[Insight-users] Frequency domain filtering

[Luis Ibanez]

Hi Emiliano,


There is not an explicit Butterworth filter in ITK.

However, you have the following two options:

1) You can compute the FFT of your image (along the
    dimensions that you need), then multiply the spectrum
    by the Butterworth equivalent transfer function, and
    finally invoke an inverse FFT.

    You may want to look at the following filters
http://www.itk.org/Insight/Doxygen/html/classitk_1_1VnlFFTComplexConjugateToRealImageFilter.html
http://www.itk.org/Insight/Doxygen/html/classitk_1_1VnlFFTRealToComplexConjugateImageFilter.html


2) You can write code for the IIR implementation of the
    Butterworth filter by following the example of the
    RecursiveGaussianImageFilter.  This filter uses a 4th
    order IIR filter in order to compute the equivalent
    of a Gaussian convolution.

    You may want to look at the code in


    Insight/Code/BasicFilters
         itkRecursiveGaussianImageFilter.h
         itkRecursiveGaussianImageFilter.txx
         itkRecursiveSeparableImageFilter.h
         itkRecursiveSeparableImageFilter.txx



Regards,


    Luis


---------------------------
Emiliano Beronich wrote:

> Hi everybody!
> 
> I need to apply a Butterworth filter, and some others too, to an image, 
> in the x-direction (unidimensional filtering to a 3D or 4D image). I 
> would like to filter on the frequency domain because a need to pass the 
> image across a ramp-filter, too.
> Is there a convenient filter to accomplish this filtering in ITK?
> 
> Thanks
> Emiliano
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