https://github.com/TheRealOrange/icer_compression is pretty neat – a mostly complete implementation of ICER, a wavelet-based, error-resilient image compression format designed by NASA 20+ years ago for Spirit and Opportunity.
It’s pretty old-fashioned now. The vanilla DWT, the Tudor-era entropy coder – with only admiration for what it was in 2003, no one would propose a new format like ICER today. But it’s interesting in itself and as a what-if. (What if JPEG 2000 were designed for normal people, basically.)