comp.lang.idl-pvwave archive: archive

Home » Public Forums » archive » Re: FFT and ROTATE

Show: Today's Messages :: Show Polls :: Message Navigator
E-mail to friend

Re: FFT and ROTATE [message #62360 is a reply to message #62358]

Wed, 03 September 2008 23:11

Kenneth P. Bowman
Messages: 585
Registered: May 2000

Senior Member

In article
<0473bd21-6928-45b9-9e59-56a8aa5ab9df@r15g2000prh.googlegroups.com>,
wheinz@gmail.com wrote:

> Hello,
>
> I have been wrestling with the FFT and ROTATE functions recently. One
> of the properties of the Fourier transform is that the transform of a
> rotated object is equal to the rotation of the transform of the
> unrotated object. To test this in IDL, I took the FFT of an nxn array
> (called image) and the FFT of that array rotated 90 degrees, image90 =
> ROTATE(image,1). Then, I sorted the real and imaginary parts of the
> coefficients of the results of the FFTs and compared the sorted
> values. I expected that the sorted list of real parts from the FFT of
> the original and rotated arrays would be identical, and that the same
> would be true for the imaginary parts. This is not the case. The sets
> of the magnitudes of the coefficients are equal, as expected.
>
> I understand that edge effects can play a role, but when I rotate a
> square array by 90 degrees, I expect that the sets of real and
> imaginary values that define the coefficients to be equal.
>
> Is this a consequence of how the FFT is calculated -- rows first then
> columns or vice versa? Or is there something else going on? Any help
> or suggestions will be greatly appreciated.

Are the differences of the order of the machine precision?

Ken Bowman

Report message to a moderator

[Message index]

		Re: FFT and ROTATE By: wheinz on Fri, 05 September 2008 14:04
		Re: FFT and ROTATE By: Vince Hradil on Thu, 04 September 2008 06:37
		Re: FFT and ROTATE By: Wox on Thu, 04 September 2008 04:55
		Re: FFT and ROTATE By: Kenneth P. Bowman on Wed, 03 September 2008 23:11
		Re: FFT and ROTATE By: Kenneth P. Bowman on Sun, 07 September 2008 05:59
		Re: FFT and ROTATE By: Wox on Sat, 06 September 2008 04:47

Previous Topic:	map plotting
Next Topic:	block fill image

-=] Back to Top [=-

[ Syndicate this forum (XML) ] [

] [

]

Current Time: Sat Oct 11 07:44:58 PDT 2025

Total time taken to generate the page: 0.00520 seconds