How to analyse video frames in parallel?

Question

Sam Duckerin 2017-4-20

2
链接

此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/336346-how-to-analyse-video-frames-in-parallel

回答： Megha Parthasarathy 2017-4-25

Hello, I'm working on processing large video files frame-by-frame. The processing for each frame is independent of other frames and is also computationally intensive, so I figured parallel processing would be a great way to speed up my analysis. While I have taught myself the basics of using parallel loops, I'm having problems combining the specifics of parfor with VideoReader objects. In my mind I imagine the code running like this

 video = VideoReader('video.mp4'); 
 parfor ii = 1 : 90000
     frame = read(video, ii);
     ...analysis on frame...
 end

However this warns me against using read() because it will be removed in a future version, so the only alternative I know of is to use frameRead(). However frameRead uses the CurrentTime property of the VideoReader object, which increments itself (according to the fps) each time frameRead is called. This works fine for reading frames in a normal loop, but it makes parfor unhappy because each frame relies on increasing the CurrentTime according to the last. Is there a way to access independent frames in a parallel loop using readFrame or otherwise? I've tried to set the CurrentTime value in each loop by using the loop index and the frame rate like this:

 video = VideoReader('video.mp4');
 fps = video.FrameRate
 results = cell(totalFrames, 1);
 parfor ii = 1 : 900000
     video.CurrentTime = ii/fps;
     frame = readFrame(video);
     results{ii} = customAnalysisFunction(frame)
 end

In this example the parfor is underlined and this message is provided:

   MATLAB runs loops in parfor functions by dividing the loop iterations into groups, 
and then sending them to MATLAB workers where they run in parallel. For MATLAB to do
this in a repeatable, reliable manner, it must be able to classify all the variables 
used in the loop. The code uses the indicated variable in a way that is incompatible 
with classification