melonjs-3d git:20261011.806ee0aA

Immutable. This exact content never changes and is served at /api/v1/blob/39cd96f167d423a6.

---
name: melonjs-3d
description: "Use this skill for anything 3D or 2.5D in melonJS — Camera3d, Mesh, InstancedMesh, Sprite3d billboards, Light3d, ground shadows, glTF/GLB scenes, and depth sorting. Covers the Y-down/+Z-forward convention that is the inverse of OpenGL, the cameraClass opt-in, clip planes, and what does not work on the Canvas fallback. Triggers on: Camera3d, Mesh, InstancedMesh, Sprite3d, Trail3d, ribbon, exhaust trail, Light3d, billboard, glTF, glb, 3D, 2.5D, depth, cameraClass, fov, setClipPlanes, setFog, fog, distance fog, height fog, heightFalloff, transparent, transparency, alpha, blendMode, fade, castGroundShadow, lit, setBasis, getBasis, lookAt, up vector, gimbal, slerp, great circle, turn a direction."
license: MIT
---

# 3D and 2.5D in melonJS

> melonJS is a 2D engine that grew a real 3D tier: perspective cameras, meshes,
> instancing, glTF scenes and 3D lights. Most of what a model "knows" about 3D
> graphics comes from the OpenGL convention, and **melonJS's is the inverse**.
> Read the conventions section before writing any 3D code.

## Coordinate conventions — the inverse of OpenGL

This is the single most important thing on this page.

| | melonJS | OpenGL convention |
|---|---|---|
| vertical | **Y-down** — higher `y` is *lower* on screen | Y-up |
| depth | **+Z forward** — higher `z` is *farther* away | −Z forward |

`Camera3d` exposes three rotation angles: `camera.pitch`
(X, look up/down), `camera.yaw` (Y, look left/right) and `camera.roll` (Z, bank
the horizon). The view is `R(yaw) ∘ R(pitch) ∘ R(roll)` inverted, and the frustum
planes come off that same matrix — so culling follows a banked view.

The basis those angles produce is **readable and writable**: `getBasis(right,
up, forward)` reads it, `setBasis(right, up, forward)` poses the camera from
one.

`camera.rotate()` is NOT the way to bank a 3D camera. It writes the inherited
`currentTransform`, which a 3D view never reads, so the call is silently inert.
Set `roll`. (On a `Camera2d` it is the other way round: roll IS that transform's
rotation, which is why `worldToLocal` / `localToWorld` compensate for it.)

The payoff is that 2D code translates directly — anywhere you used `pos.x` /
`pos.y`, add `pos.z` and the maths still holds. The cost is that every
OpenGL-shaped instinct is backwards.

**Consequence that bites immediately:** "behind everything" is the *largest*
depth when the camera looks along +Z. Put a backdrop on the wrong side and it
either paints over the whole scene (it is nearer than everything else) or
disappears without trace (it falls in front of the near plane and the GPU clips
it). Which sign means "far" depends on which side the camera sits, so check the
camera before picking a backdrop depth.

glTF assets are authored Y-up right-handed. `level.load` converts them by
rotation (`rightHanded: true` is its default), so you do not apply a flip
yourself. The **`Mesh` constructor defaults `rightHanded` to `false`**, which
bridges by mirroring instead — geometry you hand to a `Mesh` directly comes in
flipped left/right unless you pass `rightHanded: true`. See
`melonjs-3d-assets`.

## Opting in

3D starts with the camera class, and nothing else has to change. Set it once on
the `Application`:

```js
import { Application, Camera3d } from "melonjs";

const app = new Application(1024, 768, {
    parent: "screen",
    cameraClass: Camera3d,      // ← the opt-in
});
await app.init();
```

That flips the world's sort to `"depth"`, switches the broadphase to a 3D index,
and renders sprites and meshes with perspective projection. `Camera3d` extends
`Camera2d`, so follow, fade, shake and post effects all still work.

Per-stage opt-in also works, which is useful when the loading screen is 2D:

```js
class GameStage extends Stage {
    constructor() {
        super({ cameras: [new Camera3d(0, 0, 1024, 768, { fov: Math.PI / 3 })] });
    }
}
```

## Camera traps

- **There is no sky.** The frame clears to `renderer.backgroundColor` (opaque
  black unless the renderer was created transparent), and `world.backgroundColor`
  clears over it when you set one — both are flat colours. Anything more is a
  backdrop renderable, screen-fixed or placed at a far depth.
- **Set the clip planes for your scene scale.** Defaults are near 0.1 / far 1000.
  Objects beyond the far plane clip or project with bad w-divides; a `near` that
  is too small wastes depth precision and distant props z-fight. Use
  `camera.setClipPlanes(near, far)`.
- **`camera.pos.set(x, y)` is 2-argument and zeroes z.** Use
  `camera.setPosition(x, y, z)`, which leaves the depth untouched when `z` is
  omitted; `camera.depth` is the z accessor if you only want that.
- **`lookAt(target)` solves pitch and yaw only**, so on its own it cannot
  express "up is this direction" and it leaves `roll` alone. Pass an up to
  bank the horizon too: `camera.lookAt(target, up)`, where `up` is what should
  appear UP ON SCREEN. This engine renders Y-down, so a level horizon is
  `(0, -1, 0)`. There is deliberately no default, because giving one would
  silently level every camera that had banked its own horizon.
- **When the game already has all three axes, hand them over**:
  `camera.setBasis(right, up, forward)`, the writable counterpart of
  `getBasis`. The case that needs it is a view over a curved surface, where up
  is the surface normal and differs at every point, so no pitch and yaw pair
  expresses it and integrating towards one drifts and gimbals at the poles.
  `setBasis` absorbs a triple that has drifted off orthonormal rather than
  leaving the camera holding something that is not a rotation, and it does not
  care what the axes are scaled to: a direction is a direction, so anything
  from a denormal to 1e300 resolves the same way. Note its `up` is the camera's
  local +Y column, which under Y-down points DOWN the screen: the opposite
  sense to `lookAt`'s, and one negation apart.
- **`worldToLocal` does not project.** It is a 2D camera's offset subtraction.
  To pin a label or marker to a point in the scene use
  `camera.worldToScreen(x, y, z)`, which applies the projection and returns
  `null` behind the camera. See `melonjs-camera-and-drawing`.

