| Re: FFT+inverse FFT [message #73875] |
Mon, 06 December 2010 18:33  |
burton449
Messages: 15
Registered: December 2010
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Junior Member |
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On Dec 6, 10:28 am, Paolo <pgri...@gmail.com> wrote:
> On Dec 5, 3:05 pm, burton449 <burton...@gmail.com> wrote:
>
>
>
>> On Dec 5, 2:06 pm, David Fanning <n...@dfanning.com> wrote:
>
>>> Natalya Lyskova writes:
>>>> Hey! I'm a beginner at IDL and have problem with FFT. I'm trying to
>>>> perform 2d-FFT but the code doesn't work properly even on test images.
>>>> So I create an image, make the Fourier transform, then the inverse
>>>> Fourier transform and finally I expect to get the initial image. But
>>>> the resulting image is the initial one, reversed with respect to the
>>>> center.
>
>>>> My code:
>>>> nx=256L
>>>> x1=findgen(nx)-nx/2.0+1.0
>>>> x2=findgen(nx)-nx/2.0+1.0
>
>>>> ytest=fltarr(nx,nx)
>>>> for i=0l,nx-1 do begin
>>>> for j=0l,nx-1 do begin
>>>> if (x1[i] le 20.0 and x1[i] ge 0.0 and x2[j] le 20.0 and x2[j] ge
>>>> 0.0) then begin
>>>> ytest[i,j]=1.0
>>>> endif
>>>> endfor
>>>> endfor
>
>>>> ; So the initial image is a squre in the right upper corner
>
>>>> FFTtest=FFT(ytest)
>>>> sh_FFTtest=SHIFT(FFTtest,nx/2.0-1.0,nx/2.0-1.0)
>>>> inv_test=FFT(FFTtest,-1)
>
>>>> ;The result is the square in the LEFT LOWER corner.
>
>>>> I would be very grateful for comments/advices/solutions
>
>>> I think you need to read the on-line help for the FFT function. :-)
>
>>> Your code should look like this:
>
>>> FFTtest = FFT(ytest, -1)
>>> inv_test = Real_Part(FFT(FFTtest, 1))
>
>>> Now ytest and inv_test are essentially the same.
>
>>> Cheers,
>
>>> David
>
>>> --
>>> David Fanning, Ph.D.
>>> Fanning Software Consulting, Inc.
>>> Coyote's Guide to IDL Programming:http://www.dfanning.com/
>>> Sepore ma de ni thui. ("Perhaps thou speakest truth.")
>
>> Hello,
>> Im working with the FFT also but not directly in IDL, I work in ENVI.
>> I wonder how ENVI or IDL can perform a FFT on a rectangular image?
>> Theorically the image must be a square of dimension of coefficient of
>> 2. (512x512, 1024 x1024 etc... ) So what is the process that make
>> possible to do a FFT on a rectangular image? Other Image Processing
>> software like PCI Geomatica cant do that.
>
> Taking a DFT (Discrete Fourier Transformation) of an array is possible
> for any array size. There is an algorithm called FFT (Fast Fourier
> Transformation)
> that happens to be very efficient if the size is in the form 2^N for
> some N.
> However, modern incarnations of FFT can deal with other sizes too,
> albeit less
> efficiently (the smaller the factors in the prime decomposition of the
> size,
> the better).
>
> Please don't let what a particular software does or fail to do be your
> guide to what is possible or not (from a mathematical standpoint). If
> you
> want to learn a bit more about the FFT, read for instance the chapter
> about
> it on the numerical recipes book.
>
> Ciao,
> Paolo
>
>
>
>> Thank you,
>> Max
>
>
Hi Paolo,
Thank you for your comment. As a student in Remote Sensing, I have a
lot of basic things to understand. The image I would like to filter in
the frequential domain (using a Butterworth filter) is a side scan
sonar image mosaic of 9166 x 4093 pixels. Would you recommend a FFT
and if yes would you use a special algorithm?
greetings,
Max
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