Image topic subscription in ROS - Step-by-Step Execution

Concept Flow - Image topic subscription

Start Node

↓

Initialize ROS Subscriber

↓

Wait for Image Messages

↓

Receive Image Message

↓

Callback Function Executes

↓

Process Image Data

↓

Loop: Wait for Next Image

The node starts, subscribes to an image topic, waits for messages, and processes each image in a callback.

Execution Sample

ROS

import rclpy
from sensor_msgs.msg import Image

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

rclpy.init()
node = rclpy.create_node('image_listener')
sub = node.create_subscription(Image, 'camera/image', image_callback, 10)
rclpy.spin(node)
node.destroy_node()
rclpy.shutdown()

This code subscribes to the 'camera/image' topic and prints the height of each received image.

Execution Table

Step	Action	Message Received	Callback Called	Output
1	Node starts and subscriber created	No	No	No output
2	First image message arrives	Yes	Yes	Prints: Received image with height: 480
3	Second image message arrives	Yes	Yes	Prints: Received image with height: 480
4	No new messages	No	No	Waiting for messages
5	Node shutdown	No	No	Node stops

💡 Node stops when shutdown is called or program ends.

Variable Tracker

Variable	Start	After Step 2	After Step 3	Final
msg.height	N/A	480	480	N/A
Callback Called	False	True	True	False

Key Moments - 2 Insights

Why does the callback function run only when a message arrives?

Because the subscriber waits for messages and calls the callback only when a new image message is received, as shown in steps 2 and 3 of the execution table.

What happens if no image messages are published to the topic?

The node waits idly without calling the callback or printing anything, as shown in step 4 where no messages arrive.

Visual Quiz - 3 Questions

Look at the execution table, what is printed when the first image message arrives?

A No output

B Received image with height: 480

C Error: No image

D Node started

💡 Hint

Check row 2 under the Output column in the execution table.

At which step does the callback function NOT run?

A Step 2

B Step 3

C Step 4

D Step 1

💡 Hint

Look for 'Callback Called' = No in the execution table.

If the image height changes to 720, how would the output change at step 2?

A Prints: Received image with height: 720

B Prints: Received image with height: 480

C No output

D Error message

💡 Hint

Refer to variable_tracker for msg.height values and output in execution_table step 2.

Concept Snapshot

ROS Image Topic Subscription:
- Create a node and subscribe to an Image topic.
- Provide a callback function to process incoming images.
- Callback runs only when a new image message arrives.
- Use rclpy.spin(node) to keep node alive and listening.
- Process image data inside the callback.

Full Transcript

This example shows how a ROS node subscribes to an image topic named 'camera/image'. The node initializes and creates a subscriber that listens for Image messages. When an image message arrives, the callback function runs and prints the image height. If no messages arrive, the node waits silently. The process repeats for each new image until the node shuts down.

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