I use Tracker for my engineering teaching and I need to have a coordinate system with a mobile origin and a mobile axes. The mouvement of solids are simple when we observe them from a near solid (generally, two solids in contacts) but the second one must be in mouvement to.

The first point is simple : uncheck Fixed Origin in the Coordinate menu and choose a point in the Reference Frame menu.
I try to solve the second point : uncheck Fixed Angle in the Coordinate menu, but no option appears to choose 2 points or a tape measure for example.

Somebody helps me about the option Fixed angle ?
Thanks