Curvature
set point.kMin and point.kMax: minimum and maximum curvature vectors at point.
do this numerically by binary subdivision in a circle in the plane perp. to surface normal.
SegmentConverge
Suppose you get a third point within the segment of convergence.
If the three points are approx. collinear, then use regula falsi;
otherwise, assume the function is complex enough to warrant fitting the three
points with a quadratic and solving for its roots