@ampa said:
http://libspiro.sourceforge.net/
LibSpiro is an Open Source implimentation of the Spiro algorithm. It should be possible to see how it all works from the example / source code.
Also... http://www.levien.com/spiro/ is the site of the inventor.
I had a quick look, but it is very complex to understand, and I guess to reproduce. And the theory underneath is quite abstruse, based on clothoids (curves to make highway junctions), holonomic curves, and other nice mathematical theory, which however were all set out by mathematians from the 18th century (bernouilli, Euler, Fresnel, and others).
So if someone wants to give it a try....and then publish it as an extension of BezierSpline!!
As a consolation, I just realize that the Courbette allows to draw an arc of circle between 2 points which also includes a 3rd point.