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multithreading in c++, capturing thread stops after some loops

asked 2019-03-20 05:30:38 -0500

nobot gravatar image
  • opencv 3.4.2
  • OS ubuntu 16.04
  • g++ 7.4.0

    I am trying capture and process image on different threads.

    In Mcapture(), the image is captured; and after that if the processing threads completed processing, then the captured image is divided into 2 parts and moved to global variables, else it goes on capturing.

    In the other 2 threads , firstly the loop for saving the frame into global variables is paused, and after that a dummy processing is done, and after that , 2 half processed image is combined to a global UMat, to get the full image.

The code is running indefinitely, but after some 9-10 iterations , In Mcapture() it is going through the capture loop but it is failing to enter the loop (which divides the image and moves it to a global variable). And also the output of the combined processed images are not in sync with each other

Here is the link for my source_code, op_log, and the op_image. It will be very helpful, to get some help, I am getting no clue why that loop is stopping in between.

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please put (the relevant part of) your code into the question, not on an external dropbox, thank you.

berak gravatar imageberak ( 2019-03-20 06:32:38 -0500 )edit

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answered 2019-03-20 09:58:50 -0500

kbarni gravatar image

Well, your code is quite complicated, I didn't test it, and I don't understand all of it either. This is more a comment (but it's too long, so I have to put it as an answer). So a few ideas:

  • keep your code simple, it's much easier to find a bug.
  • use opencv routines to copy an image (frm.copyTo()) instead of std::move.
  • capturing from a webcam doesn't take much time, so it isn't necessary to parallelize it*.

It will be much easier to capture the image, then do the processing in parallel, and when it's ready, capture the next frame. Here's a pseudocode:

Rect roi[10];
Mat frm(480,640,CV_8UC3);
Mat subfrm[10];
for(int i=0;i<10;i++)subfrm[i]=frm(roi[i]);  //define 10 ROIs on the image
while(1){
    cap>>frm;   //get the image
    for(int i=0;i<10;i++)thread[i].run(subfrm[i]);  //launch a thread for each ROI
    for(int i=0;i<10;i++)thread[i].join();  //wait for all the threads to finish
    imshow("Result",frm); //display the result
}

This code solves the problem of combining the frames, and saves some time because it won't make copies of the image. You won't need global variables or concurrency check (mutex).

*Of course, this isn't true anymore if you want to capture a large quantity of data (e.g. high resolution RAW image with high framerate) in a time-critical application.

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Thank you for the suggestion, but i am using the threads for these exact purposes, my input is 1920*1080 of 60 fps, and it has to be done in a little endian ARM processor odroid xu4

And I do not want to do these for(int i=0;i<10;i++)thread[i].run(subfrm[i]); for(int i=0;i<10;i++)thread[i].join(); as my loop will be infinite, and so it will increase the overall thread cost, I just want it to wait for next frame.

I am getting the output but the 2 threads are not in sync with each other even, I used the lock so the particular frame will be only divided among the threads.

mut_frames.lock(); block1= std::move(frm(roic1)); block2= std::move(frm(roic2)); mut_frames.unlock();

nobot gravatar imagenobot ( 2019-03-21 13:28:34 -0500 )edit
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Asked: 2019-03-20 05:30:38 -0500

Seen: 64 times

Last updated: Mar 20