Three.js Scroll Camera Narrative

:root {
  --bg: #03060d;
  --text: #eef4ff;
  --muted: #c1d0e7;
  --accent: #8fe8ff;
}
* {
  box-sizing: border-box;
}
body {
  margin: 0;
  background: var(--bg);
  color: var(--text);
  font-family: "Avenir Next", "Segoe UI", sans-serif;
}
#scene {
  position: fixed;
  inset: 0;
  z-index: 0;
}
.topbar {
  position: fixed;
  inset: 0 0 auto 0;
  z-index: 30;
  display: flex;
  justify-content: space-between;
  align-items: center;
  padding: .75rem 1rem;
  background: rgba(0, 0, 0, 0.28);
  backdrop-filter: blur(8px);
}
.topbar a {
  color: var(--accent);
  text-decoration: none;
  font-weight: 700;
}
button {
  border: 1px solid rgba(255, 255, 255, 0.25);
  border-radius: 999px;
  padding: .45rem .8rem;
  background: rgba(255, 255, 255, 0.07);
  color: var(--text);
  cursor: pointer;
}
main {
  position: relative;
  z-index: 10;
}
.panel {
  min-height: 88vh;
  width: min(920px, 92%);
  margin: 0 auto;
  display: grid;
  align-content: center;
  gap: .7rem;
  border-bottom: 1px solid rgba(255, 255, 255, 0.12);
}
.label {
  margin: 0;
  color: var(--accent);
  text-transform: uppercase;
  letter-spacing: .1em;
}
h1,
h2,
p {
  margin: 0;
}
h1 {
  font-size: clamp(2rem, 7vw, 4.2rem);
}
p {
  color: var(--muted);
  max-width: 62ch;
}

if (!window.MotionPreference) {
  const __mql = window.matchMedia("(prefers-reduced-motion: reduce)");
  const __listeners = new Set();

  const MotionPreference = {
    prefersReducedMotion() {
      return __mql.matches;
    },
    setOverride(value) {
      const reduced = Boolean(value);
      document.documentElement.classList.toggle("reduced-motion", reduced);
      window.dispatchEvent(new CustomEvent("motion-preference", { detail: { reduced } }));
      for (const listener of __listeners) {
        try {
          listener({ reduced, override: reduced, systemReduced: __mql.matches });
        } catch {}
      }
    },
    onChange(listener) {
      __listeners.add(listener);
      try {
        listener({
          reduced: __mql.matches,
          override: null,
          systemReduced: __mql.matches,
        });
      } catch {}
      return () => __listeners.delete(listener);
    },
    getState() {
      return { reduced: __mql.matches, override: null, systemReduced: __mql.matches };
    },
  };

  window.MotionPreference = MotionPreference;
}

import * as THREE from "three";

const host = document.getElementById("scene");
const toggle = document.getElementById("toggleMotion");
let motionEnabled = !window.MotionPreference.prefersReducedMotion();

const scene = new THREE.Scene();
scene.background = new THREE.Color(0x02040a);
scene.fog = new THREE.Fog(0x02040a, 8, 70);

const camera = new THREE.PerspectiveCamera(58, window.innerWidth / window.innerHeight, 0.1, 120);
camera.position.set(0, 0.4, 12);

const renderer = new THREE.WebGLRenderer({ antialias: true });
renderer.setPixelRatio(Math.min(window.devicePixelRatio || 1, 2));
renderer.setSize(window.innerWidth, window.innerHeight);
host.appendChild(renderer.domElement);

const points = new THREE.Group();
scene.add(points);
const palette = [0x8adfff, 0xbf8cff, 0xf2e4b0, 0xef5dd9];
for (let i = 0; i < 140; i += 1) {
  const sphere = new THREE.Mesh(
    new THREE.SphereGeometry(0.11 + Math.random() * 0.28, 14, 14),
    new THREE.MeshStandardMaterial({
      color: palette[i % palette.length],
      emissive: palette[i % palette.length],
      emissiveIntensity: 0.28,
      roughness: 0.35,
      metalness: 0.2,
    })
  );
  sphere.position.set((Math.random() - 0.5) * 12, (Math.random() - 0.5) * 6, -i * 0.48);
  points.add(sphere);
}

