Opencv hough circle not detecting circles

Question

I am trying to detect the circle inside traffic light, and I am able to detect only 1 out of the 2 circle, and the size of the circle which i am getting seems to be too big

Input Image: https://i.imgur.com/VkNDt2B.png

Output image: https://i.imgur.com/BBq5tE0.png

int main()
{
    Mat src, gray;
    src = imread("C:\/test_image2.png", 1);
    resize(src, src, Size(640, 480));

    cvtColor(src, gray, CV_BGR2GRAY);

    // Reduce the noise so we avoid false circle detection
    GaussianBlur(gray, gray, Size(9, 9), 2, 2);

    vector<Vec3f> circles;

    // Apply the Hough Transform to find the circles
    HoughCircles(gray, circles, CV_HOUGH_GRADIENT, 1, 60, 200, 20, 0, 35);

    // Draw the circles detected
    for (size_t i = 0; i < circles.size(); i++)
    {
        Point center(cvRound(circles[i][0]), cvRound(circles[i][1]));
        int radius = cvRound(circles[i][2]);
        circle(src, center, 3, Scalar(0, 255, 0), -1, 8, 0);// circle center     
        circle(src, center, radius, Scalar(0, 0, 255), 3, 8, 0);// circle outline
        cout << "center : " << center << "\nradius : " << radius << endl;
    }

    // Show your results
    namedWindow("Hough Circle Transform Demo", CV_WINDOW_AUTOSIZE);
    imshow("Hough Circle Transform Demo", src);

    waitKey(0);
    return 0;
}

L.C. L.C. · Accepted Answer · 2018-11-11T08:16:48

HoughCircles works best if you know in advance the approx size of the circles you're looking for. I suggest you give a better value for min_radius and max_radius parameters. In any case, you need to play with param1 and param2 parameters. If circles are not perfect circles you can try to lower the image resolution using the dp parameter (f.ex. with dp = 2 the image is downscaled to half its resolution). Basically: play with param1 and param2 until your circles are detected, no matter if other circles are detected. Use this result to find out what radius your circles are, then fix the min and max radius to remove most circles you don't want and finally play again with param1 and param2 until only your circles are left.

Opencv hough circle not detecting circles

2 Answers