Velocity-Aware Scroll + Speed Lines

:root {
  --bg: #070a12;
  --text: #f0f4fb;
  --panel: #121a2b;
  --border: #263249;
  --accent: #86e8ff;
  --muted: #8a95a8;
}

* {
  margin: 0;
  padding: 0;
  box-sizing: border-box;
}

body {
  font-family: -apple-system, BlinkMacSystemFont, "Segoe UI", sans-serif;
  background: var(--bg);
  color: var(--text);
  line-height: 1.6;
  overflow-x: hidden;
}

/* Fixed canvas behind content */
#speed-canvas {
  position: fixed;
  inset: 0;
  z-index: 0;
  pointer-events: none;
}

section {
  position: relative;
  z-index: 1;
}

.hero {
  min-height: 100vh;
  display: flex;
  align-items: center;
  justify-content: center;
  padding: 2rem;
  background: radial-gradient(ellipse at center, rgba(134, 232, 255, 0.06) 0%, transparent 70%);
}

.hero .content {
  max-width: 600px;
  text-align: center;
}

.eyebrow {
  display: inline-block;
  font-size: 0.75rem;
  font-weight: 700;
  letter-spacing: 2px;
  text-transform: uppercase;
  color: var(--accent);
  margin-bottom: 1rem;
}

.hero h1 {
  font-size: 3.5rem;
  font-weight: 800;
  line-height: 1.1;
  margin-bottom: 1.5rem;
  letter-spacing: -1px;
}

.subtitle {
  font-size: 1.1rem;
  color: var(--muted);
}

/* Content Sections */
.section {
  max-width: 760px;
  margin: 0 auto;
  padding: 5rem 2rem;
  border-bottom: 1px solid var(--border);
}

.section h2 {
  font-size: 2rem;
  margin-bottom: 1.5rem;
  color: var(--text);
}

.section p {
  font-size: 1rem;
  color: var(--muted);
  margin-bottom: 1.5rem;
  line-height: 1.8;
}

.section ul {
  margin-left: 1.5rem;
  margin-bottom: 1.5rem;
}

.section li {
  color: var(--muted);
  margin-bottom: 0.8rem;
  line-height: 1.7;
}

code {
  background: rgba(134, 232, 255, 0.1);
  color: var(--accent);
  padding: 0.2rem 0.5rem;
  border-radius: 4px;
  font-family: "Monaco", "Courier New", monospace;
  font-size: 0.9rem;
}

.btn-back {
  display: inline-block;
  margin-top: 2rem;
  padding: 0.8rem 1.5rem;
  background: var(--accent);
  color: var(--bg);
  border-radius: 4px;
  font-weight: 600;
  text-decoration: none;
  transition: all 0.3s ease;
}

.btn-back:hover {
  background: rgba(134, 232, 255, 0.8);
  transform: translateY(-2px);
}

@media (max-width: 768px) {
  .hero h1 {
    font-size: 2.5rem;
  }
  .section h2 {
    font-size: 1.5rem;
  }
  .section {
    padding: 3rem 1.5rem;
  }
}

@media (max-width: 480px) {
  .hero h1 {
    font-size: 2rem;
  }
}

html.reduced-motion #speed-canvas {
  display: none;
}
html.reduced-motion * {
  animation-duration: 0.01ms !important;
  transition-duration: 0.01ms !important;
}

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;
}

function prefersReducedMotion() {
  return window.MotionPreference.prefersReducedMotion();
}

function initDemoShell() {
  // No-op shim in imported standalone snippets.
}

import gsap from "gsap";
import { ScrollTrigger } from "gsap/ScrollTrigger";
import Lenis from "lenis";

gsap.registerPlugin(ScrollTrigger);

initDemoShell({
  title: "Velocity-Aware Scroll + Speed Lines",
  category: "scroll",
  tech: ["lenis", "canvas-2d", "gsap", "velocity"],
});

const reduced = prefersReducedMotion();
if (reduced) document.documentElement.classList.add("reduced-motion");

