---
title: "Terrain Rider: API"
package: "@phaser-ae/rider-3d"
tier: Free
version: 3.0.0-3
released: 2026-10-14
updated: 2026-10-20
description: "Rides a board, kart, ball or walker over a height field without jitter, following the slope and catching air off lips, with the exact frame it lands."
url: https://phaser.io/packs/rider-3d/api
site: Phaser - https://phaser.io
---

# Terrain Rider: API

`@phaser-ae/rider-3d` — Ride a height field without jitter: footprint contact, slope orientation, and the airborne state machine.

## The pack

### `rider3d`

```ts
function rider3d(): Pack<{ rider: Rider3dApi }>
```

The `rider-3d` pack: bodies that ride a height field without jitter.

It owns the VERTICAL — ride height, orientation, and the grounded-versus-airborne state. The GAME still
owns HORIZONTAL motion, because that genuinely varies: a board carves, a ball rolls, a kart grips. Each
frame the game moves x and z its own way, then calls `step(dt, x, z, yaw)` and reads back `y`, `pitch`,
`roll`, `grounded`, `landed` and `airTime`.

Headless and pure — it holds no objects and draws nothing.

## Functions

### `buildFootprint`

```ts
function buildFootprint( contact: ContactShape, length: number, width: number, radius: number, ringTaps: number, ): FootTap[]
```

Build the contact footprint for a shape (exported for the tests). A
PLANK/point is a symmetric plus of fore×side taps; a SPHERE is the
centre + two rings sampling the ball's contact disc (averaged).

### `footprintTaps`

```ts
function footprintTaps(L: number): number[]
```

Fore-aft sample offsets across a footprint of length L (a symmetric
5-tap line, or a single centre tap when L = 0). Exported for the tests.

## Classes

### `TerrainRider`

```ts
class TerrainRider
```

Rides a body on a height-field surface without jitter — owns ride height,
slope orientation and the grounded/airborne state. The game moves x/z its
own way, then calls `step(dt, x, z, yaw)` and reads the outputs back.

- `x: number` — Horizontal position (the game writes these via step).
- `z: number` — Horizontal Z position, in world units (the game writes it via step).
- `yaw: number` — Heading the footprint is sampled along (radians).
- `y: number` — The smoothed ride height — set this on your mesh.
- `vy: number` — Vertical velocity (managed in the air; 0 while grounded).
- `grounded: boolean` — True while the rider is on the surface, false while airborne.
- `airTime: number` — Seconds since takeoff (frozen at flight length after landing — read it on the `landed` frame to scale a stomp/impact).
- `landed: boolean` — True ONLY on the frame the rider touched down.
- `pitch: number` — Surface-following orientation for the body (radians). Add your own lean (carve, steer) on top.
- `roll: number` — Surface-following roll for the body, in radians — the cross-slope bank.
- `slope: number` — The fitted along-heading slope (dY/dForward), smoothed.
- `groundY: number` — This frame's raw support height and last frame's — the delta is the height traded for speed if you run an energy model (v² += 2g·Δh).
- `lastGroundY: number` — Last frame's raw support height, in world units — pair with `groundY`.
- `constructor(opts: TerrainRiderOptions)`
- `sample(x: number, z: number, yaw: number): { support: number; slopeFore: number; slopeSide: number }` — Where the body rests on the terrain, and the surface slope under it. SPHERE: the average ground over the ball's contact disc (its centre and two rings), plus `radius` — averaging smooths the terrain's quad kinks, so a ball rolls over them and bridges notches without the hard support-switching a footprint max() gives. PLANK: a plane at the fitted slope, laid on the highest contact — bridges dips, pivots crests, banks to the slope. Pure (tests drive it).
- `reset(x: number, z: number, yaw = 0): void` — Place the rider on the ground at (x, z), at rest — call before the first step (and on respawn).
- `launch(vy: number): void` — Pop into the air with an initial upward velocity (a jump).
- `step(dt: number, x: number, z: number, yaw: number, speed?: number): void` — Advance one frame. The GAME has already moved the rider horizontally — pass the new (x, z, yaw). `speed` (horizontal, world u/s) tunes the suspension stiffness; omit it and it's derived from the move. Sets y, pitch, roll, grounded, landed, airTime, groundY/lastGroundY.

