threejs-builder

name	threejs-builder
description	Creates Three.js web apps with scene setup, lighting, geometries, materials, animations, and responsive rendering. Use when the user asks to create a Three.js scene, app, showcase, 3D web content, or any browser-based 3D graphics project. Supports ES modules, modern Three.js r150+ APIs, GLTF model loading, game patterns, post-processing, shaders, physics, instancing, and Capacitor iOS deployment.

Three.js Builder

A focused skill for creating simple, performant Three.js web applications using modern ES module patterns.

Reference Files

Important: Read the appropriate reference file when working on specific topics.

Topic	File	Use When
GLTF Models	gltf-loading-guide.md	Loading, caching, cloning 3D models, SkeletonUtils
Reference Frames	reference-frame-contract.md	Calibration, anchoring, axis correctness, debugging
Game Development	game-patterns.md	State machines, animation switching, parallax, object pooling
Advanced Topics	advanced-topics.md	Post-processing, shaders, physics, instancing
Capacitor iOS	capacitor-ios.md	Deploying Three.js apps to iOS via Capacitor, SPM setup, native sync
Calibration Helpers	scripts/README.md	GLTF calibration helper installation and usage

---

Philosophy: The Scene Graph Mental Model

Three.js is built on the scene graph—a hierarchical tree of objects where parent transformations affect children. Understanding this mental model is key to effective 3D web development.

Before creating a Three.js app, ask:

• What is the core visual element? (geometry, shape, model)
• What interaction does the user need? (none, orbit controls, custom input)
• What performance constraints exist? (mobile, desktop, WebGL capabilities)
• What animation brings it to life? (rotation, movement, transitions)

Core principles:

1. Scene Graph First: Everything added to scene renders. Use Group for hierarchical transforms.
2. Primitives as Building Blocks: Built-in geometries (Box, Sphere, Torus) cover 80% of simple use cases.
3. Animation as Transformation: Change position/rotation/scale over time using requestAnimationFrame or renderer.setAnimationLoop.
4. Performance Through Simplicity: Fewer objects, fewer draw calls, reusable geometries/materials.

---

Three.js Coordinate System (CRITICAL)

Understanding Three.js's right-handed coordinate system is essential to avoid inverted movement, wrong-facing models, and broken collision detection.

The Axes

      +Y (up)
       |
       |
       |_______ +X (right)
      /
     /
    +Z (toward camera/viewer)

Memory aid: Point your thumb (+X), index finger (+Y), middle finger (+Z) - that's right-handed coordinates.

Axis	Direction	Common Usage
+X	Right	Strafe right, spawn right
-X	Left	Strafe left, spawn left
+Y	Up	Jump, height
-Y	Down	Fall, gravity
+Z	Toward camera	Approach viewer, "forward" in many setups
-Z	Away from camera	Retreat, GLTF models face -Z by default

GLTF Model Default Orientation

CRITICAL: GLTF models exported from Blender/Maya face -Z (into the screen) by default.

// GLTF model faces -Z. To face +Z (toward camera):
model.rotation.y = Math.PI;  // 180° rotation

// To face +X (right):
model.rotation.y = -Math.PI / 2;  // -90°

// To face -X (left):
model.rotation.y = Math.PI / 2;   // +90°

Camera-Relative Movement (CRITICAL for Games)

PROBLEM: When camera is at an angle (e.g., isometric view), raw WASD input moves wrong!

// ❌ WRONG - Input is world-axis relative, not camera-relative
if (keyW) player.position.z -= speed;  // Moves toward -Z, not "forward" from player's view
if (keyD) player.position.x += speed;  // Moves +X, not "right" from camera's view

// ✓ CORRECT - Calculate camera-relative directions
function updateMovement(deltaTime) {
    // Get camera's forward direction, projected onto ground (XZ plane)
    const forward = new THREE.Vector3();
    camera.getWorldDirection(forward);
    forward.y = 0;
    forward.normalize();

    // Calculate right vector (cross product of forward and world up)
    const right = new THREE.Vector3();
    right.crossVectors(forward, new THREE.Vector3(0, 1, 0)).normalize();

    // Apply input relative to camera orientation
    const velocity = new THREE.Vector3();
    if (inputState.up) velocity.add(forward);
    if (inputState.down) velocity.sub(forward);
    if (inputState.right) velocity.add(right);
    if (inputState.left) velocity.sub(right);

    if (velocity.length() > 0) {
        velocity.normalize().multiplyScalar(speed * deltaTime);
        player.position.add(velocity);

        // Face movement direction
        player.rotation.y = Math.atan2(velocity.x, velocity.z);
    }
}

