Can HoughCircles function in opencv detect circles within a circle?

Question

I came across HoughCircles in OpenCV for circle detection. But it has a parameter which specifies minimum distance between detected circles. My concern is will that work in case two circles are concentric i.e. one circle within another?

Thanks Shashank

Answer 1

Hough transform will only return 2 circles as two different objects if their centers are far enough apart (fifth parameter of Hough).
So I think it's not possible to detect concentric circles that way (since their centers will be the same, or very close by).

The only way I see how to do it with Hough transform is to have an idea of the radius of the circles you're looking for, and call Hough in a loop with varying min and max radius (last and next to last parameters of Hough), each radius iteration corresponding to one of your concentric circles.

Answer 2

I tried with the shown image

But it is detecting only one circle...here is the code

Mat image_ = imread("E:/Work_DataBase/circle.jpg",3);
Mat image_temp;
cvtColor( image_, image_temp, CV_BGR2GRAY );
vector<Vec3f> circles;
HoughCircles( image_temp, circles, CV_HOUGH_GRADIENT, 1, image_temp.rows/8,100, 100, 10, 200 );
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 center
    circle( image_, center, 3, Scalar(0,255,0), -1, 8, 0 );
   // circle outline
    circle( image_, center, radius, Scalar(0,0,255), 3, 8, 0 );
}
imshow("circles",image_);
waitKey(0);

I will try other options once if i find any alternative i will update you.

Answer 3

I also think the OpenCv HougCircles detect only one radius per center. If you want to detect more radii, you will have to specify a smaller min distance between centers. But then those are not concentric circles. In short, I think openCv HoughCircle module is not designed to cater for concentric circles.