i never get an eye on that … i’m sure genete / zelgadis could answer you .
maybe you could found some info there ( warning hard math included ! 
)
- wiki.synfig.org/Dev:Linking_to_B … _Equations
- github.com/synfig/synfig/blob/m … _shape.cpp
- genete dev stuff
if you find something interesting , please share your result … we need to document this kind of “vital” informations