VideoWriter fails to open inside thread

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EDIT: I've found some interesting behaviour related to codecs and threads in OpenCV. It seems that some codecs requires that multiple object of VideoWriter have to be initialized/opened in the main thread.

After this you can use them in worker threads. My experience:

• Using uncompressed codecfourcc=0 or XVID I can open/write VideoWriter in worker threads.
• Using X264 I have to open many VideoWriters in the main thread but I can use them in worker threads.

May be some lock occours and could depends on codec/VFW/DSHOW implementation.

The following code works to me with X264 and OCV2.4.10 or OCV 3.0.0. if you comment out the line writers[i].open(fname, codec, FPS, image_size); you can test opening in worker thread using XVID.

#define FRAME_W 320
#define FRAME_H 240
#define FPS 25

// This thread uses a VideoWriter object opened in the main thread
void ThreadStd(cv::VideoWriter *writer, const char*fn, int threadNum)
{
    string fname(fn); 
    time_t end_time;
    time_t current_time = time(NULL);
    time_t clip_length = 10;
    end_time = current_time + clip_length;
    Size image_size = Size(FRAME_W, FRAME_H);
    // some codecs don't allow to open Videowriter in multiple tread instance
    if (!writer->isOpened())
    {
        //XVID allows open here
        writer->open(fname, CV_FOURCC('X', 'V', 'I', 'D'), FPS, image_size);

        //uncompressed fourcc=0 allows open here
        //writer->open(fname, 0, FPS, image_size);

        // X264 needs Videowriter already opened in the main thread
        //writer->open(fname, CV_FOURCC('X', '2', '6', '4'), FPS, image_size);
    }
    if (!writer->isOpened())
    {
        std::cout << "\nThe video file has not been opened: " + std::string(fname) << endl;
    }
    else while (current_time < end_time)
    {
        Mat  webcam_frame = Mat::zeros(image_size, CV_8UC3);
        cv::putText(webcam_frame, "TH#:" + to_string(threadNum), 
                    cv::Point(10, 10), FONT_HERSHEY_PLAIN, 1, Scalar(255,255,255));
        writer->write(webcam_frame);
        current_time = time(NULL);
    }
    cout << "\nTime's up! from thread " << threadNum << std::endl;
    return;
}

int _tmain(int argc, _TCHAR* argv[])
{
    std::cout << "This will test the VideoWriter class's ability to work in threads.\n";
    int numThreads = 5;
    //thread list
    vector<thread> threads(numThreads);
    // we create a VideoWriter object for each thread
    vector<cv::VideoWriter> writers(threads.size());
    for (int i = 0; i < threads.size(); i++)
    {
        std::string fname = "../img/" + to_string(i) + ".avi";
        DeleteFileA(fname.c_str()); //delete file if it exist

        Size image_size = Size(FRAME_W, FRAME_H);
        int codec;
        codec = CV_FOURCC('X', '2', '6', '4');
        // open the VideoWriter here but...
        writers[i].open(fname, codec, FPS, image_size);
        // ...we write it in the thread
        threads[i] = thread(ThreadStd, &(writers[i]), fname.c_str(), i);
        cout << "\n\nStarted test thread #" << to_string(i);
    }
    // wait for all treads
    for (int i = 0; i < threads.size(); i++)
        threads[i].join();
    // release all writers
    for (int i = 0; i<writers.size(); i++)
        writers[i].release();
}