Extract timeseries from rosbag with std_msgs/Bool messages

Hi,

I have recorded a rosbag with a /teleop_control topic, to which messages of type std_msgs/Bool are published. I need to read this as a timeseries object, to later synchronize with other topics. But I'm getting errors when trying to use the timeseries function, which worked fine with Twist messages.

To check it, I have done:

bag_teleop_control.MessageList

ans =

  46×4 table

       Time            Topic          MessageType     FileOffset
    __________    _______________    _____________    __________

    1.5637e+09    /teleop_control    std_msgs/Bool       208303 
    1.5637e+09    /teleop_control    std_msgs/Bool       214527 
    1.5637e+09    /teleop_control    std_msgs/Bool      1307088   
    .
    .
    .

And:

rosmsg info std_msgs/Bool
    logical Data

But this gives me an error:

timeseries(bag_teleop_control, 'Data');
    Error using robotics.ros.BagSelection/getTimeseriesArguments (line 726)
    The Data property does not exist for message type std_msgs/Bool.
    
    Error in robotics.ros.BagSelection/timeseries (line 442)
                props = obj.getTimeseriesArguments(messageType, varargin{:});

Does someone know what am I getting wrong?

[Cam Salzberger]

Hey Vinicius,

What happens when you extract the messages in the timeseries without specifying the property to extract?

boolData = timeseries(bag_teleop_control);

What about when you try to extract the messages from the rosbag using other methods?

msgList = readMessages(bag_teleop_control);
msgStructList = readMessages(bag_teleop_control, "DataFormat", "struct");

Of those options, I'd personally recommend using the struct data format. I believe that it is faster than timeseries, and I know it's faster than extracting the message objects. I'm curious if you get any other errors using these methods though.

-Cam

[Vinicius Schettino]

Hi Cam, thanks for the reply.

Extracting without the property gives me back an empty matrix:

boolData = timeseries(bag_teleop_control)
  timeseries

  Common Properties:
            Name: '/teleop_control Properties'
            Time: [46x1 double]
        TimeInfo: [1x1 tsdata.timemetadata]
            Data: [46x0 double]
        DataInfo: [1x1 tsdata.datametadata]

Using readMessages works fine, both with or without the struct option, but then I loose the time info, which I need for synchronizing this data with other topics. I know I can go and pull back the time from the original bag and then use that to create the timeseries, but it seems like a big workaround for something that worked very simply with the Twist messages. So I'm still wondering what goes wrong with the Bool messages... Is this a bug?

[Cam Salzberger]

That is pretty odd. When you did readMessages (both with and without the struct option), did the messages contain values in the Data property of the messages? Or were they empty, or all equal to the same value?

It does look like you could extract the time series to get the structure and time, then fill in the data using messages?

boolData = timeseries(bag_teleop_control);
boolMsgStructs = readMessages(bag_teleop_control, "DataFormat", "struct");
boolData.Data = [boolMsgStruct.Data]';

Sorry if the code might not work. I'm away from MATLAB right now, so can't try it out. But that's one idea for a workaround.

-Cam

[Vinicius Schettino]

Yes, the values obtained from readMessages come out correct.

Your code gave me an error, but a small change fix it:

boolData = timeseries(bag_teleop_control);
boolData.Data = cellfun(@(x) x.Data, readMessages(bag_teleop_control));

I still don't know why the timeseries function fails for Bool messages, but at leat it is a simple enough workaround. Thanks!