How to efficiently access data from ROS PoseArray Messages?

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Vikrant Shah
Vikrant Shah 2017-7-21
评论： chef13 2018-3-6
I am trying to analyse data from a rosbag from a turtlebot run. I have Pose_Array message which contains about 10,000 particles and I want to computer the mean of the X and Y position of these particles. When I try to access as follows:
rb = rosbag('test.bag');
tb_particle_bag = select(rb,'Topic','/particlecloud');
msg = readMessages(tb_particle_bag,1);
Then when I run the following I see...
b=msg{1}.Poses
b =
10000×1 ROS Pose message array with properties:
MessageType
Position
Orientation
However when I try to access the X-coordinates of all the particles I see the follwing error:
b.Position.X
Expected one output from a curly brace or dot indexing expression, but there were 10000 results.
The only work around I have found to this is to individually access each particles and read its x value in a for loop and this turns out to be extremely slow as follows:
for i = 1:tb_particle_bag.NumMessages
msg = readMessages(tb02_gt_particle_bag,i);
b=msg{1}.Poses;
sum = [0 0];
for j = 1:size(b,1)
p=b(j).Position;
sum = sum + [p.X p.Y];
end
tb_mp(i,:)=sum/size(b,1);
end
This above method takes about 2.5secs for each PoseArray message in the rosbag and makes the process excruciatingly long. Is there a more efficient way to do this?

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回答（2 个）

Sebastian Castro
Sebastian Castro 2017-7-21
I think you can do this... try:
b = [msg.Poses]
- Sebastian
  1 个评论
Vikrant Shah
Vikrant Shah 2017-7-21
b=[msg.Poses]
Throws an error
Struct contents reference from a non-struct array object.
And
b=[msg{1}.Poses]
has the same effect as
b=msg{1}.Poses
b =
10000×1 ROS Pose message array with properties:
MessageType
Position
Orientation
Just creates a Pose message array. Still doesn't let me access the data inside.

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Saurabh Gupta
Saurabh Gupta 2017-7-21
You could use arrayfun to perform the operations. I don't know whether it will be faster, but you will be able to avoid loops.
For example, for calculating the sum of all b.Position.X values, the following should work.
>> anonX = @(var)var.Position.X;
>> sumX = sum(arrayfun(anonX, b));
  2 个评论
Vikrant Shah
Vikrant Shah 2017-7-21
This makes it look nicer but doesn't help with the speed. In fact it makes slower. Time per message increased from 2.5 secs to 3.5 secs.
chef13
chef13 2018-3-6
Hi, did you reach to solve your problem ?
I am trying to use PoseArray messages in SIMULINK (but if it works I could also use matlab) but I have a lot of problems.
Thanks

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