Ubuntu Linux for Robotics · Lesson 68 of 96

OpenCV on Ubuntu

Installing OpenCV, cv_bridge, building C++ and Python examples.

Estimated time · 45–75 min

Learning goals

What you will understand

Explain opencv on ubuntu 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

OpenCV is the standard computer-vision library. Install it from apt (libopencv-dev for C++, python3-opencv for Python) rather than building it yourself, and use cv_bridge (ros-lyrical-cv-bridge) to convert between ROS Image messages and OpenCV images. In CMake use find_package(OpenCV REQUIRED).

Check the version with pkg-config --modversion opencv4 or python3 -c 'import cv2; print(cv2.__version__)'. Convert with an explicit encoding (bgr8 for colour, mono8 for grey) and avoid unnecessary copies. Mixing a pip-installed opencv-python with the apt version is a common cause of import errors: use one.

Guided lesson

Work through these steps

1.
Open a terminal and record the current state before changing anything.
2.
Read the opencv on ubuntu 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 libopencv-dev python3-opencv ros-lyrical-cv-bridge
# Install OpenCV for C++ and Python plus the ROS bridge.
pkg-config --modversion opencv4
# Print the C++ library version.
python3 -c "import cv2; print(cv2.__version__)"
# Print the Python module version.
find_package(OpenCV REQUIRED)
# CMakeLists.txt: locate OpenCV.
target_link_libraries(vision_node ${OpenCV_LIBS})
# CMakeLists.txt: link it to the node.

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: print the OpenCV version known to pkg-config for opencv4

Expected output

4.x.x

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

pkg-config --modversion opencv4

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

Install OpenCV, check both versions, then write a small node that subscribes to an image topic, converts it with cv_bridge to grey, and republishes it.

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.