#include <iostream>
#include "opencv2\opencv.hpp"

using namespace std;
using namespace cv;

int main()
{
    Mat a=imread("Capture.JPG",1);

    Mat b(a,Rect(0,0,30,30));

    line(b,Point(0,0),Point(30,30),Scalar(0,255,255));

    imshow("a",a);
    imshow("b",b);
    waitKey();
    return 0;
}