Why this matters: With camera at (8, 11, -6) looking at (0, 1, 3):

• "Forward" visually is NOT -Z, it's roughly +Z
• "Right" visually is NOT +X, it's roughly -X + Z
• Raw axis input feels completely inverted to players

---

Quick Start: Essential Setup

Minimal HTML Template

<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Three.js App</title>
    <style>
        * { margin: 0; padding: 0; box-sizing: border-box; }
        body { overflow: hidden; background: #000; }
        canvas { display: block; }
    </style>
</head>
<body>
    <script type="module">
        import * as THREE from 'https://unpkg.com/three@0.160.0/build/three.module.js';

        // Scene setup
        const scene = new THREE.Scene();
        const camera = new THREE.PerspectiveCamera(75, window.innerWidth / window.innerHeight, 0.1, 1000);
        const renderer = new THREE.WebGLRenderer({ antialias: true });

        renderer.setSize(window.innerWidth, window.innerHeight);
        renderer.setPixelRatio(Math.min(window.devicePixelRatio, 2));
        document.body.appendChild(renderer.domElement);

        // Your 3D content here
        // ...

        camera.position.z = 5;

        // Animation loop
        renderer.setAnimationLoop((time) => {
            renderer.render(scene, camera);
        });

        // Handle resize
        window.addEventListener('resize', () => {
            camera.aspect = window.innerWidth / window.innerHeight;
            camera.updateProjectionMatrix();
            renderer.setSize(window.innerWidth, window.innerHeight);
        });
    </script>
</body>
</html>

---

Geometries

Built-in primitives cover most simple app needs. Use BufferGeometry only for custom shapes.

Common primitives:

• BoxGeometry(width, height, depth) - cubes, boxes
• SphereGeometry(radius, widthSegments, heightSegments) - balls, planets
• CylinderGeometry(radiusTop, radiusBottom, height) - tubes, cylinders
• TorusGeometry(radius, tube) - donuts, rings
• PlaneGeometry(width, height) - floors, walls, backgrounds
• ConeGeometry(radius, height) - spikes, cones
• IcosahedronGeometry(radius, detail) - low-poly spheres (detail=0)

Usage:

const geometry = new THREE.BoxGeometry(1, 1, 1);
const material = new THREE.MeshStandardMaterial({ color: 0x44aa88 });
const mesh = new THREE.Mesh(geometry, material);
scene.add(mesh);

---

Materials

Choose material based on lighting needs and visual style.

Material selection guide:

• MeshBasicMaterial - No lighting, flat colors. Use for: UI, wireframes, unlit effects
• MeshStandardMaterial - PBR lighting. Default for realistic surfaces
• MeshPhysicalMaterial - Advanced PBR with clearcoat, transmission. Glass, water
• MeshNormalMaterial - Debug, rainbow colors based on normals
• MeshPhongMaterial - Legacy, shininess control. Faster than Standard

Common material properties:

{
    color: 0x44aa88,           // Hex color
    roughness: 0.5,            // 0=glossy, 1=matte (Standard/Physical)
    metalness: 0.0,            // 0=non-metal, 1=metal (Standard/Physical)
    emissive: 0x000000,        // Self-illumination color
    wireframe: false,          // Show edges only
    transparent: false,        // Enable transparency
    opacity: 1.0,              // 0=invisible, 1=opaque (needs transparent:true)
    side: THREE.FrontSide      // FrontSide, BackSide, DoubleSide
}

---

Lighting

No light = black screen (except BasicMaterial/NormalMaterial).

Light types:

• AmbientLight(intensity) - Base illumination everywhere. Use 0.3-0.5
• DirectionalLight(color, intensity) - Sun-like, parallel rays. Cast shadows
• PointLight(color, intensity, distance) - Light bulb, emits in all directions
• SpotLight(color, intensity, angle, penumbra) - Flashlight, cone of light

Typical lighting setup:

const ambientLight = new THREE.AmbientLight(0xffffff, 0.4);
scene.add(ambientLight);

const mainLight = new THREE.DirectionalLight(0xffffff, 1);
mainLight.position.set(5, 10, 7);
scene.add(mainLight);

const fillLight = new THREE.DirectionalLight(0x88ccff, 0.5);
fillLight.position.set(-5, 0, -5);
scene.add(fillLight);

Shadows (advanced, use when needed):

renderer.shadowMap.enabled = true;
renderer.shadowMap.type = THREE.PCFSoftShadowMap;

mainLight.castShadow = true;
mainLight.shadow.mapSize.width = 2048;
mainLight.shadow.mapSize.height = 2048;

mesh.castShadow = true;
mesh.receiveShadow = true;

---

Animation

Transform objects over time using the animation loop.

