Threejs Fundamentals

Three.js Fundamentals

Quick Start

import * as THREE from "three";

// Create scene, camera, renderer
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);

// Add a mesh
const geometry = new THREE.BoxGeometry(1, 1, 1);
const material = new THREE.MeshStandardMaterial({ color: 0x00ff00 });
const cube = new THREE.Mesh(geometry, material);
scene.add(cube);

// Add light
scene.add(new THREE.AmbientLight(0xffffff, 0.5));
const dirLight = new THREE.DirectionalLight(0xffffff, 1);
dirLight.position.set(5, 5, 5);
scene.add(dirLight);

camera.position.z = 5;

// Animation loop
function animate() {
  requestAnimationFrame(animate);
  cube.rotation.x += 0.01;
  cube.rotation.y += 0.01;
  renderer.render(scene, camera);
}
animate();

// Handle resize
window.addEventListener("resize", () => {
  camera.aspect = window.innerWidth / window.innerHeight;
  camera.updateProjectionMatrix();
  renderer.setSize(window.innerWidth, window.innerHeight);
});

Core Classes

Scene

Container for all 3D objects, lights, and cameras.

const scene = new THREE.Scene();
scene.background = new THREE.Color(0x000000); // Solid color
scene.background = texture; // Skybox texture
scene.background = cubeTexture; // Cubemap
scene.environment = envMap; // Environment map for PBR
scene.fog = new THREE.Fog(0xffffff, 1, 100); // Linear fog
scene.fog = new THREE.FogExp2(0xffffff, 0.02); // Exponential fog

Cameras

PerspectiveCamera - Most common, simulates human eye.

// PerspectiveCamera(fov, aspect, near, far)
const camera = new THREE.PerspectiveCamera(
  75, // Field of view (degrees)
  window.innerWidth / window.innerHeight, // Aspect ratio
  0.1, // Near clipping plane
  1000, // Far clipping plane
);

camera.position.set(0, 5, 10);
camera.lookAt(0, 0, 0);
camera.updateProjectionMatrix(); // Call after changing fov, aspect, near, far

OrthographicCamera - No perspective distortion, good for 2D/isometric.

// OrthographicCamera(left, right, top, bottom, near, far)
const aspect = window.innerWidth / window.innerHeight;
const frustumSize = 10;
const camera = new THREE.OrthographicCamera(
  (frustumSize * aspect) / -2,
  (frustumSize * aspect) / 2,
  frustumSize / 2,
  frustumSize / -2,
  0.1,
  1000,
);

ArrayCamera - Multiple viewports with sub-cameras.

const cameras = [];
for (let i = 0; i  {
  if (child.isMesh) child.material.color.set(0xff0000);
});

// Matrix updates
obj.matrixAutoUpdate = true; // Default: auto-update matrices
obj.updateMatrix(); // Manual matrix update
obj.updateMatrixWorld(true); // Update world matrix recursively

Group

Empty container for organizing objects.

const group = new THREE.Group();
group.add(mesh1);
group.add(mesh2);
scene.add(group);

// Transform entire group
group.position.x = 5;
group.rotation.y = Math.PI / 4;

Mesh

Combines geometry and material.

const mesh = new THREE.Mesh(geometry, material);

// Multiple materials (one per geometry group)
const mesh = new THREE.Mesh(geometry, [material1, material2]);

// Useful properties
mesh.geometry;
mesh.material;
mesh.castShadow = true;
mesh.receiveShadow = true;

// Frustum culling
mesh.frustumCulled = true; // Default: skip if outside camera view

// Render order
mesh.renderOrder = 10; // Higher = rendered later

Coordinate System

Three.js uses a right-handed coordinate system:

• +X points right
• +Y points up
• +Z points toward viewer (out of screen)

// Axes helper
const axesHelper = new THREE.AxesHelper(5);
scene.add(axesHelper); // Red=X, Green=Y, Blue=Z

Math Utilities

Vector3

const v = new THREE.Vector3(x, y, z);
v.set(x, y, z);
v.copy(otherVector);
v.clone();

// Operations (modify in place)
v.add(v2);
v.sub(v2);
v.multiply(v2);
v.multiplyScalar(2);
v.divideScalar(2);
v.normalize();
v.negate();
v.clamp(min, max);
v.lerp(target, alpha);

// Calculations (return new value)
v.length();
v.lengthSq(); // Faster than length()
v.distanceTo(v2);
v.dot(v2);
v.cross(v2); // Modifies v
v.angleTo(v2);

// Transform
v.applyMatrix4(matrix);
v.applyQuaternion(q);
v.project(camera); // World to NDC
v.unproject(camera); // NDC to world

Matrix4

const m = new THREE.Matrix4();
m.identity();
m.copy(other);
m.clone();

