About the Butterworth filter (relating the cutoff parameter)

Hello all, I’m working on spatial frequency filtering using code from this site.

https://www.djmannion.net/psych_programming/vision/sf_filt/sf_filt.html

But I don’t quite understand well the cutoff parameter in the butter2d_hp/lp function. I’m trying to https://psychopy.org/api/filters.html images (high-pass and low-pass) but I don’t understand how I must relate the parameter “cycles per image/cycles per degree" with this filter. What are the units?

I need to do a high-pass filter ( 24 cycles/image) and a low-pass filter (below 8 cycles/image).

The code is working fine and solving problems but I need to justify it in my undergrad project :frowning: how to convert the cutoff used in the Butterworth filter, which is a number from 0 to 1, to cycles / degree in the image when I report it :frowning:

Thank you very much and sorry If I asked a stupid question.