How to implement Sobel operator

Question

I have implemented Sobel operator in vertical direction. But the result which I am getting is very poor. I have attached my code below.

int mask_size= 3;

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

void sobel(Mat input_image)
{

/**Padding m-1 and n-1 zeroes to the result where m and n are mask_size**/

Mat result=Mat::zeros(input_image.rows+(mask_size - 1) * 2,input_image.cols+(mask_size - 1) * 2,CV_8UC1);
Mat result1=Mat::zeros(result.rows,result.cols,CV_8UC1);            
int sum= 0;

/*For loop for copying original values to new padded image **/

for(int i=0;i<input_image.rows;i++)
    for(int j=0;j<input_image.cols;j++)
        result.at<uchar>(i+(mask_size-1),j+(mask_size-1))=input_image.at<uchar>(i,j);

GaussianBlur( result, result, Size(5,5), 0, 0, BORDER_DEFAULT );
/**For loop to implement the convolution **/

for(int i=0;i<result.rows-(mask_size - 1);i++)
    for(int j=0;j<result.cols-(mask_size - 1);j++)
    {
        int counter=0;
        int counterX=0,counterY=0;
        sum= 0;
        for(int k= i ; k < i + mask_size ; k++)
        {
            for(int l= j ; l< j + mask_size ; l++)
            {
                sum+=result.at<uchar>(k,l) * mask[counterX][counterY];
                counterY++;
            }
            counterY=0;
            counterX++;
        }
        result1.at<uchar>(i+mask_size/2,j+mask_size/2)=sum/(mask_size * mask_size);
    }

/** Truncating all the extras rows and columns **/

result=Mat::zeros( result1.rows  - (mask_size - 1) * 2, result1.cols - (mask_size - 1) * 2,CV_8UC1);
for(int i=0;i<result.rows;i++)
    for(int j=0;j<result.cols;j++)                      
        result.at<uchar>(i,j)=result1.at<uchar>(i+(mask_size - 1),j+(mask_size - 1));

imshow("Input",result);
imwrite("output2.tif",result);

}

My input to the algorithm is

My output is

I have also tried using Gaussian blur before actually convolving an image and the output I got is

The output which I am expecting is

The guide I am using is: https://www.tutorialspoint.com/dip/sobel_operator.htm

Answer 1

Your convolution looks ok although I only had a quick look.

Check your output type. It's unsigned char.

Now think about the values your output pixels may have if you have negative kernel values and if it is a good idea to store them in uchar directly.

If you store -1 in an unsigned char it will be wrapped around and your output is 255. In case you're wondering where all that excess white stuff is coming from. That's actually small negative gradients.

The desired result looks like the absolute of the Sobel output values.