# How to retrieve the quaternion in (x,y,z,w) format from tf

When we use tf to retrieve the rotation relationship of two coordinate in quaternion form, according to the API of Quaternion, we can only get the rotation axis and its angle.

However, the commonly required quaternion in ROS, such as quaternion in geometry_msgs::Pose, is always in (x,y,z,w) format. Therefore, is there any function to get the quaternion from tf in (x,y,z,w) form? That will be very convienent.