documentation on imageJ's FFT bandpass filter function

Question

Does anyone here have any idea where I can find documentation on imageJ's FFT bandpass filter function?

Or even better: a precise explanation of which operations are necessary on the Powerspectrum in order to "filter large structures down to size x and small ones down to size y".

So far I have not been able to find anything here:

1. https://imagej.net/ij/docs/menus/process.html#smooth

2. Burger and Burges "Digital image processing - an algorithmic introduction with Java"

Answer 1

The following page contains some information about the fft bandpass filter function in ImageJ and some usage examples: https://imagej.net/ij/plugins/fft-filter.html.

Some more information and images can be found here as well: https://imagej.net/ij/docs/examples/FFT/

Answer 2

[…] which operations are necessary on the Powerspectrum in order to 'filter large structures down to size x and small ones down to size y'.

Literally no operations are necessary on the Fourier power-spectrum, because the Fourier power-spectrum is not involved in Fourier-filtering.