circle-detection issue - c++

Question

1.Some Information: I would like to develop a kind of circle recognition with the help of openCV. I successfully set up a connection between Swift, objc-c++, but strangely I have some problems with the circle recognition algorithm: Not all of the circles in my image gets detected!
2.Have a look at my code:
+(UIImage *)ConvertImage:(UIImage *)image {
cv::Mat matImage;
UIImageToMat(image, matImage);
cv::Mat modImage;
cv::medianBlur(matImage, matImage, 5);
cv::cvtColor(matImage, modImage, CV_RGB2GRAY);
cv::GaussianBlur(modImage, modImage, cv::Size(9, 9), 2, 2);
vector<Vec3f> circles;
cv::HoughCircles(modImage, circles, CV_HOUGH_GRADIENT, 1, 1, 100, 50, 0, 0);
for (auto i = circles.begin(); i != circles.end(); ++i)
std::cout << *i << ' ';
for( size_t i = 0; i < circles.size(); i++ )
{
cv::Point center(cvRound(circles[i][0]), cvRound(circles[i][1]));
int radius = cvRound(circles[i][2]);
circle( matImage, center, 3, Scalar(0,255,0), -1, 8, 0 );
circle( matImage, center, radius, Scalar(0,0,255), 3, 8, 0 );
}
UIImage *binImg = MatToUIImage(matImage);
return binImg;
}
As you can see in the image [click] there appears this issue :
Only 3 of 7 circles gets detected!
So in the docs I found the parameters explanation for this line:
cv::HoughCircles(modImage, circles, CV_HOUGH_GRADIENT, 1, 1, 100, 50, 0, 0);
dp = 1: The inverse ratio of resolution.
min_dist = modImage.rows/8: Minimum distance between detected centers.
param_1 = 200: Upper threshold for the internal Canny edge detector.
param_2 = 100*: Threshold for center detection.
min_radius = 0: Minimum radio to be detected. If unknown, put zero as default.
max_radius = 0: Maximum radius to be detected. If unknown, put zero as default.
3.My question
How to get rid of the issue mentioned above?
Any help would be very appreciated :)

Answer 1

For issue number 2 : The outline should be colored, not white!
What color should it be? At any rate you draw that circle in your code with this line.
circle( matImage, center, radius, Scalar(0,0,255), 3, 8, 0 );
If you want to change the color you can change the values you have declared in Scalar(0,0,255).
If you dont want the circle there at all you can remove that line of code.

Answer 2

Your images seems to be noise free. If the image is to contain circle always, You can extract the contours and fit circles using Least Squares
You can get the circle fit equations here. It is a straightforward implementation. Create a structure for the circle parameters (center and radius), fit circle and store the parameters in the structure and use it to draw circle using OpenCV.
You can also generate points on the circle using "ellipse2poly" function.