Problems in reading rosbag message type

Hello,

I'm trying to read the rosbag message in the matlab,

    bag=rosbag('path_planning.bag');
  starttime=bag.StartTime;
      endtime =bag.EndTime;
      timeinterval=[starttime endtime];
      topic_pose=select(bag,'Time',timeinterval,'Topic','/move_base/NavFnROS/path');

%     ts = timeseries(bag, 'Pose.Pose.Position.X', 'Twist.Twist.Angular.Z');
    position = timeseries(topic_pose,...
        'Poses.Pose.Position.X',...
        'Poses.Pose.Position.Y',...
        'Poses.Pose.Position.Z');

But i cannot get any data. The messsage type is nav_msgs/Path, what went wrong here in this code?

thanks in advance!

Answer 1

[Sebastian Castro]

What do you mean by "cannot get data"? Do you have an error message or does your code run fine without an error?

I think there's a level of indexing missing, because a nav_msg/Path message actually contains an array of geometry_msgs/PoseStamped messages. You're going right into the former message and treating it like the latter.

>> rosmsg info nav_msgs/Path
%An array of Poses that represents a Path for a robot to follow
std_msgs/Header Header
geometry_msgs/PoseStamped[] Poses

>> rosmsg info geometry_msgs/PoseStamped
% A Pose with reference coordinate frame and timestamp
std_msgs/Header Header
Pose Pose

Not 100% sure how the rosbag data looks (you can share a snippet if you'd like), but I would guess you either have to grab individual elements of the array of pose messages, or the timeseries contains higher-dimensional data.

What I would do is display topic_pose in the MATLAB Command Window and taking a deeper look at the contents of the Data property. What type/size/etc. is it?

- Sebastian

[ARUN ANNAIYAN]

Hi sebastin, Thanks for the info. My main objective is to play the rosbag which will be then subscribed by the simulink model. Here is my rosbag info,

path:        path_planning.bag
version:     2.0
duration:    27.1s
start:       Oct 04 2016 15:01:23.44 (1475586083.44)
end:         Oct 04 2016 15:01:50.49 (1475586110.49)
size:        2.0 MB
messages:    152
compression: none [3/3 chunks]
types:       nav_msgs/Path [6227e2b7e9cce15051f669a5e197bbf7]
topics:      /move_base/NavfnROS/plan   152 msgs    : nav_msgs/Path

It has totally 152 messages. I have seen the topic_pose but there is no data in it. what is the right way to access the data? for example Poses.Pose.Position.X doesn't work in this case

[Sebastian Castro]

Oh, haha! I see your issue. Your code says:

topic_pose=select(bag,'Time',timeinterval,'Topic','/move_base/NavFnROS/path');

... but it should be /move_base/NavFnROS/plan ... that is, PLAN and not PATH.

Try that?

[ARUN ANNAIYAN]

oh yah! i just noticed the simple error but still i can't get the data in the workspace. so i changed like this below,

bag=rosbag('/home/arun/Workspace/catkin/src/rosbag/path_planning.bag');
bagselect1 = select(bag, 'Topic', '/move_base/NavfnROS/plan')
msgs = readMessages(bagselect1);

Now i can see the data. But i don't how to plot this as time series? if i try like this,

   timeinterval=[starttime endtime];
    topic_pose=select(bagselect1,'Time',timeinterval,'Topic','/move_base/NavFnROS/plan');

it is not working....

[Sebastian Castro]

Hmm... my guess is because it's a more complex/structures message, maybe it doesn't work with timeseries but you can still extract the data as an array of messages, as in your first code snippet.

If you're willing to share your .bag file, I can take a look.

Answer 2

[poluri koundinya]

hi,

I have a bagfile in the following path path: https://github.com/ouster-lidar/ouster_example#running-the-sample-ros-node

> bag.AvailableTopics

ans =

2×3 table

                               NumMessages        MessageType         MessageDefinition
                               ___________    ____________________    _________________

    /os1_node/imu_packets          9977       ouster_ros/PacketMsg     'uint8[] Buf↵'  
    /os1_node/lidar_packets       63861       ouster_ros/PacketMsg     'uint8[] Buf↵'

bagselect1 = select(bag, 'Topic', '/os1_node/lidar_packets')

bagselect1 =

BagSelection with properties:

           FilePath: 'C:\Users\nikhil\Desktop\bag file\x.bag'
          StartTime: 1.5284e+09
            EndTime: 1.5284e+09
        NumMessages: 63861
    AvailableTopics: [1×3 table]
    AvailableFrames: {0×1 cell}
        MessageList: [63861×4 table]

>> allMsgs = readMessages(bagselect1); Error using robotics.ros.BagSelection/readMessages (line 279) Cannot find a MATLAB message class for type ouster_ros/PacketMsg.

I need to see the point cloud data in the bagfile but the topic does not contain sensor_msgs/PointCloud2.

then how am i supposed to get the point cloud data out of it? any help

[Sebastian Castro]

We responded to the email you sent us, but will answer here too.

Your /os1_node/lidar_packets topic is not of the correct type sensor_msgs/PointCloud2

So, you either need to make sure whatever is publishing to that topic is doing it with the right message type, or figure out a way to get the uint8[] data in your message converted to a point cloud by some other means.

Also, since the ouster_ros/PacketMsg is a custom ROS message definition you created, to use it in MATLAB you need to go through the custom message support workflow:

https://www.mathworks.com/help/robotics/ug/ros-custom-message-support.html

- Sebastian