image_sub.py

# Basics ROS 2 program to subscribe to real-time streaming 
# video from your built-in webcam
# Author:
# - Addison Sears-Collins
# - https://automaticaddison.com
  
# Import the necessary libraries
import rclpy # Python library for ROS 2
from rclpy.node import Node # Handles the creation of nodes
from rclpy import qos
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
import sys

class ImageSubscriber(Node):
  """
  Create an ImageSubscriber class, which is a subclass of the Node class.
  """
  def __init__(self, ros_topic_name):
    """
    Class constructor to set up the node
    """
    # Initiate the Node class's constructor and give it a name
    super().__init__('image_subscriber')
      
    # Create the subscriber. This subscriber will receive an Image
    # from the video_frames topic. The queue size is 10 messages.
    self.subscription = self.create_subscription(
      Image, 
      ros_topic_name, 
      self.listener_callback, 
      qos.qos_profile_sensor_data)
    self.subscription # prevent unused variable warning

    self.get_logger().info('Subscribed on ' + ros_topic_name)

    # Used to convert between ROS and OpenCV images
    self.br = CvBridge()
   
  def listener_callback(self, data):
    """
    Callback function.
    """
    # Display the message on the console
    self.get_logger().info('Receiving video frame')
 
    # Convert ROS Image message to OpenCV image
    current_frame = self.br.imgmsg_to_cv2(data)
    
    # Display image
    cv2.imshow("image", current_frame)
    
    cv2.waitKey(1)
  
def main():
  
  args = sys.argv

  if (len(args) < 2):
      print("Commandline : {0} ros_topic_name".format(args[0]))
      return

  # Initialize the rclpy library
  rclpy.init()
  
  # Create the node
  image_subscriber = ImageSubscriber(args[1])
  
  # Spin the node so the callback function is called.
  rclpy.spin(image_subscriber)
  
  # Destroy the node explicitly
  # (optional - otherwise it will be done automatically
  # when the garbage collector destroys the node object)
  image_subscriber.destroy_node()
  
  # Shutdown the ROS client library for Python
  rclpy.shutdown()
  
if __name__ == '__main__':
  main()