Documentation on gazebo_msgs/ContactsState

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Hi,

I am using the gazebo_ros_bumper and I don't understand how the gazebo_msgs/ContactsState (definition) message is structured. The message contains a "states" array (which usually has size 20 for me). The documentation says that it is an "array of geom pairs in contact", so I guess these are all the contacts that occur between two objects.

If we go one level lower each state is of type gazebo_msgs/ContactState (definition). Each state message then contains an array of wrenches, contact positions and contact normals. Why? Each contact should only contain one contact wrench, position and normal, right?

Any clarifications are greatly appreciated. Alex