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1. Do you just want to do a palette modification, or do you intend to go further in the future and actually change the shape of the sprites/insert different sprites, etc? Sometimes image data could be compressed but the palette data might be stored separately and uncompressed. You might be able to find and isolate palette data in an uncompressed savestate or RAM dump and then search for it in the ROM, but I'm not sure whether the utilities needed to make that process user-friendly exist for Genesis games.
2. Just a ballpark guess, but to be absolutely sure, have you tried looking for the graphics with TLP's codec set to 4BPP Linear, 2 Dimensional mode? (Or does TLP even have that setting -- not entirely sure, since I normally use TileMolester for quick graphics checks).
Generally speaking, the most common approach to figuring out decompression routines is to use an emulator's debugging and tracing features to study whatever functions are being called when graphics load. I'll leave it up to others to describe how that's done exactly because I'm sure I'd get it wrong at this point.
Sometimes it's perfectly possible to do a "brute force compare" between compressed and decompressed graphics, and literally figure out how the decompression routine is transforming the data at the byte level. Theoretically a very thorough documentation of the routine in layman's terms could be passed to a programmer and the programmer could interpret and create suitable code to reproduce the documented process. However, success will depend hugely on luck -- simple compression routines based on some variation of RLE, dictionaries, or even "sliding windows" can be rather enjoyable to figure out by comparison, but when you get into more complex compression algorithms the frustration meter rises quickly, and perhaps to the point of impossibility. Most modders will probably discourage you from brute force comparison for this reason, but I've always been rather tickled by such exercises myself.
Before addressing general methods for "brute force compares" it will be necessary to have an uncompressed copy and a compressed copy of a spritesheet from the game on-hand. That, in turn, depends on whether there's currently a convenient way of obtaining VRAM (visual RAM) from Genesis savestates, or doing straight RAM/VRAM dumps from a Genesis emulator. I'm not sure if that's currently possible; if not, some quality time with a debugger will be necessary.
EDIT: In Romhacking.net's utilities archive you can find Fuzzbuzz's DebuGens
and Korama's GSavestate
, which look most impressive for the needs of brute force comparison! A utility like Genstracer
might be recommended to you if you wanted to pursue the more usual practice of getting what you need in a debugger, though I wonder if DebuGens would suffice as well for that purpose. It appears to be quite full featured!