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Re: new user, FFT question

Posted by Michael Schmid on Mar 03, 2009; 1:30pm
URL: http://imagej.273.s1.nabble.com/Can-t-load-images-from-remote-directory-tp3693495p3693504.html

Hi Gerard,

if you want to do an inverse FFT you would need the internal FHT data  
that are hidden behind an FFT image, invisible to the user. Probably,  
it won't be very helpful for you since it is a fast Hartley transform  
(FHT), not FFT.

If you simply want to multiply the FFT with some value depending on  
the spatial frequency, use the FFT>Custom Filter. Note that the  
custom filter has 8 bits resolution only (integer values 0-255  
corresponding to factors of 0...1).

Also note that the FFT displayed on the screen as an image is a  
logarithmic version of the power spectrum. If you want to access the  
true power spectrum or the real&imaginary FFT, use FFT>FFT options.  
You can export these in any format you like (e.g. convert to 16 bit  
before saving), analyze them with your custom program and then, e.g.,  
create a filter for FFT>Custom Filter.

Hope this helps,

Michael
________________________________________________________________

On 3 Mar 2009, at 13:55, Gerard van Schip wrote:

> Hi.
>
> I downloaded imagej as it is the only package that contains the FFT
> function on the Mac (very happy with finding it).
>
> Is there a way to export the FFT image, work on it in another program
> and then reimport it to do a reverse fft?
>
> Thanks.
>
> Gerard