## Where an up vector earns its keep

Three cases. In the first two, `lookAt(target)` on its own is already right and
an up would be noise. The third is the one it exists for.

```js
// 1. ordinary third-person: world up never changes, so say nothing
camera.setPosition(x, y, z).lookAt(player);

// 2. a deliberate dutch angle: this is a ROLL, not an up vector
camera.lookAt(player);
camera.roll = Math.PI / 32;

// 3. a view over a curved surface: up is the surface normal, and it is
//    different at every point on the globe. No pitch and yaw pair expresses
//    it, and integrating towards it drifts and gimbals over the poles.
const up = player.pos.clone().sub(planetCentre).normalize();  // surface normal
camera.setPosition(eye.x, eye.y, eye.z).lookAt(player, up.negate());
```

Note the `negate()` in case 3. The surface normal points away from the planet,
which is the direction the player's head points, and `up` wants what should
appear UP ON SCREEN. This engine renders Y-down, so `(0, -1, 0)` is a level
horizon and a world-space "up" direction has to be handed over in that sense.
Get it backwards and the view is upside down, which is the single most likely
mistake with this argument.

If the game already computes all three axes, skip the solve entirely:

```js
// a surface frame the game is already maintaining
camera.setBasis(right, up, forward);
```

`setBasis` takes the camera's OWN columns, where `up` is local +Y and so points
down the screen: the opposite sense to `lookAt`'s, one negation apart. It also
absorbs a frame that has drifted off orthonormal, which one integrated over
thousands of steps will have.

**A camera posed this way and a world rotated into a fixed camera's frame are
the same picture.** If a game already transforms its world into view space by
hand, there is nothing to gain by switching; the value of `setBasis` is not
having to write that transform in the first place.

## Turning a direction: `slerp`, not `lerp`

`lerp` walks the straight line between the two tips, which passes INSIDE the
sphere. The result points the right way at both ends, lingers near them, rushes
the middle, and comes out shorter than either input as it crosses the midpoint.
On a camera swing that reads as a lurch; on a body walking over a globe it dips
into the surface.

`Vector3d#slerp` turns along the great circle instead, at a constant angular
rate, and carries the length across linearly, so two unit vectors give back a
unit vector and nothing needs renormalizing.

```js
// a turret, a camera, a ship: a facing chasing another facing
facing.slerp(toTarget, 0.25);

// frame-rate independent, the same shape as math.damp
facing.slerp(toTarget, 1 - Math.exp((-6 * dt) / 1000));
```

`ObservableVector3d` has it too and fires its callback ONCE, so
`renderable.pos.slerp(...)` is one notification rather than three.

**`Vector2d` has the same method**, and it is not a cut-down copy: in 2D the
whole turn is one signed angle, so there is no sine to divide by and no
degenerate near-parallel case at all. What it buys a 2D game is the wrap: a
heading damped as a NUMBER from 170 degrees to -170 degrees sweeps the long 340
instead of the short 20, and an arc between directions cannot do that.

Two degenerate pairs are handled inside rather than left to the caller, and
both turn up in ordinary game code:

- **the same direction**, where every path between them is the same path.
- **opposite directions**, where infinitely many great circles pass through
  both and none is the shorter way round. The textbook formula divides by a
  vanishing sine here and hands back `NaN`, which propagates into a position or
  a matrix and the object disappears. `slerp` picks a stable perpendicular and
  turns through half a circle instead.

That second case is why a hand-rolled version is a liability rather than a
shortcut: anything orbiting a planet reaches the antipode of where it started,
every run.

**`math.damp` for a scalar, `slerp` for a direction.** Same job, different
space; what `damp` cannot do is keep a vector on the sphere.

## Depth sorting

Under `Camera3d` the world sorts on `"depth"` **every frame** (recursively), so
unlike 2D you *can* assign `depth` after `addChild` and it will reorder. That is
the one place where the usual z-ordering rule is relaxed — see
`melonjs-renderables` for the 2D rules, which still apply everywhere else.

You will often *have* to. A `Container` has `autoDepth: true` by default, so
`addChild(child)` with no explicit z **overwrites `pos.z` with the child's
index** — a real world depth in a 3D scene, and never the one you wanted. Pass
it (`world.addChild(mesh, z)`), or set `mesh.depth` afterwards. The glTF
importer turns `autoDepth` off on the container it loads into for this reason.

**`floating` does not opt out of this sort.** It skips the camera *transform*,
not the depth *order*. A floating child is ordered by `|pos.z|` alone — its
`pos.x/y` are screen pixels, and the camera does not move relative to it — so
the magnitude is the distance and the sign is ignored:

```js
world.addChild(hud, -150);       // small -> nearer than anything -> on top
world.addChild(skybox, -10000);  // large -> farther -> behind everything
world.addChild(skybox, 100000);  // equally far: sign does not matter
```

Both hold at any camera position. A HUD given the huge z that would put it on
top in 2D lands at the far end of the level instead, with the scenery drawing
over it.

## Transparency

A mesh fades by setting its opacity — there is nothing else to switch on:

```js
ghost.setOpacity(0.4);
```

Meshes render in two phases. The **opaque pass** writes depth in sort order; the
**transparent pass** replays afterwards, back-to-front, blending and writing no
depth. A draw lands in the second whenever its alpha is fractional.

That default matters because the opaque path writes premultiplied colour with
blending off, so before this a faded mesh came out **darkened toward black**
rather than see-through — the background contributed nothing.

**`transparent: true`** when the transparency is in the TEXTURE rather than the
opacity — a soft-edged glow, smoke, a glTF material with `alphaMode: "BLEND"`.
The automatic check reads the draw's alpha and cannot see into a texture. Watch
`alphaCutoff` here: it discards texels *before* blending sees them, so a soft
edge needs a low cutoff (`Sprite3d` drops its own default to `1/255` when you
set `transparent: true`). The cutoff thresholds the MATERIAL's alpha, not the
drawn alpha, so a fading cutout mesh keeps its shape instead of disappearing at
its own threshold.

The glTF loader does **not** set this for you: one loaded mesh can merge several
materials and the flag routes the whole mesh, so a `"BLEND"` material sharing
geometry with an opaque one would drag the opaque half into the transparent pass.

