Oh, I know about the multi-level parallax. The water in Green Hill Zone is entirely made of "slices" a single pixel in height for example. I actually tried separating those and assembling them and found out the water had a 512 pixels wide pattern repeating itself, as did the clouds.
But the waterfalls and mountains form a much longer pattern. After capturing as much as I could, separating the slices, assembling them separately and then pasting one over the other, I managed to make out 7 different blocks 256 pixels wide that are assembled in a pattern that doesn't seem to repeat itself. The block sequence goes: 1, 2, 3, 4, 5, 6, 7, 1, 3, 5, 2, 6, 1, 7, 7, 1, 2, 3, 4, 2. That's what I mean by pattern; there doesn't seem to be a set sequence that repeats itself even when not taking multi-level parallax in account.
It's the same for Marble Zone. While the top section with mountains, clouds and trees is made of 3 different blocks, they also don't repeat themselves in a pattern: 1, 2, 3, 2, 1, 3, 3, 2, 1, 3, 2, 1, 3, 2.
And after capturing as much of the bottom inner ruins part, it also doesn't repeat itself after a fashion even though it's made of the same 9 256 x 256 blocks. So like I said, I now capture as many blocks of it as I can, and then repeat the separate sections. That's a lot of time wasted over silly little details, but I'm glad that I now understand how the game manages the stages better.
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"With the first link, the chain is forged. The first speech censored, the first thought forbidden, the first freedom denied, chains us all irrevocably." [...] The first time any man's freedom is trodden on, we're all damaged. - Captain Jean-Luc Picard
Current projects: Sonic CD (Sega CD), Mega Man V (GB), Mega Man Zero (GBA), Battletoads (NES)