OpenCV houghcircles deteting nothing C++ - c++

Question

I am having a fairly annoying issue using OpenCV's HoughCircle method to detect circles in an image. I copied the code found in the official documentation and thus far I have been unable to detect anything. The circles vector has a size of 0 after calling the function, therefore there were no circles detected.
I have tried it with multiple images, such as .ppm images, .jpg and none of which end up with circles being detected. I really have no idea what could be going wrong.
If anyone has any idea of what I should try I'd be extremely thankful.
using namespace cv;
Mat src = imread("Images/balls.jpg");
if(! src.data )
{
std::cout << "Could not open or find the image" << std::endl ;
return -1;
}
Mat src_gray2;
cvtColor(src, src_gray2, CV_BGR2GRAY );
GaussianBlur( src_gray2, src_gray2, cv::Size(9, 9), 2, 2 );
vector<Vec3f> circles;
HoughCircles(src_gray2, circles, CV_HOUGH_GRADIENT, 1, src_gray2.rows/8, 200, 100, 0, 0 );
std::cout << circles.size();

Answer 1

This works for me. I adjusted the arguments to the HoughCircles function. See below. Also, I used this book to help me out: OpenCV 2
#include <cstdio>
#include <iostream>
#include <opencv2/core/core.hpp>
#include <opencv2/highgui/highgui.hpp>
#include <opencv2/imgproc/imgproc.hpp>
#include <opencv2/features2d/features2d.hpp>
int main(int argc, char** argv) {
using namespace cv;
cv::Mat src=cv::imread("JGRiM.jpg");
if (!src.data) {
std::cout << "ERROR:\topening image" <<std::endl;
return -1;
}
cv::namedWindow("image",CV_WINDOW_AUTOSIZE);
cv::imshow("image",src);
Mat src_gray2;
cvtColor(src, src_gray2, CV_BGR2GRAY );
GaussianBlur( src_gray2, src_gray2, cv::Size(9, 9), 2, 2 );
vector<Vec3f> circles;
HoughCircles(src_gray2, circles, CV_HOUGH_GRADIENT,
2, // accumulator resolution (size of the image / 2)
5, // minimum distance between two circles
100, // Canny high threshold
100, // minimum number of votes
0, 1000); // min and max radius
std::cout << circles.size() <<std::endl;
std::cout << "end of test" << std::endl;
std::vector<cv::Vec3f>::
const_iterator itc= circles.begin();
while (itc!=circles.end()) {
cv::circle(src_gray2,
cv::Point((*itc)[0], (*itc)[1]), // circle centre
(*itc)[2], // circle radius
cv::Scalar(255), // color
2); // thickness
++itc;
}
cv::namedWindow("image",CV_WINDOW_AUTOSIZE);
cv::imshow("image",src_gray2);
cv::waitKey(0);
return 0;
}

Answer 2

You need to change param2 to a lower value to find more circles. For example in the image posted above in the comments to the question with param2 = 20 I found a circle around the tennis ball.
HoughCircles(src_gray2, circles, CV_HOUGH_GRADIENT, 1, src_gray2.rows/8, 200, 20, 0, 0 );