I’m still picking unfinished and half-finished format for na_game_tool and there were some from Access games. While supporting animations from Amazon: Guardians of Eden is rather pointless (since they simply update pixels on some scene and do not have own palette), supporting H2O and PTF formats is more interesting.
I wrote about them back in the day and about H2O specifically. And since it was a quick look and my eyes are not that good, I missed some details. But as I finally implemented a decoder for it, now I can correct that story.
As it turned out, H2O stores video data as Huffman-coded 16-bit values instead of usual bytes. And those values are used for both opcodes and colours. If top four bits of opcode are 0-3 then the top byte codes number of 4×4 blocks to process and low byte is fill value (0 means skip block instead). If top four bits of opcode are 4, then low 12 bits are number of blocks to process minus one and each block data consists of 16-bit pattern and two colours (packed as another 16-bit value). Top four bits of opcode being 5 mean partial update for the specified number of blocks, each block having 16-bit update mask and 0-16 colours to update (packed in 16-bit words).
So while the format is no ground-breaker, it still turned out to be more interesting than I expected it to be.
P.S. I guess I’ll do a bunch of FLI-based formats next. There are some curious variations of it like FLI with audio or RNC ProPack compression.