How to process video in parallel?

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Ilya Tcenov 2019-8-3

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https://ww2.mathworks.cn/matlabcentral/answers/474700-how-to-process-video-in-parallel

评论： Ilya Tcenov 2019-8-10

Hello.

I need to read a video file (N frames long), process each adjucent pair of frames (to calculate a single output frame), and write the result as a video file (N-1 frames long). Processing time is important to me, so I am trying to do this in parallel.

I can't use a gpuArray() to pass a variable that was created using VideoReader() to the gpu (I get an error). It seems pointless to pass a pair of frames to the gpu and gather the result, as the transition of data back and forth takes longer than the calculation itself. Is there another way of using the gpu in my case?

I can use multiple workers (CPU), but the problem is that I cant write the result of a processed pair as a frame (using writeVideo() ), before the previous frame was writen. I need to make the worker wait if the previous frame wasn't writen yet. What will be the correct way of doing so?

Thank you.

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Divya Gaddipati 2019-8-6

Does your current output frame depend on the previous output frame?

Ilya Tcenov 2019-8-10

编辑：Ilya Tcenov 2019-8-10

It doesn't.

Output(n) = function(Input(n),Input(n+1))

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Answer 1

Divya Gaddipati 2019-8-8

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https://ww2.mathworks.cn/matlabcentral/answers/474700-how-to-process-video-in-parallel#answer_386752

One possible approach is by using spmd. You could define spmd in such a way that each worker will operate on a chunk of the input video, and then within spmd it is possible to force each worker to write the resultant data in the specified order. Something like this:

spmd 
    myFrameIdxs = find(discretize(1:totalNumFrames, numlabs) == labindex);  
   % numlabs - Total number of workers operating in parallel on current job 
    % read chunk of data 
    myFrameData = readVideoData(videoFname, myFrameIdxs); 
    % process chunk of data 
    myOutput = processVideoFrames(myFrameData); 
    % output chunks of data in sequence, one at a time. 
    for writeIdx = 1:numlabs 
        if labindex == writeIdx 
            writeVideoData(outVideoFname, myOutput); 
        end 
    end 
end 