Why do i get different results when i perform convolution with different ways?

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Hello ,
i have an image 30x30 and i want to perform convolution with a filter 9x9 using 2 methods . Firstly , i need to perform the convolution using conv2() and then using ifft2(fft2(image)*fft2(filter)) . Can someone explain why in the first case i get back a 38x38 result as expected ( i am aware that the result after the convolution is length(image+filter-1)) but in the second i get back a 30x30 result? Isn't supposed to be the same procedure ?

Accepted Answer

Matt J
Matt J on 10 Dec 2020
Edited: Matt J on 10 Dec 2020
No, it's not the same procedure. conv2 performs linear convolution, wherease your fft method performs cyclic convolution. See also,

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