Topic remapping in ROS - Step-by-Step Execution

Concept Flow - Topic remapping

Start Node with Default Topic

↓

Apply Remapping Rules

↓

Node Publishes/Subscribes on Remapped Topic

↓

Communication Happens on New Topic Name

↓

End

The node starts with its default topic names, then remapping rules change these names before communication begins, so the node uses the new topic names.

Execution Sample

ROS

ros2 run demo_nodes_cpp talker --ros-args -r chatter:=my_topic
# Node publishes on 'my_topic' instead of 'chatter'

This command runs a talker node that normally publishes on 'chatter', but remaps it to 'my_topic' so messages go on the new topic.

Execution Table

Step	Default Topic	Remapping Rule	Effective Topic	Action
1	chatter	chatter:=my_topic	my_topic	Node prepares to publish on 'my_topic'
2	chatter	none	my_topic	Node publishes message on 'my_topic'
3	listener subscribes to 'chatter'	chatter:=my_topic	my_topic	Listener receives messages from 'my_topic'
4	none	none	none	Communication established on remapped topic

💡 Remapping applied, node uses 'my_topic' instead of 'chatter' for communication

Variable Tracker

Variable	Start	After Remapping	Final
topic_name	chatter	my_topic	my_topic

Key Moments - 2 Insights

Why does the node publish on 'my_topic' instead of 'chatter'?

Because the remapping rule 'chatter:=my_topic' changes the topic name before the node starts publishing, as shown in execution_table step 1.

Does remapping change the node's internal code?

No, remapping only changes the topic names at runtime without modifying the node's source code, as seen in the variable_tracker where only the topic_name changes.

Visual Quiz - 3 Questions

Look at the execution table, what is the effective topic name at step 2?

A listener

B chatter

C my_topic

D none

💡 Hint

Check the 'Effective Topic' column in execution_table row for step 2.

At which step does the listener subscribe to the remapped topic?

A Step 1

B Step 3

C Step 2

D Step 4

💡 Hint

Look for the row mentioning listener subscription in execution_table.

If the remapping rule was removed, what would the final topic_name be in variable_tracker?

A chatter

B my_topic

C listener

D none

💡 Hint

Without remapping, the topic_name stays as the default in variable_tracker start value.

Concept Snapshot

Topic remapping in ROS lets you change topic names at runtime.
Use the syntax '-r old_name:=new_name' when launching nodes.
This does not change node code, only topic names.
Remapping helps connect nodes flexibly.
Communication happens on remapped topic names.

Full Transcript

Topic remapping in ROS allows nodes to communicate on different topic names than those hardcoded in their source. When a node starts, remapping rules are applied to its default topic names. For example, a talker node that normally publishes on 'chatter' can be remapped to publish on 'my_topic' by using the command line argument '-r chatter:=my_topic'. This changes the topic name before the node begins publishing. The listener node can also remap its subscription to the same new topic name to receive messages correctly. Remapping does not modify the node's internal code; it only changes the topic names used at runtime. This flexibility helps connect nodes in different configurations without changing their source code.

Practice

1. What is the main purpose of topic remapping in ROS?

easy

A. To create new topics dynamically during runtime

B. To change the message type of a topic

C. To stop a node from publishing messages

D. To change topic names without modifying the node's source code

Solution

1. Step 1: Understand topic remapping concept

   Topic remapping allows changing the topic names nodes subscribe or publish to without changing the node's code.

2. Step 2: Identify the correct purpose

   Changing message types or stopping nodes are unrelated to remapping. Creating new topics dynamically is not the main purpose.

3. Final Answer:

   To change topic names without modifying the node's source code -> Option D

4. Quick Check:

   Topic remapping = rename topics without code change [OK]

Hint: Remapping means renaming topics without touching code [OK]

Common Mistakes:

• Thinking remapping changes message types
• Confusing remapping with stopping nodes
• Assuming remapping creates new topics automatically

2. Which of the following is the correct syntax to remap a topic when launching a ROS node from the command line?

easy

A. rosrun package node _old_topic:=new_topic

B. rosrun package node __old_topic:=new_topic

C. rosrun package node old_topic:=new_topic

D. rosrun package node _old_topic=new_topic

Solution

1. Step 1: Recall ROS remapping syntax

   ROS uses the syntax old_topic:=new_topic to remap topics on the command line.

