void circle(Mat img, Point center, int radius, Scalar color, int thickness=1, int lineType=8, int shift=0)

#include <iostream> 
#include <opencv2/core/core.hpp>
#include <opencv2/imgproc/imgproc.hpp>
#include <opencv2/opencv.hpp>
using namespace std;
using namespace cv;
int main()
{
  Mat src = imread("C:\\tupian\\test1.jpg", 3);
  circle(src, Point(src.cols/ 2, src.rows / 2), 30, Scalar(0, 0, 255));
  imshow("src", src);
  waitKey(0);
  return 0;
}