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entry variable: $mmid -- MRI force zeropoint
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default: 45.0

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

The default value (45.0) causes the moveable
surface to move toward a point near the pial
surface, where a standard 'T1-weighted' image
transitions to the nearly zero (black) CSF.

This variable is consulted in forcemode=0/1/2

For example, in forcemode=0, the normal force
is calculated as:

   force = mstrength* tanh((val - mmid)*mslope);

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