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i trial code based on convexityDefects maybe you will improve it.

result image :

#include <opencv2/highgui.hpp>
#include <opencv2/imgproc.hpp>
#include <iostream>
using namespace cv;
using namespace std;

int main( int argc, char** argv )
{
    char* filename = argc >= 2 ? argv[1] : (char*)"Obj_Sep.png";
    Mat src = imread(filename);
    if (src.empty())
        return -1;

    Mat bw;
    cvtColor( src, bw, COLOR_BGR2GRAY );
    bw = bw > 127;

    // Find contours
    vector<vector<Point> > contours;
    vector<int> contoursHull;
    vector<Vec4i> defects;
    findContours( bw, contours, CV_RETR_EXTERNAL, CV_CHAIN_APPROX_NONE );

    for ( size_t i = 0; i < contours.size(); i++)
    {
        if( contourArea(contours[i]) > 500 )
        {
            approxPolyDP(contours[i],contours[i],9,true);
            convexHull(contours[i], contoursHull,true);
            convexityDefects(contours[i], contoursHull,defects);

            for ( size_t j = 0; j <  defects.size(); j++)
            {
                Vec4i defpoint = defects[j];
                circle(src,contours[i][defpoint[2]],3,Scalar(0,0,200),3);
            }
           imshow("bw", src);
            waitKey();
        }
    }
    return 0;
}