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variable: $wn -- normal force shrink,sphere_morph,etc
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default: 0.5

The floating point parameter, $wn ("weight of
normal"), (F3 panel, lower left) controls the
strength of the vertex-normal 'force' in
shrink(), area_shrink(), and sphere_morph().

The first effect of the normal force is to reduce
curvature.  Since the surface is typically
smoothly convex, this will also gradually reduce
its area.

If $MRIflag is set to 1 (and a 3D data set is
loaded), the normal force will also vary as a
function of local image brightness ($forcemode
provides several different functions).