**`transparent: false`** pins a mesh to the opaque pass however it is faded.

### `depthTest: false` for something with no surface to be occluded on

`transparent` decides the PASS, and the transparent pass turns depth *writing*
off on its own so overlapping blended draws blend instead of fighting. Whether
geometry in front of a mesh **hides** it is a separate question, and
`depthTest` is where you answer it. The default, `true`, is what you want for
anything genuinely in the scene — a muzzle flash behind a wall should still be
behind the wall, and this is not a synonym for "additive".

Set it `false` for a mesh standing in for a screen-space effect: an additive
glow carrying a world position only so it can sort and move with the thing it
belongs to. A world-space caption billboarded over a prop wants it for the
same reason, since it has no surface for the prop's own foliage to be in front
of. Left depth tested, a flat billboard is sliced along a hard straight
line the moment any geometry is nearer at some pixel. That shows up worst
around something round — a glow on a rock near a planet's limb gets cut,
and no offset escapes it, because the near surface bulges further toward the
camera than any offset you would dare apply.

```js
const halo = new Sprite3d(0, 0, {
    image: glowTexture,
    width: rock.radius * 3, height: rock.radius * 3,
    billboard: "spherical",
    blendMode: "additive",
    transparent: true,     // the blended pass
    alphaCutoff: 0,        // do not cut the soft falloff
    depthTest: false,      // and do not let the world slice it
});
```

Honoured in the transparent pass on both GPU backends; the Canvas renderer has
no depth buffer and ignores it.

Blending uses the renderable's existing `blendMode`, so a glow is one property.
The advanced modes (`"overlay"`, `"difference"`, and the rest that need a
compositing pass) fall back to `"normal"` here, on both backends:

```js
const glow = new Mesh(0, 0, {
    ...quad, texture: glowTexture,
    transparent: true, blendMode: "additive", alphaCutoff: 0,
});
```

| | |
| --- | --- |
| sorting | **per object**, by distance from the camera |
| intersecting transparent meshes | may pop as the camera moves — split them, or accept it |
| `InstancedMesh` | sorts as **one** object; instances draw in buffer order |
| needs | a GPU backend and a `Camera3d` |

Ground shadows ride the same pass — a blob is a decal, and decals are its first
client rather than a feature of their own.

**`Trail3d`** is the transparent pass's other obvious client: a camera-facing
ribbon for an exhaust streak, a speed trail or a tyre mark. It is a `Mesh`, so
it sorts and fogs with everything else here, and like the rest of this tier it
needs a `Camera3d`. `Trail` is the 2D one and stays flat under a perspective
camera. See `melonjs-particles-and-trails`.

## Distance fog

Off until you ask for it, and one call on the camera:

```js
camera.setFog({ near: 2000, far: 7000 });   // linear: name the two distances
camera.setFog({ mode: "exp2", density: 4e-4 }); // or one density
camera.setFog(null);                         // off
```

It is the cheapest thing that stops a 3D scene reading as flat cut-outs, and it
lets props arrive at the far plane without a visible edge.

**Every parameter is optional, and the omitted ones track live.** Distances
default to the camera's own clip planes, so fog cannot silently disagree with
them after a later `setClipPlanes`. The colour defaults to
`renderer.backgroundColor` and follows it, so geometry dissolves into whatever
sky you already set — including through a day/night fade. Pass `color` only
when the fog should deliberately differ from the backdrop:

```js
camera.setFog({ far: 5000, color: "#8899aa" });
```

A `Color` is held by reference, so mutating it animates the fog.

**Height falloff** makes mist pool in low ground instead of hanging at every
altitude equally — the difference between fog reading as weather and as a global
desaturation:

```js
camera.setFog({ near: 1200, far: 7000, fogHeight: 0, heightFalloff: 0.0015 });
```

`heightFalloff` defaults to **0**, which is uniform fog — not a special case,
the same integral with the dial at zero, so leaving it out changes nothing.
Render space is **Y-down**, so `fogHeight` is the floor and density rises
*below* it; every published form of this formula assumes Y-up and has the
opposite sign.

Fog is measured **radially** from the camera and applied **per fragment**, so
it does not slide as the camera turns and does not band across large triangles.
It lives on the camera, so a split-screen or minimap view fogs independently —
and a `Camera2d` never fogs at all.

**Per object:** `fog: false` exempts a mesh however far away it is — for a
waypoint or objective marker that has to stay readable. It exempts the mesh and
not the ground shadow it casts: a blob is a mark on the floor and fogs with the
floor. Emissive surfaces fog
like everything else (light travelling through fog is attenuated too), so a
neon sign that should punch through wants `fog: false`, not a brighter
emissive.

Only meshes fog. 2D content, HUDs and `floating` renderables never reach the
mesh shaders, so a screen-space overlay stays clean with no work.

**A custom mesh shader is not fogged unless it asks to be.** Fog is compiled
into the engine's own mesh programs — `#define FOG` on WebGL, an `enable_fog`
overridable constant on WebGPU — and a shader you supply is yours: the engine
binds it as written and never substitutes a fogged variant. So a mesh given its
own `GLShader` keeps full contrast while the scene around it recedes. It is
safe — nothing throws, and the camera's fog is simply not applied — but it is
usually surprising.

A hosted `ShaderEffect` is the other case and needs nothing: its body is
spliced into the engine's own mesh program as a colour hook, and fog is applied
to the hook's result, so such a mesh fogs like any other.

To opt in, declare the same uniforms and the engine will feed them, because the
fog values are pushed to any mesh program that declares them rather than only to
the built-in ones:

```glsl
uniform vec3 uFogColor;   // straight (unpremultiplied) fog colour
uniform vec4 uFogParams;  // x = mode (0 off / 1 linear / 2 exp2),
                          // y = near, z = 1/(far - near), w = density
```

Your vertex stage computes the distance itself — `length((uViewMatrix *
uModelMatrix * vec4(aVertex, 1.0)).xyz)`, radially so it does not swim as the
camera turns — and the blend must scale the fog colour by the fragment's own
alpha, `mix(uFogColor * a, rgb, f)`, because `vColor` arrives premultiplied.
Mixing toward the unscaled colour haloes every alpha-cutout edge.

