How can we use 'SPMD' for 'VideoReader'?

Question

MOHIT 2016-6-16

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https://ww2.mathworks.cn/matlabcentral/answers/290312-how-can-we-use-spmd-for-videoreader

编辑： MOHIT 2016-6-17

hi, I want to do do some video quality reduction fastly, I am using parallel computing , but this error is coming: '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'

my code is

videoSrc = VideoReader('rhinos.avi');
% v = VideoWriter('newfile.avi');
% open(v)
spmd 
        if labindex==1
            v = VideoWriter('newfile.avi');
open(v)
            for sr = 1:700
              huo = read(videoSrc,sr);
              RGB2 = imresize(huo, [300 NaN]);
          writeVideo(v,RGB2)
  % waitbar(sr / videoSrc.NumberOfFrames)
              end
              close(v)
        else
          R = rand(4,4);
          end
  end

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Answer 1

Sean de Wolski 2016-6-16

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在 MATLAB Online 中打开

Use readFrame() instead of read(). I'd also recommend building the videoSrc on the worker.

spmd
    if labindex==1
        videoSrc = VideoReader('rhinos.avi');
        for ii = 1:700
            if hasFrame(videoSrc)
                huo = readFrame(videoSrc);
            else
                break
            end
        end    
    end
end

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Answer 2

Steven Lord 2016-6-16

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Writing video to a file seems like an inherently serial operation since you care about the order in which the frames are written to the file. So I wouldn't try to parallelize this.

Picture watching Star Wars if the Death Star exploded first, then Princess Leia was rescued, then Luke and Obi-Wan met Han Solo in the cantina, then Luke was flying his X-Wing toward the Death Star trench. It doesn't really make sense out of order, does it?

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MOHIT 2016-6-17

编辑：MOHIT 2016-6-17

在 MATLAB Online 中打开

hi, yes you are right, output video needs to be in a sequence and this is the reason I am using 'spmd' not 'parfor'. Because 'spmd' gives results in an organised way.

see this:

spmd
      if labindex==1
      dataToSend=1:10
      elseif labindex==2
      dataToSend=1:2:10
       elseif labindex==3
      dataToSend=1:3:10
       elseif labindex==4
      dataToSend=1:4:10
      end
end