Posted by
Michael Schmid on
Aug 06, 2015; 7:46am
URL: http://imagej.273.s1.nabble.com/FFT-and-Inverse-FFT-problem-for-an-Electron-Microscope-Image-tp5013915p5013920.html
Hi Mark,
this is a restriction of the way how FFTs are handled in ImageJ.
What you see as an image is not the FFT but an 8-bit version of the power spectrum with logarithmic grayscale and pixel values 0 and 255 omitted (reserved for filtering/masking). This information is not sufficient for an inverse transform. The FFT (actually an FHT) is a hidden property of the image, not visible for the user.
If you save the image as "FFT of..." it actually saves the FFT (strictly speaking, the FHT), not the displayed image, and recovers the displayed image from the FHT data. You may notice that any modifications to the displayed image (e.g. by painting on it, cropping, etc.) are not saved, you will always get what 'Redisplay power spectrum' would show you. If you save it under a different name, ImageJ assumes that you want to save the displayed image (including any modifications by the user), not the FHT.
If you want to do any analysis or further processing of FFTs in a different program, apart from measuring peak positions etc., you need a quantitative relationship between the pixel value and the power spectrum. You get this when you select 'Raw Power Spectrum' in the FFT options. Nevertheless, also the power spectrum is insufficient for the inverse transform because the it contains only the amplitude (squared), not the phase of the Fourier components.
To summarize, if you want to do further processing on an FFT in ImageJ, save it as "FFT of..." as .tif or .zip, but beware that modifications of the displayed image will get lost. For analysis in other programs, save the power spectrum, but you won't be able to transform it back.
Michael
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On Aug 5, 2015, at 22:21, Rosenberg, Mark F wrote:
> I would be so grateful for any direction to this problem I have encountered with Image J please.
>
> I am using OS X 10.10.3 and Image J 1.50 a.
>
> If I open an Electron Microscope Image in Image J and generate an FFT and then save the FFT as “FFT of IMAGENAME” in tif or zip format, reopen the FFT in Image J and do an Inverse FFT I can regenerate the original image as expected.
>
> However, if I rename “FFT of IMAGENAME” to any other name but exclude “FFT of ” in the filename I cannot do an Inverse FFT and if I do an FFT the Image is "lower resolution” .
>
> I have looked at the Info for FFT and all I can see is a change in ID number from -79 to -80 and a change in “Screen Location” coordinates. I don’t know how to edit these.
>
> Does anyone have any suggestions how to get round this renaming problem please because I would like to use the filename in a subsequent program which requires a different format ?
>
> Thank you for your help.
>
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