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Bone Rotations to Joint Angles

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I would like to know what would be the best approach to converting bone rotations (quaternions) to joint angles (ideally a single angle value based on the axis of rotation for the joint rather than euler angles).


Currently, I have data from a Quest 2 app that has several bone rotation quaternions which I assume each correspond to the joints in this diagram 


To get the joint angle, I follow the euler axis representation of a quaternion

and simply use this calculation: joint angle = 2*acos(real or scalar part of bone rotation quaternion) to get a joint angle from a bone rotation.


I would like to know if 1) I'm interpreting bone rotations and how they are organized properly 2) if the calculation makes sense 3) if there's any conversion that's available that can be used. Thanks!