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

[Cory Quammen]

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