Create ROS2 launch package for subscribing/publishing image topic

0. Package Structure

image_subscriber/
├── launch/
│ └── image_subscriber_launch.py
├── config/
│ └── rviz_image_subscriber.rviz
│ └── params.yaml
├── src/
│ └── image_subscriber.cpp
├── CMakeLists.txt
├── package.xml

1. Install dependencies

$ sudo apt update
$ sudo apt install ros-humble-image-transport

2. Create a package

$ cd ~/robot_ws/src
$ ros2 pkg create --build-type ament_cmake image_subscriber --dependencies rclcpp sensor_msgs cv_bridge

3. C++ source code (src/image_subscriber.cpp)

#include <rclcpp/rclcpp.hpp>
#include <cv_bridge/cv_bridge.h>
#include <opencv2/opencv.hpp>
#include <opencv2/highgui/highgui.hpp>
#include <image_transport/image_transport.hpp>
#include <sensor_msgs/msg/image.hpp>

rclcpp::Publisher<sensor_msgs::msg::Image>::SharedPtr pub_img;

double canny_threshold1 = 100.0;
double canny_threshold2 = 200.0;


void image_callback(const sensor_msgs::msg::Image::ConstSharedPtr image)
{
    RCLCPP_INFO(
        rclcpp::get_logger("image_subscribe_node"),
        "Subscribed image! Canny thresholds: threshold1 = %.2f, threshold2 = %.2f",
        canny_threshold1, canny_threshold2
    );

    // ROS → OpenCV
    cv::Mat image_opencv = cv_bridge::toCvShare(image, "bgr8")->image;

    /* Applying OpenCV algorithm */
    cv::Mat image_gray;
    cv::cvtColor(image_opencv, image_gray, cv::COLOR_BGR2GRAY);
    cv::Mat edges;
    cv::Canny(image_gray, edges, canny_threshold1, canny_threshold2);


    // OpenCV → ROS
    auto image_msg = cv_bridge::CvImage(image->header, "mono8", edges).toImageMsg();

    // Publish
    pub_img->publish(*image_msg);
}

int main(int argc, char **argv)
{
    rclcpp::init(argc, argv);
    auto node = rclcpp::Node::make_shared("image_subscribe_node");

    // Declare parameters with defaults
    node->declare_parameter<double>("canny_threshold1", 100.0);
    node->declare_parameter<double>("canny_threshold2", 200.0);

    // Get params
    node->get_parameter("canny_threshold1", canny_threshold1);
    node->get_parameter("canny_threshold2", canny_threshold2);

    // Publisher
    pub_img = node->create_publisher<sensor_msgs::msg::Image>("/camera_array/left/image_output", 10);

    // Subscriber
    auto sub_img = node->create_subscription<sensor_msgs::msg::Image>(
        "/camera_array/left/image_raw", 10, image_callback);

    rclcpp::spin(node);
    rclcpp::shutdown();
    return 0;
}

4. CMakeLists.txt

cmake_minimum_required(VERSION 3.8)
project(image_subscriber)

find_package(ament_cmake REQUIRED)
find_package(rclcpp REQUIRED)
find_package(sensor_msgs REQUIRED)
find_package(image_transport REQUIRED)
find_package(cv_bridge REQUIRED)
find_package(OpenCV REQUIRED)


add_executable(image_subscriber_node src/image_subscriber.cpp)
ament_target_dependencies(image_subscriber_node rclcpp sensor_msgs cv_bridge image_transport)

target_include_directories(image_subscriber_node PRIVATE ${OpenCV_INCLUDE_DIRS})
target_link_libraries(image_subscriber_node ${OpenCV_LIBS})

install(TARGETS
  image_subscriber_node
  DESTINATION lib/${PROJECT_NAME}
)

install(DIRECTORY launch
  DESTINATION share/${PROJECT_NAME}/
)

ament_package()

5. rviz configuration

locate ‘rviz_image_subscriber.rviz’ in ‘config’ folder

6. params.yaml file

image_subscriber:
  ros__parameters:
    canny_threshold1: 50.0
    canny_threshold2: 300.0

7. launch file

from launch import LaunchDescription
from launch_ros.actions import Node
import os


def generate_launch_description():

    pkg_name = 'image_subscriber'
    pkg_dir = os.popen('/bin/bash -c "source /usr/share/colcon_cd/function/colcon_cd.sh && \
        colcon_cd %s && pwd"' % pkg_name).read().strip()
    
    return LaunchDescription([
        Node(
            package='image_subscriber',
            executable='image_subscriber_node',
            name='image_subscriber',
            output='screen',
            parameters= [os.path.join(pkg_dir, 'config', 'params.yaml')]
        ),

        Node(
            package='rviz2',
            namespace='',
            executable='rviz2',
            name='rviz2',
            arguments=['-d', [os.path.join(pkg_dir, 'config', 'rviz_image_subscriber.rviz')]]
        )
    ])

8. Run

$ colcon build --packages-select image_subscriber
$ source install/setup.bash
$ ros2 launch image_subscriber image_subscriber_launch.py

9. Play ROS Bag File

$ ros2 bag play {bagfile}

10. Results

Read canny edge detection parameters from ‘config/params.yaml’ and publish image topic

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