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you can try the code below which finds a center point between all contours and sorts contours circularly ( maybe this will be a starting point )

#include "opencv2/core.hpp"
#include "opencv2/imgproc.hpp"
#include "opencv2/highgui.hpp"
#include <iostream>

using namespace cv;
using namespace std;

Point center;

struct contour_sorter
{
    bool operator ()(const vector<Point>& a, const vector<Point>& b)
    {
        double angle1 = atan2(a[0].y - center.y, a[0].x - center.x) * 180.0 / CV_PI;
        double angle2 = atan2(b[0].y - center.y, b[0].x - center.x) * 180.0 / CV_PI;
        return (angle1 < angle2);
    }
};

int main(int argc, char** argv)
{
    Mat src = imread("15931824191786043.jpg");
    Mat bw;

    cvtColor(src, bw, COLOR_BGR2GRAY);
    bw = bw < 127;

    Rect r = boundingRect(bw);
    center = (r.tl() + r.br()) * 0.5;

    // Find contours
    vector<vector<Point> > contours;
    vector<Point> allcontours;
    findContours(bw, contours, RETR_EXTERNAL, CHAIN_APPROX_NONE);

    sort(contours.begin(), contours.end(), contour_sorter());

    for (size_t i = 0; i < contours.size()-1; i++)
    {
        drawContours(src, contours, i, Scalar(0, 0, 255), 1);

        imshow("src", src);
        waitKey(0);
    }
    waitKey(0);
}