Battle Background

Figured out the palettes at least.

From the 6 bytes data at 270200 take the least significant 7 bits of the 6th byte and multiply it by 3. That will give you the starting palette of a 6 palette set from the battle bg palettes.

Individual tiles only use one of three palettes. To get which one take the 3rd and 4th least significant bits from the 2nd byte of the corresponding tile assembly data and subtract 1 from it.

Now I just have to figure out what the hell is happening with those tilesets with an index of FF. I checked out what the code does when it tries to load an FF tileset and... it literally just skips over it (well really it just does a plx and ply then moves on, but I think that's because before that check happens x and y get pushed, so I think it's just undoing what was just done). Maybe I've got to follow the code farther, but I'm pretty stumped for the moment.

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Okay wow, I did not thing that was going to work, but it did.

So when I said,

but for right now I'm working on the assumption that everything below
bank E7 is uncompressed. It seems to be working, but more testing is
needed

Turns out I was right. However I didn't take it far enough. The uncompressed tilesets are 256 items instead of 128, and a tileset of FF just means to use the second half of the uncompressed tileset (which is apparently always tileset 0). So basically, tileset FF means, tileset[0][tile_index + 128]

http://ff3.herokuapp.com/battle is updated if you want to check out the backgrounds.