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i modified /samples/cpp/contours2.cpp

i hope it will be a good start to solve your problem.

try to find a way for better result or wait my update.

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

using namespace cv;
using namespace std;

const int w = 500;
int levels = 3;

vector<vector<Point> > contours;
vector<Vec4i> hierarchy;

static void on_trackbar(int, void*)
{
    Mat cnt_img = Mat::zeros(w, w, CV_8UC3);
    int _levels = levels - 3;
    drawContours( cnt_img, contours, _levels <= 0 ? 3 : -1, Scalar(128,255,255),
                  1, LINE_AA, hierarchy, std::abs(_levels) );

    imshow("contours", cnt_img);
}

int main( int, char** argv)
{
    Mat img = imread(argv[1], 0);
    img = img < 170;
    imshow( "image", img );
    //Extract the contours so that
    vector<vector<Point> > contours0;

    findContours( img, contours0, hierarchy, RETR_TREE, CHAIN_APPROX_SIMPLE);

    contours.resize(contours0.size());
    for( size_t k = 0; k < contours0.size(); k++ )
        approxPolyDP(Mat(contours0[k]), contours[k], 3, true);

    namedWindow( "contours", 1 );
    createTrackbar( "levels+3", "contours", &levels, 7, on_trackbar );

    on_trackbar(0,0);
    waitKey();

    return 0;
}

i modified /samples/cpp/contours2.cpp

i hope it will be a good start to solve your problem.

try to find a way for better result ( you can filter contours using their roundness ) or wait my update.

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

using namespace cv;
using namespace std;

const int w = 500;
int levels = 3;

vector<vector<Point> > contours;
vector<Vec4i> hierarchy;

static void on_trackbar(int, void*)
{
    Mat cnt_img = Mat::zeros(w, w, CV_8UC3);
    int _levels = levels - 3;
    drawContours( cnt_img, contours, _levels <= 0 ? 3 : -1, Scalar(128,255,255),
                  1, LINE_AA, hierarchy, std::abs(_levels) );

    imshow("contours", cnt_img);
}

int main( int, char** argv)
{
    Mat img = imread(argv[1], 0);
    img = img < 170;
    imshow( "image", img );
    //Extract the contours so that
    vector<vector<Point> > contours0;

    findContours( img, contours0, hierarchy, RETR_TREE, CHAIN_APPROX_SIMPLE);

    contours.resize(contours0.size());
    for( size_t k = 0; k < contours0.size(); k++ )
        approxPolyDP(Mat(contours0[k]), contours[k], 3, true);

    namedWindow( "contours", 1 );
    createTrackbar( "levels+3", "contours", &levels, 7, on_trackbar );

    on_trackbar(0,0);
    waitKey();

    return 0;
}