Hello, I am also fighting hard to grasp the concepts of quaternions. Could you explain how to get back the Pich, Roll and Yaw values from a quaternion? I understand I should come back to the axis-angle representation, but how to extract the roll value in radians for example? Raph -- View this message in context: http://ros-users.122217.n3.nabble.com/tf-and-quaternions-tp2206128p2228313.html Sent from the ROS-Users mailing list archive at Nabble.com.