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variable: $mslope -- steepness normal img force sigmoid
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default: 0.05

This floating point parameter (F3 panel, lower
right) controls the steepness of the the normal
image force tanh sigmoid in shrink() and
shrink_to_fill().

For the default forcemode (0), the normal image
force is calculated as:

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

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