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here is my result

here is my suggest code,may it help you .

//answerOpenCV OpenCV / C++ - Filling holes
#include "stdafx.h"
#include <iostream>
#include <vector>


using namespace cv;
using namespace std;

//find the biggest contour
vector<Point> FindBigestContour(Mat src){    
    int imax = 0;  
    int imaxcontour = -1;  
    std::vector<std::vector<Point> >contours;    
    findContours(src,contours,CV_RETR_LIST,CV_CHAIN_APPROX_SIMPLE);
    for (int i=0;i<contours.size();i++){
        int itmp =  contourArea(contours[i]);
        if (imaxcontour < itmp ){
            imax = i;
            imaxcontour = itmp;
        }
    }
    return contours[imax];
}

//remove Light difference by using top hat
Mat moveLightDiff(Mat src,int radius){
    Mat dst;
    Mat srcclone = src.clone();
    Mat mask = Mat::zeros(radius*2,radius*2,CV_8U);
    circle(mask,Point(radius,radius),radius,Scalar(255),-1);
    //top hat
    erode(srcclone,srcclone,mask);
    dilate(srcclone,srcclone,mask);
    dst =  src - srcclone;
    return dst;
}

int main( void )
{
    Mat src = imread("e:/sandbox/question.png");
    Mat src_hsv;
    Mat bin;
    Mat src_h;

    cvtColor(src,src_hsv,COLOR_BGR2HSV);
    vector<Mat> rgb_planes;
    split(src_hsv, rgb_planes );
    src_h = rgb_planes[0]; // h channel is useful

    src_h = moveLightDiff(src_h,40);
    threshold(src_h,bin,100,255,THRESH_OTSU);

    //find and draw the biggest contour
    vector<Point> bigestcontrour =  FindBigestContour(bin);
    vector<vector<Point> > controus;
    controus.push_back(bigestcontrour);
    cv::drawContours(src,controus,0,Scalar(0,0,255),3);

    waitKey();
    return 0;
}