Image Publishers and Subscribers in ROS2

The goal of this repository is to introduce students to image publishers and subscribers using ROS2 and OpenCV.

0 CMakeList.txt and package.xml

When we build and install a package in ROS2, we need to give the compiler some information regarding names and libraries. This guide explains how to use the CMakeLists.txt and package.xml files to configure and build a package in ROS2.

0.1 package.xml

The package.xml file contains package metadata and dependency declarations.

• Key Sections:
  • <name>: Defines the package name (robovision_images).
  • <version>: Specifies the package version.
  • <maintainer>: Provides the maintainer and contact email.
  • <license>: Placeholder for the license.
  • <buildtool_depend>: Tools for building the package, such as ament_cmake and ament_cmake_python.
  • <depend>: Runtime dependencies like rclcpp, cv_bridge, sensor_msgs, and std_msgs.
  • <test_depend>: Dependencies for testing, such as ament_lint_auto.

0.2 CMakeLists.txt

The CMakeLists.txt file defines how the package is built.

• Key Components:
  • Specifies the minimum CMake version and compiler options.
  • Locates dependencies (rclcpp, cv_bridge, OpenCV, etc.).
  • Declares include directories for OpenCV and cv_bridge.
  • Adds executables for C++ nodes like my_publisher and my_camera_publisher and links their dependencies.
  • Installs Python scripts and C++ executables to the appropriate directories.
  • Integrates the package with ROS2 using ament_package().

0.3 Example: robovision_images

0.3.1 package.xml

We declare the name of our project

<name>robovision_images</name>

We are using both C++ and Python code in our project, and therefore, we need to declare it:

<buildtool_depend>ament_cmake</buildtool_depend>
<buildtool_depend>ament_cmake_python</buildtool_depend>

<depend>rclcpp</depend>
<depend>rclpy</depend>

Additionally, we need to add all the ros-based dependencies we will use in our project

<depend>cv_bridge</depend>
<depend>sensor_msgs</depend>
<depend>std_msgs</depend>

0.3.2 CMakeList.txt

First, we declare the name of our project

project(robovision_images)

Similarly, we need to declare that we are using both C++ and Python code

find_package(ament_cmake REQUIRED)
find_package(ament_cmake_python REQUIRED)
find_package(rclcpp REQUIRED)

and the dependencies we are using

find_package(OpenCV REQUIRED)
find_package(cv_bridge REQUIRED)
find_package(sensor_msgs REQUIRED)
find_package(std_msgs REQUIRED)

To build a project, we need to specify which files we want to compile and how we are going to refer them to our package. For C++ files, we declare them as in the example here

add_executable(my_publisher src/my_publisher.cpp)
ament_target_dependencies(my_publisher 
    rclcpp 
    cv_bridge 
    sensor_msgs 
    OpenCV
)

and for Python, we use

ament_python_install_package(${PROJECT_NAME})

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