The flip side is the reason fog costs nothing when unused: with no camera fog,
the mesh programs are compiled without any of it, on both backends. It is not a
branch that is skipped at runtime — the code is not there.

## Meshes

```js
const cube = new Mesh(x, y, {
    model: "cube",          // a preloaded OBJ name — no built-in primitives
    texture: atlas,         // TextureAtlas, image, or a preloaded image name
    width: 64, height: 64,
    cullBackFaces: true,    // the default
    lit: true,
});
```

**A mesh pivots about its model origin `(0, 0, 0)`, not an anchor point.** Under
`Camera3d` it opts out of the anchor offset entirely, so `anchorPoint` is inert
there — place the origin where you want the pivot at authoring time, or nest the
mesh under a transformed parent. The anchor is only honoured on the legacy
2D-camera path.

### InstancedMesh, and when it is the wrong tool

**`InstancedMesh`** draws one mesh many times in a single draw call — the
difference between a hundred trees and a hundred thousand. glTF scenes using
`EXT_mesh_gpu_instancing` load as one automatically; by hand it is a `Mesh`
with a count:

```js
const trees = new InstancedMesh(0, 0, { ...treeGeometry, instanceCount: 400 });
const at = new Matrix3d();               // one scratch, reused
for (let i = 0; i < trees.instanceCount; i++) {
    at.identity().translate(x, y, z);
    trees.setInstance(i, at);
}
world.addChild(trees, 0);

trees.visibleInstanceCount = 120;        // draw fewer, without re-uploading
```

It is not a free upgrade. One `InstancedMesh` is **one geometry and one
material**, and four things move from per-object to per-group:

| | with `Mesh` | with `InstancedMesh` |
| --- | --- | --- |
| depth sort | each object sorts on its own `pos` | the whole set has **one** sort key |
| ground shadow | one blob per object | one instanced draw for the set |
| removal | `removeChild`, indices unaffected | `removeInstance(i)` swaps the **last** instance into the hole, so any index you were holding is now wrong |
| colour | `tint` per object | needs `instanceColors: true` and `setInstanceColor(i, …)` |

So the question is not "how many are there" but **"does the game address them
individually"**:

- **Scenery — instance it.** Trees, rocks, grass, debris: the game never asks
  about one of them.
- **Collision-tested props — still fine.** You test against positions you
  already own; instancing only changes how they are *drawn*.
- **Collectibles and enemies — usually not.** Anything removed one at a time
  makes `removeInstance`'s swap your problem: you have to keep an index↔object
  map and repair it on every removal. At small counts a pooled `Mesh` each is
  less code and no slower.

Under a few hundred objects the draw-call saving is not what limits you
anyway — reach for it when the count is in the thousands, or when the objects
are pure scenery and it costs nothing to.

## Normals are generated for you

A `lit` mesh needs per-vertex normals for the shader to light with. Supply them
if you have them; **omit them and the engine computes them from the geometry**:

```js
const mesh = new Mesh(x, y, { vertices, uvs, indices, lit: true });
// normals derived from the triangles — nothing else to do
```

You do not choose flat or smooth, because the geometry already decides. Face
normals accumulate into their vertices weighted by area, so where faces **share**
a vertex they average and the surface shades smoothly, and where every triangle
carries its **own** three vertices — a triangle soup, which is how most
hand-built geometry comes out — each vertex belongs to one face and the result
is that face's normal, so it shades flat. Want faceted edges: duplicate the
vertices. Want smooth: share them.

An explicit `settings.normals` always wins, and an unlit mesh gets none — there
would be nothing to read them.

**A lit mesh with no normals used to render fullbright**, which looks like the
lighting is broken rather than absent. If an older scene suddenly picks up
shading, that is why.

## Colouring a mesh

There are four levels, and picking the wrong one is the usual reason a colour
"does not apply". They all multiply together.

| level | how | use for |
|---|---|---|
| whole object | `mesh.setTint("#ff8800")`, or `mesh.tint.setColor(r, g, b)` | flash on hit, team colour, fading one object |
| per vertex | `settings.vertexColors`, or `mesh.setVertexColor(i, color)` | a gradient *within* one mesh — distance haze, a darker crease |
| per material | `textureGroups`, from a multi-material OBJ + MTL | a model whose parts differ, in one draw call |
| per instance | `new InstancedMesh(…, { instanceColors: true })` then `setInstanceColor(i, color)` | a thousand copies that differ |

**`tint` is per object.** That is the trap: build a terrain as one big mesh and
you can tint the whole valley or none of it. Anything that varies *across* a
single mesh is per-vertex.

```js
// fade a procedural terrain toward the sky the further out it goes
const ground = new Color(217, 230, 244);
const sky = new Color(207, 230, 247);
const haze = new Color();
for (let i = 0; i < mesh.vertexCount; i++) {
    const t = Math.min(1, mesh.originalVertices[i * 3 + 2] / 6000);
    mesh.setVertexColor(i, haze.copy(ground).lerp(sky, t));
}
```

Supply the whole array at construction when you already have it —
`vertexColors` takes a packed `Uint32Array` (the form the batchers read, so no
conversion) or one `Color` per vertex. A length that does not match
`vertexCount` **throws**; it is not padded, because a short array would leave
the tail of the mesh mis-coloured and that reads as a lighting bug.

Mutating the array directly is fine, but say so afterwards:

```js
mesh.vertexColors[i] = color.toUint32(color.alpha);
mesh.needsUpdate = true;   // the retained Camera3d path uploads once
```

`setVertexColor` does that for you. Skip it and the colour applies under a 2D
camera and silently does not under `Camera3d`.

On a lit mesh the colour multiplies the **lit** result, so it behaves as albedo
rather than as an emissive override — a vertex colour will not make an unlit
face bright.

`alpha` hides a mesh as it hides anything else: at 0 the draw is skipped. There
is no *partial* mesh transparency though — the mesh path renders opaque, so a
mesh at `alpha = 0.5` draws fully opaque rather than half see-through. Fade a
mesh out and it will stay solid until it vanishes.

