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entry variable: $forcemode -- normal img force eq. in shrink()
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default: 0: force = mstrength*tanh((val - mmid)*mslope);

This integer parameter (F3 panel, lower right) controls
the equation used to calculate the surface-normal image
force in shrink().  The current options are:

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

 1:  force = mstrength* tanh((inval - mmid)*mslope);

 2:  force = mstrength*(tanh((inval - mmid)*mslope)
      - ptanh((outval - mmid)*mslope));

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

The 0-255 brightness of the image voxel (val) is
nearest-neighbor sampled at the vertex location
(val), at a distance $cthk inward along the
normal (inval), and/or at a distance $ostilt
outward along the normal (outval).

The other adjustable C/tcl parameters are:

  $mstrength -- strength of normal force
  $mslope -- steepness of ramp part of tanh sigmoid
  $mmid -- zero point of normal force
  $whitemid -- (forcemode=3) zero point inward sample
  $graymid -- (forcemode=3) zero point vertex sample

The tksurfer.c function, ptanh(), truncates
negative outputs of tanh() to 0.0
