Sobel-Filter using C++

Using OpenCV to achieve Sobel Filter without using the built-in function

Getting Started

Edge dectection is one of the most fundamental algorithm used in computer vision. It can easily detect the edges in the horizontal and vectical direction by convolving the masks.

Method

Creating two matrics

x-direction mask

    sobel_x[0][0] = -1; sobel_x[0][1] = 0; sobel_x[0][2] =1;
    sobel_x[1][0] = -2; sobel_x[1][1] = 0; sobel_x[1][2] =2;
    sobel_x[2][0] = -1; sobel_x[2][1] = 0; sobel_x[2][2] =1;

y-direction mask

    sobel_y[0][0] = -1; sobel_y[0][1] = -2; sobel_y[0][2] = -1;
    sobel_y[1][0] = 0; sobel_y[1][1] = 0; sobel_y[1][2] = 0;
    sobel_y[2][0] = 1; sobel_y[2][1] = 2; sobel_y[2][2] = 1;

Convolving every pixel in the image in x and y direction

for (int j = 0; j<img.rows-2; j++)
    {
        for (int i = 0; i<img.cols-2; i++)
        {
            int pixval_x =
            (sobel_x[0][0] * (int)img.at<uchar>(j,i)) + (sobel_x[0][1] * (int)img.at<uchar>(j+1,i)) + (sobel_x[0][2] * (int)img.at<uchar>(j+2,i)) +
            (sobel_x[1][0] * (int)img.at<uchar>(j,i+1)) + (sobel_x[1][1] * (int)img.at<uchar>(j+1,i+1)) + (sobel_x[1][2] * (int)img.at<uchar>(j+2,i+1)) +
            (sobel_x[2][0] * (int)img.at<uchar>(j,i+2)) + (sobel_x[2][1] * (int)img.at<uchar>(j+1,i+2)) + (sobel_x[2][2] * (int)img.at<uchar>(j+2,i+2));
            
            int pixval_y =
            (sobel_y[0][0] * (int)newimg.at<uchar>(j,i)) + (sobel_y[0][1] * (int)newimg.at<uchar>(j+1,i)) + (sobel_y[0][2] * (int)newimg.at<uchar>(j+2,i)) +
            (sobel_y[1][0] * (int)newimg.at<uchar>(j,i+1)) + (sobel_y[1][1] * (int)newimg.at<uchar>(j+1,i+1)) + (sobel_y[1][2] * (int)newimg.at<uchar>(j+2,i+1)) +
            (sobel_y[2][0] * (int)newimg.at<uchar>(j,i+2)) + (sobel_y[2][1] * (int)newimg.at<uchar>(j+1,i+2)) + (sobel_y[2][2] * (int)newimg.at<uchar>(j+2,i+2));
            
            int sum = abs(pixval_x) + abs(pixval_y);
            if (sum > 255)
            {
                sum = 255; //for best performance
            }
            cout << sum << endl;
            newimg.at<uchar>(j,i) = sum;
        }
    }

Results

Applied only x-direction mask

Applied only y-direction mask

Final result