Get one circle after Hough transform method - c++

Question

I use Hough transform method so I get 2 circles, how can I get just the zone of the big circle from the for loop?
vector<Vec3f> circles;
/// Apply the Hough Transform to find the circles;
HoughCircles(openImg, circles, CV_HOUGH_GRADIENT, 1,1,67, 17,35, 80);
/// 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 center
circle(openImg, center, 1, Scalar(255, 255, 255), -1, 8, 0);
// circle outline
circle(openImg, center, radius, Scalar(255, 255, 255), 1, 4, 0);
}
/// Show your results
namedWindow("Hough Circle Transform Demo", CV_WINDOW_AUTOSIZE);
imshow("Hough Circle Transform Demo", openImg);

Answer 1

The documentation of opencv states:
circles – Output vector of found circles. Each vector is encoded as a 3-element floating-point vector (x, y, radius)
https://docs.opencv.org/3.4.1/d3/de5/tutorial_js_houghcircles.html
You are reading the radius (int radius = cvRound(circles[i][2]);). That is actually the size of the circle.
So, you need to loop through your array circles and pick the circle with the biggest radius:
// remember biggest radius
float radiusBiggest = 0;
// remember the index of the biggest radius / circle
int indexBiggest = -1;
// loop through all circles
for (size_t i = 0; i < circles.size(); i++)
{
// get the radius
float radius = circles[i][2];
// check if this radius is bigger than any previous
if (radius > radiusBiggest)
{
// this radius/circle is the biggest so far. remember it
radiusBiggest = radius;
indexBiggest = i;
}
}
// if we found a circle then draw it
if (indexBiggest != -1)
{
Point center(cvRound(circles[indexBiggest][0]), cvRound(circles[indexBiggest][1]));
int radius = cvRound(circles[indexBiggest][2]);
// circle center
circle(openImg, center, 1, Scalar(255, 255, 255), -1, 8, 0);
// circle outline
circle(openImg, center, radius, Scalar(255, 255, 255), 1, 4, 0);
}