Increasing filter size of edge detector

Question

Andrew 2014-4-7

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评论： DGM 2022-11-6

Hello,

I am looking at using edge detectors to detect edges in my images but I as wondering if there was a way of increasing the size of the filters? For example increase Sobel or Prewitt from a 3 x 3 array to a 5 x 5 array or bigger. I am looking to see what the effect of a larger window will have on my edge detection.

Thanks, Andrew

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Answer 1

Nitin 2014-4-7

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编辑：Nitin 2014-4-7

There is a nice thread on stackoverflow which might give you the values of the filter you are looking for.

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Nitin 2014-4-8

Sorry, I have updated my answer. Hopefully this will help

DGM 2022-11-6

在 MATLAB Online 中打开

For sake of making things local and elaborating, let's consider Daniel's Sobel-esque filters.

For a 5x5 filter, you can do

r = 2; % filter radius
[x y] = meshgrid(-r:r);
fk = x./(x.^2 + y.^2);
fk(r+1,r+1) = 0
fk = 5×5
   -0.2500   -0.2000         0    0.2000    0.2500
   -0.4000   -0.5000         0    0.5000    0.4000
   -0.5000   -1.0000         0    1.0000    0.5000
   -0.4000   -0.5000         0    0.5000    0.4000
   -0.2500   -0.2000         0    0.2000    0.2500

You can simply change r to get a different size filter. If you want, you can transpose it.

If you want your output image to stay within a convenient range, you can just prenormalize the kernel depending on how you intend to apply it. If you intend to apply the filter once to get a directional derivative estimate, you can do

fk = fk/-sum(sum(fk(:,1:r))); % for single-pass

... and your image will conveniently stay within [-1 1].

If you intend to do two orthogonal filter passes to calculate the gradient magnitude, you can normalize like so:

fk = fk/(sqrt(5)*r); % for 2-pass

... and your image will stay within [0 1]

... or you can do whatever you want with it as is.

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