OpenCV's VideoCapture::open Video Source Dialog - c++

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;
}