[ITK] Valid FFT Sizes

[Wood, Tobias]

Thanks Brad & Corey. That all makes sense. No, I hadn’t seen FFTPadImageFilter, I will add it to my pipeline now.

Toby


On 17/09/2015 14:20, "Bradley Lowekamp" <blowekamp at mail.nih.gov> wrote:

>Hello Toby,
>
>Have you looked into the FFTPadImageFilter [1], this automatically pad the image as needed.
>
>This error message may need to be improved/corrected. If I recall this may be a a restriction on the prime factorization of the image dimensions, not simply that the size is a multiple of one of those numbers but that they are the only valid number for prime factorization...
>
>416 prime factorization is 2^5 * 13. Does not work since 13 is a factor
>450 prime factorization is 5^2*3^2*2  ( eyeball a close number)
>
>I think the FFTPad filter should help with this computation.
>
>HTH,
>Brad
>
>
>[1] http://www.itk.org/Doxygen/html/classitk_1_1FFTPadImageFilter.html
>
>On Sep 17, 2015, at 7:10 AM, Wood, Tobias <tobias.wood at kcl.ac.uk> wrote:
>
>> Hello,
>> 
>> I am trying to FFT a moderately large 3D image but am receiving a contradictory error message:
>> 
>> itk::ERROR: VnlForwardFFTImageFilter(0x3038500): Cannot compute FFT of image with size [416, 416, 60]. VnlForwardFFTImageFilter operates only on images whose size in each dimension is a multiple of 2, 3, or 5.
>> 
>> All the image dimensions are a multiple of 2!  Are the dimensions restricted to be powers of 2/3/5 instead of multiples?
>> 
>> I can pad the image out to a sensible size (e.g. 512x512x64), but would like to know if there are any other restrictions I should be aware of before going around in circles? 
>> 
>> Thanks,
>> Toby
>> 
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