Is it possible to use parallel computing with VideoReader?

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Nathan Pucheril 2015-7-20

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评论： Walter Roberson 2017-7-5

I am trying to read frames from a video in a parallel fashion, possibly using parfor. But parfor states that it cannot be used do to the nature of VideoReader. Any ways around this?

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Dinesh Iyer 2015-7-21

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Nathan, I am getting the following error:

Cannot call READ method after using READFRAME or HASFRAME methods or setting the CURRENTTIME property. Recreate the object to read a frame at a specific frame index.

when I execute:

vidObj = VideoReader('xylophone.mp4');
numFrames = vidObj.NumberOfFrames
parfor cnt = 1:numFrames
    data = read(vidObj, cnt);
end

Are you getting the same error?

I think this is a bug that is addressed by the patch: Bug Report 1209823

Hope this helps.

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Sam Duckerin 2017-7-5

Walter could you please give an example about how you might make a copy of a VideoReader object that would allow the frames to be read in parallel?

Walter Roberson 2017-7-5

Hmmmm, it appears that VideoReader objects are handle class, and it appears that they do not have a copy semantics built in.

https://www.mathworks.com/matlabcentral/newsreader/view_thread/257925

Ummm, it just might work to use https://www.mathworks.com/help/distcomp/parallel.pool.constant.html parallel.pool.Constant> from R2015b onwards

If you are doing a lot of reading from the same file, it might be worth using parfevalOnAll() to create the objects on each of the workers, which you would do once each time the file changed, rather than once for each parfor iteration.

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