## Moving a mesh in 3D

`Mesh` and `Sprite3d` inherit the ordinary `Renderable` transform helpers, and
both are already 3D:

```js
mesh.rotate(Math.PI / 2, new Vector3d(1, 0, 0));   // axis overload
mesh.scale(2, 1, 0.5);                             // z is a real argument
```

`rotate(angle)` alone is the 2D case (about Z); pass a `Vector3d` and it
rotates about that axis. Both write to `currentTransform`, a **`Matrix3d`**,
which `Mesh` folds into its model matrix — so orienting a mesh costs a matrix
update, not new geometry.

This matters most for the case it is least obvious for: a horizontal plane.
`Sprite3d` with `billboard: false` is a quad in the sprite plane — upright —
and the way to lay one flat (water, a road, a shadow decal) is to rotate it a
quarter turn about X, **not** to hand-build vertices. The one reason to build
geometry instead is tiling: a `Sprite3d` bakes its UVs 0..1 from its atlas
frame, so a surface that needs `textureRepeat` across many world units still
wants an explicit `vertices`/`uvs` mesh.

## The mesh pass is opaque

Worth stating plainly, because the symptom does not look like a blending
problem: **the mesh pass disables blending**. A soft-edged texture — a glow, a
sun, a halo, anything with a gradient alpha — composites as a hard-edged disc
with a grey rim, which reads as a broken texture rather than as a missing
feature.

`transparent: true` routes it through the transparent pass, and `blendMode:
"additive"` on top of that makes a light source add to the sky instead of
sitting on it. This is the same feature a translucent water surface needs.

## Textures under a perspective camera

A tiling texture on a large ground plane is seen at a grazing angle, and that
changes which detail survives:

- **Nothing straight and parallel to the direction of travel.** Every such run
  converges on the vanishing point, so a few current bands on a river become a
  fan of rays radiating out of the horizon. Narrowing them and tiling more
  often only multiplies the rays. Short, scattered, direction-free detail
  minifies into an even shimmer instead.
- **`textureFilter: "linear"`** for a floor — `"nearest"` opts out of mipmaps
  and anisotropy, and detail aliases into a wash exactly where the camera looks
  most.

## Ground must outlast the props standing on it

Scenery scattered up a slope is placed by its own rule, and terrain is built by
another. When the planting reaches further than the mesh does, a low camera
sees trees standing over open sky at the corners of the frame. Build the ground
wider than the widest thing planted on it — past the profile's clamp it is a
flat plateau, so the extra columns cost two triangles and no silhouette.

Recycling has the same trap in Z: an endless runner that recycles tiles a fixed
distance behind the *player* forgets that the camera trails further back still,
and the bottom of the frame falls off the world.

## Sprite3d and billboards

`Sprite3d` is the 2.5D workhorse: a flat sprite living at a real depth, with
`billboard` controlling how it faces the camera.

| `billboard` | behaviour |
|---|---|
| `false` (default) / `"none"` | no rotation — a flat plane in the world |
| `true` / `"cylindrical"` | yaws to face the camera, stays upright (characters, trees) |
| `"spherical"` | always fully faces the camera (particles, impostors) |

`"cylindrical"` is what you want for paper-thin characters in a 2.5D game.
Billboarding needs a `Camera3d` drawing the frame; under a 2D camera the quad
renders fixed-orientation. **Any other string falls through to the spherical
branch**, so a guessed value like `"upright"` silently gives you spherical.

A billboard takes its **orientation** from the camera, so `rotate()` has no
effect on one: a card already turned to face the camera has no free rotation
left to give. Everything that is not an orientation still applies, `scale()`
and `meshScale` included, so animating the size of a shockwave or a pickup is
the ordinary call and the cull bounds grow with it:

```js
const wave = new Sprite3d(x, y, { image: "ring", width: 64, height: 64,
                                  billboard: "spherical", transparent: true });
wave.scale(1 + 3 * t);          // grows; `scale` accumulates, so drive it
                                 // from the authored size, not per frame
```

For an in-plane rotation, a bolt streak or an exhaust, use `billboard: false`
and orient the quad yourself.

## Lighting

`Light3d` takes **one argument — the options object** (it has no `x, y, z`
pair; a light carries its own `position`). Types are `"directional"` (the
default), `"ambient"`, `"point"` and `"spot"`.

```js
world.addChild(new Light3d({ type: "directional", direction: [0.3, 1, 0.2] }));
world.addChild(new Light3d({ type: "ambient", intensity: 0.3 }));
// a Vector3d works too, wherever the game already has one
world.addChild(new Light3d({ type: "spot", position: torch.pos, range: 400 }));
```

`direction` and `position` take an `[x, y, z]` array **or** a `Vector3d`, and
`color` takes a `Color`, a CSS string or an `[r, g, b]` array — so a value the
game already holds can go straight in. Each is read, not retained: move your
vector afterwards and the light stays where it was.

Two things still bite, both silently:

- `new Light3d(0, 0, {…})` — the constructor takes **options alone**. JavaScript
  drops the extra arguments, so `options` becomes the number `0` and every
  setting in your literal is discarded. The light still appears, on pure
  defaults: a `type: "ambient"` written this way is a second DIRECTIONAL light,
  and the scene looks plausible enough that nobody checks. TypeScript stops
  checking a literal once the argument count is wrong, so nothing inside it is
  verified either.
- **`direction` is where the light GOES, and this is a Y-down space** — so a sun
  overhead travels *downward* and its Y is **positive**. Get the sign wrong and
  the scene is lit from underneath: faces that should be in shade are bright,
  the ground glows and the sky-facing surfaces go dark.

  ```js
  direction: [-0.35, 0.8, 0.45]   // sun overhead, late afternoon
  direction: [-0.35, -0.8, 0.45]  // lit from below — almost never what you want
  ```

  `position` follows the same convention: a lamp above the floor has a
  **smaller** y than the floor.

Use **both halves**: with a key light but no ambient, the shadow side of a mesh
goes black. With no `Light3d` in the world at all, a `lit: true` mesh falls back
to a white ambient and renders fullbright — indistinguishable from unlit, which
is why "I added `lit: true` and nothing changed" is the usual first report.

Other defaults worth knowing: `intensity` 1, `color` white, `range` 1000 (point
and spot, a stylised quadratic falloff, not inverse-square),
`innerConeAngle` 0 and `outerConeAngle` π/4 (spot).

