CODEWITHSUNDEEP
matlabopencv
Mzk
Mzk

Reputation: 1120

Confusion on Sobel mask

What is the exact mask for Sobel (Gx and Gy)? What I saw is there are two types on how people wrote it such as below,

Style 1

Gx = [-1 -2 -1
        0 0 0
        1 2 1]

Gy = [-1 0 1
      -2 0 2
      -1 0 1]

Style 2

Gx = [-1 0 1
      -2 0 2
      -1 0 1]

Gy = [-1 -2 -1
        0 0 0
        1 2 1]

Edited

@Aurelis

In Matlab --> (row x col)

In OpenCV --> (col x row)

However, the diagram below is correct for both

-->column
^
|row
|

Probably in Matlab will output Gx == horizontal edge, Gy == vertical edge if Style 1 is used and Gx == horizontal edge, Gy ==vertical edge if Style 2 is used. Both will produce same output (internal operation might be different due to the col-row major order).

@Abhishek You are using style 1 to compute the horizontal and vertical edge? and Gx correspond to horizontal edge while Gy correspond to vertical edge? Does that mean style 2 is complement of that? For eg. computing Gx will gives vertical edge and Gy gives horizontal edge?

Upvotes: 0

Views: 1417

Answers (2)

Abhishek Thakur
Abhishek Thakur

Reputation: 17015

Style 2 is correct. However, using both the styles we will get the same result, as the kernels are convolved with the image

Gx = [-1 -2 -1 0 0 0 <--- will extract features in Y direction and not in X direction. 1 2 1]

Gy = [-1 0 1 -2 0 2 <--- will extract features in X direction and not in Y direction. -1 0 1]

This can be verified by using a simple 2-D convolution.

original image: original image

using Style1,Gx: enter image description here

using style1, Gy: enter image description here

Upvotes: 3

Aurelius
Aurelius

Reputation: 11329

If you are using mathematical notation, the proper mask is Style 2 (see here).

Your confusion may be arising from the difference between matrices in MATLAB and OpenCV. MATLAB matrices are specified in column-major order, while OpenCV matrices are specified in row-major order.

Style 1 is representing the Sobel mask in a column-major fashion, and Style 2 represents the same mask in row-major order.

Upvotes: 1

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