How to get individual image from video file with the .mat format?

Hello,

I have video files of the color image (480x640x3 uint 8) and depth image (480x640 uint 16) with the .mat format both. I need to extract frames and save individual frames to separate image of jpeg files.

I tried a few ways to extract those files as suggested but they just didn't work such cell2mat and cat. I failed to convert using VideoReader since the .mat is not supported.

Am I need to convert the files to .mp4 video format first? If yes, how to do it? Any tips?

Thank you very much

Answer 1

[Jan]

MAT files are created by Matlab and can contain data of any class and shape. Knowing the class of the single images is not sufficient to know, how the movie is store inside the file. The readers cannot guess this detail, but you can check it easily:

Data = load('YourFile.mat')
disp(Data)

What do you see? After a load command, we are not talking about importing from a MAT file, but now the inputs are the variables found in this files. And if you post, what this is, we can create a solution.

Usually It is useful if you post the failing code and the corresponding error messages.

[hana razak]

Thanks for the quick response.

Applying your code,

Data = load('squat.mat');
disp(Data);

gives this,

imagecolor: {1x61 cell}
imagedepth: {1x61 cell}

and

In both Data.imagecolor and Data.imagedepth contain 61 cells of color image (480x640x3 uint 8) and depth image (480x640 uint 16).

This is the failing code,

Folder='Z:\v_frames';
vid=VideoReader('Z:\data2\squat.mat');
numFrames = vid.NumberOfFrames;
n=numFrames;
for iFrame = 1:n
  frames = read(vid, iFrame);
  imwrite(frames, fullfile(Folder, sprintf('%06d.jpg', iFrame)));
end

This is the error message,

Unable to determine the codec required.

Error in VideoReader (line 172)
            obj.init(fileName);

Error in video_frame (line 4)
vid=VideoReader('Z:\data2\squat.mat');

I'm hoping we can find the solution for this.

Thank you so much

[Jan]

Why do you use VideoReader, when there is no hint anywhere, that it supports .mat files? The solution is easy:

Data   = load('YourFile.mat');
nFrame = numel(Data.imagecolor);
for iFrame = 1:nFrame
   imwrite(Data.imagecolor{iFrame}, ...  % [EDITED], was {k}
           fullfile(Folder, sprintf('%06d.jpg', iFrame)));
end

And equivalently for Data.imagedepth also.

Do you see it? .mat means that it contains a set of Matlab variables. The VideoReader cannot know, in which format the data are stored, e.g. which fieldnames you use. But with load the variables are directly available.

[hana razak]

Thank you so much for the explanation.

But what is {k} in Data.imagecolor{k}? What does it refer to?

Thank you once again

[Jan]

After Data = load('File.mat') you get a struct with the fields:

Data.imagecolor: {1x61 cell}
Data.imagedepth: {1x61 cell}

This means, that these two fields contain cell arrays with the fieldnames imagecolor and imagedepth. Then

Data.imagecolor{k}

is the k.th element of the cell array Data.imagecolor.

[hana razak]

Thanks a lot. It works but how to display all elements of cell array at once instead of the k.th element only?

I tried to read for all elements,

Data.imagecolor{:}

but it failed with the error messages,

A filename must be supplied as either the second or the third argument.

Error in imwrite (line 418)
[data, map, filename, format, paramPairs] = parse_inputs(varargin{:});

I don't really understand what it means.

Please help me with this.

Thank you

[Jan]

I do not understand what "display all elements of cell array at once" means. The posted code does not display anything.

The line

Data.imagecolor{:}

cannot produce the show error message, which concerns imwrite. So please post the failing code.

[hana razak]

The code that you've given only show k.th element and it repeated for 61 times. One cell should be for one frame. I asked about to "display all elements of cell array at once" meaning that I wish to have the first frame till the last frame in a folder instead of k.th element repeating for 61 times.

I'm using your given code and it works well for each frame until I changed it from {k} to {:} while wishing to get all elements in a folder,

Folder='Z:\v_frames';
Data   = load('Z:\data2\a3.mat');
nFrame = numel(Data.imagecolor);
for iFrame = 1:nFrame
   imwrite(Data.imagecolor{:}, ...
         fullfile(Folder, sprintf('%06d.jpg', iFrame)));
end

And, this code gives the error message earlier.

Thank you

[hana razak]

Hello Mr. Jan,

It's been over a week since your last response. I hope you are well and healthy so you can come back to help us here as usual.

I sincerely pray for your health because this is not your usual response.

I really need the last answer so I can process my video way much faster instead of doing it one by one.

If Mr. Jan is not available, I hope someone else can help me with this. I'm waiting for my question to be answered as soon as possible.

Thank you so much.

[Stephen23]

@hana razak: Jan made one trivial mistake with the indexing, which is easy to fix:

Data   = load('YourFile.mat');
nFrame = numel(Data.imagecolor);
for kk = 1:nFrame
    imwrite(Data.imagecolor{kk}, ...
        fullfile(Folder, sprintf('%06d.jpg',kk)));
end

"I really need the last answer so I can process my video way much faster instead of doing it one by one."

Given that your goal is to save the frames individually, I doubt that there is any convenient way to speed up this process: the slowest part is going to be the file reading/writing, not the very simple loop that occurs in MATLAB. Any attempt to speed up your code should be focused on the hard-drive (e.g. use an SSD rather than a HD, or a combo drive). Rather than guessing how to speed up this process you need to learn where the bottle-necks are. Start by using the profiler:

https://www.mathworks.com/help/matlab/ref/profile.html

https://www.mathworks.com/help/matlab/matlab_prog/techniques-for-improving-performance.html

[hana razak]

Thank you so much.

It's work well for Data.imagecolor but it doesn't work for Data.imagedepth.

When I run for Data.imagedepth, it gives this error messages.

Error using writejpg>set_jpeg_props (line 183)
UINT16 image data requires bitdepth specifically set to either 12 or 16.

Error in writejpg (line 49)
props = set_jpeg_props(data,varargin{:});

Error in imwrite (line 472)
        feval(fmt_s.write, data, map, filename, paramPairs{:});

Error in test_squat (line 13)
   imwrite(Data.imagedepth{kk}, ...

I hope you can help me in solving this error.

Thank you so much.