MATLAB Answers

0

Normalization of gradient image

Asked by Kyle Wang on 13 Feb 2019
Latest activity Answered by Shunichi Kusano on 21 Feb 2019
https://au.mathworks.com/help/images/ref/imgradient.html#bthgcil-method
imgradient does not normalize the gradient output. If the range of the gradient output image has to match the range of the input image, consider normalizing the gradient image, depending on the method argument used. For example, with a Sobel kernel, the normalization factor is 1/8, for Prewitt, it is 1/6, and for Roberts it is 1/2.
I'm confused with the last sentence. So excatly how to calculate the normalised gradient image with a sobel kernel for example?

  0 Comments

Sign in to comment.

1 Answer

Answer by Shunichi Kusano on 21 Feb 2019
 Accepted Answer

For the case using sobel kernel, multiply 1/8 to the resulted values:
[Gmag,Gdir] = imgradient(I, 'sobel');
Gmag_normalized = Gmag / 8;
The window (kernel) used in sobel filter is:
w = [1 0 -1; 2 0 -2; 1 0 -1]; % for vertical edge detection
The sum of the absolute value is 8 (=abs(1+2+1)+abs(-1-2-1)). Hence, the resulted values are enhanced by this factor due to the convolution. One needs to compensate this for the normalization. The idea is the same as moving average.

  0 Comments

Sign in to comment.