about use Mat to make binary image [closed]

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I want make a binary image . the code is the following:

#include<opencv\cv.h>
#include<stdlib.h>
#include<opencv2\opencv.hpp>
#include<opencv\highgui.h>
using namespace cv;
int main()
{
    int i=0,j=0;
    Mat img = imread("hh.jpg");
    if(img.empty())
    {
        printf("无法打开\n");
        return 0;
    }
    for(i=0;i<img.rows;++i)
        for(j=0;j<img.cols;++j)
        {
            if(img.at<uchar>(i,j)>=230)
                img.at<uchar>(i,j)=255;
            else
                img.at<uchar>(i,j)=0;
        }

    imshow("图片浏览",img);
    waitKey();
    return 0;
}