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entry variable: $fslope -- statistical contrast
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default: 1.5
keyboard shortcut:
  Cmd-s,S  =>  adjust fslope 0.5 up/down

The "fslope:" entry controls steepness of the
saturation sigmoid (or double sigmoids with
two-ended/signed color scales), whose midpoint is
set by $fmid.

Small values of $fslope result in a nearly linear
relation between statistic and color saturation
while large values result in a step function
cutoff at the value of $fmid (i.e., same as a
hard threshold).

N.B.: The meaning of "small" and "large" above
depends on the range of the data.

For Fourier-analyzed, complex-valued data such as
the sqrt(F) found in the {_r,_i} files, the data
will range from 0-10.  In this case, an $fslope
of 1.5 will provide a smooth roll-off, while an
$fslope of 10 will result in a hard threshold.

For Fourier-analyzed, complex-valued data such as
the raw Fourier amplitude found in the {_x,_y}
files, the data will typically range from 0-1000.
In this case, an $fslope of 0.01 will provide a
smooth roll-off, while an $fslope of 0.1 will
result in a hard threshold.

The equation mapping data (0-whatever)to color
saturation (0-1) is:

  saturation = (1 + tanh(fslope*(data - fmid)))/2;

Thus, in general, $fslope should be inversely
related to the data range to obtain the same
visual effect.
