How to get list of all tf frames programatically?

asked 2020-03-30 04:15:04 -0600

146 ●8 ●18 ●18

updated 2020-03-30 05:34:00 -0600

Hi all,

I have a question on how to obtain tf frames in a ros node (preferrably Python). I am aware of the method tf2_ros.Buffer.lookup_transform(), however, one has to specify source and target frame for that.

Lets say I have an array of known markers, but only a subset of them are detected by the camera. I want to know which of these markers are detected by looking at the existing tf frames. Is there a more elegant way to do so instead of looping over all possible markers and try to lookup the transform in a try catch block?

Thanks in advance!

P.S I am using kinetic on Ubuntu 16.04

EDIT:
In the tutorial TfUsingPython I found the method allFramesAsString() but it seems to use tf, not tf2. would there be an equivalence in tf2? Or is it generally a bad practice to mix the use of tf and tf2?

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answered 2020-03-30 05:38:26 -0600

gvdhoorn
86574 ●283 ●1432 ●1054 http://cor.tudelft.nl/

It seems #q261460 is at least a partial duplicate of your question.

In the tutorial TfUsingPython I found the method allFramesAsString() but it seems to use tf, not tf2. would there be an equivalence in tf2? Or is it generally a bad practice to mix the use of tf and tf2?

TF has been implemented on-top of TF2 for quite some time now, so you cannot really "mix" them: it would all be using TF2 in the end.

As to the method you found: there is tf2::BufferCore::allFramesAsYAML(double current_time) const in TF2 (and allFramesAsStringNoLock() is available as well).

The Python API of TF2 is essentially a mirror of the C++ API, so you should be able to call that from Python as well (I have not verified this).

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Since the Python API is rather poorly documented, here is sth if someone has the same problem as me: After some further research I found some example usage of the Python API here: examples_tf2_py. Probably this still does not contain all methods of the C++ API, but imo it's a good start.

rfn123 ( 2020-03-30 07:30:21 -0600 )edit

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answered 2020-03-30 07:50:44 -0600

rfn123

146 ●8 ●18 ●18

I ended up searching alternative to native tf library functions to solve my problem. The reason was that after getting allFramesAsYAML() (or in Python tfBuffer_ros.all_frames_as_yaml()) to work i found out that it will return _all_ frames that have been in the tf buffer, regardless of whether the frame is in the current tf topic or not. The output was:

marker1: 
  parent: 'camera'
  broadcaster: '/marker_tracker'
  rate: 30.401
  most_recent_transform: 1585572193.003
  oldest_transform: 1585572190.503
  buffer_length: 2.500
marker2: 
  parent: 'camera'
  broadcaster: '/marker_tracker'
  rate: 30.457
  most_recent_transform: 1585572192.703
  oldest_transform: 1585572190.503
  buffer_length: 2.200
marker3: 
  parent: 'camera'
  broadcaster: '/marker_tracker'
  rate: 27.709
  most_recent_transform: 1585572195.538
  oldest_transform: 1585572191.568
  buffer_length: 3.970

I occluded marker2 after a while, and it still appeared in the output. The only difference is that "most_recent_transform" stopped updating.

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The only difference is that "most_recent_transform" stopped updating.

doesn't that align with what you were trying to achieve? You wrote:

I want to know which of these markers are detected by looking at the existing tf frames

Thing is: TF frames "don't exist", so seeing whether they "still exist" is not really something you can do.

Even TF itself (ie: the internals of the provided libraries) will look at most_recent_transform to determine whether a transform is still up-to-date and relevant.

In your case you could see how long ago the transform was updated and determine -- based on some threshold -- whether the last detected pose for a marker is "too old".

gvdhoorn ( 2020-03-30 07:54:25 -0600 )edit

You are right, by looking at the update time I can filter the tfs. I was just rather looking for sth. simpler (e.g. boolean) that shows if a certain tf frame is updated or not, i.e. "existing", but I was not aware that the internal libraries use the mechanics of update time. Thanks for clarifying that!

rfn123 ( 2020-03-30 13:32:48 -0600 )edit

I was just rather looking for sth. simpler (e.g. boolean) that shows if a certain tf frame is updated or not

well, think about it: what would it mean for a frame to "be updated or not"? How would TF determine this to set that boolean true or false?

Probably be comparing the time at which a frame was last updated to some sort of maximum allowable age .. :)

gvdhoorn ( 2020-03-30 14:38:27 -0600 )edit

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answered 2022-07-06 22:32:21 -0600

lindzey
1775 ●18 ●31 ●42

updated 2022-07-06 22:32:44 -0600

Building off the other answers, here's a snippet of runnable code that I wound up with:

import rospy
import tf2_ros
import time
import yaml

rospy.init_node('ipython')

tf_buffer = tf2_ros.Buffer()
tf_listener = tf2_ros.TransformListener(tf_buffer)
time.sleep(5.0)

frames_dict = yaml.safe_load(tf_buffer.all_frames_as_yaml())
frames_list = list(frames_dict.keys())

(Tested on Ubuntu 20.04, running noetic)

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I tried porting your snippet into ROS2 but haven't had success so far. Spun off a new question https://robotics.stackexchange.com/qu... FYI.

130s ( 2023-05-04 03:57:53 -0600 )edit

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answered 2023-05-04 03:51:37 -0600

130s

10937 ●203 ●339 ●330 http://www.linkedin.co...

updated 2023-05-04 11:30:53 -0600

CLI though, this returns the list of frames and some associated data (the Service type is what rqt_tf_tree uses. Related #q349175). Since it's ROS2 Service you should be able to write in Python or C++ in standard way (I just haven't had time to do that yet).

ros2 service call /tf2_frames tf2_msgs/srv/FrameGraph

In my case I got:

response:                                                             
tf2_msgs.srv.FrameGraph_Response(frame_yaml="breakbeam_0_frame: \n  parent: 'world'\n  broadcaster: 'default_authority'\n  rate: 10000.000\n  most_recent_transform: 0.000000\n  oldest_transform: 0.000000\n  buffer_length: 0.000\nadvanced_logical_camera_0_frame: \n  parent: 'world'\n  broadcaster: 'default_authority'\n  ...

Btw I also tried in Python tf_buffer.all_frames_as_yaml() way, but no success. Opened a separate question robotics.stackexchange.com#24731.

Update. Hm, the output of ROS2 service cmd above looks the same as what ros2 run tf2_tools view_frames, which is introduced in the very beginning of tf tutorial, returns, so nothing new to anyone :/

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