turtlebot2-tutorials

OpenCV with ROS using Python

Using OpenCV with ROS is possible using the CvBridge library.
This tutorial will show you how to get a message from an Image topic in ROS, convert it to an OpenCV Image, and manipulate the image.

Prequisites

This example requires an image stream on the /camera/rgb/image_raw topic.

  1. On the turtlebot, run 3dsensor.launch:
    1. roslaunch turtlebot_bringup 3dsensor.launch

This section requires the catkin_ws to be initialized and the turtlebot_dabit package created.
Please click here to learn how to initialize the catkin workspace

Getting an Image from a ROS Topic using Python

The following example code can be used on either the master or turtlebot computers.

  1. Create a new Python file in your scripts folder in your workspace:
    1. mkdir -p ~/catkin_ws/src/turtlebot_dabit/scripts
    2. gedit ~/catkin_ws/src/turtlebot_dabit/scripts/rospy_example.py

  2. Start the script by specifying which Python version to use:

     #!/usr/bin/env python2.7

 # Print "Hello!" to terminal
 print "Hello!"
    • Test the script in your terminal:
      • python ~/catkin_ws/src/turtlebot_dabit/scripts/opencv_example_1.py

  3. Import the neccesary ROS and Computer Vision libraries and Initialize the ROS Node

     #!/usr/bin/env python2.7
 # Import ROS libraries and messages
 import rospy

 # Print "Hello!" to terminal
 print "Hello!"

 # Initialize the ROS Node named 'opencv_example', allow multiple nodes to be run with this name
 rospy.init_node('opencv_example', anonymous=True)

 # Print "Hello ROS!" to the Terminal and ROSLOG
 rospy.loginfo("Hello ROS!")
    • Test the script in your terminal:
      • python ~/catkin_ws/src/turtlebot_dabit/scripts/opencv_example_1.py

  4. Create a subscriber for an Image topic, and define a callback function
    Use CTRL+C to stop the program

     ...

 import rospy
 from sensor_msgs.msg import Image

 ...

 # Define a callback for the Image message
 def image_callback(img_msg):
     # log some info about the image topic
     rospy.loginfo(img_msg.header)

 # Initalize a subscriber to the "/camera/rgb/image_raw" topic with the function "image_callback" as a callback
 sub_image = rospy.Subscriber("/camera/rgb/image_raw", Image, image_callback)

 # Loop to keep the program from shutting down unless ROS is shut down, or CTRL+C is pressed
 while not rospy.is_shutdown():
     rospy.spin()
    • Test the script in your terminal:
      • python ~/catkin_ws/src/turtlebot_dabit/scripts/opencv_example_1.py

  5. Import OpenCV and cv_bridge, create a window to show a live image in
    Use CTRL+C in the terminal to stop the program

     ...
 # Import ROS libraries and messages
 ...

 # Import OpenCV libraries and tools
 import cv2
 from cv_bridge import CvBridge, CvBridgeError

 ...

 # Print "Hello ROS!" to the Terminal and to a ROS Log file located in ~/.ros/log/loghash/*.log
 rospy.loginfo("Hello ROS!")

 # Initialize the CvBridge class
 bridge = CvBridge()

 # Define a function to show the image in an OpenCV Window
 def show_image(img):
     cv2.imshow("Image Window", img)
     cv2.waitKey(3)

 # Define a callback for the Image message
 def image_callback(img_msg):
     # log some info about the image topic
     rospy.loginfo(img_msg.header)

     # Try to convert the ROS Image message to a CV2 Image
     try:
         cv_image = bridge.imgmsg_to_cv2(img_msg, "passthrough")
     except CvBridgeError, e:
         rospy.logerr("CvBridge Error: {0}".format(e))

     # Show the converted image
     show_image(cv_image)

 # Initalize a subscriber to the "/camera/rgb/image_raw" topic with the function "image_callback" as a callback
 sub_image = rospy.Subscriber("/camera/rgb/image_raw", Image, image_callback)

 # Initialize an OpenCV Window named "Image Window"
 cv2.namedWindow("Image Window", 1)

 # Loop to keep the program from shutting down unless ROS is shut down, or CTRL+C is pressed
 while not rospy.is_shutdown():
     rospy.spin()
    • Test the script in your terminal:
      • python ~/catkin_ws/src/turtlebot_dabit/scripts/opencv_example_1.py

  6. Rotate the image 90 degrees
    Use CTRL+C in the terminal to stop the program

     ...
 # Define a callback for the Image message
 define image_callback(img_msg):
     ...

     # Flip the image 90deg
     cv_image = cv2.transpose(cv_image)
     cv_image = cv2.flip(cv_image,1)

     # Show the converted image
     show_image(cv_image)

 ...
    • Test the script in your terminal:
      • python ~/catkin_ws/src/turtlebot_dabit/scripts/opencv_example_1.py
  7. Allow execution permissions for your code, and use rosrun to run it!
    In a new terminal:
    1. chmod +x ~/catkin_ws/src/turtlebot_dabit/scripts/opencv_example_1.py
    2. source ~/catkin_ws/devel/setup.sh
    3. rosrun turtlebot_dabit opencv_example_1.py

First Example Complete

Additional Resources

ROSPY Tutorials
CV_Bridge Tutorials

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