// --- Canvas Speed Lines Setup ---
const canvas = document.getElementById("speed-canvas");
const ctx = canvas.getContext("2d");
let velocity = 0;
let decayedVelocity = 0; // Smoothly decays when not scrolling

function resizeCanvas() {
  canvas.width = window.innerWidth;
  canvas.height = window.innerHeight;
}
resizeCanvas();
window.addEventListener("resize", resizeCanvas);

function drawSpeedLines(vel) {
  const cx = canvas.width / 2;
  const cy = canvas.height / 2;
  const absVel = Math.abs(vel);
  const lineCount = Math.floor(Math.min(60, absVel * 4));
  const opacity = Math.min(0.7, absVel * 0.04);
  const maxLength = Math.min(canvas.width, canvas.height) * 0.5 * (absVel * 0.03);

  ctx.clearRect(0, 0, canvas.width, canvas.height);

  if (lineCount < 1 || opacity < 0.01) return;

  for (let i = 0; i < lineCount; i++) {
    const angle = (i / lineCount) * Math.PI * 2;
    const minDist = 80;
    const startX = cx + Math.cos(angle) * minDist;
    const startY = cy + Math.sin(angle) * minDist;
    const endX = cx + Math.cos(angle) * (minDist + maxLength);
    const endY = cy + Math.sin(angle) * (minDist + maxLength);

    const gradient = ctx.createLinearGradient(startX, startY, endX, endY);
    gradient.addColorStop(0, `rgba(134, 232, 255, ${opacity})`);
    gradient.addColorStop(1, `rgba(134, 232, 255, 0)`);

    ctx.beginPath();
    ctx.moveTo(startX, startY);
    ctx.lineTo(endX, endY);
    ctx.strokeStyle = gradient;
    ctx.lineWidth = Math.max(0.5, absVel * 0.1);
    ctx.stroke();
  }
}

// Animation loop
function tick() {
  // Smooth decay toward 0 when not scrolling
  decayedVelocity += (velocity - decayedVelocity) * 0.12;

  if (!reduced) {
    drawSpeedLines(decayedVelocity);
  }

  // Fade velocity toward 0
  velocity *= 0.9;
}

// --- Lenis Setup ---
const lenis = new Lenis({ lerp: 0.1, smoothWheel: true });

lenis.on("scroll", (e) => {
  // Track velocity from Lenis scroll event
  velocity = e.velocity;
  ScrollTrigger.update();
});

gsap.ticker.add((time) => {
  lenis.raf(time * 1000);
  tick();
});
gsap.ticker.lagSmoothing(0);

// --- Hero Entrance ---
if (!reduced) {
  gsap.set(".hero .eyebrow", { opacity: 0, y: 20 });
  gsap.set(".hero h1", { opacity: 0, y: 40 });
  gsap.set(".hero .subtitle", { opacity: 0, y: 25 });

  gsap
    .timeline({ defaults: { ease: "expo.out" } })
    .to(".hero .eyebrow", { opacity: 1, y: 0, duration: 0.7, delay: 0.3 })
    .to(".hero h1", { opacity: 1, y: 0, duration: 0.9 }, "-=0.4")
    .to(".hero .subtitle", { opacity: 1, y: 0, duration: 0.7 }, "-=0.5");
}

// --- Section Reveals ---
document.querySelectorAll(".section").forEach((section, i) => {
  if (!reduced) {
    gsap.set(section, { opacity: 0, y: 40 });
    gsap.to(section, {
      opacity: 1,
      y: 0,
      duration: 0.8,
      ease: "expo.out",
      scrollTrigger: {
        trigger: section,
        start: "top 75%",
        toggleActions: "play none none reverse",
      },
    });
  }
});

document.querySelectorAll(".section h2").forEach((el) => {
  if (!reduced) {
    gsap.set(el, { opacity: 0, x: -20 });
    gsap.to(el, {
      opacity: 1,
      x: 0,
      duration: 0.7,
      ease: "expo.out",
      scrollTrigger: { trigger: el, start: "top 75%", toggleActions: "play none none reverse" },
    });
  }
});

document.querySelectorAll(".section ul li").forEach((li, i) => {
  if (!reduced) {
    gsap.set(li, { opacity: 0, x: -15 });
    gsap.to(li, {
      opacity: 1,
      x: 0,
      duration: 0.5,
      ease: "expo.out",
      delay: i * 0.06,
      scrollTrigger: { trigger: li, start: "top 75%", toggleActions: "play none none reverse" },
    });
  }
});

