How do I get readImage (ROS) to work?

Question

Ulrik Söderström 2019-4-10

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此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/455547-how-do-i-get-readimage-ros-to-work

编辑： Cam Salzberger 2020-1-2

I have a file recorded with a RealSense-camera and it is saved as a rosbag file .bag.

I want to access the individual frames as images through Matlab and I have found information how to do from different forums. One example is from Matlab answers: https://se.mathworks.com/matlabcentral/answers/407981-access-images-from-within-a-rosbag

The problem is that that the function readImage doesn't work.

Depending on how I use it I get different error messages.

Either:

Array indices must be positive integers or logical values.

The error is generated with this code:

bag = rosbag('20190402_093734.bag');

bagInfo = rosbag('info', '20190402_093734.bag')

bSel = select(bag, 'Topic', '/device_0/sensor_1/Color_0/image/data');

msgs = readMessages(bSel,"DataFormat","struct");

img = readImage(msgs{1});

Or:

Undefined function or variable 'readImage'.

The error is generated with this code:

bag = rosbag('20190402_093734.bag');

bagselect1 = select(bag, 'Topic', '/device_0/sensor_0/Depth_0/image/data')

bagselect2 = select(bag, 'Time', [1 5], 'Topic', '/device_0/sensor_0/Depth_0/image/data')

bagselect2.AvailableTopics{1,end}

bagselect3 = select(bagselect2, 'Time', [1 5])

msgs = readMessages(bagselect3,"DataFormat","struct");

img = readImage(msgs{1});

I thought there was a problem with my installation of ROS so I tried using the online version of MATLAB but I got the same error message there.

I am assuming that there is a very simple, logical solution to this but I have no idea of what it might be. Could it be a problem with the .ros-file?

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Answer 1

Cam Salzberger 2019-4-11

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https://ww2.mathworks.cn/matlabcentral/answers/455547-how-do-i-get-readimage-ros-to-work#answer_370197

Hello Ulrik,

It's good that you are experimenting with the 'DataFormat','struct' option for extracting messages from rosbags, since it is a big performance improvement. Unfortunately, the convenience functions (readImage, readPointCloud, etc.) don't yet support accepting structs as input, and are expecting ROS message objects. If you want to use readImage, I would suggest leaving off the 'DataFormat','struct' option, and passing the output to readImage. Otherwise, you can manually extract the image from the ROS message structure.

-Cam

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Mohamed Atia 2019-12-27

Could you please post the code that you used to "create an empty message and copy all the content (except Header) " so you can use readImage ? Thanks

Cam Salzberger 2020-1-2

编辑：Cam Salzberger 2020-1-2

在 MATLAB Online 中打开

Something like this should work, though I haven't tested it:

msgStructs = readMessages(bagSel, 'DataFormat', 'struct');
msg = copyImage(msgStructs{1});
I = readImage(msg);
function msg = copyImage(msgStruct)
    msg = rosmessage(msgStruct.MessageType);
    fNames = fieldnames(msg);
    for k = 1:numel(fNames)
        if ~any(strcmp(fNames{k}, {'MessageType', 'Header'}))
            msg.(fNames{k}) = msgStruct.(fNames{k});
        end
    end
end