Test work for a video encoding using Atari 800-family GTIA grayscale mode on top of text cells. This allows a "block dictionary" of 128 2x8-pixel blocks on an 80x160 grayscale image, at cost of 1840 bytes per full frame or 800 bytes to reuse previous blocks.
Currently just converts to grayscale and counts up unique blocks. Next step: decimate if > 128 unique blocks per image, and combine the most similar blocks in the output.