What's the differences between Opencv Sobel operator and my Sobel operator, the output is different

Question

I am tring to find edges in colour image using Sobel masks, i implemented the Sobel function but the outputs between Opencv Sobel and my are defferent.

void Sobel(const Mat &image, Mat &new_image) {
    int gx[3][3] = { -1,0,1,
                     -2,0,2,
                     -1,0,1 };
    int gy[3][3] = { 1,2,1,
                     0,0,0,
                    -1,-2,-1 };
    for (int i = 1; i < image.rows - 1; i++)
        for (int j = 1; j < image.cols - 1; j++) {
            int XR = 0, XG = 0, XB = 0, YR = 0, YG = 0, YB = 0;
            for (int r = -1; r < 2; ++r) {
                for (int c = -1; c < 2; ++c) {
                    XR += gx[r + 1][c + 1] * image.at<Vec3b>(i + r, j + c)[0];
                    YR += gy[r + 1][c + 1] * image.at<Vec3b>(i + r, j + c)[0];

                    XG += gx[r + 1][c + 1] * image.at<Vec3b>(i + r, j + c)[1];
                    YG += gy[r + 1][c + 1] * image.at<Vec3b>(i + r, j + c)[1];

                    XB += gx[r + 1][c + 1] * image.at<Vec3b>(i + r, j + c)[2];
                    YB += gy[r + 1][c + 1] * image.at<Vec3b>(i + r, j + c)[2];
                }
            }
            int sumR = std::abs(XR) + std::abs(YR);
            int sumG = std::abs(XG) + std::abs(YG);
            int sumB = std::abs(XB) + std::abs(YB);

            new_image.at<Vec3b>(i, j)[0] = (sumR < 255 ? sumR>0 ? sumR : 0 : 255);
            new_image.at<Vec3b>(i, j)[1] = (sumG < 255 ? sumG>0 ? sumG : 0 : 255);
            new_image.at<Vec3b>(i, j)[2] = (sumB < 255 ? sumB>0 ? sumB : 0 : 255);
        }
}

int main()
{
    Mat image = imread("valve.png");
    Mat new_image = image.clone();

    //Sobel(image, new_image);
    cv::Sobel(image, new_image, -1, 1, 1);

    namedWindow("Original", WINDOW_NORMAL);
    imshow("Original", image);

    namedWindow("Sobel", WINDOW_NORMAL);
    imshow("Sobel", new_image);

    waitKey();
    return 0;
}

What are your results? I cannot see them in the link you provided. — Piotr Siekański, Jul 26 '19 at 11:52
I cannot see them: "Your temporary album 0 photos" Can you upload them directly on Stack Overflow? — Piotr Siekański, Jul 26 '19 at 11:55

score 3 · Accepted Answer · answered Jul 26 '19 at 13:33

cv::Sobel internally creates the filter coefficients using getDerivKernels which create 1D filter masks for derivatives in both directions. The first order derivatives are in the form [-1, 0, 1]. Later they are multiplied via outer product and form a single matrix:

`1 0 -1
 0 0 0
-1 0 1

If you apply this matrix to your Sobel function and remove the abs function in your gradient calculation, you'll get the same results as OpenCV.

void Sobel2(const Mat &image, Mat &new_image) {

double gy[3][3] = { 1,0,-1,
                 0,0,0,
                -1,0,1 };

for (int i = 1; i < image.rows - 1; i++)
    for (int j = 1; j < image.cols - 1; j++) {
        double XR = 0, XG = 0, XB = 0, YR = 0, YG = 0, YB = 0;
        for (int r = -1; r < 2; ++r) {
            for (int c = -1; c < 2; ++c) {
                YR += gy[r + 1][c + 1] * image.at<Vec3b>(i + r, j + c)[0];

                YG += gy[r + 1][c + 1] * image.at<Vec3b>(i + r, j + c)[1];

                YB += gy[r + 1][c + 1] * image.at<Vec3b>(i + r, j + c)[2];
            }
        }

        new_image.at<Vec3b>(i, j)[0] = (YR < 255 ? YR>0 ? YR : 0 : 255);
        new_image.at<Vec3b>(i, j)[1] = (YG < 255 ? YG>0 ? YG : 0 : 255);
        new_image.at<Vec3b>(i, j)[2] = (YB < 255 ? YB>0 ? YB : 0 : 255);
    }
}

Here's the code that helps you understand how the filter matrix is computed:

cv::Mat kx, ky;
cv::getDerivKernels(kx, ky, 1, 1, 3);
cv::Mat k = kx * ky.t(); // k is the filter matrix that is used internally in cv::Sobel function

What's the differences between Opencv Sobel operator and my Sobel operator, the output is different

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