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Re: FFT and ROTATE [message #62414 is a reply to message #62326]

Sun, 07 September 2008 05:59

Kenneth P. Bowman
Messages: 585
Registered: May 2000

Senior Member

In article
<0fc5b913-ae5d-4a1c-afb3-ba1184f174ce@c58g2000hsc.googlegroups.com>,
wheinz@gmail.com wrote:

> Thanks for the suggestions. I am still confused by a few things, but
> with Wox's code I think I can clarify my question. The problem is that
> although Wox's code shows that the FFT of the rotated image and the
> rotated FFT of the original image give you back images that look like
> the original when inverse -transfored, the values of the coefficients
> in those two FFTs are not the same. I added some print statements to
> Wox's code to show what is confusing me.

I find that the only way to really understand FFTs is to work with a
example where I know the answer exactly. I suggest that you create
a highly simplified image by combining a few harmonic components
in the x- and y-directions, then look at the FFTs with and
without rotation.

For example

f(x,y) = sin(2 * !pi * x) # sin(4 * !pi * y)

where x = y = FINDGEN(16)/16.0

You can use try different (distinct) sine and cosine components
to investigate how they translate into real and imaginary
parts.

Also, you can look at dimensions sizes that are not powers of 2.

Ken Bowman

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[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

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