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Re: IDL 5.5, 2D FFT indexing confusion. [message #44821]

Tue, 19 July 2005 13:49

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

Senior Member

"Pitufa" <c.c.calderon@gmail.com> wrote in message
news:1121772201.952005.96070@g43g2000cwa.googlegroups.com...
> Hi,
>
> I have been trying to generate an real even function in fourier space
> that I can INVERSE FFT in order to get a function which has no
> imaginary part. I have no problems when the function is a vector, but I
> get an imaginary part when it is a two dimensional array.
...

Hello Pitufa,
I shrunk the size of the array and took a look.
I think your nyquist rows and columns should all be positive (i.e. don't
flip the signs).
(by nyquist rows/columns i mean the npix/2+1 columm and the npix/2+1 row)

Here is a npix=6 example that i fixed
f = [ $
[0.00, 0.00, 0.00, 0.00, 0.00, 0.00],$
[0.00, 1.00, 0.80, 0.60, -0.80, -1.00],$
[0.00, 0.80, 1.00, 0.92, -1.00, -0.80],$
[0.00, 0.60, 0.92, 1.00, 0.92, 0.60],$
[0.00,-0.80, -1.00, 0.92, 1.00, 0.80],$
[0.00,-1.00, -0.80, 0.60, 0.80, 1.00] $
]

Cheers,
bob

PS
Note that if your "npix" is odd, you have both a positive and negative
nyquist points
and they are both complex.

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[Message index]

		Re: IDL 5.5, 2D FFT indexing confusion. By: R.G. Stockwell on Tue, 19 July 2005 13:49
		Re: IDL 5.5, 2D FFT indexing confusion. By: Pitufa on Tue, 19 July 2005 09:44
		Re: IDL 5.5, 2D FFT indexing confusion. By: Benjamin Hornberger on Tue, 19 July 2005 07:29
		Re: IDL 5.5, 2D FFT indexing confusion. By: Pitufa on Thu, 21 July 2005 10:28

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