Gratitude Day 14: Apple Video Codec

I'm continuing to explore lossy compression algorithms in my "Unsung Heroes, Human and Algorithmic" sub-theme of my September Gratitude series. The Apple Video codec from 1991 is one of my favorite codecs: not because it's particularly efficient/effective (it's horrible), nor because it's popular (you'd be insane to use it today), but because it's one of the few lossy algorithms that's actually quite easy to understand and doesn't involve any heavy mathematics. Apple Video saves data in two lossy ways: by reducing the color depth from 24-bits to 16-bits universally, and by replacing pixels with pixels from a previous frame, or a flat color, or with pixels from a further reduced color palette. After reducing the color depth down to 16 bits, Apple Video separates each video frame into 4x4 pixel blocks, and encodes each block in one of four ways (for simlicity, I'm going to use my own terminology for the names of the modes): "0-color": No 4x4 pixel block is encoded, but rather, the…