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I have got this image :

with this program :

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

using namespace cv;
using namespace std;

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

int main(int argc, char **argv)
{
Mat mThresh;
Mat m=imread("C:/Users/Laurent.PC-LAURENT-VISI/Downloads/rectangle.png",CV_LOAD_IMAGE_GRAYSCALE);
Mat mc;
threshold(m,mThresh,80,255,THRESH_BINARY);
findContours(mThresh,contours,hierarchy, cv::RETR_CCOMP, cv::CHAIN_APPROX_NONE, cv::Point(0,0));
imshow("Image",m);
mc = Mat::zeros(m.size(),CV_8UC3);
drawContours(mc,contours,0,Scalar(255,0,0),1);
vector<Point> approx;
double d=0;
do
{
    d=d+1;
    approxPolyDP(contours[0],approx,d,true);
    cout << approx.size() << " " <<d<<endl;
}
while (approx.size()>4);
contours.push_back(approx);
drawContours(mc,contours,contours.size()-1,Scalar(0,0,255),1);
imshow("Ctr",mc);
waitKey();

return 0;
}

result is