| Re: FFT+inverse FFT [message #73940 is a reply to message #73875] |
Thu, 09 December 2010 10:58  |
pgrigis
Messages: 436
Registered: September 2007
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Senior Member |
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On Dec 6, 9:33 pm, burton449 <burton...@gmail.com> wrote:
> 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
Well if it's just a single image, then you can certainly go ahead
and implement filtering with the FFT - the array is pretty big
and it will take a little while. If performance becomes an
issue, you can expand it to 9216x4096 and it should run a bit
faster.
Ciao,
Paolo
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