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Re: FFT OF A NON RECTANGULAR IMAGE [message #63079 is a reply to message #63075]

Tue, 28 October 2008 11:58

R.G. Stockwell
Messages: 363
Registered: July 1999

Senior Member

<legall_alice@yahoo.fr> wrote in message
news:4a2c3474-def0-48fc-8611-05635d75f05d@v39g2000pro.google groups.com...
> Hi all:
>
> How can we do a FFT on a 2D-function that defines a non-rectangular
> image?
>
> Here is an example: the region of interest is an inclined ellipse. To
> be able to apply FFT(array,1), I created an array where all the pixels
> around the ellipse are set to the value zero. I would like to exclude
> from the FFT process the black area (zero value pixels) surrounding
> the ellipse.

No. The typical FFT requires uniform sampling, with constant length
rows and columns.

So if you put your ellipse of data into this form, then you are essentially
multiplying the rectangle with an eclipse shaped window (= 1 where you
have data, and 0 where you don't), and you are therefore convolving the
2D spectrum with the spectrum of that window.

So, you can do it, but there are consequences.

Cheers,
bob

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[Message index]

		Re: FFT OF A NON RECTANGULAR IMAGE By: R.G. Stockwell on Tue, 28 October 2008 14:07
		Re: FFT OF A NON RECTANGULAR IMAGE By: pgrigis on Tue, 28 October 2008 13:15
		Re: FFT OF A NON RECTANGULAR IMAGE By: R.G. Stockwell on Tue, 28 October 2008 11:58
		Re: FFT OF A NON RECTANGULAR IMAGE By: pgrigis on Tue, 28 October 2008 14:21

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