having done my own experiments with indexed pallets I suspect method #2 is responsible for most of the compression here. most photographs only have half a milli

having done my own experiments with indexed pallets I suspect method #2 is responsible for most of the compression here. most photographs only have half a million distinct colors and it drops considerably as you focus in on an area. if you're willing to relax the single-pass requirement, partitioning the whole image by pallet similarly to how one encodes high-color images into multi-framed gifs can yield surprisingly good compression.