Image topic subscription in ROS - Mini Project: Build & Apply

Image topic subscription

📖 Scenario: You are building a simple ROS node that listens to an image stream from a camera. This is common in robotics where you want to process or display images coming from a robot's camera.

🎯 Goal: Create a ROS node that subscribes to the /camera/image_raw topic and processes incoming images.

📋 What You'll Learn

Create a ROS node named image_listener

Subscribe to the /camera/image_raw topic with message type sensor_msgs/Image

Implement a callback function named image_callback that receives the image messages

Print a message inside the callback confirming receipt of an image

💡 Why This Matters

🌍 Real World

Robots often use cameras to see their environment. Subscribing to image topics lets software process these images for tasks like navigation or object detection.

💼 Career

Understanding how to subscribe to sensor data topics in ROS is essential for robotics software engineers and developers working with robot perception.

1

Create the ROS node and import necessary modules

Write code to import rclpy and sensor_msgs.msg.Image. Then create a ROS node named image_listener using rclpy.init() and rclpy.create_node('image_listener').

ROS

# Import rclpy and sensor_msgs.msg.Image
# Initialize ROS and create node named 'image_listener'
# Your code here

Hint

Use import rclpy and from sensor_msgs.msg import Image. Then call rclpy.init() and create a node with Node('image_listener').

2

Define the image callback function

Define a function named image_callback that takes one parameter msg. Inside the function, write a line that prints "Received an image message".

ROS

import rclpy
from rclpy.node import Node
from sensor_msgs.msg import Image

rclpy.init()
node = Node('image_listener')

# Define image_callback function
# Your code here

Hint

Define def image_callback(msg): and inside it write print("Received an image message").

3

Subscribe to the /camera/image_raw topic

Use the node.create_subscription method to subscribe to the /camera/image_raw topic. Use Image as the message type, image_callback as the callback function, and 10 as the queue size.

ROS

import rclpy
from rclpy.node import Node
from sensor_msgs.msg import Image

rclpy.init()
node = Node('image_listener')

def image_callback(msg):
    print("Received an image message")

# Subscribe to /camera/image_raw topic
# Your code here

Hint

Call node.create_subscription with Image, '/camera/image_raw', image_callback, and 10.

4

Spin the node to process callbacks

Add code to spin the node using rclpy.spin(node) so it keeps running and processing incoming image messages.

ROS

import rclpy
from rclpy.node import Node
from sensor_msgs.msg import Image

rclpy.init()
node = Node('image_listener')

def image_callback(msg):
    print("Received an image message")

subscription = node.create_subscription(Image, '/camera/image_raw', image_callback, 10)

# Spin the node to keep it alive
# Your code here

Hint

Use rclpy.spin(node) to keep the node running and processing callbacks.

Practice

1. What is the main purpose of subscribing to an image topic in ROS?

easy

A. To save images directly to disk without processing

B. To send commands to the robot's motors

C. To create new image topics automatically

D. To receive live camera data for processing or display

Solution

1. Step 1: Understand ROS image topics

   Image topics in ROS provide live camera data streams from sensors or cameras.

2. Step 2: Purpose of subscribing

   Subscribing allows a node to receive this live data to process or display images.

3. Final Answer:

   To receive live camera data for processing or display -> Option D

4. Quick Check:

   Image subscription = receive live camera data [OK]

Hint: Image topics deliver live camera data to subscribers [OK]

Common Mistakes:

• Confusing subscription with publishing
• Thinking subscription sends commands
• Assuming images are saved automatically

2. Which of the following is the correct way to create a ROS subscriber for an image topic named /camera/image_raw in Python?

easy

A. rospy.Subscriber('/camera/image_raw', Image, image_callback)

B. rospy.Publisher('/camera/image_raw', Image, image_callback)

C. rospy.Subscriber('/camera/image_raw', String, image_callback)

D. rospy.Subscriber('/camera/image_raw', Image)

Solution

1. Step 1: Identify subscriber syntax

   In ROS Python, a subscriber is created with rospy.Subscriber(topic, message_type, callback).

