How to use more than two cameras?

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How to use more than two cameras?

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I tried on with C++, on Java, result one. More than two cameras bug. I try to just save images from the camera to a folder.

void CameraThread(int id) {

    int sensivity = 30;
    double maxArea = 30;

    int widthFrame = 320;
    int heightFrame = 160;

    // для работы со списком камер
    Mat *frameI = new Mat(heightFrame, widthFrame, CV_8UC3);
    Mat *frameCurrentI = new Mat(heightFrame, widthFrame, CV_8UC3);
    Mat *framePreviousI = new Mat(heightFrame, widthFrame, CV_8UC3);
    Mat *frameResultI = new Mat(heightFrame, widthFrame, CV_8UC3);


    Size *size = new Size(3, 3);
    Mat *v = new Mat();

    // open camera
    std::cout << "thread camera id " << id << std::endl;

    VideoCapture cap(id);

    cap.set(CAP_PROP_FRAME_WIDTH, widthFrame);
    cap.set(CAP_PROP_FRAME_HEIGHT, heightFrame);


    std::string filename; 
    filename.append("camera_");
    filename.append(std::to_string(id));
    filename.append(".jpg");

    Mat frame;

    while (true) {

        if (!cap.grab()) {
            printf("failed to grab from camera\n");
        }
        else
        {
            cap.read(frame);

            if (frame.empty()) {
                printf("failed to grab from camera\n");
            }
            else {
                imwrite(filename, frame);
            }
        }

        /*if (cap.read(frame)) {
            m.lock();
            cap >> frame;
            imwrite(filename, frame);
            m.unlock();
        } */

        //Sleep(1000);

    }
}

int main()
{
    std::thread t0(CameraThread, 2);
    t0.detach();

    std::thread t1(CameraThread, 1);
    t1.detach();

    std::thread t2(CameraThread, 2);
    t2.detach();

    std::thread t3(CameraThread, 3);
    t3.detach();

    std::thread t4(CameraThread, 4);
    t4.detach();

    std::thread t5(CameraThread, 5);
    t5.detach();

    std::thread t6(CameraThread, 6);
    t6.join();

    waitKey();
    return 0;
}