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variable: $wbn -- boundary normal force: shrink_2d()
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default: 0.5

The floating point parameter, $wbn ("weight of
boundary normal"), (was F3 panel, lower left, now
only available by tcl script) controls the
strength of the 'force' that moves vertices at a
cut edge of a flattened patch outward to prevent
them from curling under (compensates unbalanced
distribution of neighboring vertices at a cut
edge).

This is only relevant to shrink_2d() (SHRINK with
"2d" ticked).
