Use the original handling, walls and projection from the executable

Decoded from SKIDMARKS_CD32's machine code:

- Car physics: the per-frame engine/grip/steering update, gravity and
  jumps, the hill effect on the throttle, and body tilt. Replaces the
  reconstructed handling model.
- The meaning of every .set value (separate drag, grip and power for
  human and computer drivers; steering, gravity, tow hitch), and the
  six handling presets, offered by a new Handling menu option.
- Walls: the road edges are quadratic B-splines through the gates'
  ends. They match the painted road edges.
- The real projection, which lifts points by their height; the height
  map is indexed by world cells, not screen pixels.
- Tracks have one more gate than the stored count: the last gate was
  being dropped on every track.

Computer drivers now follow the road between the walls and steer like
the original's. Vehicle assets store the values as `handling`; older
files still load. Docs describe the decoded logic.

assets/README.md

@@ -18,8 +18,9 @@ The track picture: 1024 × 512, a 16-colour indexed (palette) image.
 
 - **Palette:** the PNG's palette is the track's palette. Change a palette
   entry to recolour every pixel that uses it.
-- **Road vs scenery:** colours 8–15 are road (full grip, full speed).
-  Colours 0–7 are scenery (less grip, slower).
+- **Road vs scenery:** colours 8–15 are road; colours 0–7 are scenery,
+  where cars throw up dust. The picture doesn't affect handling. The
+  walls come from the gates (below).
 - **Painting a new route:** paint it with the upper eight colours and
   update the gates to match.
 - **Saving:** keep the image indexed if your editor allows it. If it saves
@@ -32,16 +33,26 @@ Plain text; lines starting with `#` are comments.
 
 - **`gate x1 y1 x2 y2`:** lap gates in race order.
   - Gate 0 is the start/finish line, and the grid forms up behind it.
-  - Cars must cross every gate, in order, to complete a lap. Computer
-    drivers steer from one gate to the next.
+  - Cars must cross every gate, in order, to complete a lap.
+  - **Walls:** the gates' ends also shape the road. One edge is a smooth
+    curve through every gate's first end (`x1 y1`), the other through
+    every second end (`x2 y2`). Those curves are the walls, and computer
+    drivers follow the road between them. The curves pass halfway
+    between neighbouring gates' ends and round off the corners, as in
+    the original.
   - Coordinates are on the isometric ground plane. To find where a point
-    sits in the picture:
-    `picture x = wx - wy + 512`, `picture y = (wx + wy) × 31/64 − 252`.
-  - `skids trace --data assets --track N --out t.png` draws the gates
-    (gate 0 in yellow) and the computer cars' paths over the picture.
-- **`height`:** 64 rows of 64 numbers giving ground height × 256. Each
-  number covers a 16 × 8 pixel cell of the picture. Slopes push cars
-  downhill and tilt the car sprites.
+    on the ground sits in the picture, with `z` its height in pixels
+    (from `height`):
+    `picture x = wx - wy + 512`, `picture y = (wx + wy) / 2 − 156 − z`.
+  - `skids trace --data assets --track N --out t.png` draws the walls
+    (grey), the gates (gate 0 in yellow) and the computer cars' paths over
+    the picture.
+- **`height`:** 64 rows of 64 numbers giving ground height in pixels × 64.
+  - **Layout:** the rows run along world y and the numbers within a row
+    along world x. Each covers 16 × 16 world units.
+  - **Effect:** climbing slows cars and descending speeds them up. Cars
+    leave the ground over crests that drop away faster than gravity, and
+    the car sprites tilt with the slope.
 - **`set`:** the name of the set of tracks this one belongs to, shown under
   the menu thumbnail.
 - **`start`, `flags`:** values from the original data that the game
@@ -91,17 +102,35 @@ The display name, driver and handling:
 | `name`    | name shown in the menu |
 | `driver`  | driver's name, shown next to it |
 | `trailer` | `yes` for something towed behind cars (the caravan) rather than driven |
-| `original`| the 16 values from the original `.set` file, kept for reference; the game doesn't use them yet |
-| `top`     | top speed on tarmac |
-| `accel`   | acceleration |
-| `brake`   | braking |
-| `grip`    | sideways grip; higher means less sliding |
-| `turn`    | steering rate |
-| `mass`    | weight in collisions |
-| `offroad` | speed and grip multiplier on scenery |
+| `handling`| the 16 values from the original `.set` file, used as the original game uses them (below) |
+| `mass`    | weight when cars collide; not from the original data |
 
-The handling values are a reconstruction, so tune them freely. To add a
-car, add a new `<id>.png` / `<id>.txt` pair.
+The `handling` values, counting from 1:
+
+| Value | Meaning |
+|-------|---------|
+| 1, 2, 6 | computer drivers: drag, grip, power |
+| 9, 10, 11 | human drivers: drag, grip, power |
+| 3 | steering: 16 = 1/64 of a turn per frame at full lock |
+| 5 | gravity: 8 = 1 pixel per frame² |
+| 7 | tow hitch distance behind the car: 2 = 1 world unit |
+| 4, 8, 12–16 | unused |
+
+What the driving values do:
+- **Drag** (below 64): how much of the engine's speed survives each frame.
+  Low drag gives fast pick-up and fast slowing.
+- **Power:** what the throttle adds.
+- **Top speed:** `power × drag / (64 − drag) / 32` world units per frame,
+  at 50 frames a second.
+- **Grip:** how quickly the car's movement turns to follow where it's
+  pointing. Low grip means long slides.
+
+The menu's **Handling** option can replace every car's values with one of
+the original's presets.
+
+Older files with `original`, `top`, `accel`, `brake`, `grip`, `turn` and
+`offroad` lines still load: `original` is read as `handling`, and the
+others are ignored. To add a car, add a new `<id>.png` / `<id>.txt` pair.
 
 ## Sounds: `sounds/*.wav`