Question
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.
Expert Answer
Prashant Kumar
PhD Expert
Answered Aug 26, 2026
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)
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