andy
c36572f8dc
- Track files: decode the selector thumbnails (a 640x64 16-colour picture, four sequential bitplanes, six 123x32 tiles in a 3x2 grid) into tracks/<name>/thumb.png, and show them in the menu at double height (they are Amiga hi-res) - gfx/ball.spr: decode the 64-frame attached-sprite bank (spinning ball with the Acid logo) into gfx/ball.png; its colours are not in the file - Command-line output no longer panics when piped into e.g. head
229 lines
9.1 KiB
Markdown
229 lines
9.1 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), worked out from all six disks. The one thing not
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yet decoded is the executable's game logic. 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
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bytes). Disk 1 is FFS (`DOS\1`); disks 2–6 are OFS (`DOS\0`). The `[HD]` in
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the file names refers to the hard-drive-installable release.
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| Disk | Archive | Game files |
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|------|---------------|------------|
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| 1 | `smcd32a.lha` | Executables, `sfx/`, `gfx/`, the tune, `vehicles/*.set`, `Camaro.aga`, `caravan.aga` |
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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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Disk 1 also carries an Installer script that installs from all six disks,
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and a few AmigaDOS commands.
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Each archive stands on its own; the set isn't one archive split across
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disks. They use level-0 headers under a top-level `super_skidmarks_cd³²\`
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directory.
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- **Compression:** disks 2–6 store their members (`-lh0-`), which are
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themselves CrunchMania-packed. Disk 1 mostly uses `-lh5-` (LZSS with
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static Huffman blocks, decoded by `src/formats/lzh.rs`).
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- **Integrity:** every member is checked against its header CRC-16. One
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file, `c/iconx` (an AmigaDOS utility the game doesn't use), fails its CRC
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in this dump. The reference `lha` tool reports the same failure.
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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: thumbnails and palette for the whole file (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.
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The last 142 bytes of section 0 hold:
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| Offset | Size | Contents |
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|--------|------|----------|
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| 0 | 2 | File number (1–4) |
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| 2 | 96 | The palette shared by the file's six tracks: 16 colours × R, G, B, each a `u16` from 0 to 15 (Amiga 12-bit colour) |
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| 98 | 12 | Unknown |
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| 110 | … | The name of the set of tracks, NUL-terminated |
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The first 20480 bytes of section 0 are the track-selector thumbnails:
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- **Picture:** one 640 × 64, 16-colour picture in the file's palette,
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stored as four bitplanes one after another (80 bytes per row, 5120 bytes
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per plane).
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- **Layout:** the six tracks' thumbnails, 123 × 32 each, sit in a 3 × 2
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grid at its top left, in track order: top row tracks 1–3, bottom row
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4–6.
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- **Display:** the picture is Amiga hi-res, whose pixels are half as wide
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as low-res ones. So a thumbnail shows at 123 × 64 next to the low-res
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track pictures, the same shape as the track.
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Each track section unpacks to 281240 (`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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### Road and scenery
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Colours 8–15 are the driving surface on every track, so the fourth
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bitplane is effectively a road mask. Off the road means less grip and
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lower speed.
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## Vehicles
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### Sprite banks (`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. Block
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12 is the car on flat ground.
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The caravan (`caravan.aga`) uses the same layout. It's a trailer with a tow
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bar, not a car.
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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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### Settings (`vehicles/*.set`)
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- **Header:** the car's name and its driver's name, each followed by a
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newline.
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- **Settings:** 16 `u16` values. Eleven are used by the cars; the rest are
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zero. The caravan's values differ in kind; two of them are −4.
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- **Menu picture:** 512 bytes. A 64 × 32 picture in two bitplanes, plane 0
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(256 bytes) then plane 1.
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The settings are clearly handling values. The first is smallest for the
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Cow, the second largest for the F1, and the Police car and Porsche are
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nearly identical. What each one means lives in the executable and isn't
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decoded yet.
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## Sounds (`sfx/*.iff`, `gfx/*.iff`)
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Uncompressed IFF 8SVX, 8-bit signed, mostly at Amiga rates (8363 Hz and
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others). Looping sounds use the repeat part of `VHDR`. `gfx/` holds a few
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extra and alternative sounds (`crash1`, `crash2`, `engine`, `motor`,
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`screech`); `gfx/crash4.iff` isn't IFF.
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## Pictures (`gfx/*.iff`)
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IFF ILBM with ByteRun1 compression. `skidtoon.iff` is the 608 × 180,
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16-colour title picture; `chat2`, `mouse` and `sign` are small odds and
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ends.
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`ball.spr` is a bank of 64 saved hardware sprites: a spinning ball with
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the Acid logo. Each 544-byte frame is:
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- **Header** (16 bytes): `u32` saved memory pointer, `u16` lines (33),
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`u16` strips (4), `u16` flags (`0x8020`), `u16` bytes per strip (132),
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4 unused bytes.
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- **Strips:** four 16-pixel sprites, each line two bitplane words.
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Flag bit 15 marks attached sprites. The strips pair up into two 16-colour
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columns, so a frame is 32 × 33 pixels. The bank carries no colours.
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## Music (`sfx/tune.crn`)
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A CrunchMania-packed OctaMED `MMD1` module: 4 tracks, 21 plain 8-bit
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sample instruments, 70 blocks played in order. The main song is positions
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0–65, ending with a jump back to the start. Positions 66–69 hold separate
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short pieces that end with "stop".
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The song uses these commands:
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- `1`, `2` (slide up, slide down)
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- `3` (portamento)
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- `9` (speed)
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- `B` (jump)
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- `C` (volume, decimal)
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- `D` (volume slide)
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- `F00` (break), `F1` (play twice), `F2` (delay), `FD` (set pitch),
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`FE` (stop)
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`src/music.rs` plays these. Its loudness tracks OpenMPT's rendering of the
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module second by second through the whole song (correlation 0.996 over the
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first minute), and it loops at the same point.
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## Executables
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- **`SKIDMARKS_CD³²`** (237 KB): a single-hunk compiled Blitz Basic 2
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program. It includes IFF ILBM/8SVX/ANIM loaders and a tracker replay
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routine with note-period tables.
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- **`skidmarks2`** (90 KB): a three-hunk program whose code is packed.
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## Gaps
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- **Game logic.** The handling, AI and rules in the executable aren't
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decoded; this port's versions are reconstructions. Decoding the `.set`
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values would be the first step towards the original handling.
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- **Unknown data:** the 12 bytes after each track file's palette, the bit
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pattern after the gates in each track section, and the colours of
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`ball.spr` (probably set by the executable).
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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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