Image topic subscription in ROS - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What happens when subscribing to a ROS image topic without a callback?

Consider a ROS node subscribing to an image topic but without defining a callback function. What will be the behavior of this node?

ROS

import rospy
from sensor_msgs.msg import Image

def main():
    rospy.init_node('image_listener')
    rospy.Subscriber('/camera/image_raw', Image, None)
    rospy.spin()

if __name__ == '__main__':
    main()

A The node will subscribe but never process any images because no callback is defined.

B The node will throw a runtime error because the callback cannot be None.

C The node will automatically print image data to the console.

D The node will unsubscribe immediately after starting.

💡 Hint

Think about what a callback function does in a subscription.

In ROS, a subscriber needs a callback function to process incoming messages. If None is passed, the subscriber is created but no processing happens.

📝 Syntax

intermediate

Which code snippet correctly subscribes to a ROS image topic and converts the message to OpenCV format?

Select the code that correctly subscribes to '/camera/image_raw' and converts the ROS Image message to an OpenCV image using cv_bridge.

A

import rospy
from sensor_msgs.msg import Image
from cv_bridge import CvBridge
bridge = CvBridge()
def callback(msg):
    cv_image = bridge.imgmsg_to_cv2(msg, 'mono8')
rospy.Subscriber('/camera/image_raw', Image, callback)

B

import rospy
from sensor_msgs.msg import Image
from cv_bridge import CvBridge
bridge = CvBridge()
def callback(msg):
    cv_image = bridge.cv2_to_imgmsg(msg, 'bgr8')
rospy.Subscriber('/camera/image_raw', Image, callback)

C

import rospy
from sensor_msgs.msg import Image
from cv_bridge import CvBridge
bridge = CvBridge()
def callback(msg):
    cv_image = bridge.imgmsg_to_cv2(msg)
rospy.Subscriber('/camera/image_raw', Image, callback)

D

import rospy
from sensor_msgs.msg import Image
from cv_bridge import CvBridge
bridge = CvBridge()
def callback(msg):
    cv_image = bridge.imgmsg_to_cv2(msg, 'bgr8')
rospy.Subscriber('/camera/image_raw', Image, callback)

💡 Hint

Check the correct function name and encoding for color images.

The correct function to convert ROS Image to OpenCV is imgmsg_to_cv2. For color images, 'bgr8' encoding is standard.

❓ state_output

advanced

What is the value of 'image_received' after receiving one image message in this ROS subscriber?

Given the following code, what will be the value of the variable 'image_received' after the first image message is processed?

ROS

import rospy
from sensor_msgs.msg import Image

image_received = False
def callback(msg):
    global image_received
    image_received = True

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

A False

B True

C None

D Raises a NameError

💡 Hint

Consider what happens when the callback is called and the global keyword is used.

The callback sets the global variable image_received to True when an image message arrives.

🔧 Debug

advanced

Why does this ROS image subscriber node fail to process images?

Examine the code and identify why the subscriber does not process any images.

ROS

import rospy
from sensor_msgs.msg import Image
from cv_bridge import CvBridge

bridge = CvBridge()

def callback():
    print('Image received')

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

A The callback function lacks the required message parameter.

B The CvBridge instance is not used inside the callback.

C The subscriber topic name is incorrect.

D The node is missing rospy.spin() to keep running.

💡 Hint

Check the callback function signature for ROS subscribers.

ROS subscriber callbacks must accept the message parameter. Without it, the callback will not be called properly.

🧠 Conceptual

expert

What is the main advantage of using image_transport for subscribing to image topics in ROS?

Why would a ROS developer choose to use the image_transport package instead of a standard subscriber for image topics?

A It allows subscribing to multiple topics simultaneously with one subscriber.

B It converts images to OpenCV format without extra code.

C It automatically handles image compression and decompression, optimizing bandwidth usage.

D It provides built-in image filtering and enhancement.

💡 Hint

Think about network efficiency when transmitting images.

image_transport manages compressed image streams transparently, reducing bandwidth and improving performance.

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