I don't know what parameter I should enter here.

asked 2018-02-06 10:40:31 -0600

anonymous user


updated 2018-03-04 14:27:24 -0600

jayess gravatar image

Trying to use this function to get (x,y,z) out of a bag file. However, I don't know what I should enter as "joint_name" input. Function won't work unless i supply this input.

I try to use: get_joint_data(somefile.bag, ?). I don't know what to put there. base_link, base_footprint, start_of_device don't work.

def get_joint_data(bagFile, joint_name, convert_to_sec=True):
        Function that filters bag files to obtain data from a joint that is 
        published to the /tf topic.
        Only x-, y- and z-coordinates are returned
        if convert_to_sec is set to True (default), then the first timestamp 
        will be taken as zero seconds, and the following timesteps that are 
        retured will be relative to this timestamp.
    x = np.array([])
    y = np.array([])
    z = np.array([])
    all_t = np.array([])
    bag = rosbag.Bag(bagFile) # Initialize rosbag object
    first = True # True on first iteration to take first timestamp

    # Add message values to collections
    for topic, msg, t in bag.read_messages(topics=['/tf']):

        joint = msg.transforms[0].child_frame_id
        translation = msg.transforms[0].transform.translation

            if joint == joint_name: 

                # Get timestamp in seconds

    t = msg.transforms[0].header.stamp
        t_sec = t.to_sec()
        if (first and convert_to_sec):
            t_0 = t_sec
            first = False
        all_t = np.append(all_t, t_sec-t_0)        

        # Get x, y and z coordinates
            pose = [translation.x , translation.y, translation.z]

            x = np.append(x, pose[0])
            y = np.append(y, pose[1])
            z = np.append(z, pose[2])

    pose = [x,y,z]

    return pose, all_t
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what do you mean with

base_link, base_footprint, start_of_device don't work.

As long as base_link is in the bag, and does not contain a leading /, it seems to produce what you want (I'm assuming the messed up indentation is only here and not in the original file...) Maybe have a look at ..

mgruhler gravatar image mgruhler  ( 2018-02-07 01:06:48 -0600 )edit

the topic manually and check what you have in the child_frame_id field of some of the messages.

Also, you should probably rename one of the two pose variables to make sure you don't run into any issues there...

mgruhler gravatar image mgruhler  ( 2018-02-07 01:08:12 -0600 )edit