Issues in Working with quaternion [closed]

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I am new to ROS and its application in robot. I have just executed the Openni skeleton tracker. I would like to know (A) how do we transform each joint’s rotation matrix so that the rotation is given with respect to the person’s torso so as to more compactly represent the joint’s orientation. (B) from

rotation.GetQuaternion(qx, qy, qz, qw);

how do I record the values of the quaternion for each joint?

(C) What is the difference between joint angles and quaternions that also represent joint angles? Thank you for kind help