CODEWITHSUNDEEP
c++opencvcomputer-visionsobel
Abhishek
Abhishek

Reputation: 680

Implementing Sobel Operator

I am trying to implement a sobel operator in both horizontal and vertical direction. But somehow I am getting the reverse output. The code I have attached below. For the horizontal mask

char mask [3][3]=  {{-1,-2,-1},{0,0,0},{1,2,1}};

void masking(Mat image){

Mat temImage= image.clone();
for (int i = 1; i < image.rows-1; i++)
{
    for (int j = 1; j < image.cols-1; j++)
    {   
        for(int k=0;k<3;k++)
        {
            int pixel1 = image.at<Vec3b>(i-1,j-1)[k] * -1;
            int pixel2 = image.at<Vec3b>(i,j-1)[k] * -2;
            int pixel3 = image.at<Vec3b>(i+1,j-1)[k] * -1;

            int pixel4 = image.at<Vec3b>(i-1,j)[k] * 0;
            int pixel5 = image.at<Vec3b>(i,j)[k] * 0;
            int pixel6 = image.at<Vec3b>(i+1,j)[k] * 0;

            int pixel7 = image.at<Vec3b>(i-1,j+1)[k] * 1;
            int pixel8 = image.at<Vec3b>(i,j+1)[k] * 2;
            int pixel9 = image.at<Vec3b>(i+1,j+1)[k] * 1;

            int sum = pixel1 + pixel2 + pixel3 + pixel4 + pixel5 + pixel6 + pixel7 + pixel8 + pixel9;
            if(sum < 0)
            {
                sum = 0;
            }

            if(sum > 255)
                sum = 255;

            temImage.at<Vec3b>(i,j)[k] = sum;


        }
    }
}
//printf("conter = %d",counter);
imshow( "Display", temImage );
imwrite("output1.png",temImage);

}

I am getting the output as

enter image description here

where as for the vertical mask

char mask [3][3]=  {{-1,0,1},{-2,0,2},{-1,0,1}}; 

void masking(Mat image){

Mat temImage= image.clone();
for (int i = 1; i < image.rows-1; i++)
{
    for (int j = 1; j < image.cols-1; j++)
    {   
        for(int k=0;k<3;k++)
        {
            int pixel1 = image.at<Vec3b>(i-1,j-1)[k] * -1;
            int pixel2 = image.at<Vec3b>(i,j-1)[k] * 0;
            int pixel3 = image.at<Vec3b>(i+1,j-1)[k] * 1;

            int pixel4 = image.at<Vec3b>(i-1,j)[k] * -2;
            int pixel5 = image.at<Vec3b>(i,j)[k] * 0;
            int pixel6 = image.at<Vec3b>(i+1,j)[k] * 2;

            int pixel7 = image.at<Vec3b>(i-1,j+1)[k] * -1;
            int pixel8 = image.at<Vec3b>(i,j+1)[k] * 0;
            int pixel9 = image.at<Vec3b>(i+1,j+1)[k] * 1;

            int sum = pixel1 + pixel2 + pixel3 + pixel4 + pixel5 + pixel6 + pixel7 + pixel8 + pixel9;
            if(sum < 0)
            {
                sum = 0;
            }

            if(sum > 255)
                sum = 255;

            temImage.at<Vec3b>(i,j)[k] = sum;


        }
    }
}
//printf("conter = %d",counter);
imshow( "Display", temImage );
imwrite("output1.png",temImage);

}

I am getting output as

enter image description here

The main function is attached below

int main( int argc, char** argv ){
Mat input_image = imread("sobel1.jpg",1);
masking(input_image);
waitKey(0);
return 0;

}

According the the guide https://www.tutorialspoint.com/dip/sobel_operator.htm I should get reverse output. Can anyone help me in this

The original image is

enter image description here

Upvotes: 11

Views: 2627

Answers (1)

fireant
fireant

Reputation: 14538

No, the tutorial is not wrong, it talks about masks and not gradients. The weak point of that tutorial is that it doesn't mention we are calculating horizontal gradients using what they call the vertical mask.

Upvotes: 1

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