Manual Implementation of Sobel Operator in OpenCV

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!

geza · Accepted Answer · 2017-06-28 17:53:23Z

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)

answered Jun 28, 2017 at 17:53

geza

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3 Comments

ecatalano Over a year ago

Perfect! Using abs(sum) when setting my pixel value fixed it. I'm just curious as to what you mean by clamping the value. Are you saying if the value is > 255, then set it to 255?

geza Over a year ago

@ecatalano: yes, as sum can be larger than 255

Micka Over a year ago

you can use opencv's saturate_cast function to prevent over- and underflows.

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