**`Light2d` is 2D-only** and produces visible artifacts under perspective
projection. Do not combine it with `Camera3d`.

**`lit: true` only does anything under a `Camera3d`.** Lighting a mesh needs
world-space normals and a world-space fragment position, and the 2D-camera path
has neither: its vertices are already the projected output, so there is no world
space left to light in. Such a mesh degrades to unlit and warns once, naming the
fix. If you want lit 3D props in an otherwise 2D game, set
`cameraClass: Camera3d` on the application. (For 2D **sprites**, lighting is a
separate system that works under a 2D camera as normal, see the lighting skill.)

A `lit` mesh takes a **tangent-space normal map**, so relief comes from the
material rather than from geometry — `settings.normalMap` (a loader key or any
image-like source), or MTL `map_bump` / `bump` / `norm`, which the MTL loader
fetches and applies per material. The tangent frame is derived per fragment, so
the model needs no tangent attribute, and an unmapped mesh is unaffected. This
is the 3D counterpart of the `Sprite` normal maps in the lighting skill; they
are separate systems and a `Mesh` uses this one.

### What the ground-shadow tier can and cannot do

Check this table before claiming a setting is unsupported. Every one of these
is read **every draw**, so all of them can be animated.

| setting | `Mesh` / `Sprite3d` / `GLTFModel` part | `InstancedMesh` |
|---|---|---|
| `castGroundShadow` | yes | yes |
| `shadowGroundY` | yes, the caster's own floor | yes, **one plane** through the SET's origin |
| `shadowGroundPoint` | yes on a `Mesh` or `Sprite3d`; on a `GLTFModel` part only by setting it after the load, since it is neither a model nor a `level.load` option | **no**, warns and ignores |
| `shadowGroundNormal` | yes | yes, one plane for the set |
| `shadowOpacity` | yes | yes |
| `shadowScale` | yes | yes |
| `shadowOffset` | yes | yes, one throw for the set |
| `shadowStretch` | yes | yes, one stretch for the set |
| `shadowDirectionX` / `Z` | yes | yes |
| `shadowLight` | yes | yes |
| height fade as the caster rises | yes | **no** |
| a different value per instance | n/a | **no**, and there is no plan for one |

The last two "no"s are the same limitation seen twice: a set gets ONE shadow
plane and ONE shared quad, so the instance's own height is dropped and nothing
can vary between instances. Everything else is one value for the whole set,
which is what a set means. A scatter over curved ground is therefore one set
per flat facet, and a flying scatter wanting a height fade has to be
per-object meshes.

`shadowGroundPoint` (20.9) is the one genuinely per-object setting, and the
reason is structural rather than missing work: the instanced tier builds its
plane THROUGH the set's own origin so the translation's x and z drop out of
the intercept, which is then `groundY + lift / nY`. An arbitrary point does
not drop out, and that same `nY` divisor is why the 75 degree tilt ceiling
cannot be lifted there either. `InstancedMesh` warns and ignores rather than
storing it, so a game cannot think it took.

Apart from that one, if a claim that a setting "does not work on an
`InstancedMesh`" is in your head, it is out of date: `shadowOffset` and
`shadowStretch` both reached the instanced tier in 20.8.

Ground shadows are **on by default** (the `castGroundShadow` application
setting), and need a GPU backend and a `Camera3d`. As a blanket default they
skip geometry with no vertical extent — a ground plane. Per object,
`castGroundShadow: true`/`false` overrides the app setting and is obeyed as
given, safeguard included; `shadowGroundY` names the floor the blob lands on,
`shadowOpacity` how dark it is and `shadowScale` how big it is.

On ground that is not flat, name the floor with `shadowGroundPoint` instead:
a point and `shadowGroundNormal` are a plane, and the blob leaves the caster's
vertical column to land on it. `shadowGroundY` cannot express that, because
the blob is always directly below the caster in world Y, which is only "below"
on level ground. It is also what lifts the tilt ceiling, since that ceiling
measures tilt from world up and so assumes a camera looking down the Y axis.

The height fade follows whichever floor you named. With a point **and** a
normal it is measured along that normal; with a point alone it is the
straight-line distance to the point. So on a sphere the blob fades with
ALTITUDE rather than with the vertical drop, which is the whole reason the
point exists. Two consequences worth knowing, because they fail in opposite
directions: a `shadowGroundNormal` facing inward makes the height negative, so
the fade clamps to full strength and the blob never fades at all; and a point
on the wrong side of the caster reads as a large height, so the blob
disappears outright rather than thinning to a ring the way a backwards
`shadowGroundY` does. It is also the one shadow setting that THROWS rather
than clamping or hiding: a non-finite component is rejected on the spot.

The blob is an ellipse sized to the caster's own footprint and placed at the
caster's x/z, or at `shadowGroundPoint` when one is set. So a tall or narrow object (a character, a tree, a pickup) shows
its shadow clearly, while a wide, flat-bottomed one resting on the floor covers
its own completely from a camera looking down at it. That is the shadow
behaving correctly, not a bug.

**Do not chase it by raising `shadowGroundY`.** Lifting the plane does not slide
the blob out from under the object, it floats the blob *up* — and past a few
units it projects over the top of the caster as a dark halo ringing it.

The simplest fix is a bigger blob, so its edge shows past the object:

```js
mesh.shadowScale = 1.5;        // 1 is the default; 0 hides the shadow
```

`shadowScale` works everywhere, `InstancedMesh` included. Changing it on a
set rewrites the shared quad's four corners in place rather than rebuilding
it, as does a `shadowLight` that moves, so an animated sun costs no
allocation.

Or, for the look of a sun that is not overhead, use the shape controls:

```js
mesh.shadowOffset = 1.5;       // slide it out, in blob radii
mesh.shadowDirectionX = 1;     // the ground direction it is cast along
mesh.shadowDirectionZ = 0;
mesh.shadowStretch = 2;        // and lengthen it along the same line
```

`shadowOffset` is measured in multiples of the blob's **own radius**, not in
world units, so one value serves a boulder and a pebble and every part of a
glTF model. About `stretch - 1` is what keeps the shadow attached to its
caster. It is honoured **only when the floor has been named**, by
`shadowGroundY` or by `shadowGroundPoint`, because the
blob is a flat quad on one named plane: sliding it across a plane the game has
not named puts it where there may be no floor. `shadowStretch` is clamped to 3
and the blob fades as it pulls, so an extreme value degrades to nothing rather
than to a smear.