Animation patterns:

1. Continuous rotation:

renderer.setAnimationLoop((time) => {
    mesh.rotation.x = time * 0.001;
    mesh.rotation.y = time * 0.0005;
    renderer.render(scene, camera);
});

2. Wave/bobbing motion:

renderer.setAnimationLoop((time) => {
    mesh.position.y = Math.sin(time * 0.002) * 0.5;
    renderer.render(scene, camera);
});

3. Mouse interaction:

const mouse = new THREE.Vector2();

window.addEventListener('mousemove', (event) => {
    mouse.x = (event.clientX / window.innerWidth) * 2 - 1;
    mouse.y = -(event.clientY / window.innerHeight) * 2 + 1;
});

renderer.setAnimationLoop(() => {
    mesh.rotation.x = mouse.y * 0.5;
    mesh.rotation.y = mouse.x * 0.5;
    renderer.render(scene, camera);
});

---

Camera Controls

Import OrbitControls from examples for interactive camera movement:

<script type="module">
    import * as THREE from 'https://unpkg.com/three@0.160.0/build/three.module.js';
    import { OrbitControls } from 'https://unpkg.com/three@0.160.0/examples/jsm/controls/OrbitControls.js';

    // ... scene setup ...

    const controls = new OrbitControls(camera, renderer.domElement);
    controls.enableDamping = true;
    controls.dampingFactor = 0.05;

    renderer.setAnimationLoop(() => {
        controls.update();
        renderer.render(scene, camera);
    });
</script>

---

Common Scene Patterns

Rotating Cube (Hello World)

const geometry = new THREE.BoxGeometry(1, 1, 1);
const material = new THREE.MeshStandardMaterial({ color: 0x00ff88 });
const cube = new THREE.Mesh(geometry, material);
scene.add(cube);

renderer.setAnimationLoop((time) => {
    cube.rotation.x = time * 0.001;
    cube.rotation.y = time * 0.001;
    renderer.render(scene, camera);
});

Floating Particle Field

const particleCount = 1000;
const geometry = new THREE.BufferGeometry();
const positions = new Float32Array(particleCount * 3);

for (let i = 0; i < particleCount * 3; i += 3) {
    positions[i] = (Math.random() - 0.5) * 50;
    positions[i + 1] = (Math.random() - 0.5) * 50;
    positions[i + 2] = (Math.random() - 0.5) * 50;
}

geometry.setAttribute('position', new THREE.BufferAttribute(positions, 3));
const material = new THREE.PointsMaterial({ color: 0xffffff, size: 0.1 });
const particles = new THREE.Points(geometry, material);
scene.add(particles);

Animated Background with Foreground Object

// Background grid
const gridHelper = new THREE.GridHelper(50, 50, 0x444444, 0x222222);
scene.add(gridHelper);

// Foreground object
const mainGeometry = new THREE.IcosahedronGeometry(1, 0);
const mainMaterial = new THREE.MeshStandardMaterial({
    color: 0xff6600,
    flatShading: true
});
const mainMesh = new THREE.Mesh(mainGeometry, mainMaterial);
scene.add(mainMesh);

---

Colors

Three.js uses hexadecimal color format: 0xRRGGBB

Common hex colors:

• Black: 0x000000, White: 0xffffff
• Red: 0xff0000, Green: 0x00ff00, Blue: 0x0000ff
• Cyan: 0x00ffff, Magenta: 0xff00ff, Yellow: 0xffff00
• Orange: 0xff8800, Purple: 0x8800ff, Pink: 0xff0088

---

Animation Index Contract

For projects with multiple characters and animations, use a JSON metadata contract to decouple UI/logic from raw GLB clip names.

Contract Shape (assets_index.json)

{
  "schemaVersion": 1,
  "characters": {
    "hero": {
      "id": "hero",
      "displayName": "Hero",
      "skeleton": { "url": "/assets/glb/hero/hero_skeleton.glb" },
      "animationSource": { "url": "/assets/glb/hero/hero_animations.glb" },
      "animations": [
        {
          "id": "idle",
          "displayName": "Idle",
          "sourceClipName": "Armature|Idle",
          "durationSec": 2.0,
          "loop": "repeat",
          "tags": ["idle"]
        },
        {
          "id": "run",
          "displayName": "Run",
          "sourceClipName": "Armature|Run",
          "durationSec": 0.8,
          "loop": "repeat",
          "tags": ["locomotion", "run"]
        }
      ],
      "defaults": {
        "defaultAnimationId": "idle",
        "crossFadeSec": 0.2
      }
    }
  }
}

