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CV_16US means signed short this line is wrong

    cout << grad_x.at<int>(i, j) << endl;

It should be

    cout << grad_x.at<short>(i, j) << endl;

CV_16US CV_16S means signed short this line is wrong, also you got i,j wrong (opencv is row-major)

    cout << grad_x.at<int>(i, j) << endl;

It should be

    cout << grad_x.at<short>(i, j) grad_x.at<short>(j,i) << endl;

CV_16S means signed short this line is wrong, also you got i,j wrong wrong.

(opencv is row-major)

    cout << grad_x.at<int>(i, j) << endl;

It should be

    cout << grad_x.at<short>(j,i) << endl;

CV_16S means signed short this line is wrong, also you got i,j wrong.

(opencv is row-major)

    cout << grad_x.at<int>(i, j) << endl;

It should be

    cout << grad_x.at<short>(j,i) << endl;

{
    Mat prev_frame=imread("g:/lib/opencv/samples/data/fruits.jpg",IMREAD_GRAYSCALE);
    resize(prev_frame, prev_frame, Size(8, 8));
    cout << prev_frame << " \n";
    Mat grad_x, grad_y;
    int scale = 1;
    int delta = 0;
    int ddepth = CV_16S;
    Sobel(prev_frame, grad_x, ddepth, 1, 0, 3, scale, delta, BORDER_DEFAULT);
    Sobel(prev_frame, grad_y, ddepth, 0, 1, 3, scale, delta, BORDER_DEFAULT);
    cout << grad_x << endl;

    double min, max;
    minMaxLoc(grad_x, &min, &max);
    cout << min << endl;
    cout << max << endl;

    for (int i = 0; i < grad_x.cols; i++)
    {
        for (int j = 0; j < grad_x.rows; j++)
        {
            cout << grad_x.at<short>(j, i)<<"\t" ;
        }
        cout << endl;
    }
}

CV_16S means signed short this line is wrong, also you got i,j wrong.

(opencv is row-major)

    cout << grad_x.at<int>(i, j) << endl;

It should be

    cout << grad_x.at<short>(j,i) << endl;

I use this program to test ( frame unused) grad_x empty don't need to print....

{
    Mat prev_frame=imread("g:/lib/opencv/samples/data/fruits.jpg",IMREAD_GRAYSCALE);
    resize(prev_frame, prev_frame, Size(8, 8));
    cout << prev_frame << " \n";
    Mat grad_x, grad_y;
    int scale = 1;
    int delta = 0;
    int ddepth = CV_16S;
    Sobel(prev_frame, grad_x, ddepth, 1, 0, 3, scale, delta, BORDER_DEFAULT);
    Sobel(prev_frame, grad_y, ddepth, 0, 1, 3, scale, delta, BORDER_DEFAULT);
    cout << grad_x << endl;

    double min, max;
    minMaxLoc(grad_x, &min, &max);
    cout << min << endl;
    cout << max << endl;

    /* ALREADY in cout <<grad_x
    for (int j = 0; j < grad_x.rows; j++)
    {
           for (int i = 0; i < grad_x.cols; i++)
    {
        for (int j = 0; j < grad_x.rows; j++)
        {
            cout << grad_x.at<short>(j, i)<<"\t" ;
        }
        cout << endl;
    }
}*/
}