// --- Motion Preference ---
window.addEventListener("motion-preference", (e) => {
  if (e.detail.reduced) {
    ctx.clearRect(0, 0, canvas.width, canvas.height);
    gsap.globalTimeline.paused(true);
  } else {
    gsap.globalTimeline.paused(false);
  }
});

<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Velocity-Aware Scroll + Speed Lines — stealthisdesign</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>
  <style>html.lenis,
html.lenis body {
  height: auto;
}

.lenis:not(.lenis-autoToggle).lenis-stopped {
  overflow: clip;
}

.lenis [data-lenis-prevent],
.lenis [data-lenis-prevent-wheel],
.lenis [data-lenis-prevent-touch] {
  overscroll-behavior: contain;
}

.lenis.lenis-smooth iframe {
  pointer-events: none;
}

.lenis.lenis-autoToggle {
  transition-property: overflow;
  transition-duration: 1ms;
  transition-behavior: allow-discrete;
}</style>
</head>
<body>
  <canvas id="speed-canvas"></canvas>

  <section class="hero">
    <div class="content">
      <span class="eyebrow">Demo 39</span>
      <h1>Velocity-Aware Scroll</h1>
      <p class="subtitle">Scroll faster to see speed lines intensify. The canvas layer reacts to scroll velocity — the faster you scroll, the stronger the visual feedback.</p>
    </div>
  </section>

  <section class="section">
    <div class="content">
      <h2>Scroll Speed Detection</h2>
      <p>Lenis provides real-time velocity data through its <code>scroll</code> event. We map that velocity to visual intensity — opacity, line count, and blur all scale with how fast you scroll.</p>
      <p>Slow scrolling produces minimal effect. Fast scrolling creates a dramatic speed-lines tunnel on the Canvas overlay.</p>
    </div>
  </section>

  <section class="section">
    <div class="content">
      <h2>Canvas Speed Lines</h2>
      <p>A Canvas 2D layer sits fixed behind the page. On each animation frame, lines radiate from the center at lengths proportional to the scroll velocity magnitude.</p>
      <p>The visual effect is inspired by motion blur in photography and the speed lines used in action manga. Faster scroll = more lines, higher opacity, longer streaks.</p>
    </div>
  </section>

  <section class="section">
    <div class="content">
      <h2>Parallax Intensity</h2>
      <p>Parallax layers in the background also react to velocity. Faster scrolling temporarily increases the parallax offset multiplier, creating a sense of dimensional acceleration through space.</p>
      <p>This effect is commonly used in gaming landing pages, sportswear brands, and esports tournament sites.</p>
    </div>
  </section>

  <section class="section">
    <div class="content">
      <h2>Performance</h2>
      <p>The effect is optimized for 60fps using:</p>
      <ul>
        <li>Canvas <code>clearRect</code> + redraw on each rAF tick (no DOM mutations)</li>
        <li>Velocity decay — speed lines fade naturally when scrolling slows</li>
        <li>Line count cap to prevent overdraw on slow devices</li>
        <li>Disabled entirely if <code>prefers-reduced-motion</code> is active</li>
      </ul>
    </div>
  </section>

  <section class="section">
    <div class="content">
      <h2>Implementation Details</h2>
      <p>Access Lenis velocity via <code>lenis.on('scroll', ({ velocity }) => ...)</code>. The velocity value is a float between roughly -30 and +30 depending on scroll speed.</p>
      <p>Map the absolute value to your visual parameters:</p>
      <ul>
        <li><code>lineCount</code> = <code>Math.min(maxLines, Math.abs(velocity) * 5)</code></li>
        <li><code>lineLength</code> = <code>Math.abs(velocity) * 20</code> pixels</li>
        <li><code>opacity</code> = <code>Math.min(0.8, Math.abs(velocity) * 0.05)</code></li>
      </ul>
      <a href="/" class="btn-back">Back to Showcase</a>
    </div>
  </section>

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