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Using ROS 2? Check out the [[https://docs.ros.org/en/rolling/Tutorials/Intermediate/Tf2/Tf2-Main.html|ROS 2 tf2 tutorials]].
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## title = Time travel with tf2 (Python)
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## description = This tutorial teaches you about advanced time travel features of tf2
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<<TableOfContents(4)>>

In the [[tf2/Tutorials/tf2 and time (Python)|previous tutorial]] we discussed the basic concept of tf2 and time. This tutorial will take this one step further, and expose one of the most powerful tf2 tricks.

== Time travel ==
So let's go back to where we ended in the [[tf2/Tutorials/tf2 and time (Python)|previous tutorial]]. Go to your package for the tutorial:

{{{
  $ roscd learning_tf2
}}}
Now, instead of making the second turtle go to where the  carrot is '''now''', make the second turtle go to where the first carrot was '''5 seconds ago'''. Edit '''`nodes/turtle_tf2_listener.py`''':

{{{#!python
        try:
            past = rospy.Time.now() - rospy.Duration(5.0)
            trans = tfBuffer.lookup_transform(turtle_name, 'carrot1', past, rospy.Duration(1.0))
        except (tf2_ros.LookupException, tf2_ros.ConnectivityException, tf2_ros.ExtrapolationException):
}}}
<<Include(tf2/Tutorials/Time travel with tf2 (C++),,from="##explainstart",to="##explainend")>>

{{{#!python
        try:
            past = rospy.Time.now() - rospy.Duration(5.0)
            trans = tfBuffer.lookup_transform_full(
                target_frame=turtle_name,
                target_time=rospy.Time.now(),
                source_frame='carrot1',
                source_time=past,
                fixed_frame='world',
                timeout=rospy.Duration(1.0)
            )
}}}
<<Include(tf2/Tutorials/Time travel with tf2 (C++),,from="##advancestart",to="##advancedend")>>

{{{
  $ roslaunch learning_tf2 start_demo.launch
}}}
And yes, the second turtle is directed to where the first carrot was 5 seconds ago!

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