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assets/ now holds every track as a 16-colour indexed PNG (its palette is the track palette) plus a text file of lap gates and height map, and every vehicle as a PNG sprite sheet plus a text file with its name and handling (moved out of the code). The build embeds assets/ instead of unpacking the ADFs, so the disks are no longer needed to build either. skids extract converts original disks to assets and checks each file reads back identically; benchmark results from the assets match the disks exactly. Assets win over disk data with the same name, and --data assets picks up edits without a rebuild. assets/README.md describes how to edit everything.
139 lines
5.6 KiB
Markdown
139 lines
5.6 KiB
Markdown
# Super Skidmarks data formats
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Notes from reverse engineering the AGA/CD32 release of *Super Skidmarks*
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(Acid Software, 1995). Everything here was worked out from the data on disks
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2–6. Disk 1, which holds the executable, is missing from this archive, so
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anything that lives only in the executable (palettes, sound, handling) is
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unknown. See [Gaps](#gaps).
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All multi-byte values are big-endian unless noted.
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## Disks
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Each disk is a standard 880 KB double-density ADF (1760 blocks of 512 bytes)
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formatted as OFS (`DOS\0`). The `[HD]` in the file names refers to the
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hard-drive-installable release, not high-density disks. Each disk holds a
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single file:
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| Disk | File | Contents |
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|------|---------------|------------------------------------|
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| 1 | `smcd32a.lha` | *missing* |
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| 2 | `smcd32b.lha` | `vehicles/cow.aga`, `track/TRACK1` |
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| 3 | `smcd32c.lha` | `track/TRACK2`, `vehicles/TRUCK.aga` |
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| 4 | `smcd32d.lha` | `track/TRACK3`, `vehicles/PORSCHE.aga` |
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| 5 | `smcd32e.lha` | `track/track4`, `vehicles/vw.aga` |
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| 6 | `smcd32f.lha` | `vehicles/F1.aga`, `Midget.aga`, `mini.aga`, `Police.aga` |
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Each disk's archive stands on its own (the set isn't one archive split
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across disks). They use level-0 headers and the `-lh0-` method (stored),
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under a top-level `super_skidmarks_cd³²\` directory. Most members are
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individually packed with CrunchMania.
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## CrunchMania
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```
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0 4 id: "CrM!" (LZ + fixed codes) or "CrM2" (LZ + Huffman)
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lower-case "Crm" = delta-encoded samples (running byte sum)
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4 2 unused
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6 4 unpacked size
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10 4 packed size (excludes this 14-byte header)
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14 … packed stream
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```
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The stream is read **backwards** from `14 + packed - 6`, a byte at a time,
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LSB first. The last six bytes seed the bit buffer: a 32-bit word shifted
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right by `16 - shift` with `16 + shift` valid bits, where `shift` is the
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final 16-bit word. Output is also written from the end towards the start.
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`src/formats/crm.rs` implements both variants. Every packed file on the
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disks decodes exactly: the output buffer fills completely and the input is
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consumed to its first byte.
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## Tracks (`track/TRACKn`)
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```
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0x00 7 × u32 end offset of sections 0..6
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0x1C … section 0: track-selector thumbnails (20622 bytes, raw)
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… sections 1..6: one track each, CrunchMania-packed
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```
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So the four files hold 24 tracks. Each track section unpacks to 281240
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(`0x44A98`) bytes:
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| Offset | Size | Contents |
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|----------|--------|----------|
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| `0x0000` | 8192 | Height map: 64 × 64 `u16`, 8.8 fixed point, row-major. Each cell covers 16 × 8 screen pixels. |
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| `0x2000` | 8192 | Unused, zero on every track. |
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| `0x4000` | 8 | `u16` gate count, `u16` start x, `u16` start y, `u16` flags. |
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| `0x4008` | 8 × n | Gates: `x1 y1 x2 y2`, world coordinates × 32. The list repeats after the last gate. |
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| … | | Unknown data, mostly a bit pattern, up to `0x4A98`. |
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| `0x4A98` | 4 × 65536 | Four non-interleaved 1024 × 512 bitplanes, giving 16 colours. |
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### Track geometry
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Gates are line segments across the road and run in race order; gate 0 is
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the start/finish line. They're defined on an isometric ground plane rotated
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45° to the picture. The projection that best fits the data is:
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```
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screen x = wx - wy + 512
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screen y = (wx + wy) × 31/64 - 252
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```
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With these constants, 96% of points sampled along the gates land on road
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pixels across all 24 tracks (92% minimum). A plain screen-aligned 2:1 squash
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gives 40–70%. Adding a height term doesn't improve the fit, so the height
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map is used only for slope physics and body tilt.
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The stored start point is on the road for 14 of the 24 tracks read as screen
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coordinates, and 10 read as world coordinates. It isn't used; the grid forms
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up behind gate 0.
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### Palette use
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The track pixel indices are ordered from dark to light. Indices 0–7 are
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scenery and 8–15 the driving surface, so the fourth bitplane is effectively
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a road mask. The game uses it as such here: off-road means less grip and
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lower speed.
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## Vehicles (`vehicles/*.aga`)
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CrunchMania-packed, 409600 bytes unpacked:
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25 tilt blocks × 32 headings × 512-byte frames.
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A frame is a 64 × 32 AGA hardware sprite. Each line holds 8 bytes of plane
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0, then 8 bytes of plane 1, giving 3 colours plus transparent:
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| Index | Use |
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|-------|---------------------|
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| 0 | transparent |
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| 1 | tyres, shadow, dark |
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| 2 | trim / accent |
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| 3 | bodywork |
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- **Heading:** frame 12 faces screen-right, and frames advance clockwise in
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11.25° steps. In world terms, frame = 12 + (angle + 45°) / 11.25°.
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- **Tilt:** block = roll × 5 + pitch, each in 0..4, with 2 = level.
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Block 12 is the car on flat ground.
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`Police.aga` reads as all zeros with one third-party ADF tool, but it's
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intact when read directly from the disk image.
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## Gaps
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These almost certainly live on the missing disk 1:
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- The main executable, and with it the real handling model, AI, rules,
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menus and fonts. This port's versions are reconstructions.
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- Colour palettes. Neither the track nor the vehicle files carry any. The
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engine uses a reconstructed palette, and real ones can be dropped in as
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`.pal` files (see the README).
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- Sound effects and music.
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- Any tracks or vehicles beyond the 24 and 8 above.
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The engine's own editable formats, which `skids extract` converts all of
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the above into, are described in [assets/README.md](../assets/README.md).
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If disk 1 turns up, put it next to the others and run `skids info`.
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Anything in a `track/` or `vehicles/` directory inside its archive is
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picked up automatically.
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