remap to part of message

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Hi. I have written my first node - a Quaternion to euler conversion node (using the tf library). The node subscribes to geometry_msgs::Quaternion and publishes geometry_msgs::Vector3.

I now want to connect this node to the quaternion part of a pose message that's published by another node.

So i used this command:

rosrun mypkg quat_to_euler_conv quaternion:=ground_truth/pose/orientation

because pose/orientation is of type geometry_msgs::Quaternion i think this should work. but it doesn't connect. using rostopic echo and pub i can make the node work fine though.

Is it maybe not possible to remap a topic to a message part of another topic?