#### toggle sphere norm, linear/Gaussian ####
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tickbox variable (left-click):
  $sphnormflag -- toggle ramp vs. sph
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tickbox variable (middle-click):
  $gaussnormflag -- toggle linear vs. gauss
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The "sph" checkbutton (lower left on F3 panel)
toggles 1D vs. 3D ramp ($spherenormflag) and
linear versus Gaussian 3D ramp ($gaussnormflag).
This affects the operation done by the TEST and
APPLY buttons.

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Left-click (toggle $sphnormflag)
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A default left-click on the F3-panel-only "sph"
checkbutton toggles the state of manual
normalization done by the TEST and APPLY buttons
between:

  OFF=0:  a piecewise 4-control-point ramp along
  one major (sagittal/coronal/horizontal) axis

  ON=1:  a spherical field around the most recent
  click/cursor

In the first case, normalization is controlled by
4 control points (lim0-3, intrinsic coords) and 4
corresponding normalization factors (ffrac0-3,
where 1.0 is no change).

In the second case, the current cursor sets the
center of a spherical normalization field.  The
radius of the field is set by lim0 (directly to
the left of "lim0").  The shape of the field
(linear vs. Gaussian) is controlled by the next
flag ($gaussnormflag).  The default is linear
($gaussnormflag=0).

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Middle-click (toggle $gaussnormflag)
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A non-default middle-click on the F3-panel "sph"
checkbutton toggles the shape of the spherical
normalization field between:

  OFF=0:  linear ramp
  ON=1:  Gaussian ramp

Since $gaussnormflag has no effect unless
$sphnormflag is ON, turning it on will also
auto-set $sphnormflag to ON.

In both cases, the normalization factor at the
center of the field (most recent left-click), is
set by ffrac0.

When $gaussnormflag if OFF, the normalization
field is a linear ramp starting from no change at
a radius of $lim0 to a factor of $ffrac0 at the
center (a 3D "cone").

When $gaussnormflag is ON, the normalization
field is a gaussian ramp starting again from no
change at a radius of $lim0 to a factor of
$ffrac0 at the center.  The gaussian function
used is:

  existing *= 1.0 + (ffrac0-1.0)*exp(-(d/flim0*2.3)^2);

where d is the distance in pixels from the
center/cursor.  The factor of 2.3 results in the
Gaussian scaling reaching no change (in a 0-255
byte image) when d equals $ffrac0, which results
in a FWHM of 0.36198*ffrac0.
