---
name: threejs
description: Use when building or debugging interactive 3D scenes on the web with Three.js (scene/camera/renderer, lights/materials, GLTF loading, controls, performance). Helpful for designers shipping 3D UI moments.
---

# Three.js — WebGL 3D Scenes Skill

## When to use
- Real 3D: product spins, interactive hero scenes, shaders/material effects, 3D data viz
- You need full control beyond “background effects”
- You can budget time for asset pipeline + performance tuning

## Core mental model
- Create:
  - `Scene` (root graph)
  - `Camera` (Perspective/Orthographic)
  - `Renderer` (`WebGLRenderer`)
  - `Mesh` = `Geometry` + `Material`
  - Lights (if using non-unlit materials)
- Render loop:
  - `requestAnimationFrame(animate)`
  - Update time-based animations, controls, mixers, then `renderer.render(scene, camera)`

## Key APIs/patterns
- Setup:
  - `const renderer = new THREE.WebGLRenderer({ canvas, antialias, alpha })`
  - `renderer.setSize(width, height, false)`
  - `renderer.setPixelRatio(Math.min(window.devicePixelRatio, 2))`
- Camera:
  - `camera.aspect = width / height; camera.updateProjectionMatrix()`
- Scene graph:
  - `scene.add(object)` / `object.position/rotation/scale`
- Loading assets:
  - `GLTFLoader` (models), `TextureLoader` (images), `DRACOLoader` (compressed glTF)
- Controls (common):
  - `OrbitControls` (debug/product), `PointerLockControls` (FPS), custom pointer handlers
- Cleanup (important in SPAs):
  - `geometry.dispose()`, `material.dispose()`, `texture.dispose()`, `renderer.dispose()`
  - Remove event listeners; cancel RAF.

## Common pitfalls
- Not handling resize → stretched/cropped rendering
- Too high devicePixelRatio → mobile GPU meltdown
- Leaking WebGL resources (not disposing) → crashes after route changes
- Loading huge textures/models → slow start; use compressed textures, Draco/KTX2, smaller maps
- Using too many lights/shadows → expensive; fake lighting with baked textures when possible

## Quick recipes

### 1) Minimal spinning cube
```js
import * as THREE from "three";

const canvas = document.querySelector("#c");
const renderer = new THREE.WebGLRenderer({ canvas, antialias: true, alpha: true });

const scene = new THREE.Scene();
const camera = new THREE.PerspectiveCamera(45, 1, 0.1, 100);
camera.position.set(0, 0, 4);

const geom = new THREE.BoxGeometry(1, 1, 1);
const mat = new THREE.MeshStandardMaterial({ color: 0x7c3aed });
const mesh = new THREE.Mesh(geom, mat);
scene.add(mesh);

scene.add(new THREE.AmbientLight(0xffffff, 0.8));
const dir = new THREE.DirectionalLight(0xffffff, 0.8);
dir.position.set(2, 2, 2);
scene.add(dir);

function resize() {
  const w = canvas.clientWidth;
  const h = canvas.clientHeight;
  renderer.setSize(w, h, false);
  renderer.setPixelRatio(Math.min(window.devicePixelRatio, 2));
  camera.aspect = w / h;
  camera.updateProjectionMatrix();
}
window.addEventListener("resize", resize);
resize();

function animate(t) {
  mesh.rotation.y = t * 0.0006;
  renderer.render(scene, camera);
  requestAnimationFrame(animate);
}
requestAnimationFrame(animate);
```

### 2) Respect reduced motion
- If `prefers-reduced-motion: reduce`, render a still frame (no RAF) or slow updates.

## What to ask the user
- Is this decorative (hero) or functional 3D (product viewer)?
- Target devices: mobile? older iPhones?
- Asset format availability (glTF, HDRI, textures) and file size constraints
- Accessibility/reduced motion requirements