samples/cpp/tutorial_code/ImgTrans/houghcircles.cpp

An example using the Hough circle detector

#include "opencv2/imgcodecs.hpp"
#include "opencv2/highgui.hpp"
#include "opencv2/imgproc.hpp"

using namespace cv;
using namespace std;

int main(int argc, char** argv)
{
    const char* filename = argc >=2 ? argv[1] : "../data/smarties.png";

    // Loads an image
    Mat src = imread( filename, IMREAD_COLOR );

    // Check if image is loaded fine
    if(src.empty()){
        printf(" Error opening image\n");
        printf(" Program Arguments: [image_name -- default %s] \n", filename);
        return -1;
    }

    Mat gray;
    cvtColor(src, gray, COLOR_BGR2GRAY);

    medianBlur(gray, gray, 5);

    vector<Vec3f> circles;
    HoughCircles(gray, circles, HOUGH_GRADIENT, 1,
                 gray.rows/16,  // change this value to detect circles with different distances to each other
                 100, 30, 1, 30 // change the last two parameters
            // (min_radius & max_radius) to detect larger circles
    );

    for( size_t i = 0; i < circles.size(); i++ )
    {
        Vec3i c = circles[i];
        Point center = Point(c[0], c[1]);
        // circle center
        circle( src, center, 1, Scalar(0,100,100), 3, LINE_AA);
        // circle outline
        int radius = c[2];
        circle( src, center, radius, Scalar(255,0,255), 3, LINE_AA);
    }

    imshow("detected circles", src);
    waitKey();

    return 0;
}