[Insight-developers] FFT implementations handle spacing differently

[Williams, Norman K]

This is code I wrote years ago, so I'd have stop what I'm doing and spend
a couple hours reading the code to know what was really going on.  But it
sounds like the pos/orientation/spacing isn't being propogated from the
input to the FFT and by the IFFT to the output.

I would argue that this should always happen -- an image converted to the
frequency domain doesn't transform it spatially; no matter what you do in
the frequency domain, when you transform back to the spatial domain, it
should be oriented and positioned the same.

--
Kent Williams norman-k-williams at uiowa.edu






On 9/27/12 7:13 AM, "Bradley Lowekamp" <blowekamp at mail.nih.gov> wrote:

>Hello,
>
>I have compiled SimpleITK with FFTW and I am getting the following
>regression test failue:
>
>81: [ RUN      ] BasicFilters.ForwardFFT_defaults
>81: Starting image compare on BasicFilters_ForwardFFT_defaults_
>81: Loading baseline
>/Users/blowekamp/src/SimpleITK/Testing/Data/Baseline/BasicFilters_ForwardF
>FT_defaults.nrrd
>81:
>/scratch/blowekamp/build/SimpleITK/SimpleITK-build/Testing/Unit/sitkForwar
>dFFTImageFilterTest.cxx:128: Failure
>81: Value of: imageCompare.compare( output, "", "" )
>81:   Actual: false
>81: Expected: true
>81: ImageCompare: Failed to subtract image
>/Users/blowekamp/src/SimpleITK/Testing/Data/Baseline/BasicFilters_ForwardF
>FT_defaults.nrrd because:
>/scratch/blowekamp/build/SimpleITK/ITK-prefix/include/ITK-4.2/itkImageToIm
>ageFilter.hxx:243:
>81: itk::ERROR: SubtractImageFilter(0x7ffe943009e0): Inputs do not occupy
>the same physical space!
>81: InputImage Origin: [0.0000000e+00, 0.0000000e+00, 0.0000000e+00],
>InputImageIndexedDataObject1 Origin: [2.8345293e-01, 2.8345293e-01,
>2.8345293e-01]
>81:    Tolerance: 1.0000000e-06
>81: InputImage Spacing: [1.0000000e+00, 1.0000000e+00, 1.0000000e+00],
>InputImageIndexedDataObject1 Spacing: [7.5587445e-01, 7.5587445e-01,
>7.5587445e-01]
>81:    Tolerance: 1.0000000e-06
>
>
>This is telling me that the vnl and the FFTW implementation of the
>ForwardFFT image filter are handling the spacing different. The second
>input is the baseline originating from the Vnl FFT implementation, while
>the one from FFTW still has the default origin and spacing. However, I am
>uncertain that this actually make sense, as the FFTForward filter does
>converts from the spacial domain to the frequency this physical
>information may not be very logical.
>
>What should the correct behavior be?
>
>Thanks,
>Brad
>
>ps
>Please note that this exception is coming from the cumbersome standard
>check in the ImageToImageFilter during comparison to the baseline. And
>it's revealing a true bug.
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