An `InstancedMesh` honours both, as **one** offset and **one** stretch for the
whole set, which is all a shared set can carry. Both ride the quad the blobs
share rather than the matrix that also places them: a stretch in the matrix
would scale the instance positions too and smear the scatter instead of
lengthening its blobs. So a scatter can sit next to loose props under the same
sun without looking lit from somewhere else.

`shadowLight` takes the direction from a `Light3d` you name, re-read every
draw, so a moving sun carries the shadows with it. Nothing is inferred: the
engine never picks a dominant light, because a scene with several has no
non-arbitrary answer and one with none has no answer at all.

### A floor that is not level

`shadowGroundY` says where the floor is; `shadowGroundNormal` says which way
it faces:

```js
prop.shadowGroundY = floorY;
prop.shadowGroundNormal = [Math.sin(0.35), -Math.cos(0.35), 0];  // a 20 deg slope
```

World up is `(0, -1, 0)`, because render space is Y-down, and that is the
default. The blob is **rotated** onto the plane rather than projected, so it
keeps its size; it sits directly under the caster, since the normal turns it
rather than moving it, unless `shadowGroundPoint` says where instead. It needs
the floor named, by `shadowGroundY` or `shadowGroundPoint`, for the same
reason the offset does: tilting the bounds fallback has no anchor to turn
about.

The tilt is clamped at 75 degrees alongside `shadowGroundY`, past which an
edge-on blob has nothing left to show. That ceiling measures tilt from WORLD
up, which stands in for "tilted away from the viewer" only while the camera
looks down the Y axis, so it is lifted alongside `shadowGroundPoint`, where a
point and a normal state the plane outright.

On an `InstancedMesh` one plane serves the **whole set**: it passes through
the set's origin at `shadowGroundY`, faces the normal, and every blob is
projected onto it whatever height its own instance sits at, exactly as the
level case puts every blob at one height. A scatter over curved ground is
therefore one set per flat facet, each with the facet's own plane, not one
set with an averaged normal.

All three compose on a set: the stretch turns the shared quad, the offset
slides the whole set along the light, and both are carried onto the tilted
plane rather than applied flat, so a bank's planting lies along its slope
instead of hovering across it.

Give blobs to the plants that stand apart. Dense low ground cover is better
left without: a thousand blobs each as wide as its own fern sum to a darkened
hillside, not to a thousand shadows, and ground cover is the ground.

These are **art direction, not a projection**. A stretched ellipse is not a
silhouette and has no contact with terrain, so on ground that is not the plane
you named it will not lie on it. Direction-correct shadows want a shadow map,
which this tier does not do.

### Get the sign right: the floor is a GREATER y

Render space is **Y-down**, so the floor an object stands on is a *larger* y
than the object itself. Nudging a shadow plane below a surface is therefore a
**plus**:

```js
mesh.shadowGroundY = WATER_LEVEL + 8;   // just under the surface — correct
mesh.shadowGroundY = mesh.pos.y - 8;    // eight units ABOVE its own base — wrong
```

Get it backwards and the blob becomes a horizontal quad slicing through the
caster's own body. It is still drawn at full strength — the height fade sees a
*negative* height and clamps to 1, so nothing warns you — but the depth test
hides every part of it that falls inside the caster's silhouette. What survives
is a thin ring, only as wide as the blob overhangs the object.

The symptom is distinctive and easy to misread: **shadows appear to switch on as
casters approach the camera.** That ring is a few pixels on the nearest object
and sub-pixel further out, so a scene reads as "only nearby things have
shadows" — which looks like culling, an LOD stage or a fog problem, none of
which it is. Before chasing any of those, dump `shadowGroundY` against the
caster's own y and check which one is larger.

## Collision in 3D

Use a body with a depth shape, not a hand-rolled distance check. Two shapes
carry depth — `Box3d` and `Sphere` — and any pair of them is resolved in three
dimensions; the response carries `overlapNZ` / `overlapZ` for the depth axis.

Pick by whether the thing turns. `Box3d` is world-axis-aligned and cannot be
oriented, so it fits walls, floors and crates and misfits anything that
tumbles. `Sphere` has no orientation at all, which makes it the shape for
spinning debris, projectiles, and anything spread over a curved surface: on a
globe each object's own "up" points a different way, so one axis-aligned box
cannot fit any of them.

A hand-rolled check is the usual mistake here and it costs twice — it throws
away the penetration vector the response already carries, and it is invisible
to the debug plugin's hitbox overlay, which draws from `body.shapes`. If the
red boxes never appear, the reason is usually that there is no body to draw.

Against a planar shape, either 3D shape degrades to its XY footprint with the
planar side read as unbounded along Z, so a sphere or box body still collides
with an ordinary collision layer, just without a depth result. And mind the
normal: a box pair separates along one world axis, a sphere along the line
between two centres, so a sphere contact fills the 2D and depth halves of the
response at the same time. See the physics skill for the full contract.

## glTF / GLB scenes

Loaded through the same level director as everything else:

```js
await loader.preload([{ name: "diorama", type: "glb", src: "data/diorama.glb" }]);
level.load("diorama", { scale, castGroundShadow, shadowGroundY, onLoaded });

// every ground-shadow setting travels except shadowGroundPoint, which is
// per object and dropped by a scene load: a loaded scene can
// be told where its sun is, and `shadowLight` is re-read every draw
level.load("diorama", {
    scale, castGroundShadow: true, shadowGroundY: 0,
    shadowLight: sun, shadowOffset: 0.6, shadowStretch: 1.8, async: true,
});
```

`.obj` / `.mtl` are also supported loader types.

An **animated** asset instantiates as a `GLTFModel`, which is a game object as
much as a scene: construct it straight from the parsed descriptor, and place it
with `pos` / `depth` / `rotate` / `scale` like any other renderable — the
placement drives the whole rig and composes with the playing clip.

```js
const model = new GLTFModel(loader.getGLTF("boat"), { scale: 40, lit: false });
model.setCurrentAnimation("paddle", { loop: true });
model.pos.set(x, y);
model.depth = z;
```

To scatter an authored mesh yourself, read the geometry off the descriptor and
hand it to an `InstancedMesh` — `loader.getGLTF(name).nodes[0]` carries
`vertices` / `uvs` / `normals` / `indices`. That wants a single merged
primitive exported at the origin; see `melonjs-3d-assets`.

