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[Bruno Chareyre <bruno.chareyre@xxxxxxxxxxx>]

> how to compute twist and bending (i.e. relative rotation) increment between
> two spheres?

Maybe using an equation similar to the total formulation (ScGeom.cpp:94) but with spin
instead of rotations?
It would give :

relativeSpin = angularVelocity2 - angularVelocity1
twistVel = relativeSpin.dot(normal)
bendVel = relativeSpin - twistVel
and then you multiply by dt.

Alternatively, you could do this at each timestep :
ScGeom6D::precomputeRotations()
ScGeom6D::initRotations()

then it would use orientations of the previous timestep instead of initial orientations,
and precomputed angles would be increments. It would really give increments, not vel*dt.

Bruno