Ubuntu Linux for Robotics · Lesson 69 of 96

Image transport

Compressed images, republishing and measuring bandwidth on the network.

Estimated time · 30–45 min

Learning goals

What you will understand

Explain image transport in your own words
Use it in a small workstation example
Verify the result instead of assuming it worked

Understand the idea first

The mental model

image_transport lets image topics be sent in several formats. A raw 640×480 colour stream at 30 Hz is around 27 MB/s, too much for Wi-Fi; a compressed version is a small fraction of that. Install the plugins (ros-lyrical-image-transport-plugins) and use the republish node to convert between formats.

Measure before and after with ros2 topic bw. Subscribe to the compressed topic on the operator laptop and keep raw images on the robot only. Compression adds CPU load on the robot, so check htop.

Guided lesson

Work through these steps

1.
Open a terminal and record the current state before changing anything.
2.
Read the image transport commands and predict what each one does.
3.
Run the commands one at a time and compare each output with your prediction.
4.
Write the commands that worked in a setup note you can repeat later.

Worked example

Read it before you run it

sudo apt install ros-lyrical-image-transport-plugins
# Install the compressed and theora plugins.
ros2 run image_transport list_transports
# List the transports that are available.
ros2 run image_transport republish raw compressed --ros-args -r in:=/image_raw -r out/compressed:=/image_raw/compressed
# Publish a compressed copy.
ros2 topic bw /image_raw
# Bandwidth of the raw stream.
ros2 topic bw /image_raw/compressed
# Bandwidth of the compressed stream.

What the example is doing

1.Lines without sudo only inspect or change files you own; sudo authorizes a system-wide change.
2.Read command output before moving to the next line. Stop if names, versions, disks, or paths differ from the lesson.
3.Comments beginning with # explain safety checks and are not executed by the shell.

Watch it run · Sensor view

See what this code does

The terminal types this lesson's practice solution and runs it. The scene shows the kind of robot behaviour that code drives. It plays by itself and loops.

Practice code

Now type it yourself

Open a terminal, run the starter line by line (or save it as practice.sh and run bash practice.sh), and replace every TODO with your own command.

# TODO: measure the bandwidth of the compressed image topic /image_raw/compressed

Expected output

The compressed stream uses far less bandwidth

Stuck? Show the full solution (try it yourself first)

ros2 topic bw /image_raw/compressed

Expected result

How you know it worked

Each command completes without an unexplained error.
Your notes contain the command, the important output, and what it proves.

Practical exercise

Apply the lesson

Compare the bandwidth of the raw and compressed streams of a camera, compute the compression ratio, and check the CPU cost of compression with htop.

Before you continue

You recorded the starting state
Every command and output is understood
The change can be repeated from your notes

If something goes wrong

A command says permission denied.

Check ownership and permissions first. Do not add sudo unless the command changes a documented system location.

A command or package is not found.

Check spelling, Ubuntu version, enabled repositories, and whether the previous installation command succeeded.