Well, in the beginning of the course, I told you that nodes can interact with other nodes, irrespective of the package they are in (as long as you publish and subscribe to a topic).

So, I prefer to create a new package for my called opencv_tools that will contain my OpenCV nodes. Nothing new, we have already created a package earlies so this part should not bother you much.

FAQ! Instead of creating an entirely new package, can I just create another node in my existing package to undergo OpenCV applications? Yes, of course you can! The whole point of creating a separate package is to make sure we can use this later in our other projects.

Create a pkg

cd ~/ros2_ws/src

Then, we will create a package named opencv_tools but you may name it anything.

ros2 pkg create --build-type ament_python opencv_tools --dependencies rclpy image_transport cv_bridge sensor_msgs std_msgs opencv2

Now, we will build this package using colcon

cd ~/ros2_ws/
colcon build

Image Publisher Node

In order to access images from the webcam, we need to initialize publisher/subscriber nodes which publish/subscribe image messages to a common topic. Confusing? I hope not. Because we have done it before. Remember Talker/Listener? It is exactly same, but instead of sending/receiving msg of type ‘str’ we use something else. Still confused? Don’t worry, just tag along and you will get it fully.

cd ~/ros2_ws/src/opencv_tools/opencv_tools
gedit basic_image_publisher.py

In this basic_image_publisher.py file, let’s import necessary libraries.

import rclpy # Python Client Library for ROS 2
from rclpy.node import Node # Handles the creation of nodes
from sensor_msgs.msg import Image # Image is the message type
from cv_bridge import CvBridge # Package to convert between ROS and OpenCV Images
import cv2 # OpenCV library

Note: You may need to install the libraries that aren’t already installed on your Ubuntu. You may look into that specific library’s documentation to do so.

After that, let’s create a ImagePublisher class, which is a subclass of the Node class.

class ImagePublisher(Node):
  def __init__(self):
    # Initiate the Node class's constructor and give it a name
    super().__init__('image_publisher')

__init__ is the Class constructor to set up the node.

class ImagePublisher(Node):
  def __init__(self):
    super().__init__('image_publisher')
    
    # Create the publisher. This publisher will publish an Image
    # to the video_frames topic. The queue size is 10 messages.
    self.publisher_ = self.create_publisher(Image, 'video_frames', 10)