## Types

### `ContactShape`

```ts
type ContactShape = 'point' | 'plank' | 'sphere'
```

How the body meets the ground. `point` = a single sample (a marker, a
foot). `sphere` = a ball: the average ground under its contact disc
(rolls over quad kinks and small notches without the
hard support-switching a footprint max() gives — the right model for a
round thing). `plank` = a flat footprint (board/kart/box/GLB) that rests
on its highest contact and, if `tilt`, banks to the slope. Auto-picked:
`radius` → sphere, `length` → plank, else point.

### `Rider3dApi`

```ts
interface Rider3dApi
```

`game.rider`.

- `add(opts: TerrainRiderOptions): TerrainRider` — Make a rider over a surface. `groundAt` is the one required input.
- `readonly all: readonly TerrainRider[]` — Every rider made, so a game can step them together.
- `clear(): void` — Forget them all.

### `TerrainRiderOptions`

```ts
interface TerrainRiderOptions
```

Options for the TerrainRider constructor — `groundAt` is the one required input.

- `groundAt: (x: number, z: number) => number` — Surface height at world (x, z) — the ONE required input. Ride terrain with `(x, z) => terrain.heightAt(x, z)` (or the fresh field, or a seabed…). The rider adds the body's own thickness (`radius` / the plank plane), so groundAt returns the bare surface. Keep it smooth (the fresh field, not the carved surface, if the rider carves — else it rides its own cut).
- `contact?: ContactShape` — Contact shape (default auto from radius/length — see ContactShape).
- `radius?: number` — SPHERE radius — a ball/wheel. The centre rides the spherical envelope, so it bridges notches narrower than R and never jitters on inclines.
- `ringTaps?: number` — Sphere envelope resolution — samples on a ring (default 12). More = smoother over rough ground, costs more groundAt calls.
- `length?: number` — Fore-aft footprint length (PLANK; default 0 = point). A board/kart/box BRIDGES dips and PIVOTS crests over this length — the biggest anti-jitter win for a flat-bottomed body.
- `width?: number` — Lateral footprint width (PLANK; default 0). Set it to bank to the cross-slope (a wide vehicle).
- `tilt?: boolean` — Bank/pitch the body to the surface slope (default: true for a plank, false for sphere/point). Set false for a box that stays level, resting on its highest corner.
- `settle?: number` — How far the contact settles INTO the surface (default 0). A board compacts the snow it rides; the trench opens up behind it.
- `gravity?: number` — Air gravity, world units/s² (default 30). Ignored while glued.
- `detach?: number` — Detach into the air when the surface drops more than this below the rider — a lip throws you (default 0.9). `Infinity` = GLUED (never airborne; e.g. a ball stuck to a seabed).
- `followK?: number` — Suspension stiffness = `followK + speed·followKGain`, capped at `followKMax`. Soft enough to filter terrain-quad steps, stiff enough not to float off a fast descent — but the cap matters: uncapped it goes near-rigid at speed and transmits every support switch raw. Defaults 25 / 0.5 / 55. Set followK = Infinity for a hard follow.
- `followKGain?: number` — Extra suspension stiffness per unit of speed. Default 0.5.
- `followKMax?: number` — The cap on the speed-scaled suspension stiffness. Default 55.
- `slopeK?: number` — Orientation smoothing: the fitted slope eases at `slopeK`, then the pitch/roll ease toward it at `orientK` (defaults 14 / 10).
- `orientK?: number` — Ease rate of pitch/roll toward the fitted slope, per second. Default 10.
- `pitchGain?: number` — Surface pitch/roll = atan(slope) × gain (defaults 0.9).
- `rollGain?: number` — Roll = atan(cross-slope) × this gain. Default 0.9.
- `maxFall?: number` — Terminal fall speed (default 200).

---

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