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Work with rosbag Logfiles

Introduction

A rosbag or bag is a file format in ROS for storing message data. These bags are often created by subscribing to one or more ROS topics, and storing the received message data in an efficient file structure. MATLAB® can read these rosbag files and help with filtering and extracting message data. See ROS Log Files (rosbags) for more information about rosbag support in MATLAB.

In this example, you will load a rosbag and learn how to select and retrieve the contained messages.

Prerequisites: Work with Basic ROS Messages

Load a rosbag

  • Load an example file using the rosbag command. Specify the path to the bag file either as an absolute or relative path:

filepath = fullfile(fileparts(which('ROSWorkingWithRosbagsExample')), 'data', 'ex_multiple_topics.bag');
bag = rosbag(filepath)
bag = 

  BagSelection with properties:

           FilePath: '/mathworks/devel/bat/BR2017bd/build/matlab/toolbox/robotics/robotexamples/ros/data/ex_multiple_topics.bag'
          StartTime: 201.3400
            EndTime: 321.3400
        NumMessages: 36963
    AvailableTopics: [4x3 table]
        MessageList: [36963x4 table]

The object returned from the rosbag call is a BagSelection object, which is a representation of all the messages in the rosbag.

The object display shows details about how many messages are contained in the file (NumMessages) and the time when the first (StartTime) and the last (EndTime) message were recorded.

  • Evaluate the AvailableTopics property to see more information about the topics and message types that are recorded in the bag:

bag.AvailableTopics
ans =

  4x3 table

                           NumMessages         MessageType                                        MessageDefinition                               
                           ___________    ______________________    ______________________________________________________________________________

    /clock                 12001          rosgraph_msgs/Clock       '# roslib/Clock is used for publishing simulated time in ROS. ...'            
    /gazebo/link_states    11999          gazebo_msgs/LinkStates    '# broadcast all link states in world frame...'                               
    /odom                  11998          nav_msgs/Odometry         '# This represents an estimate of a position and velocity in free space.  ...'
    /scan                    965          sensor_msgs/LaserScan     '# Single scan from a planar laser range-finder...'                           

The AvailableTopics table contains the sorted list of topics that are included in the rosbag. The table stores the number of messages, the message type, and the message definition for the topic. For more information on the MATLAB table data type and what operations you can perform on it, see the documentation for Tables (MATLAB).

Initially the rosbag is only indexed by MATLAB and no actual message data is read.

You might want to filter and narrow the selection of messages as much as possible based on this index before any messages are loaded into MATLAB memory.

Select Messages

Before you retrieve any message data, you must select a set of messages based on criteria such as time stamp, topic name, and message type.

  • You can examine all the messages in the current selection with:

bag.MessageList
ans =

  36963x4 table

     Time            Topic                MessageType          FileOffset
    ______    ___________________    ______________________    __________

    201.34    /clock                 rosgraph_msgs/Clock           4524  
    201.34    /odom                  nav_msgs/Odometry             7666  
    201.34    /gazebo/link_states    gazebo_msgs/LinkStates        9866  
    201.35    /clock                 rosgraph_msgs/Clock          10962  
    201.35    /clock                 rosgraph_msgs/Clock          12876  
    201.35    /odom                  nav_msgs/Odometry            12112  
    201.35    /gazebo/link_states    gazebo_msgs/LinkStates       11016  
    201.36    /odom                  nav_msgs/Odometry            14026  
    201.36    /gazebo/link_states    gazebo_msgs/LinkStates       12930  
    201.37    /clock                 rosgraph_msgs/Clock          14790  
    201.37    /odom                  nav_msgs/Odometry            14844  
    201.37    /gazebo/link_states    gazebo_msgs/LinkStates       15608  
...

The MessageList table contains one row for each message in the bag (there are over 30,000 rows for the bag in this example). The rows are sorted by the time stamp the first column that represents the time (in seconds) when this message was recorded.

  • Since the list is very large, you can also display a selection of rows with the familiar row and column selection syntax:

bag.MessageList(500:505,:)
ans =

  6x4 table

    Time           Topic                MessageType          FileOffset
    ____    ___________________    ______________________    __________

    203     /clock                 rosgraph_msgs/Clock       339384    
    203     /odom                  nav_msgs/Odometry         336328    
    203     /odom                  nav_msgs/Odometry         337092    
    203     /odom                  nav_msgs/Odometry         337856    
    203     /odom                  nav_msgs/Odometry         338620    
    203     /gazebo/link_states    gazebo_msgs/LinkStates    331944    

Use the select function to narrow the selection of messages. The select function operates on the bag object.

You can filter the message list by time, topic name, message type, or any combination of the three.

  • To select all messages that were published on the /odom topic, use the following select command:

bagselect1 = select(bag, 'Topic', '/odom')
bagselect1 = 

  BagSelection with properties:

           FilePath: '/mathworks/devel/bat/BR2017bd/build/matlab/toolbox/robotics/robotexamples/ros/data/ex_multiple_topics.bag'
          StartTime: 201.3400
            EndTime: 321.3300
        NumMessages: 11998
    AvailableTopics: [1x3 table]
        MessageList: [11998x4 table]

Calls to the select function return another BagSelection object, which can be used to make further selections or retrieve message data. All selection objects are independent of each other, so you can clear them from the workspace once you are done.

