To access a computer's webcam or camera device, you can simply call the open() function on a cv::VideoCapture object (OpenCV's method of accessing your camera device), and pass 0 as the default camera ID number. Some computers have multiple cameras attached, or they do not work as default camera 0, so it is common practice to allow the user to pass the desired camera number as a command-line argument, in case they want to try camera 1, 2, or -1, for example. We will also try to set the camera resolution to 640x480 using cv::VideoCapture::set() to run faster on high-resolution cameras.

You can put this code in the main() function of your main.cpp file:

    int cameraNumber = 0; 
    if (argc> 1) 
    cameraNumber = atoi(argv[1]); 

    // Get access to the camera. 
    cv::VideoCapture camera; 
    camera.open(cameraNumber); 
    if (!camera.isOpened()) { 
      std::cerr<<"ERROR: Could not access the camera or video!"<< 
      std::endl; 
      exit(1); 
    } 

    // Try to set the camera resolution. 
    camera.set(cv::CV_CAP_PROP_FRAME_WIDTH, 640); 
    camera.set(cv::CV_CAP_PROP_FRAME_HEIGHT, 480);

After the webcam has been initialized, you can grab the current camera image as a cv::Mat object (OpenCV's image container). You can grab each camera frame by using the C++ streaming operator from your cv::VideoCapture object into a cv::Mat object, just like if you were getting input from a console.