A question about sharpen image

Hello everyone.
Here is the code from the book OpenCV 2 Computer Vision Application Programming Cookbook and I used vs2010 in Windows system to compile it.

#include <opencv2/opencv.hpp>

using namespace std;
using namespace cv;

void sharpen(const cv::Mat &image, cv::Mat &result) {

result.create(image.size(), image.type());
    for (int j= 1; j&lt;image.rows-1; j++) 
{    // for all rows
        // (except first and last)
        const uchar* previous=
        image.ptr&lt;const uchar&gt;(j-1); // previous row
        const uchar* current=
        image.ptr&lt;const uchar&gt;(j); // current row
        const uchar* next=
        image.ptr&lt;const uchar&gt;(j+1); // next row
        uchar* output= result.ptr&lt;uchar&gt;(j); // output row
        for (int i=1; i&lt;image.cols-1; i++)
    {
            *output++= cv::saturate_cast&lt;uchar&gt;(5*current[i]-current[i-1]-current[i+1]-previous[i]-next[i]);
        }
    }

}

int main(int argc, char* argv[]) {
const char* imagename = "E:\\library\\Pictures\\grayLena.jpg";

Mat img = imread(imagename);

if(img.empty())
{
        fprintf(stderr, "Can not load image %s\n", imagename);
        system("pause");
        return -1;
    }
    Mat result;
    result.create(img.size(),img.type());
    sharpen(img,result);

imshow("image", img);
    imshow("result",result);

waitKey();

return 0;
}

2/3 of the result image is a gray rectangle. I don't know why and how to solve it.
Thank you for your attention.

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A question about sharpen image