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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
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
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
485
Sobel filtering involves applying two 3 x 3 convolutional kernels (also called filters) to an image. The kernels are usually called G x and G y, and they are shown in the following figure. These two kernels detect the edges in the image in the horizontal and vertical directions.
The Sobel method, or Sobel filter, is a gradient-based method that looks for strong changes in the first derivative of an image. The Sobel edge detector uses a pair of 3 × 3 convolution masks, one estimating the gradient in the x-direction and the other in the y-direction.
The primary advantages of the Sobel operator lie in its simplicity. The Sobel method provides a approximation to the gradient magnitude. Another advantage of the Sobel operator is it can detect edges and their orientations.
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.
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