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variable: $cthk -- inward normal sample position
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default: 1.0

This floating point parameter (F3 panel, lower
right) controls the inward distance to move along
the vertex normal before sampling the 'inval' 
from the "im:" dataset in shrink().

This is relevant for forcemode=1-3 (ignored in
forcemode=0).

For example, in forcemode=1, the normal image
force is calculated as:

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

Another C/tcl variable, not currently exposed in
the interface (so script-only usage), is $ostilt
(default=1.0).

This is the 'outward along the normal' equivalent
of $cthk.  It is used to find 'outval', which is
only used in forcemode=2 as follows:

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

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