Regressive JPEGs

Let me just say why this is cool, because the title may not make it clear: you've heard about animated GIFs. You know that JPEGs don't support animations. But... the 30+ year old JPEG standard supports progressive JPEGs, in which coefficients for different frequencies are sent in separate batches ("scans") so that you get a blurry image more quickly, and then more detail is resolved as your internet connection permits.

Several days ago, I noted that nothing really requires these subsequent scans to really describe the same image, and posted an idle question of whether it's possible to have a "regressive JPEG" that turns into something worse / different as it loads: https://infosec.exchange/@lcamtuf/116922305016859176

Maurycy noticed that you can do even better: you can send the same spectral data more than once, so you can actually overwrite previously-rendered content and build... animated JPEGs.

This is very cool! I love seeing creative ways to get software to do something it was not designed to do.