/**
 * Count current camera number
 */
int countCamera() {
    int maxCamNum = 5;
    int count = 0;
    for(int device = 0; device < maxCamNum; device++) 
    {
        CvCapture* capture;
        if (_capture[device]) {
            ++count;
        }
        else {
            capture = cvCaptureFromCAM(CV_CAP_DSHOW + device);
            if (capture) {
                ++count;
            }
            cvReleaseCapture(&capture);
        }
    }
    return count;
}