`melonjs-3d-assets` covers the rest: every `level.load` option, which material
features are imported, imported lights and their intensity units, node-TRS
animation (and the skinning that is out of scope), and OBJ/MTL.

## Requires a GPU backend

The whole 3D tier needs WebGPU or WebGL 2. The Canvas renderer has no depth
buffer, no perspective path and no `drawMesh`, so a `Camera3d` scene does not
render correctly there — usually a black canvas. `Light3d` and shader effects
are inert too. A `Mesh` under a *2D* camera still renders, CPU-projected by
painter's algorithm and unlit.

`Application` does warn: constructing with a `cameraClass` whose
`defaultSortOn` is `"depth"` on a renderer with no depth buffer logs a
`console.warn`. But `video.AUTO` falls back silently to Canvas, so the strong
gate is to ask for a GPU backend by name — `renderer: video.WEBGL` (or
`video.WEBGPU`) makes `await app.init()` reject instead of misrendering:

```js
const app = new Application(1024, 768, {
    parent: "screen",
    renderer: video.WEBGL,      // init() rejects if WebGL 2 is unavailable
    cameraClass: Camera3d,
});
await app.init();
```

To branch rather than fail, read `app.renderer.supportsDepthBuffer` after
`init()`.

## Symptom → cause

| symptom | cause |
|---|---|
| scene lit from underneath | `direction` Y sign — this is Y-down, so a sun overhead is **+Y** |
| a `lit` mesh renders fullbright | it had no normals — supply them, or let the engine generate them |
| a gradient across one mesh is impossible | `tint` is per object — use `vertexColors` / `setVertexColor` |
| a mesh stays solid as you fade it out | meshes render opaque; only `alpha` 0 (hidden) and 1 differ |
| a glow is cut along a hard straight line, worst near a round object's limb | a flat billboard being depth tested against the geometry behind it — `depthTest: false` |
| vertex colour applies under a 2D camera but not `Camera3d` | wrote the array directly without setting `needsUpdate` |
| `Mesh: vertexColors has N entries, expected M` | one colour per *vertex*, not per triangle or per index |
| nothing renders, or a backdrop covers everything | wrong depth sign — "far" is *larger* z when looking along +Z |
| distant objects vanish or warp | scene exceeds the default far plane; `setClipPlanes` |
| distant surfaces z-fight | `near` too small for the scene scale |
| black canvas under `Camera3d` | Canvas renderer (no depth buffer) — check the `console.warn` |
| everything flat and unlit | `lit: true` with no `Light3d` in the world (falls back to fullbright), or a mesh under a 2D camera |
| a faded mesh goes dark instead of see-through | `transparent: false` on it, or a 2D camera — the transparent pass needs a `Camera3d` |
| a soft-edged glow has hard edges | `alphaCutoff` discarded the soft texels; lower it |
| a glTF `alphaMode: "BLEND"` material draws opaque | the loader does not set `transparent` — one mesh can merge several materials, so set it yourself |
| two transparent objects pop as the camera moves | per-object sorting cannot order intersecting geometry |
| a `floating` HUD draws behind the scenery | a large \|z\| is *far* under `Camera3d` — use a small depth |
| fog hangs in the sky as thickly as in the valley | uniform fog — add `heightFalloff` so it pools low |
| mist sits on the ridges instead of the valley floor | the `fogHeight` sign — Y is DOWN here, density rises below it |
| distant geometry pops in against the sky | no fog — `camera.setFog({})` picks up the clip planes and background colour |
| fog does not match the sky after a background fade | an explicit `color` was passed; omit it to track `renderer.backgroundColor` |
| geometry clips before it has finished fading | fog `far` beyond the clip far — omit the distances and they default to the clip planes |
| one marker must stay readable in fog | `fog: false` on that mesh |
| an object casts no visible shadow | wide and flat-bottomed — its own blob is underneath it; raise `shadowScale` (or slide it out with `shadowOffset`), not `shadowGroundY`, which haloes it instead of revealing it |
| a dark ring around the top of an object | `shadowGroundY` lifted too far, floating the blob up into the caster |
| a tilted blob never fades as its caster rises | `shadowGroundNormal` faces inward, so the height came out negative and clamped to full strength |
| a blob vanishes completely after setting `shadowGroundPoint` | the point is on the wrong side of the caster; the fade reads that as height and hides it |
| `shadowGroundPoint` throws | it is the one shadow setting that rejects a bad value instead of clamping: all three components must be finite |
| a scatter ignores `shadowGroundPoint` | `InstancedMesh` shares one plane across the set and warns; use one set per facet, or per-object meshes |
| a mesh sits at the wrong depth after being added | `autoDepth` overwrote `pos.z` with the child index — pass `addChild(mesh, z)` |
| a mesh sits half its size off | `anchorPoint` — only on the 2D-camera path; a `Camera3d` mesh pivots on its model origin |
| a billboard tips over when the camera looks down | `"spherical"`, or a mistyped mode string falling through to it — use `true` / `"cylindrical"` |
| `rotate()` does nothing on a billboard | the camera owns its orientation; `billboard: false` and orient the quad yourself |
| the camera will not tilt its horizon to an arbitrary up | `lookAt(target)` solves pitch and yaw only; pass the up as the next argument, or hand all three axes to `setBasis` |
| the view is upside down after `lookAt(target, up)` | `up` is the direction that should appear UP ON SCREEN, and this is Y-down, so a level horizon is `(0, -1, 0)` |
| `setBasis` throws "forward must have a non-zero length" | `forward` is all zeros, the one case that names no direction; a short one is fine, so a surface frame that shrank as it drifted still works |
| roll is ignored when the target is straight up or down | no rotation about the view axis is determined there, so `lookAt` keeps the roll it had and `setBasis` falls back to the `right` axis |
| an object jumps to the camera plane | `Vector3d.set(x, y)` / `camera.pos.set(x, y)` zeroed its z |
| lighting looks wrong under a 3D camera | `Light2d` used instead of `Light3d` |

## Related skills

- `melonjs-renderables` — the 2D z-ordering rules, anchors, custom draw
- `melonjs-getting-started` — Application settings and the renderer fallback