A rosbag or bag is a file format in ROS for storing ROS 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. The following sections detail the structure of rosbags in MATLAB® and the workflow for extracting data from them.
When accessing rosbag log files, call
specify the file path to the object. MATLAB then creates a
that contains an index of all the messages from the rosbag.
BagSelection object has the following
properties related to the rosbag:
FilePath: a character vector of
the absolute path to the rosbag file.
StartTime: a scalar indicating
the time the first message was recorded
EndTime: a scalar indicating the
time the last message was recorded
NumMessages: a scalar indicating
how many messages are contained in the file
AvailableTopics: a list of what
topic and message types were recorded in the bag. This is stored as
table data that lists the number of messages, message type, and message
definition for each topic. For more information on table data types,
see Access Data in a Table (MATLAB). Here is an example output
of this table:
ans = NumMessages MessageType MessageDefinition ___________ ______________________ _________________ /clock 12001 rosgraph_msgs/Clock [1x185 char] /gazebo/link_states 11999 gazebo_msgs/LinkStates [1x1247 char] /odom 11998 nav_msgs/Odometry [1x2918 char] /scan 965 sensor_msgs/LaserScan [1x2123 char]
MessageList: a list of every message
in the bag with rows sorted by time stamp of when the message was
recorded. This list can be indexed and you can select a portion of
the list this way. Calling
you to select subsets based on time stamp, topic or message type.
Also, note that the
BagSelection object contains
an index for all the messages. However, you must still use functions
to extract the data. For extracting this information, see
readMessages for getting messages based
on indices as a cell array or see
reading the data of specified properties as a time series.
When working with rosbags, there is a general procedure of how you should extract data.
Load a rosbag: Call
rosbag and the file path to load file
Examine available messages:
BagSelection properties (
MessageList) to determine how to select a subset
of messages for analysis.
select to create a selection
of messages based on your desired properties.
Visualize, analyze or process data: Use the extracted data for your specific application. You can plot data or develop algorithms to process data.
The following figure also shows the workflow.
There are a few limitations in the rosbag support within MATLAB:
MATLAB can only parse uncompressed rosbags. See the ROS Wiki for a tool to decompress a compressed rosbag.
Only rosbags in the v2.0 format are supported. See the ROS Wiki for more information on different bag formats
The file path to the rosbag must always be accessible. Because the message selection process does not retrieve any data, the file needs to be available for reading when the message data is accessed.