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Need to iterate through an array faster

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hector martinez
hector martinez 2019 年 1 月 29 日
コメント済み: hector martinez 2019 年 1 月 29 日
I'm currently reading frames from one video and writing them to another, I have a huge bottleneck in my for loop.
vidObj = VideoReader('inputVideo.wmv');
outputVideo = VideoWriter('outputVideo.avi');
outputVideo.FrameRate = 24;
%Change FrameRate without changing video length
numberFrames=vidObj.Duration * vidObjB.FrameRate;
for i=1:length(Frames)
Is there a way to pass Frames directly without indexing? Python allows you to do 'for item in list....' is there something in matlab that does the same?
  2 件のコメント
hector martinez
hector martinez 2019 年 1 月 29 日
In messing with my code some more I've realized indexing is not the issue. Passing the frame number directly
for k=1:vidObj.FrameRate/24:numberFrames
makes no difference.
In a previous exercise the individual frames were already available in a folder. I didn't realize reading from a video file would take a lot longer.


回答 (1 件)

OCDER 2019 年 1 月 29 日
Do not use read. Use readFrame instead, since read will re-read everything from beginning to end.
i = 1;
while hasFrame(vidObjB)
writeVideo(outputVideo, readFrame(vidObjB));
fprintf('%d\n', i);
i = i + 1;
If that doesn't work, are you using a solid state hard drive, or a spinning magnetic hard drive? The SSD might speed things up as this could be a read/write speed issue. Otherwise the compression/decompression could be the slow step.
  1 件のコメント
hector martinez
hector martinez 2019 年 1 月 29 日
The reason I am using read instead of readFrame is because I need to access specific frames to be able to change the framerate without changing the video length. As far as I know readFrame doesn't allow reading specific frames.
But your comment about ssd vs magnetic drives is a valid point. I was doing all my testing on a machine with a magnetic drive with an elapsed time of 337.96 sec, the final version will run on a machine with an ssd. I tested my this code on that machine and the elapsed time is 13.03 seconds. So not really an issue anymore.
Thank you for making that point.


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