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Thread Subject:
2D fourier transform filter

Subject: 2D fourier transform filter

From: JK

Date: 10 Dec, 2012 01:34:22

Message: 1 of 2

I am trying to remove low frequency bands from an image after transforming it using 2D fast fourier. I can identify individual components of the transformed image, but cannot yet work out how to filter only components higher than a certain frequency threshold and then reconstruct the original image using only those frequencies.

Say, for example, I have the following 5 x 5 image matrix, x:

1 2 3 4 5
1 2 3 4 5
1 2 3 4 5
1 2 3 4 5
1 2 3 4 5

I subtract out the mean and compute the FFT:
x = x - mean(mean(x)); X = fft(x);

But then how would I filter only frequencies of X above a a threshold, t, and reconstruct the image using only those frequencies to give Y?

Thanks for your help!

Subject: 2D fourier transform filter

From: JK

Date: 10 Dec, 2012 21:47:21

Message: 2 of 2

"JK" wrote in message <ka3e6u$gi4$1@newscl01ah.mathworks.com>...
> I am trying to remove low frequency bands from an image after transforming it using 2D fast fourier. I can identify individual components of the transformed image, but cannot yet work out how to filter only components higher than a certain frequency threshold and then reconstruct the original image using only those frequencies.
>
> Say, for example, I have the following 5 x 5 image matrix, x:
>
> 1 2 3 4 5
> 1 2 3 4 5
> 1 2 3 4 5
> 1 2 3 4 5
> 1 2 3 4 5
>
> I subtract out the mean and compute the FFT:
> x = x - mean(mean(x)); X = fft(x);
>
> But then how would I filter only frequencies of X above a a threshold, t, and reconstruct the image using only those frequencies to give Y?
>
> Thanks for your help!

Sorry I meant X = fft2(x);

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