[ITK] Valid FFT Sizes

[Wood, Tobias]

For posterity (and my own sanity) I eventually found the problem - it was the negative image index from the FFT padding. I was not taking account of that when constructing my Fourier space filter, which meant the filter was offset from where it should be. Once I take account of the index it all works fine.

Toby



On 17/09/2015 16:20, "Community on behalf of Wood, Tobias" <community-bounces at itk.org on behalf of tobias.wood at kcl.ac.uk> wrote:

>(Sorry - if you want to see the code try the development branch https://github.com/spinicist/QUIT/blob/development/Source/qiunwrap.cpp)
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>
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>On 17/09/2015 16:05, "Community on behalf of Wood, Tobias" <community-bounces at itk.org on behalf of tobias.wood at kcl.ac.uk> wrote:
>
>>Hello again,
>>
>>I added FFTPadImageFilter to my pipeline. The Forward FFT now works, however something weird happens with my Inverse FFT later in the pipeline. My whole pipeline roughly looks like:
>>
>>1. Load Image
>>2. Pad
>>3. Forward FFT
>>4. Multiply by a filter
>>5. Inverse FFT
>>6. Save Image
>>
>>The pipeline works fine if the input image size is a power of 2. But if it’s not (i.e. pad FFT image is needed), then the output of the Inverse FFT is just blank (all zeros). I checked this two ways:
>>
>>1. Saving the input of the of the Inverse FFT (output of stage 4) as a complex image and looking at the magnitude and phase. This looks correct (i.e. not blank)
>>2. Cropping the input image to a subregion that has dimensions that are powers of 2. This produced the expected output.
>>
>>So does the Inverse FFT have stricter requirements than the Forward FFT? One odd thing I noticed is that FFTPadImageFilter sets the index of the output region to negative values, i.e. for my image with size [416,416,60] the output of padding has size [432,432,60] and index [-8,-8,0], but I assume this is how the padding for x/y<0 works.
>>
>>My code is on github at https://github.com/spinicist/QUIT/blob/master/Source/qiunwrap.cpp. Apologies for the (ab)use of the C++11 auto keyword.
>>
>>Toby
>>
>>
>>On 17/09/2015 14:28, "Community on behalf of Wood, Tobias" <community-bounces at itk.org on behalf of tobias.wood at kcl.ac.uk> wrote:
>>
>>>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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