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Re: Correction: 2D FFt [message #7294 is a reply to message #7290]

Tue, 22 October 1996 00:00

agraps
Messages: 35
Registered: September 1994

Member

Walid Atia <atia@wam.umd.edu> writes:

> I just realized that FFT(data,-1) takes the FFT of an array of up to 7
> dimensions. What confused me was that when I plotted, say, the FFT of a
> gaussian, I got values only near the sides. However, the FFT of a
> gaussian is a gaussian, so I thought that the FFT routine must not be
> working correctly for my array. Just in case anyone out there is
> interested, the problem lies in the way IDL stores the FFT data. The
> edges are taken as 0 frequency, rather than the more intuitively obvious
> center to be zero.

To be fair, I don't think that IDL is the only software package that
performs the ordering that way. I believe that many others order the
output into quadrants as well. One should always check the output of
the FFT first and then reorder it (or the filter) if necessary before
applying any kind of filter.

Well I learned this little fact too, about a year ago. I have some
illustrations showing some high-pass filtering of solar images with
the swapped gaussian filter I used to remove some noisy components of
my images. (All done in IDL). The results were nice. See

http://quake.stanford.edu/~amara/movdev.html

(Warning: that page has about 500 kB of images)

Amara

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************************************************************ *************
Amara Graps email: agraps@netcom.com
Computational Physics vita: finger agraps@best.com
Multiplex Answers URL: http://www.amara.com/
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[Message index]

		Re: Correction: 2D FFt By: steinhh on Tue, 29 October 1996 00:00
		Re: Correction: 2D FFt By: Graeme K Harkness on Wed, 23 October 1996 00:00
		Re: Correction: 2D FFt By: peter on Tue, 22 October 1996 00:00
		Re: Correction: 2D FFt By: thompson on Tue, 22 October 1996 00:00
		Re: Correction: 2D FFt By: agraps on Tue, 22 October 1996 00:00

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