fft paint
I consider myself Fourier-pilled. Learning to think of things in terms of their frequency decomposition was one of the most significant takeaways from my EEE degree. I love the Fourier transform.
As a toy project for playing with 2D FFT and spatial spectra, I wrote a small program that opens two windows: (1) an image of your choice, (2) the 2D Fourier transform of the image.
The 2nd screen with the FFT allows you to "draw" black or white, where drawing black blocks frequencies and drawing white passes them. This allows you to experiment, "drawing" filters of different kinds. You can get some educational results.
Blocking all but horizontal frequency components on an image of a zebra makes only horizontally oscillating stripes appear.
and the same vice-versa:
