Image Subscriber Guide

This guide will walk you through the process of creating a subscriber node that subscribes to a the camera topic.

Import the Required Packages
Create a Subscriber Node
Create a Callback Function
Create the Node
Full Code

Import the Required Packages

First be sure to import the required packages.

import rospy
from sensor_msgs.msg import Image
import cv2
import numpy as np

Create a Subscriber Node

Now we need to create a subscriber node. This node will subscribe to the /camera/depth/image_raw topic and call the callback function whenever a new message on this topic is received.

image_sub = rospy.Subscriber('/camera/depth/image_raw', Image, callback)

You can change the topic to any other image topic that you want to subscribe to, for example /camera/color/image_raw.

Create a Callback Function

Now we need to create the callback function that will be called whenever a new message is received on the topic. This function will convert the image message to a NumPy array and display it using OpenCV.

The image message format is sensor_msgs/Image, details about this message can be found here.

We take the data field of the message and convert it to a NumPy array using np.frombuffer. Remember to choose the correct data type and number of channels for the image. For example, if the image is a color image, then the data type should be np.uint8 and number of channels ‘3’. If the image is a depth image, then the data type should be np.float32 and number of channels ‘1’.

def callback(msg):
	try:
		# Convert the image message to a NumPy array
		img_np = np.frombuffer(msg.data, dtype=np.float32).reshape(msg.height, msg.width, 1) # For depth image
	except Exception as e:
		rospy.logerr(e)
		return

	# Display the image using OpenCV
	cv2.imshow('Image', img_np)
	cv2.waitKey(1)

Create the Node

Finally we need to create the node and initialize it.

rospy.init_node('image_subscriber')

Don’t forget to add the following line to the end of the file to keep the node running.

rospy.spin()

Full Code

Here is the full code for a full subscriber node.

#!/usr/bin/env python3

import rospy
from sensor_msgs.msg import Image
import cv2
import numpy as np

class ImageSubscriber:
    def __init__(self):
        self.image_sub = rospy.Subscriber('/camera/depth/image_raw', Image, self.callback)

    def callback(self, msg):
        try:
            # Convert the image message to a NumPy array
            img_np = np.frombuffer(msg.data, dtype=np.float32).reshape(msg.height, msg.width, 1)
        except Exception as e:
            rospy.logerr(e)
            return

        # Display the image using OpenCV
        cv2.imshow('Image', img_np)
        cv2.waitKey(1)

if __name__ == '__main__':
    rospy.init_node('image_subscriber', anonymous=True)
    image_subscriber = ImageSubscriber()
    rospy.spin()