snarlly

Say curve C is interpolated quadraticly by (from random sets of (x,y)
in space)

parabola A that concave up for x= a to b
parabola B that concave down for x = b to c

The two parabolas join smoothly at b with continuous first degree
derivative.

At b, C changes concavity, which suddenly causes the tangent vectors
to be 'mirrored' against the curve. (Graphicly, instead of being
always above parabola A, now the tangents are always below parabola B)

This causes problem for me because in 3d, the Normal calculated from
the Tangent suddenly become mirrored against the curve. (from the TNB
coordinate's view, everything is now upside down after passing the
change in concavity).

Basicly I need an interpolation method that prevents the TNB system
from these sudden flips. Thanks.