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tickbox var: $chkselfintflag -- consider self-intersect
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default: 0 (FALSE)

The "ckself" tickbox (F3 panel, lower middle)
toggles whether a self interect volume is
consulted during shrink operations.

If ticked, the shrink() function updates the
self-intersect volume at the top of the loop over
vertices.  The normal force is then zeroed if the
vertex is inside the self-intersect volume.

The update of the self-intersect volume can be
refined by adjusting the value in the "selfd:"
entry, controls the how far to move along the
surface normal from a vertex before marking the
self-intersection volume.
