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From transform to pose?

asked 2014-08-18 10:38:07 -0500

silentwf gravatar image

Hi all, I'm very new to Python and the TF package. Given a transform, I am trying to calculate its pose. How would I do that?

I've seen this link and saw the code

  tf::Transform grasp_tf(gripperRotation, gripperOrigin);
  tf::Stamped<tf::Pose> grasp_tf_pose(grasp_tf);
  geometry_msgs::PoseStamped msg;
  tf::poseStampedTFToMsg    (grasp_tf_pose, msg);

However, I don't know how I should convert this into Python code. Is there more documentation than the one given in the wiki?

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answered 2014-08-18 10:57:28 -0500

paulbovbel gravatar image

You actually linked to the API documentation, which you should definitely read through AFTER going through the tutorials .

A transform represents a transformation from frame A to frame B. So could you be more specific about what you mean by 'calculating its pose'. If you mean get the vectors representing the translation and rotation from A to B, there's a few ways (in C++ anyways, python will be somewhat similar):

1&2) First waitForTransform, to make sure the transform is available.

1) lookupTransform, then getOrigin and getRotation on the transform object

2) Initialize a Stamped<pose> in frame B with a position vector of (0,0,0), and a quaternion of (0,0,0,1), then transformPose to frame A.

tf::poseStampedTFToMsg and similar are helper methods in C++ to convert between the tf and ROS message representation of various objects. A tf representation is used to perform actual mathematical operations, while the ROS message representation is used for communication (publishing/subscribing/broadcasting tf).

A Pose contains a position and an orientation, a Stamped<pose> also contains frame and time information needed to transform the pose from one frame to another. Similar pattern applies to vectors, quaternions etc.

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Yes. What I meant was if I had an original Pose information and then a Transform information, how would I use these two information to calculate its resulting Pose?

silentwf gravatar image silentwf  ( 2014-08-20 19:44:39 -0500 )edit


transformPose(target_frame, pose_msg) -> pose_msg

paulbovbel gravatar image paulbovbel  ( 2014-08-21 09:34:53 -0500 )edit

However, I dont understand how exactly to calculate the transform which can be used later as in the question. I have two messages coming to me from two different topics. Both of geometry_msgs/poseStamped type. I need to calculate the transform.

gk gravatar image gk  ( 2016-08-04 10:46:05 -0500 )edit

answered 2014-08-18 11:04:09 -0500

dj0405 gravatar image

updated 2014-08-18 11:04:45 -0500

The tf function you're looking for is

lookupTransform(target_frame, source_frame, time) -> (position, quaternion)

The function returns the translation and rotation of the transformation between the two frames:

(translation,rotation) = t.lookupTransform("tf_1", "tf_2", rospy.Time())

Now, you have the translation and rotation values, you can construct the pose msg.

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Yes, this is it. I think this is the correct answer. If it is a Pose to a Goal in an autonomous base, just set "Z" to 0, for example

Enriq gravatar image Enriq  ( 2017-05-31 04:46:39 -0500 )edit

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Asked: 2014-08-18 10:38:07 -0500

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Last updated: Aug 18 '14