Topic Remapping¶

A node isn't really much more than an executable file from a package, and topic remapping isn't much more than simply changing the names of node properties. In our Lesson 2: Starting Nodes, we've learned how to reassign the name of a node. In this tutorial, we're going to remap topics with command-line interface.

Usage:

 ros2 run [package_name] [node_name] --ros-args --remap [from]:=[to]

Example-1: Connecting Joy and Turtlesim¶

NOTE: If you don't have a joystick, you could skip this example and jump to Example-2 and Example-3.

Example-1 Prerequisites¶

For this tutorial we will usethe joy package. Acquire a joystick and ensure the joy and teleop_twist_joy package are installed:

 sudo apt-get install ros-jazzy-joy
 sudo apt-get install ros-jazzy-teleop-twist-joy

Read about the "joy" package: ROS2 Joy Package and ROS Driver for Generic Linux Joysticks.

joy outputs the messages of type sensor_msgs/msg/Joy. turtlesim requires the input messages of type geometry_msgs/msg/Twist. In order for joy to send messages to turtlesim we need to be able to convert messages of type sensor_msgs/msg/Joy to geometry_msgs/msg/Twist.

Let's take a look more in depth at the differences between the two message types. Try:

 ros2 interface show sensor_msgs/msg/Joy

# Reports the state of a joystick's axes and buttons.

# The timestamp is the time at which data is received from the joystick.
std_msgs/Header header
    builtin_interfaces/Time stamp
        int32 sec
        uint32 nanosec
    string frame_id

# The axes measurements from a joystick.
float32[] axes

# The buttons measurements from a joystick.
int32[] buttons
 ```

And try:
``` bash
 ros2 interface show geometry_msgs/msg/Twist

# This expresses velocity in free space broken into its linear and angular parts.

Vector3  linear
    float64 x
    float64 y
    float64 z
Vector3  angular
    float64 x
    float64 y
    float64 z

As you can see, the contents (as well as type) of these two messages are fundamentally different, hence we need a node that can convert one to the other.

The node we will use to convert sensor_msgs/msg/Joy messages to geometry_msgs/msg/Twist messages is teleop_twist_joy.

In different terminals we run:

 ros2 run turtlesim turtlesim_node

 ros2 run joy joy_node

 ros2 run teleop_twist_joy teleop_node

Now try using the joystick to move the turtle around. You'll notice that it doesn't work. Let's investigate this problem using rqt:

rqt

You should see something similar to this: ROS2 joy rqt 1

As seen above, all three nodes are running, but teleop_twist_joy_node is not communicating with turtlesim. Let's look at what topic teleop_twist_joy_node is publishing on and turtlesim is subscribing to.

Let's start with teleop_twist_joy_node:

 ros2 node info /teleop_twist_joy_node

/teleop_twist_joy_node
  Subscribers:
    /joy: sensor_msgs/msg/Joy
    /parameter_events: rcl_interfaces/msg/ParameterEvent
  Publishers:
    /cmd_vel: geometry_msgs/msg/Twist
    /parameter_events: rcl_interfaces/msg/ParameterEvent
    /rosout: rcl_interfaces/msg/Log
  Service Servers:
    /teleop_twist_joy_node/describe_parameters: rcl_interfaces/srv/DescribeParameters
    /teleop_twist_joy_node/get_parameter_types: rcl_interfaces/srv/GetParameterTypes
    /teleop_twist_joy_node/get_parameters: rcl_interfaces/srv/GetParameters
    /teleop_twist_joy_node/get_type_description: type_description_interfaces/srv/GetTypeDescription
    /teleop_twist_joy_node/list_parameters: rcl_interfaces/srv/ListParameters
    /teleop_twist_joy_node/set_parameters: rcl_interfaces/srv/SetParameters
    /teleop_twist_joy_node/set_parameters_atomically: rcl_interfaces/srv/SetParametersAtomically
  Service Clients:

  Action Servers:

  Action Clients:seperately

It appears teleop_twist_joy_node is publishing on the /cmd_vel topic. Let's see what turtlesim is subscribing to:

