# Problem with ForwardFFTImageFilter in SimpleFilters

**URL:** <https://discourse.slicer.org/t/problem-with-forwardfftimagefilter-in-simplefilters/5657>\
**Category:** Support\
**Created:** [February 6, 2019, 10:18am UTC](https://discourse.slicer.org/t/problem-with-forwardfftimagefilter-in-simplefilters/5657 "2019-02-06T10:18:41Z")\
**Posts on this page:** 3\
**Page:** 1

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**Author:** ![torquil](https://avatars.discourse-cdn.com/v4/letter/t/e79b87/32.png) [@torquil](https://discourse.slicer.org/u/torquil)\
**Post date:** [February 6, 2019, 10:18am UTC](https://discourse.slicer.org/t/problem-with-forwardfftimagefilter-in-simplefilters/5657/1 "2019-02-06T10:18:41Z")

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Hi!

In an attempt to determine the real 3d resolution of an X-ray scan, I’m tried to use the ForwardFFTImageFilter function in the SimpleFilters module, but I’m getting an error message saying that it does not support 32-bit signed integers:

![slicer_forward_fft_bug](https://us1.discourse-cdn.com/flex002/uploads/slicer/original/3X/7/f/7ffc6f8293cc1ee565165841278f0fbd6b3d7db5.png)

Is this a bug, or do I need to convert to pixel values to something that this filter supports first?

Best regards,  
Torquil Sørensen

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**Author:** ![lassoan](https://sea2.discourse-cdn.com/flex002/user_avatar/discourse.slicer.org/lassoan/32/13_2.png) [@lassoan](https://discourse.slicer.org/u/lassoan)\
**Post date:** [February 6, 2019, 4:31pm UTC](https://discourse.slicer.org/t/problem-with-forwardfftimagefilter-in-simplefilters/5657/2 "2019-02-06T16:31:16Z")

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The error message tells that 32-bit signed integer voxel type is not supported by the selected filter. You can use “Cast scalar volume” module to change the voxel type.

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<div class="post-metadata">

**Author:** ![pieper](https://sea2.discourse-cdn.com/flex002/user_avatar/discourse.slicer.org/pieper/32/8_2.png) [@pieper](https://discourse.slicer.org/u/pieper)\
**Post date:** [February 6, 2019, 4:36pm UTC](https://discourse.slicer.org/t/problem-with-forwardfftimagefilter-in-simplefilters/5657/3 "2019-02-06T16:36:24Z")

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See this line:

`This filter works only for real single-component input image types`

from [https://itk.org/Doxygen/html/classitk\_1\_1ForwardFFTImageFilter.html](https://itk.org/Doxygen/html/classitk_1_1ForwardFFTImageFilter.html)