Image topic subscription in ROS - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to subscribe to an image topic named '/camera/image_raw'.

ROS

image_sub = rospy.Subscriber('[1]', Image, image_callback)

Drag options to blanks, or click blank then click option'

A/camera/depth

B/odom

C/camera/image_raw

D/scan

Attempts:

3 left

💡 Hint

Common Mistakes

Using a non-image topic like '/scan' or '/odom'.

Missing the leading slash '/' in the topic name.

The topic name '/camera/image_raw' is the standard topic for raw camera images in ROS.

2fill in blank

medium

Complete the code to import the correct message type for image data.

ROS

from sensor_msgs.msg import [1]

Drag options to blanks, or click blank then click option'

AImage

BLaserScan

COdometry

DPointCloud2

Attempts:

3 left

💡 Hint

Common Mistakes

Importing 'LaserScan' instead of 'Image'.

Forgetting to import the message type.

The 'Image' message type from 'sensor_msgs.msg' is used for image data in ROS.

3fill in blank

hard

Fix the error in the callback function definition to accept the image message.

ROS

def image_callback([1]):
    rospy.loginfo('Received image')

Drag options to blanks, or click blank then click option'

Aimage

Bself

Cdata

Dmsg

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'self' in a non-class function.

Not defining any parameter in the callback.

The callback function must accept the message parameter, commonly named 'msg'.

4fill in blank

hard

Fill both blanks to initialize the ROS node and start listening to the image topic.

ROS

rospy.[1]('image_listener')
rospy.[2]()

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Ainit_node

Bspin

Csleep

Dshutdown

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'sleep' instead of 'spin' to keep the node alive.

Forgetting to initialize the node before subscribing.

Use 'init_node' to start the node and 'spin' to keep it running and processing callbacks.

5fill in blank

hard

Fill all three blanks to convert the ROS image message to OpenCV format using cv_bridge.

ROS

bridge = CvBridge()
cv_image = bridge.[1](msg, desired_encoding=[2])
cv2.imshow('Image', cv_image)
cv2.[3](1)

Drag options to blanks, or click blank then click option'

Aimgmsg_to_cv2

B'bgr8'

CwaitKey

DcvtColor

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'cvtColor' instead of 'imgmsg_to_cv2' for conversion.

Omitting 'waitKey' causing the image window to close immediately.

Use 'imgmsg_to_cv2' to convert the ROS image message, specify 'bgr8' encoding for color images, and 'waitKey' to display the image window properly.

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