tf2/Tutorials/tf2 and time (Python)

Note: This tutorial assumes you have completed the adding a frame tutorial (Python) (C++).

Learning about tf2 and time (Python)

Description: This tutorial teaches you to use the waitf2orTransform 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             now = rospy.Time.now()
   2             (trans,rot) = listener.lookupTransform("/turtle2", "/carrot1", now)

So, all of the sudden lookupTransform() is failing, telling you:

Traceback (most recent call last):
  File "~/ros/pkgs/wg-ros-pkg-trunk/sandbox/learning_tf2/nodes/turtle_tf2_listener.py", line 25, in <module>
    (trans,rot) = listener.lookupTransform('/turtle2', '/carrot1', now)
tf2.ExtrapolationException: Extrapolation Too Far in the future: target_time is 1253830476.460, but the closest tf2  data is at 1253830476.435 which is 0.024 seconds away.Extrapolation Too Far in the future: target_time is 1253830476.460, but the closest tf2  data is at 1253830476.459 which is 0.001 seconds away.Extrapolation Too Far from single value: target_time is 1253830476.460, but the closest tf2  data is at 1253830476.459 which is 0.001 seconds away. See http://pr.willowgarage.com/pr-docs/ros-packages/tf2/html/faq.html for more info. When trying to transform between /carrot1 and /turtle2. See http://www.ros.org/wiki/tf2#Frequently_Asked_Questions

   1     listener.waitf2orTransform("/turtle2", "/carrot1", rospy.Time(), rospy.Duration(4.0))
   2     while not rospy.is_shutdown():
   3         try:
   4             now = rospy.Time.now()
   5             listener.waitf2orTransform("/turtle2", "/carrot1", now, rospy.Duration(4.0))
   6             (trans,rot) = listener.lookupTransform("/turtle2", "/carrot1", now)

The waitf2orTransform() takes four arguments:

Wait for the transform from this frame...
... to this frame,
at this time, and
timeout: don't wait for longer than this maximum duration

So waitf2orTransform() will actually block until the transform between the two turtles becomes available (this will usually take a few milli-seconds), OR --if the transform does not become available-- until the timeout has been reached.

So why the two waitf2orTransform() calls? At the beginning of code we spawned turtle2 but tf2 may have not ever seen the /turtle2 frame before waiting for the transform. The first waitf2orTransform() will wait until the /turtle2 frame is broadcast on tf2 before trying to waitf2orTransform() at time now.

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Learning about tf2 and time (Python)