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Note: This tutorial assumes you have completed the adding a frame tutorial (Python) (C++).
(!) Please ask about problems and questions regarding this tutorial on answers.ros.org. Don't forget to include in your question the link to this page, the versions of your OS & ROS, and also add appropriate tags.

Learning about tf2 and time (Python)

Description: This tutorial teaches you to use the timeout in lookup_transform function to wait for a transform to be available on the tf2 tree.

Tutorial Level: INTERMEDIATE

Next Tutorial: Time travel with tf2 (Python) (C++)

Contents

Edit nodes/turtle_tf2_listener.py and change the lookupTransform() call to:

   1 while not rospy.is_shutdown():
   2         try:
   3             trans = tfBuffer.lookup_transform('turtle2', 'carrot1', rospy.Time.now())

Also, edit the exception handling by adding raise to be able to see the exception:

   1 except (tf2_ros.LookupException, tf2_ros.ConnectivityException, tf2_ros.ExtrapolationException):
   2         raise
   3         rate.sleep()
   4         continue

So, all of the sudden lookup_transform() is failing, telling you that the frame does not exist or that the data is in the future. To fix this, edit your code as shown below. Note: once this change is made, remove the raise line from the except() block that we added above or the code will continue to fail.

   1     while not rospy.is_shutdown():
   2         try:
   3             trans = tfBuffer.lookup_transform('turtle2',
   4                                               'carrot1',
   5                                               rospy.Time.now(),
   6                                               rospy.Duration(1.0))

The lookup_transform takes four arguments. The 4th is an optional timeout. It will block for up to that duration waiting for it to timeout.

So lookup_transform() will actually block until the transform between the two turtles becomes available (this will usually take a few milli-seconds). Once the timeout has been reached (1 second in this case), an exception will be raised if the transform is still not available.


2020-02-15 13:15