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entry variable: $whitemid -- MRI force zeropnt, inward sample
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default: 45.0

Only relevant for forcemode=3 (ignored by other
forcemode's).

This C/tcl variable (F3 panel, lower right), sets
the value of a loaded 3D image data ("im:"
entry), sampled at a distance of $cthk inward
along the local normal, at which no normal force
will be applied to the moveable surface vertex
during shrink().

This behaves like $mmid except for the sample point.

In forcemode=3, the normal force is calculated as
follows:

 force = 0.5*(mstrength*tanh((inval - whitemid)*mslope)
      + mstrength*tanh((val - graymid)*mslope));

See R-click help for $forcemode for more details.
