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I want to ask why everywhere is used to do edge detection in spatial domain and not in frequency domain with FFT of image and apply HP filter. Thank you.

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What you claim is simply not true. One step in edge detection is often a simple high-pass (or band-pass) filter, and those are commonly applied using fast convolution, which is a frequency-domain algorithm.

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    $\begingroup$ Then why is everywhere in edge detection posted info about Sobel operator, Perwitt and etc? Is it easier to do it in spatial domain with convolution or what ? I cant find obvious reason to do edge detection only in spatial domain. $\endgroup$ – Olo Dec 8 '20 at 19:53
  • $\begingroup$ It's easy to explain. It's not easier to do, necessary. For some things it is. For everything nonlinear, it's usually necessary. $\endgroup$ – Marcus Müller Dec 8 '20 at 19:54
  • $\begingroup$ So for linear transformation it is necessary to do it in spatial domain? and in nonlinear it can be done in both (freq and spatial)? Thx for ans u helped me so much $\endgroup$ – Olo Dec 8 '20 at 19:57
  • $\begingroup$ No, exactly the other way around. If something is linear, and you're using a linear transform like the Fourier transform, it can be done in whichever domain is easier/faster/elegant. Non-linear operations don't "translate". $\endgroup$ – Marcus Müller Dec 8 '20 at 20:02
  • $\begingroup$ I dont understand nevertheless. $\endgroup$ – Olo Dec 8 '20 at 20:11

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