How Sobel Operator works in edge function?

What does the edge function do when using the Sobel operator? How to automatically choosing a threshold? The edge is different from my own Sobel function.

Answer 1

[Sarvesh Kale]

Hi C J ,

The sobel operator is used in Image processing edge detection, it works in the following manner

1.) Compute gradient in X direction

This can be done by multiple methods, simplest is to use conv2 function, call the resultant image Gx

2.) Compute gradient in Y direction

This can be done by multiple methods, simplest is to use conv2 function, call the resultant image Gy

3.) Obtain magnitude of the resultant image

I = sqrt(Gx*Gx + Gy*Gy)

4.) Choose threshold based on application

the following lines of code does all the above steps

G  = imread('cameraman.tif')
Gx = conv2(G , [1 0 -1;...
                2 0 -2;...
                1 0 -1]); 
Gy = conv2(G , [1  2  1;...
                0  0  0;...
               -1 -2 -1]);
I = sqrt(Gx.*Gx + Gy.*Gy);
I_final = I > 150  % choose the thresold on your application need, experiment with different value to pick the best 
imshow(255*I_final)

OR

you can use the inbuilt function in MATLAB that does the same functionality

I = imread('cameraman.tif');
I = edge(I, 'sobel') % there are also other operators like canny
imshow(I)