How to calculate the difference between two geometry_msgs::Pose

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See the tutorial on calculating a relative rotation between quaternions. After you have the difference as a quaternion, you can convert it to roll/pitch/yaw. For Python, use quaternion_to_euler() for that. Finally, it's no problem to convert between tf and geometry_msg quaternions. The tutorial also has some examples of that.

Please add to the tutorial if it's not explicit enough. For example, I see that it shows how to convert tf->geometry_msgs but not the other way around.