HOW TO USE SIFT WITH VIDEO [closed]

edit

I WANT TO RUN SIFT (OPENCV 2.9 VERION), BUT NOT ONLY on ONE frame. I want to use my webcam live and see the result on the video

how to change my code ?

#include <opencv2/opencv.hpp>
#include <opencv2/nonfree/nonfree.hpp>
using namespace cv;

int main()
{

Mat img = imread("C:\\Users\\user\\Documents\\Visual Studio 2015\\Projects\\yakir-baby-analistic\\yakir1\\255.jpg", CV_LOAD_IMAGE_COLOR);   // Read the file

SIFT sift;
vector<KeyPoint> key_points;

Mat descriptors;
sift(img, Mat(), key_points, descriptors);

Mat output_img;
drawKeypoints(img, key_points, output_img);

namedWindow("Image");
imshow("Image", output_img);
waitKey(0);
destroyWindow("Image");



return 0;
}