scene.add(new THREE.AmbientLight(0x9dbfff, 0.44));
const key = new THREE.PointLight(0x9be0ff, 1.3, 40);
key.position.set(5, 6, 6);
scene.add(key);

const chapters = [
  { pos: new THREE.Vector3(0, 0.4, 12), look: new THREE.Vector3(0, 0, -2) },
  { pos: new THREE.Vector3(2.6, 0.8, -1), look: new THREE.Vector3(0, 0, -12) },
  { pos: new THREE.Vector3(-1.8, 0.6, -16), look: new THREE.Vector3(0.5, 0, -26) },
  { pos: new THREE.Vector3(0, 1.2, -33), look: new THREE.Vector3(0, 0, -46) },
];

function progress() {
  const max = Math.max(document.body.scrollHeight - window.innerHeight, 1);
  return Math.min(1, Math.max(0, window.scrollY / max));
}

function setLabel() {
  toggle.textContent = motionEnabled ? "Disable motion" : "Enable motion";
}

function interpolateChapter(p) {
  const scaled = p * (chapters.length - 1);
  const i = Math.floor(scaled);
  const t = Math.min(1, scaled - i);
  const from = chapters[i];
  const to = chapters[Math.min(chapters.length - 1, i + 1)];
  const pos = from.pos.clone().lerp(to.pos, t);
  const look = from.look.clone().lerp(to.look, t);
  return { pos, look };
}

function animate() {
  requestAnimationFrame(animate);

  const p = motionEnabled ? progress() : 0;
  const rail = interpolateChapter(p);
  camera.position.lerp(rail.pos, 0.08);
  camera.lookAt(rail.look);

  points.children.forEach((m, i) => {
    if (!motionEnabled) return;
    m.position.y += Math.sin(performance.now() * 0.001 + i) * 0.0009;
    m.rotation.y += 0.004;
  });

  renderer.render(scene, camera);
}

function onResize() {
  camera.aspect = window.innerWidth / window.innerHeight;
  camera.updateProjectionMatrix();
  renderer.setSize(window.innerWidth, window.innerHeight);
}

toggle.addEventListener("click", () => {
  motionEnabled = !motionEnabled;
  setLabel();
});

window.addEventListener("resize", onResize);
setLabel();
animate();

<!doctype html>
<html lang="en">
  <head>
    <meta charset="UTF-8" />
    <meta name="viewport" content="width=device-width, initial-scale=1.0" />
    <title>Demo 13 - Scroll Camera Narrative</title>
    <link rel="stylesheet" href="style.css" />
    <script type="importmap">{"imports":{"gsap":"https://esm.sh/gsap@3.13.0","gsap/ScrollTrigger":"https://esm.sh/gsap@3.13.0/ScrollTrigger","gsap/SplitText":"https://esm.sh/gsap@3.13.0/SplitText","gsap/Flip":"https://esm.sh/gsap@3.13.0/Flip","gsap/ScrambleTextPlugin":"https://esm.sh/gsap@3.13.0/ScrambleTextPlugin","gsap/TextPlugin":"https://esm.sh/gsap@3.13.0/TextPlugin","gsap/all":"https://esm.sh/gsap@3.13.0/all","gsap/":"https://esm.sh/gsap@3.13.0/","lenis":"https://esm.sh/lenis@1.1.13/dist/lenis.mjs","three":"https://esm.sh/three@0.171.0","three/addons/":"https://esm.sh/three@0.171.0/examples/jsm/"}}</script>
</head>
  <body>
    <header class="topbar">
      <a href="../">Back to demos</a>
      <button id="toggleMotion"></button>
    </header>

    <div id="scene"></div>
    <main>
      <section class="panel"><p class="label">Demo 13</p><h1>Scroll Camera Narrative</h1><p>Use scroll to move through scene chapters.</p></section>
      <section class="panel"><h2>Chapter 1</h2><p>Approach foreground cluster.</p></section>
      <section class="panel"><h2>Chapter 2</h2><p>Glide into core tunnel axis.</p></section>
      <section class="panel"><h2>Chapter 3</h2><p>Finish with composed hero framing.</p></section>
    </main>

    <script type="module" src="script.js"></script>
      </body>
</html>