  • You can make a different selection that combines two criteria. To get the list of messages that were recorded within the first 30 seconds of the rosbag and published on the /odom topic, enter the following command:

start = bag.StartTime
bagselect2 = select(bag, 'Time', [start start + 30], 'Topic', '/odom')
start =

  201.3400


bagselect2 = 

  BagSelection with properties:

           FilePath: '/mathworks/devel/bat/BR2017bd/build/matlab/toolbox/robotics/robotexamples/ros/data/ex_multiple_topics.bag'
          StartTime: 201.3400
            EndTime: 231.3200
        NumMessages: 2997
    AvailableTopics: [1x3 table]
        MessageList: [2997x4 table]

  • Use the last selection to narrow down the time window even further:

bagselect3 = select(bagselect2, 'Time', [205 206])
bagselect3 = 

  BagSelection with properties:

           FilePath: '/mathworks/devel/bat/BR2017bd/build/matlab/toolbox/robotics/robotexamples/ros/data/ex_multiple_topics.bag'
          StartTime: 205.0200
            EndTime: 205.9900
        NumMessages: 101
    AvailableTopics: [1x3 table]
        MessageList: [101x4 table]

The selection in this last step operated on the existing bagselect2 selection and returned a new bagselect3 object.

  • If you want to save a set of selection options, store the selection elements in a cell array and then re-use it later as an input to the select function:

selectOptions = {'Time', [start, start+1; start+5, start+6], 'MessageType', {'sensor_msgs/LaserScan', 'nav_msgs/Odometry'}};
bagselect4 = select(bag, selectOptions{:})
bagselect4 = 

  BagSelection with properties:

           FilePath: '/mathworks/devel/bat/BR2017bd/build/matlab/toolbox/robotics/robotexamples/ros/data/ex_multiple_topics.bag'
          StartTime: 201.3400
            EndTime: 207.3300
        NumMessages: 209
    AvailableTopics: [2x3 table]
        MessageList: [209x4 table]

Read Selected Message Data

After you narrow your message selection, you might want to read the actual message data into MATLAB. Depending on the size of your selection, this can take a long time and consume a lot of your computer's memory.

  • To retrieve the messages in you selection as a cell array, use the readMessages function:

msgs = readMessages(bagselect3);
size(msgs)
ans =

   101     1

The resulting cell array contains as many elements as indicated in the NumMessages property of the selection object.

  • In reading message data, you can also be more selective and only retrieve messages at specific indices. Here is an example of retrieving 4 messages:

msgs = readMessages(bagselect3, [1 2 3 7])
msgs{2}
msgs =

  4x1 cell array

    {1x1 Odometry}
    {1x1 Odometry}
    {1x1 Odometry}
    {1x1 Odometry}


ans = 

  ROS Odometry message with properties:

     MessageType: 'nav_msgs/Odometry'
          Header: [1x1 Header]
    ChildFrameId: 'base_footprint'
            Pose: [1x1 PoseWithCovariance]
           Twist: [1x1 TwistWithCovariance]
...

Each message in the cell array is a standard MATLAB ROS message object. For more information on messages, see the Work with Basic ROS Messages example.

Extract Message Data as Time Series

Sometimes you are not interested in the complete messages, but only in specific properties that are common to all the messages in a selection. In this case, it is helpful to retrieve the message data as a time series instead. A time series is a data vector that is sampled over time and represents the time evolution of one or more dynamic properties. For more information on the MATLAB time series support, see the documentation for Time Series (MATLAB).

In the case of ROS messages within a rosbag, a time series can help to express the change in particular message elements through time. You can extract this information through the timeseries function. This is memory-efficient, since the complete messages do not have to be stored in memory.

  • Use the same selection, but use the timeseries function to only extract the properties for x-position and z-axis angular velocity:

ts = timeseries(bagselect3, 'Pose.Pose.Position.X', 'Twist.Twist.Angular.Z')
  timeseries

  Timeseries contains duplicate times.

  Common Properties:
            Name: '/odom Properties'
            Time: [101x1 double]
        TimeInfo: tsdata.timemetadata
            Data: [101x2 double]
        DataInfo: tsdata.datametadata

The return of this call is a timeseries object that can be used for further analysis or processing.

Note that this method of extracting data is only supported if the current selection contains a single topic with a single message type.

  • To see the data contained within the time series, access the Data property:

ts.Data
ans =

    0.0003    0.0003
    0.0003    0.0003
    0.0003   -0.0006
    0.0003   -0.0006
    0.0003   -0.0010
    0.0003   -0.0010
    0.0003   -0.0003
    0.0003   -0.0003
    0.0003   -0.0003
    0.0003   -0.0003
    0.0003   -0.0016
    0.0003   -0.0016
    0.0003   -0.0001
    0.0003   -0.0001
    0.0003    0.0003
    0.0003    0.0003
    0.0003   -0.0006
...
  • There are many other possible ways to work with the time series data. Calculate the mean of the data columns:

mean(ts)
ans =

   1.0e-03 *

    0.3213   -0.4616

  • You can also plot the data of the time series:

figure
plot(ts, 'LineWidth', 3)

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