Well on the path to getting car texture support done, but there have been a few trials along the way.
Firstly the game uses a format I haven’t run into before:
The indices are stored in interleaved order (all even rows, then all odd rows) and the odd rows are word swapped (xxxxxxxx yyyyyyyy is stored as yyyyyyyy xxxxxxxx), very strange.
Second problem I have yet to resolve, I haven’t found how the game keeps track of the texture sizes. This may be hard-coded for car data, as they seem to use 80×38 textures for everything except the wheels, and the wheels have a separate block of texture pointers to the rest of the car parts. Hopefully looking at the track data which has a variety of texture sizes will shed more light on this.
Finally, and perhaps most frustrating, the game appears to use a slightly modified version of zlib 1.0.4 (there’s a string referring to this version in the ROM) to inflate the compressed data. You can use vanilla zlib to decompress and compress data and more often than not you’ll get the same output as the game, buuut not 100% of the time. Hoping that it was perhaps just some breaking change in zlib (which is currently v2.x) I grabbed the source for 1.0.4 and tried that, but it gives the exact same output. So now I have to reverse the game’s inflate routine to be able to extract correct data. Reversing an ASM routine is kinda fun, but can be painful as well.
Such is life.