[Insight-developers] Consistency in the output of forward/inverse FFT image filters

[Johnson, Hans J]

Cory,

I think that the default should be just the 1/2 output.  Many optimized
FFT routines generate only the 1/2 output, and for a "production quality"
application it is likely that they will need this version.

I am supportive of you working on this.  It would be a very nice addition.

Hans


On 8/18/11 2:14 PM, "Cory Quammen" <cquammen at cs.unc.edu> wrote:

>Kevin Hobbs brought up a good point in a recent post to the ITK users
>list regarding the output of the forward FFT filter. Right now, the
>output of this filter is quite different if FFTW is enabled. This is
>because FFTW computes only about half of the output image because of
>the Hermitian symmetry inherent in the Fourier transform of a real
>signal, while the FFT routine in VNL computes the full output image.
>Kevin pointed out that to effectively work with the output of this
>filter, one must know whether the output is in the half or full image
>format, and one must explicitly check in the code whether FFTW is
>enabled to check that. I agree with him that this is undesirable.
>
>To make the FFT filters' behaviors consistent regardless of whether
>FFTW is enabled, I propose that all FFT filters include an option to
>produce the full output image. By default, all the FFT filters would
>produce the half output (what FFTW produces). The inverse filters
>would also have an option to specify whether their input is in the
>half or full output format.
>
>Does this sound like a good idea? I'm happy to work on a patch for
>this change if there are no objections.
>
>Thanks,
>Cory
>
>--
>Cory Quammen
>Research Associate
>Department of Computer Science
>The University of North Carolina at Chapel Hill
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