 ros2 node info /turtlesim

/turtlesim
  Subscribers:
    /parameter_events: rcl_interfaces/msg/ParameterEvent
    /turtle1/cmd_vel: geometry_msgs/msg/Twist
  Publishers:
    /parameter_events: rcl_interfaces/msg/ParameterEvent
    /rosout: rcl_interfaces/msg/Log
    /turtle1/color_sensor: turtlesim/msg/Color
    /turtle1/pose: turtlesim/msg/Pose
  Service Servers:
    /clear: std_srvs/srv/Empty
    /kill: turtlesim/srv/Kill
    /reset: std_srvs/srv/Empty
    /spawn: turtlesim/srv/Spawn
    /turtle1/set_pen: turtlesim/srv/SetPen
    /turtle1/teleport_absolute: turtlesim/srv/TeleportAbsolute
    /turtle1/teleport_relative: turtlesim/srv/TeleportRelative
    /turtlesim/describe_parameters: rcl_interfaces/srv/DescribeParameters
    /turtlesim/get_parameter_types: rcl_interfaces/srv/GetParameterTypes
    /turtlesim/get_parameters: rcl_interfaces/srv/GetParameters
    /turtlesim/get_type_description: type_description_interfaces/srv/GetTypeDescription
    /turtlesim/list_parameters: rcl_interfaces/srv/ListParameters
    /turtlesim/set_parameters: rcl_interfaces/srv/SetParameters
    /turtlesim/set_parameters_atomically: rcl_interfaces/srv/SetParametersAtomically
  Service Clients:
seperatelysim/action/RotateAbsolute
  Action Clients:

Look at the "Subscriber" list. turtlesim is subscribing to /turtle1/cmd_vel instead of /cmd_vel. Let's shut the turtlesim and fix this problem by remapping the input of turtlesim to be /cmd_vel.

 ros2 run turtlesim turtlesim_node --ros-args --remap turtle1/cmd_vel:=cmd_vel

Now refresh your rqt. It should now look like: ROS2 joy rqt 2

If you hold down the activation button (for Xbox 360 controllers it's the A button) and move the joystick you should be able to control your turtle!

Example-1 Summary¶

The joy package is for interfacing generic joysticks to ROS2. We want to use a joystick to control the turtle in turtlesim. However, joy outputs messages of type sensor_msgs/msg/Joy, and turtlesim requires input messages of type geometry_msgs/msg/Twist.

Therefore, we need a special node teleop_twist_joy which is used for converting sensor_msgs/msg/Joy messages to geometry_msgs/msg/Twist.

The teleop_twist_joy_node is publishing to the topic /cmd_vel. However, turtlesim, by default, is subscribing to the topic /turtle1/cmd_vel. Therefore, we need to remap the input topic of turtlesim to be /cmd_vel instead of /turtle1/cmd_vel.

Example-2: Controling Two Turtles Separately¶

Run two turtlesim nodes and rename one of them as "turtlesim2":

 ros2 run turtlesim turtlesim_node

In a new terminal:

 ros2 run turtlesim turtlesim_node --ros-args --remap __node:=turtlesim2

In new terminals, run two teleop_turtle nodes:

 ros2 run turtlesim turtle_teleop_key

No matter which teleop_turtle node you're using, the two turtles always move the same way. ROS2 topic remapping example2-1

In a new terminal, use the rqt tool and we'll see why:

rqt

At this moment, both two turtlesim nodes are subscribing to the topic /turtle1/cmd_vel published by the teleop_turtle nodes. If we want to control two turtles separately, using two different teleop_turtle nodes, we need to remap the topics. Close one turtlesim node and one teleop_turtle node. Instead, we run:

 ros2 run turtlesim turtlesim_node --ros-args --remap __node:=turtlesim2 -r turtle1/cmd_vel:=turtle2/cmd_vel

In another terminal, run:

 roson__node:=telelop_turtle_2 -r turtle1/cmd_vel:=turtle2/cmd_vel

The turtlesim2 node is now subscribing to the topic /turtle2/cmd_vel and the teleop_turtle_2 node is publishing to this topic. Now you should be able to control the two turtles to do different movement. ROS2 topic remapping example2-2

In a new terminal, use the rqt tool and we'll see that the two turtlesim nodes are communicating with the teleop_turtle nodes through two different topics: /turtle1/cmd_vel and /turtle2/cmd_vel.

rqt

Example-2 Summary¶

We're trying to control two turtlesim nodes separately with two teleop_turtle nodes, so we need two separate topics /turtle1/cmd_vel and /turtle2/cmd_vel for the communication between two pairs of nodes.

The first turtlesim node subscribes to the topic /turtle1/cmd_vel, and the first teleop_turtle node publishes to this topic. The turtlesim2 node subscribes to the topic /turtle2/cmd_vel, and the teleop_turtle_2 node publishes to the same topic.

Example-3: Controlling Two Separate Turtles in One Window¶

If you'd love to try another example, you could go to the "5 Use rqt" and "6 Remapping" parts of the ROS2 official documentation "Using turtlesim, ros2, and rqt" and give a try to the /spawn service in the Service Caller of the rqt tool and practice on this remapping command:

 ros2 run turtlesim turtle_teleop_key --ros-args --remap turtle1/cmd_vel:=turtle2/cmd_vel