registry.gimp.org). (I chose the Windows distribution, 32 bits).
From this gimpchat.com forum post I learned that both fourier.exe and libfftw3-3.dll must be placed into the GIMP plugins folder (specifically
Unfortunately, when starting GIMP again, I got the error that the file
libgcc_s_sjlj-1.dllwas not found, so this meant that the filter wouldn't work.
To get the libgcc_s_sjlj-1.dll file, this instruction on another GIMP plugin suggested how to get that file: download the x32-4.8.0-release-win32-sjlj-rev2.7z package from Sourceforge (36.1 Mb); then you need to open the compressed file with 7-Zip (if you don't have it, download and install it separately beforehand), move to directory
x32-4.8.0-release-win32-sjlj-rev2.7z\mingw32\bin\, and there's the file.
I became interested in the FFT plugin from a Wikimedia Commons Help section offering instructions about image manipulation, introducing the plugin here and in the next section.