Runtime Pattern

// 1. Load index
const indexResp = await fetch('/assets/assets_index.json');
const index = await indexResp.json();
const charData = index.characters.hero;

// 2. Load skeleton and animation GLBs
const skeleton = await loadGLTF(charData.skeleton.url);
const animSource = await loadGLTF(charData.animationSource.url);

// 3. Build action map from metadata
const mixer = new THREE.AnimationMixer(skeleton.scene);
const actions = {};

for (const entry of charData.animations) {
    const clip = animSource.animations.find(c => c.name === entry.sourceClipName);
    if (!clip) {
        console.warn(`Missing clip: ${entry.sourceClipName}`);
        continue;
    }
    const action = mixer.clipAction(clip);
    action.setLoop(
        entry.loop === 'repeat' ? THREE.LoopRepeat :
        entry.loop === 'once' ? THREE.LoopOnce : THREE.LoopPingPong
    );
    if (entry.loop === 'once') action.clampWhenFinished = true;
    actions[entry.id] = action;
}

// 4. Play default
const defaultId = charData.defaults.defaultAnimationId;
if (actions[defaultId]) actions[defaultId].play();

// 5. Generate UI buttons from metadata (not hardcoded)
for (const entry of charData.animations) {
    const btn = document.createElement('button');
    btn.textContent = entry.displayName;
    btn.onclick = () => switchToAction(actions[entry.id]);
    uiContainer.appendChild(btn);
}

Why this matters:

• A renamed clip in GLB won't silently break UI
• Animation buttons are generated from data, not hardcoded
• Multiple characters share the same loading/UI pattern
• Cross-fade timing is configurable per-character

---

Anti-Patterns to Avoid

Basic Setup Mistakes

❌ Not importing OrbitControls from correct path Why bad: Controls won't load, THREE.OrbitControls is undefined in modern Three.js Better: Use import { OrbitControls } from 'three/addons/controls/OrbitControls.js' or unpkg examples/jsm path

❌ Forgetting to add object to scene Why bad: Object won't render, silent failure Better: Always call scene.add(object) after creating meshes/lights

❌ Using old requestAnimationFrame pattern instead of setAnimationLoop Why bad: More verbose, doesn't handle XR/WebXR automatically Better: renderer.setAnimationLoop((time) => { ... })

Performance Issues

❌ Creating new geometries in animation loop Why bad: Massive memory allocation, frame rate collapse Better: Create geometry once, reuse it. Transform only position/rotation/scale

❌ Using too many segments on primitives Why bad: Unnecessary vertices, GPU overhead Better: Default segments are usually fine. SphereGeometry(1, 32, 16) not SphereGeometry(1, 128, 64)

❌ Not setting pixelRatio cap Why bad: 4K/5K displays run at full resolution, poor performance Better: Math.min(window.devicePixelRatio, 2)

Code Organization

❌ Everything in one giant function Why bad: Hard to modify, hard to debug Better: Separate setup into functions: createScene(), createLights(), createMeshes()

❌ Hardcoding all values Why bad: Difficult to tweak and experiment Better: Define constants at top: const CONFIG = { color: 0x00ff88, speed: 0.001 }

---

Variation Guidance

IMPORTANT: Each Three.js app should feel unique and context-appropriate.

Vary by scenario:

• Portfolio/showcase: Elegant, smooth animations, muted colors
• Game/interactive: Bright colors, snappy controls, particle effects
• Data visualization: Clean lines, grid helpers, clear labels
• Background effect: Subtle, slow movement, dark/gradient backgrounds
• Product viewer: Realistic lighting, PBR materials, smooth orbit

Vary visual elements:

• Geometry choice: Not everything needs to be a cube. Explore spheres, tori, icosahedra
• Material style: Mix flat shaded, glossy, metallic, wireframe
• Color palettes: Use complementary, analogous, or monochromatic schemes
• Animation style: Rotation, oscillation, wave motion, mouse tracking

Avoid converging on:

• Default green cube as first example every time
• Same camera angle (front-facing, z=5)
• Identical lighting setup (always directional light at 1,1,1)

---

Remember

Three.js is a tool for interactive 3D on the web.

Effective Three.js apps:

• Start with the scene graph mental model
• Use primitives as building blocks
• Keep animations simple and performant
• Vary visual style based on purpose
• Import from modern ES module paths

Modern Three.js (r150+) uses ES modules from three package or CDN. CommonJS patterns and global THREE variable are legacy.

These patterns guide the way—they don't limit the result.

For specific topics, see the Reference Files table at the top of this document.