Unable to access ROSbag topics

I have a bag file and would like to work with it in MATLAB.

I can view the messages for one of my topics using:

    >> bag = rosbag('HMB_1.bag');
    >> bagselect_c = select(bag, 'Topic', '/center_camera/image_color/compressed');
    >> msgs_c = readMessages(bagselect_c);

when I try to do the same for another topic, I get an error:

    >> bagselect= select(bag, 'Topic', '/vehicle/steering_report');
    >> msgstemp = readMessages(bagselect);
    Error using robotics.ros.BagSelection/deserializeMessages (line 445)
    Java exception occurred:
    org.ros.exception.RosMessageRuntimeException: java.lang.ClassNotFoundException: dbw_mkz_msgs.SteeringReport

      at org.ros.internal.message.definition.MessageDefinitionReflectionProvider.get(MessageDefinitionReflectionProvider.java:66)

      at org.ros.internal.message.DefaultMessageFactory.newFromType(DefaultMessageFactory.java:42)

      at org.ros.internal.message.DefaultMessageDeserializer.deserialize(DefaultMessageDeserializer.java:42)

    Caused by: java.lang.ClassNotFoundException: dbw_mkz_msgs.SteeringReport

      at java.net.URLClassLoader$1.run(Unknown Source)

      at java.net.URLClassLoader$1.run(Unknown Source)

      at java.security.AccessController.doPrivileged(Native Method)

      at java.net.URLClassLoader.findClass(Unknown Source)

      at java.lang.ClassLoader.loadClass(Unknown Source)

      at sun.misc.Launcher$AppClassLoader.loadClass(Unknown Source)

      at java.lang.ClassLoader.loadClass(Unknown Source)

      at org.ros.internal.message.definition.MessageDefinitionReflectionProvider.get(MessageDefinitionReflectionProvider.java:62)

      ... 2 more

    Error in robotics.ros.BagSelection/readMessages (line 195)
                msgs = obj.deserializeMessages(obj.MessageList, rows);

I'm not sure how to troubleshoot.

Answer 1

[Kushagr Gupta]

This error is a result of the rosbag containing custom message types (e.g. 'dbw_mkz_msgs.SteeringReport') that are not supported out of the box.

However,there is a separate support package which can help with that. Please refer to the following link to access the support package:

Robotics System Toolbox Interface for ROS Custom Messages

Note:This support package will make custom message types available in MATLAB.

• For the next steps, please note that you will need the "rt_msgs ROS package" (since it contains the message definition for the custom message). This package is not publicly available, but the full rt_msgs package can be obtained from the machine which recorded the bag file.
• Create a new folder in a writable location, e.g. "custom_messages"
• Copy the "rt_msgs" package folder into "custom_messages"
• Execute "rosgenmsg('custom_messages')". This will generate all necessary files that MATLAB needs to handle dbw_mkz_msgs.SteeringReport.
• Once it finishes successfully, "rosgenmsg" will display three steps (modify javaclasspath, change MATLAB path, restart MATLAB). Please follow these steps.
• After restarting MATLAB, you should be able to read the rosbag and extract the messages.

Hope this helps!

[poluri koundinya]

Hi, I have a bag file which i received from the internet and it has custom bag file. I have installed robotic system toolbox interface to read custom bag file. link:https://github.com/ouster-lidar/ouster_example#running-the-sample-ros-node but how do i download "rt_msgs ROS package" as you said? please help Regards, Nikhil

[Cemre Kavvasoglu]

I have same problem. Where can I find "rt_msgs ROS package"?