I found these resources helpful when I was actively working on a method to identify the screen printing frequency of printed materials for the purposes of denoising and eliminating moire from scans of printed materials (yea I'm talking about pokemon cards again). a lot of DFT/FFT resources are focused on auditory or other signals in a 1D domain, but you can do similar analyses of spatial data and do some interesting image editing in a 2D domain.
Someday, I want to give a more in-depth writeup of what exactly I used all this stuff for and my process of learning it, but today I'm just dumping links. (also I never quite finished my project! I gotta get back to that... I found an academic paper on higher-order moire that blew my mind open so hard that I put the project down until I could understand that paper...!)
Some broadly scoped videos that are great for understanding the D/FT in general:
https://www.youtube.com/watch?v=spUNpyF58BY - Excellent primer video by 3b1b
https://www.youtube.com/watch?v=MBnnXbOM5S4 - This one is a lot less relevant for image processing in general, but I still found it really enlightening and interesting. 3b1b could tell me about anything and I'd be happy for it.
https://www.youtube.com/watch?v=nmgFG7PUHfo - I'm not always a big fan of veritasium, but DFTs are confusing enough that I'll take what I can get, and this is a fun video. well, "fun" if you like learning about DFTs.
https://www.youtube.com/watch?v=h7apO7q16V0 - This one is inspired by 3b1b and I'll be honest, I don't remember it as well but youtube insists I've watched it, so on the list it goes.
way, way more nerdier stuff below the fold:
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Stuff about image processing and 2D DFT/FFTs:
Stuff very narrowly related to screen printing moire:
https://www.youtube.com/watch?v=3137dDa6P4s - A small tutorial video for GIMP showing how to eliminate film grain using FFTs. Short and sweet, if you've never thought about using FFTs for image editing, I think this video is extremely enlightening for seeing the practical implications of all the math in more academic texts. (It blew my mind apart, at least!)
https://www.youtube.com/watch?v=yyox358zIRw - This video is about using FFTs for film and scanner grain in general, it's much less focused on screen printing noise. Also extremely enlightening for a practical demo.