threejs-builder

name	threejs-builder
description	Creates simple Three.js web apps with scene setup, lighting, geometries, materials, animations, and responsive rendering. Use for: "Create a threejs scene/app/showcase" or when user wants 3D web content. Supports ES modules, modern Three.js r150+ APIs.

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
Anti-Patterns	anti-patterns.md	Common mistakes, performance issues, code organization
Variation Guidance	variation-guidance.md	Visual variety, color palettes, animation styles
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

---

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.

Claude is capable of creating elegant, performant 3D web experiences. 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.

Boundaries

• Focus on Three.js-based 3D web experiences.
• Do not switch to Phaser, Unity, or backend-heavy architecture unless the task clearly exceeds this skill's scope.
• Prefer modern ES module patterns and web delivery constraints.

When NOT to Use

• 2D game development → use phaser-build
• Native mobile 3D apps → use developer
• Backend or API development → use developer
• Console game development → use developer

Escalation Rules

Pause and ask the owner before:

• introducing heavy dependencies, advanced rendering stacks, or non-web deployment assumptions
• changing visual direction in a way that conflicts with an established product style
• making performance-costly scene design decisions without a clear user-facing benefit

Final Output Contract (MANDATORY)

Every use of this skill should end with:

1. Skill Fit - why Three.js is the right implementation path
2. Primary Deliverable - scene/app design or implementation artifact
3. Execution Evidence - files touched, references consulted, and validation performed
4. Risks / Open Questions - rendering, asset, or performance concerns
5. Next Action - the next concrete build, polish, or verification step