[Insight-users] Forward + Inverse FFT != Identity transform

[Julien Jomier]

Jordi, Stefan,

We had the same issue here. One solution was to use FFTW and the 
corresponding FFTW classes in ITK.

We didn't have time to dig into the possible bug using the VNL classes. 
Did you submit a bug report on the bug tracker (www.itk.org/bug) yet?

Julien

Jordi Inglada wrote:
> Dear Stefan, all,
> 
> Has there been a proposed solution to this problem? We are having 
> similar problems : the FFT of a sinusoidal surface has not the expected 
> shape (see the real, imaginary, and module parts of the FFT). We have 
> used the FFTImageFilter.cxx example.
> 
> Thanks for any advice.
> 
> J. Inglada
> 
> group at spellgen.com wrote:
> 
>> Trying to debug a convolution routine for two images based on Fourier
>> transforms, I find that a forward FFT immediately followed by its 
>> inverse does
>> not produce the image I started with. The code looks straightforward 
>> enough -
>> it does the following:
>> 1. Read the image from a grayscale char-type file
>> 2. Resample to width and height both being powers of two.
>> 3. Forward transform (float)
>> 4. Another forward transform.
>> 5. Flip data in both dimensions
>> 6. Rescale to bring it into char range again
>> 7 Write the image.
>>
>> urls for the input (black with a white dot at the origin) and the output
>> images:
>> http://neuromagnetics.mc.vanderbilt.edu/out/itk/point.png
>> http://neuromagnetics.mc.vanderbilt.edu/out/itk/out.png
>>
>> The source code is there as well:
>> http://neuromagnetics.mc.vanderbilt.edu/out/itk/four2-example.cxx
>>
>> Any suggestions as to what is going on would be appreciated...
>>
>> Cheers,
>>
>> Stefan
>>
>>
>>
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>>
> 
> 
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