2. Step 2: Match correct message type and parameters

   The image topic uses sensor_msgs/Image, so message type is Image, and callback is required.

3. Final Answer:

   rospy.Subscriber('/camera/image_raw', Image, image_callback) -> Option A

4. Quick Check:

   Subscriber needs topic, message type, callback [OK]

Hint: Subscriber needs topic, message type, and callback function [OK]

Common Mistakes:

• Using Publisher instead of Subscriber
• Wrong message type like String instead of Image
• Omitting the callback function

3. Given this ROS Python callback code snippet, what will be printed when an image message is received?

def image_callback(msg):
    print(f"Received image with height: {msg.height} and width: {msg.width}")

medium

A. Received image with height: 0 and width: 0

B. Received image with height: [actual height] and width: [actual width]

C. Error: msg has no attribute height

D. No output because callback is not called

Solution

1. Step 1: Understand the callback parameter

   The callback receives a sensor_msgs/Image message with attributes height and width representing image size.

2. Step 2: Analyze the print statement

   The print uses f-string to output the actual height and width values from the message.

3. Final Answer:

   Received image with height: [actual height] and width: [actual width] -> Option B

4. Quick Check:

   Image message attributes printed correctly [OK]

Hint: Image message has height and width attributes [OK]

Common Mistakes:

• Assuming height and width are zero by default
• Confusing message type without these attributes
• Thinking callback is not triggered

4. What is the error in this ROS Python subscriber code snippet?

import rospy
from sensor_msgs.msg import Image

def callback(image):
    print("Image received")

rospy.Subscriber('/camera/image_raw', Image)
rospy.spin()

medium

A. Missing rospy.init_node() before Subscriber

B. Incorrect import of Image message

C. Missing callback function argument in Subscriber

D. Using rospy.spin() without Subscriber

Solution

1. Step 1: Check Subscriber syntax

   rospy.Subscriber requires three arguments: topic, message type, and callback function.

2. Step 2: Identify missing callback

   The code calls Subscriber without the callback argument, causing an error.

3. Final Answer:

   Missing callback function argument in Subscriber -> Option C

4. Quick Check:

   Subscriber needs callback argument [OK]

Hint: Subscriber must include callback function [OK]

Common Mistakes:

• Forgetting to pass callback to Subscriber
• Not calling rospy.init_node() before Subscriber
• Confusing import statements

5. You want to convert a ROS image message to an OpenCV image inside a callback using cv_bridge. Which code snippet correctly performs this conversion?

hard

A. bridge = CvBridge()
cv_image = bridge.imgmsg_to_cv2(msg, desired_encoding='bgr8')

B. cv_image = CvBridge.imgmsg_to_cv2(msg, 'mono8')

C. bridge = CvBridge()
cv_image = bridge.cv2_to_imgmsg(msg, 'bgr8')

D. cv_image = cv2.imread(msg)

Solution

1. Step 1: Understand cv_bridge usage

   cv_bridge provides a CvBridge class instance to convert ROS images to OpenCV format using imgmsg_to_cv2.

2. Step 2: Identify correct method and parameters

   Create a CvBridge object, then call imgmsg_to_cv2 with the message and encoding like 'bgr8'.

3. Final Answer:

   bridge = CvBridge() cv_image = bridge.imgmsg_to_cv2(msg, desired_encoding='bgr8') -> Option A

4. Quick Check:

   CvBridge instance + imgmsg_to_cv2 with encoding [OK]

Hint: Use CvBridge instance and imgmsg_to_cv2 for conversion [OK]

Common Mistakes:

• Calling imgmsg_to_cv2 as class method without instance
• Using cv2_to_imgmsg instead of imgmsg_to_cv2
• Trying to read ROS message directly with cv2.imread