About the approximation I have read this here on wikipedia. but maybe I've haven't read all the paragraph, only the first part. https://en.wikipedia.org/wiki/Non-uniform_rational_B-spline#Example:_a_circle.




About the program can do that, If I remember well Solidedge I'm sure NX and the others and can insert a dynamic radius on a curve and also FreeCad, and I'm pretty sure Autocad but I don't remember well if was it or an other at my previous job

These image attached is a SubD cylinder with imported in Moi and then exported in FreeCAD and I was warned about the curve isn't circle and the measure can be irregular







And this image is on FreeCad and show the curvature normal on B-spline and is possible to get the visualization of this on Moi3d maybe with a script?