Step 1 - getting the files off the disk
The disk is a normal 10 sector per track TOS disk with a protection track at 79, so once the STX image was decoded, all the files came straight off it. There is a small loader in the AUTO folder, the main game (CARS.REL) and about a hundred files called FILExx.SCR.
Step 2 - why rippers find nothing
Every graphics file is packed with Magnetic Fields' own packers, which start with "SP", "SA" or "SF" headers. On top of that, the SP graphics unpack into whole separate bitplanes rather than the normal ST interleaved screen layout, so even unpacked they do not look like ST graphics. No wonder the rippers gave up, they were looking for something that was never there.
The unpack routines were taken straight out of the game code and ported line for line. Every file now unpacks without errors, and the title screens come out perfectly, which proved the depackers were right.
- SC2_1.PNG (169.82 KiB) Viewed 34 times
Step 3 - the collision / height map
Each course has a 32KB map with 4 cells per 16 pixel tile. You can clearly see the track shape, bridges and crossovers in it. The best guess so far is two height values (upper and lower layer) plus a surface type, but that part is NOT confirmed yet and needs more digging in the race code.
- SC2_2.PNG (46.49 KiB) Viewed 34 times
Step 4 - the scenery tiles
There are three scenery sets, each made of sheets of 256 tiles at 16x16 pixels. The game code shows which files get loaded for each race and which palette goes with each set.
- SC2_3.PNG (58.34 KiB) Viewed 34 times
Step 5 - the track maps
Each course also has an 8KB tile map, a 64x64 grid of tile numbers, with the actual course using part of it (from 20x50 up to 64x16 tiles). Put that together with the right tile set and palette and out pop all 21 courses in full colour. Some of the maps even still have the designer's size notes like "32 x 32" drawn in tiles outside the track area, which was a nice touch from 1991.
- SC2_4.PNG (1.05 MiB) Viewed 34 times
Here is Bagley Marsh, the first race, at full size:
- SC2_5.PNG (62.31 KiB) Viewed 34 times
There is also a course database file which still has the original symbol names in it. It holds the race order, which tile set each race uses, lap counts, AI settings, and each course's size, name and description text.
Step 6 - the cars
The cars are pre-rotated sprites, 32 frames per car, with the missiles and explosions in a separate bank that carries its own mask plane. The colours here are only roughly right because I used the track palette for the preview.
- SC2_6.PNG (64.31 KiB) Viewed 34 times
What this means for a level editor
The good news is the loader happily accepts files that are not packed, so an edited map can simply be saved back as raw data without having to write a packer. There is about 154KB free on the disk, which is enough for a couple of new or modified tracks.
What is still left to work out is exactly what the values in the collision map mean, and a few tables in the course data that are probably AI routes and gate positions. Once those are pinned down, an editor is very doable.
Not bad for a game that has kept its secrets for 35 years, it only took slightly longer than loading Elite off cassette.
