HOG feature

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I learned about HOG feature from here following this tutorial I am trying to create HOG feature...I have approached till here..

#include "stdafx.h"
#include <conio.h>
#include <iostream>
#include <iomanip>
#include <opencv2/opencv.hpp>

using namespace cv;
using namespace ml;
using namespace std;


void main()
{
    Mat h1,h2,h3;
    Mat image=imread("lena.jpg");
    image.convertTo(image,CV_32F,1/255.0);
    imshow("out",image);
    Mat dx,dy; //image axial derivatives..
    Sobel(image,dx,CV_32F,1,0,1);
    Sobel(image,dy,CV_32F,0,1,1);
    imshow("dx",dx);
    imshow("dy",dy);
    Mat mag,angle;
    cartToPolar(dx,dy,mag,angle);
    imshow("magnitude",mag);
    imshow("angle",angle);

    waitKey(0);
    _getch();


}

I have obtained image~derivative of orignal image and mapped to polar coordinate system.Now I obtained two matrices represent gradient and its orientation namely mag and angle respectively.Now I have to obtain histogram of gradients over 8x8 patch of image...how shall I proceed next

I AM NOT FOND OF USING PRE BUILD HOG DESCRIPTOR..