-------------------------------------------
entry variable: $graymid -- MRI force zeropoint
-------------------------------------------
default: 30.0

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)
at which no normal force will be applied to the
moveable surface vertex during shrink().

The default value (30.0) 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.

In forcemode=3, this force is calculated as
follows:

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

Thus, this is a forcemode=3 synonym for
forcemode=0 $mmid.
