)
  4) "S/V" (swap .stat and .val)
  5) type name real-valued wfile in "val:" field
  6) "R" (auto-reads matching imaginary pair of real wfile)
  7) opt. SMOOTH (auto-detects complex-val'd, does both)
   
or from a tcl script as follows:

  ### read/smooth/load stat
  set val /some/stat/wfile
  read_binary_values
  smooth_val <steps>
  swap_stat_val
  ### read/smooth/push imaginary
  set val /some/imaginary/wfile
  read_binary_values
  smooth_val <steps>
  push_val_val2
  ### read/smooth real
  set val /some/real/wfile
  read_binary_values
  smooth_val <steps>
  ### show
  redraw
