I got really good compression using this technique with JPEG XL, I'm sure there's even a good reason why it works so well but it's been a long time and I don't seem to remember why.
Could it be possible that jpg also exploits the repetition at the wavelength of the width of a single picture, so to say? E.g. 4 pictures side-by-side with the same black dot in the center, can all 4 dots be encoded with a single sine wave (simplifying a lot here..) that has peaks at each dot?
