I want to find the "distance" between two quaternions. By "distance" I mean a single float or int, not another quaternion (that would be the difference, i.e. inverse(q1)*q2). I guess you could call what I want "angular magnitude".
I need to apply more torque to a physics object the further it's rotated from its original angle.
I don't understand the maths involved in quaternions, so a code-based example would be most helpful. I've looked at several other questions but I don't believe any answer my question, or at least not in a way I understand it.
Find the difference quaternion qd = inverse(q1)*q2).
Than find the angle between q1 and q2 by angle = 2 * atan2(qd.vec().length(), qd.w()) // NOTE: signed
The "angle" here, is the angle of rotation from q1 to q2 by shortest arc.
Also you can use this lib function from pyquaternion.
Quaternion.absolute_distance(q0, q1)
来源：https://stackoverflow.com/questions/23260939/distance-or-angular-magnitude-between-two-quaternions