// Build transforms
m.makeTranslation(x, y, z);
m.makeRotationX(theta);
m.makeRotationY(theta);
m.makeRotationZ(theta);
m.makeRotationFromQuaternion(q);
m.makeScale(x, y, z);

// Compose/decompose
m.compose(position, quaternion, scale);
m.decompose(position, quaternion, scale);

// Operations
m.multiply(m2); // m = m * m2
m.premultiply(m2); // m = m2 * m
m.invert();
m.transpose();

// Camera matrices
m.makePerspective(left, right, top, bottom, near, far);
m.makeOrthographic(left, right, top, bottom, near, far);
m.lookAt(eye, target, up);

Quaternion

const q = new THREE.Quaternion();
q.setFromEuler(euler);
q.setFromAxisAngle(axis, angle);
q.setFromRotationMatrix(matrix);

q.multiply(q2);
q.slerp(target, t); // Spherical interpolation
q.normalize();
q.invert();

Euler

const euler = new THREE.Euler(x, y, z, "XYZ"); // Order matters!
euler.setFromQuaternion(q);
euler.setFromRotationMatrix(m);

// Rotation orders: 'XYZ', 'YXZ', 'ZXY', 'XZY', 'YZX', 'ZYX'

Color

const color = new THREE.Color(0xff0000);
const color = new THREE.Color("red");
const color = new THREE.Color("rgb(255, 0, 0)");
const color = new THREE.Color("#ff0000");

color.setHex(0x00ff00);
color.setRGB(r, g, b); // 0-1 range
color.setHSL(h, s, l); // 0-1 range

color.lerp(otherColor, alpha);
color.multiply(otherColor);
color.multiplyScalar(2);

MathUtils

THREE.MathUtils.clamp(value, min, max);
THREE.MathUtils.lerp(start, end, alpha);
THREE.MathUtils.mapLinear(value, inMin, inMax, outMin, outMax);
THREE.MathUtils.degToRad(degrees);
THREE.MathUtils.radToDeg(radians);
THREE.MathUtils.randFloat(min, max);
THREE.MathUtils.randInt(min, max);
THREE.MathUtils.smoothstep(x, min, max);
THREE.MathUtils.smootherstep(x, min, max);

Common Patterns

Proper Cleanup

function dispose() {
  // Dispose geometries
  mesh.geometry.dispose();

  // Dispose materials
  if (Array.isArray(mesh.material)) {
    mesh.material.forEach((m) => m.dispose());
  } else {
    mesh.material.dispose();
  }

  // Dispose textures
  texture.dispose();

  // Remove from scene
  scene.remove(mesh);

  // Dispose renderer
  renderer.dispose();
}

Clock for Animation

const clock = new THREE.Clock();

function animate() {
  const delta = clock.getDelta(); // Time since last frame (seconds)
  const elapsed = clock.getElapsedTime(); // Total time (seconds)

  mesh.rotation.y += delta * 0.5; // Consistent speed regardless of framerate

  requestAnimationFrame(animate);
  renderer.render(scene, camera);
}

Responsive Canvas

function onWindowResize() {
  const width = window.innerWidth;
  const height = window.innerHeight;

  camera.aspect = width / height;
  camera.updateProjectionMatrix();

  renderer.setSize(width, height);
  renderer.setPixelRatio(Math.min(window.devicePixelRatio, 2));
}
window.addEventListener("resize", onWindowResize);

Loading Manager

const manager = new THREE.LoadingManager();

manager.onStart = (url, loaded, total) => console.log("Started loading");
manager.onLoad = () => console.log("All loaded");
manager.onProgress = (url, loaded, total) => console.log(`${loaded}/${total}`);
manager.onError = (url) => console.error(`Error loading ${url}`);

const textureLoader = new THREE.TextureLoader(manager);
const gltfLoader = new GLTFLoader(manager);

Performance Tips

1. Limit draw calls: Merge geometries, use instancing, atlas textures
2. Frustum culling: Enabled by default, ensure bounding boxes are correct
3. LOD (Level of Detail): Use THREE.LOD for distance-based mesh switching
4. Object pooling: Reuse objects instead of creating/destroying
5. Avoid getWorldPosition in loops: Cache results

// Merge static geometries
import { mergeGeometries } from "three/addons/utils/BufferGeometryUtils.js";
const merged = mergeGeometries([geo1, geo2, geo3]);

// LOD
const lod = new THREE.LOD();
lod.addLevel(highDetailMesh, 0);
lod.addLevel(medDetailMesh, 50);
lod.addLevel(lowDetailMesh, 100);
scene.add(lod);

See Also

• threejs-geometry - Geometry creation and manipulation
• threejs-materials - Material types and properties
• threejs-lighting - Light types and shadows

Source & license

This open-source skill is cataloged on AgentStack and links to its original source — we do not rehost the code.

• Author: Miaoge-Ge
• Source: Miaoge-Ge/coding-agent-skills
• License: MIT

Install and usage instructions live in the source repository linked above.