Manual Implementation of Sobel Operator in OpenCV - c++

Question

I am trying to apply a sobel operator by iterating through an image and applying a mask to surrounding pixels.
For now, I am trying to apply the vertical portion of the mask, which is:
-1 0 1
-2 0 2
-1 0 1
In my implementaiton, I am iterating through the rows and columns as follows:
for (int i = 1; i < image.rows-1; i++){
for (int j = 1; j < image.cols-1; j++){
int pixel1 = image.at<Vec3b>(i-1,j-1)[0] * -1;
int pixel2 = image.at<Vec3b>(i,j-1)[0] * 0;
int pixel3 = image.at<Vec3b>(i+1,j-1)[0] * 1;
int pixel4 = image.at<Vec3b>(i-1,j)[0] * -2;
int pixel5 = image.at<Vec3b>(i,j)[0] * 0;
int pixel6 = image.at<Vec3b>(i+1,j)[0] * 2;
int pixel7 = image.at<Vec3b>(i-1,j+1)[0] * -1;
int pixel8 = image.at<Vec3b>(i,j+1)[0] * 0;
int pixel9 = image.at<Vec3b>(i+1,j+1)[0] * 1;
int sum = pixel1 + pixel2 + pixel3 + pixel4 + pixel5 + pixel6 + pixel7 + pixel8 + pixel9;
verticalSobel.at<Vec3b>(i,j)[0] = sum;
verticalSobel.at<Vec3b>(i,j)[1] = sum;
verticalSobel.at<Vec3b>(i,j)[2] = sum;
}
}
Where the pixels are labeled as:
1 2 3
4 5 6
7 8 9
However, the resulting image is far off of what it should look like.
For reference, the resulting image is
Where it should look similar to:
The guide I am using is: https://www.tutorialspoint.com/dip/sobel_operator.htm
I am not sure if I am simply implementing the operator incorrectly, or just iterating through the image incorrectly.
Any help would be greatly appreciated. Thanks!

Answer 1

You seem to have problems where the sum is negative. Take the absolute value of sum, and clamp it to 255 (or instead of absolute value, clamp it to 0 - depending of what you want to achieve. A "full" sobel operator usually uses 2d distance formula, so a horizonal/vertical only variant should use the absolute value)