OpenCV's VideoCapture::open Video Source Dialog

Question

In my current project, when I call VideoCapture::open(camera device index) and the camera is in used by another program, it shows a Video Source dialog and returns true when I select a device that is already in use.

However, in my [previous] experiment project, when I called VideoCapture::open(camera device index), it doesn't show this dialog.

I want to know what is causing the Video Source dialog to show and the program to behave differently from the experimental project.

This is the source code to the experiment project:

int main (int argc, char *argv[])
{

    //vars
    time_duration td, td1;
    ptime nextFrameTimestamp, currentFrameTimestamp, initialLoopTimestamp, finalLoopTimestamp;
    int delayFound = 0;
    int totalDelay= 0;

    // initialize capture on default source
    VideoCapture capture;
    std::cout << "capture.open(0): " << capture.open(0) << std::endl;
    std::cout << "NOOO" << std::endl;
    namedWindow("video", 1);

    // set framerate to record and capture at
    int framerate = 15;

    // Get the properties from the camera
    double width = capture.get(CV_CAP_PROP_FRAME_WIDTH);
    double height = capture.get(CV_CAP_PROP_FRAME_HEIGHT);

    // print camera frame size
    //cout << "Camera properties\n";
    //cout << "width = " << width << endl <<"height = "<< height << endl;

    // Create a matrix to keep the retrieved frame
    Mat frame;

    // Create the video writer
    VideoWriter video("capture.avi",0, framerate, cvSize((int)width,(int)height) );

    // initialize initial timestamps
    nextFrameTimestamp = microsec_clock::local_time();
    currentFrameTimestamp = nextFrameTimestamp;
    td = (currentFrameTimestamp - nextFrameTimestamp);

    // start thread to begin capture and populate Mat frame
    boost::thread captureThread(captureFunc, &frame, &capture);
    // loop infinitely
    for(bool q=true;q;)
    {
        if(frame.empty()){continue;}
        //if(cvWaitKey( 5 ) == 'q'){ q=false; }
        // wait for X microseconds until 1second/framerate time has passed after previous frame write
        while(td.total_microseconds() < 1000000/framerate){
            //determine current elapsed time
            currentFrameTimestamp = microsec_clock::local_time();
            td = (currentFrameTimestamp - nextFrameTimestamp);
            if(cvWaitKey( 5 ) == 'q'){
                std::cout << "B" << std::endl;
                q=false;
                boost::posix_time::time_duration timeout = boost::posix_time::milliseconds(0);
                captureThread.timed_join(timeout);
                break;
            }
        }

        // determine time at start of write
        initialLoopTimestamp = microsec_clock::local_time();

        // Save frame to video
        video << frame;
        imshow("video", frame);

        //write previous and current frame timestamp to console
        cout << nextFrameTimestamp << " " << currentFrameTimestamp << " ";

        // add 1second/framerate time for next loop pause
        nextFrameTimestamp = nextFrameTimestamp + microsec(1000000/framerate);

        // reset time_duration so while loop engages
        td = (currentFrameTimestamp - nextFrameTimestamp);

        //determine and print out delay in ms, should be less than 1000/FPS
        //occasionally, if delay is larger than said value, correction will occur
        //if delay is consistently larger than said value, then CPU is not powerful
        // enough to capture/decompress/record/compress that fast.
        finalLoopTimestamp = microsec_clock::local_time();
        td1 = (finalLoopTimestamp - initialLoopTimestamp);
        delayFound = td1.total_milliseconds();
        cout << delayFound << endl;

        //output will be in following format
        //[TIMESTAMP OF PREVIOUS FRAME] [TIMESTAMP OF NEW FRAME] [TIME DELAY OF WRITING]
        if(!q || cvWaitKey( 5 ) == 'q'){
            std::cout << "C" << std::endl;
            q=false;
            boost::posix_time::time_duration timeout = boost::posix_time::milliseconds(0);
            captureThread.timed_join(timeout);
            break;
        }
    }

    // Exit
    return 0;
}