2. Step 2: Identify correct option

   rosrun package node old_topic:=new_topic matches the correct syntax without extra underscores or wrong symbols.

3. Final Answer:

   rosrun package node old_topic:=new_topic -> Option C

4. Quick Check:

   Remap syntax = old_topic:=new_topic [OK]

Hint: Use := between old and new topic names for remapping [OK]

Common Mistakes:

• Adding underscores before topic names
• Using = instead of :=
• Confusing remapping syntax with parameters

3. Given the launch command:
rosrun turtlesim turtlesim_node turtle1/cmd_vel:=robot1/cmd_vel
What will happen when the node publishes velocity commands to turtle1/cmd_vel?

medium

A. The commands will be sent to robot1/cmd_vel topic instead

B. The commands will be ignored because the topic name changed

C. The node will publish to both turtle1/cmd_vel and robot1/cmd_vel

D. The node will fail to start due to invalid remapping

Solution

1. Step 1: Understand remapping effect on publishing

   Remapping turtle1/cmd_vel to robot1/cmd_vel means the node publishes to robot1/cmd_vel instead of the original topic.

2. Step 2: Analyze options

   Commands are not ignored or duplicated; remapping redirects the topic. The node starts normally with valid remapping.

3. Final Answer:

   The commands will be sent to robot1/cmd_vel topic instead -> Option A

4. Quick Check:

   Remapping redirects publishing topic [OK]

Hint: Remapping changes topic destination for publishing [OK]

Common Mistakes:

• Thinking remapping duplicates topics
• Assuming remapping disables publishing
• Believing remapping causes node failure

4. You tried to remap a topic using the command:
rosrun my_package my_node /input:=/sensor_data
but the node still subscribes to /input instead of /sensor_data. What is the likely error?

medium

A. Using a leading slash in the remapping name causes remapping to fail

B. The remapping syntax is incorrect; it should be input:=sensor_data without slashes

C. The node does not support topic remapping

D. The ROS master is not running

Solution

1. Step 1: Check remapping syntax rules

   Remapping uses relative topic names without leading slashes; slashes can cause remapping to fail.

2. Step 2: Identify correct syntax

   Correct syntax is input:=sensor_data without slashes. Leading slashes make remapping ineffective.

3. Final Answer:

   The remapping syntax is incorrect; it should be input:=sensor_data without slashes -> Option B

4. Quick Check:

   Use relative names without slashes for remapping [OK]

Hint: Avoid leading slashes in remapping topic names [OK]

Common Mistakes:

• Using absolute topic names with slashes in remapping
• Assuming node does not support remapping without checking
• Ignoring ROS master status

5. You have two nodes: nodeA publishes to /camera/image_raw and nodeB subscribes to /image. You want nodeB to receive images from nodeA without changing their code. How can you achieve this using topic remapping in a launch file?

hard

A. inside nodeB's launch tag

B. inside nodeA's launch tag

C. inside nodeB's launch tag

D. inside nodeA's launch tag

Solution

1. Step 1: Understand remapping direction

   NodeB subscribes to /image, but nodeA publishes /camera/image_raw. To connect them, remap nodeB's /image to /camera/image_raw.

2. Step 2: Apply remapping in nodeB's launch tag

   Use <remap from="/image" to="/camera/image_raw"/> inside nodeB's launch tag to redirect subscriptions.

3. Final Answer:

   <remap from="/image" to="/camera/image_raw"/> inside nodeB's launch tag -> Option A

4. Quick Check:

   Remap subscriber topic to publisher's topic [OK]

Hint: Remap subscriber's topic to publisher's topic in launch file [OK]

Common Mistakes:

• Remapping publisher's topic instead of subscriber's
• Swapping from and to attributes
• Changing code instead of using remapping