How to get an array from a Rosbag

Hi, I'm trying to plot a data from a rosbag, here what i did until now:

bag=rosbag('/home/paolino/bagfiles/subset.bag');
wh_gaz=select(bag,'Topic','/joint_states');
wh_gazStruct=readMessages(wh_gaz,'DataFormat','struct');

This struct is a 19153x1 cell and each element is a 1x1 struct with a velocity record, a 2x1 array.

Now, what i need is to create 1 column array with each wh_gazStruct.Velocity first element, but I can't find a solution. Until now I tryed:

wh_gaz_mat=cellfun(@(m) double(m.Velocity),wh_gazStruct);

but since the .Velocity record has two elements it doesn't work. How may I solve my problem?

Answer 1

[Cam Salzberger]

I provided some guidance and sample code to a similar question here. That should help get you moving in the right direction!

-Cam

[Paolino De Risi]

It doesn't work for me, in my case the record .Velocity has two elements inside since it is a float64 array with two elements. they are not indexed as subrecords as in the following image:

This seems not work:

function [wh1, wh2] = getwheelsvelocities(msg)
% Extract internal message first for efficiency
% (matters more for more nested messages)
wh_vect = msg.Velocity(1);
wh_vect1=msg.Velocity(2);
wh1 = wh_vect;
wh2 = wh_vect1;
end

[Cam Salzberger]

Well, I don't know exactly what your messages look like. But if I set up my messages like this (to simulate using readMessages with DataFormat='struct'):

msg = rosmessage('sensor_msgs/JointState', 'DataFormat', 'struct');
n = 4;
msgs = cell(n, 1);
for k = 1:n
msgs{k} = msg;
msgs{k}.Velocity = [k ; -k];
end

Then I can do this:

[wh1, wh2] = cellfun(@getwheelsvelocities, msgs);

with your code as-provided, and it works just fine.

If you have any messages that do not have two entries in the Velocity field, then this code will fail.

-Cam