I have solved my BSPline-segmentation problem and thought that maybe someone else would be interested in general, how !
Imagine that you have a curve (BSPlineCurve or what ever) and you are interested for a specific segment from it confinded by two points.You seek the parameter u1 and u2 for both points, because you calculate the segment with them via Geom_BSPlineCurve::Segment(Standard_Real u1, Standard_Real u2).
Now just cut (see GeomAPI_IntCS)the Curve with a BSPlineSurface created by the GeomFill-package, that contains the interesting point and get the Parameter via GeomAPI_IntCS::Parameters();
An unstable alternative would be Geom_ProjectPointOnCurve, but I do not recommend that ! It is not working properly.
I hope this may help someone out in the world struggling with comparable questions like me.
Besides, how can one calculate a normal-vector in an arbitrary 3-dimensional (not 2) point (gp_Pnt or whatever) ?
You first want to calculate the tangent ? How, using D0-methods or what ?????
But it must be very precise, others it is better to calculate it by hand.
Hints are always welcome.