xfig has x-splines (cross splines), and I've always wondered why they never got more popular elsewhere. Basically, they have an extra parameter at each endpoint that allows you to smoothly pick between approximation, smooth interpolation, or sharp interpolation, as odd as that may sound:
Confusingly, there seem to be other splines called "X-splines" that appear to be completely unrelated, where the X seems to refer to the varying degree of the spline instead of just cubic.
http://wiki.povray.org/uploaded/5/5e/LeForgeronXSplines-10.1...
Confusingly, there seem to be other splines called "X-splines" that appear to be completely unrelated, where the X seems to refer to the varying degree of the spline instead of just cubic.