09Noise flow distortion — the scroll value pushes a 2D noise field's offset value so particle paths bend into a smooth flowing path

<main class="stage">
<div class="sp" id="sp">
  <p class="sp__hint">
    <svg class="sp__ico" viewBox="0 0 256 256" aria-hidden="true"><g fill="currentColor"><path d="M224 128a96 96 0 1 1-96-96a96 96 0 0 1 96 96" opacity=".2"/><path d="M232 128a104 104 0 0 1-208 0c0-41 23.81-78.36 60.66-95.27a8 8 0 0 1 6.68 14.54C60.15 61.59 40 93.27 40 128a88 88 0 0 0 176 0c0-34.73-20.15-66.41-51.34-80.73a8 8 0 0 1 6.68-14.54C208.19 49.64 232 87 232 128"/></g></svg>
    <span>Scroll velocity changes the flow field's speed, and position shifts the wave's phase.</span>
  </p>
  <div class="sp__viewport" id="viewport" tabindex="0" role="region" aria-label="Particle noise flow scroll area">
    <canvas class="sp__cv" id="cv" aria-hidden="true"></canvas>
    <b class="sp__val" id="val" aria-hidden="true">0%</b>
    <div class="sp__track" aria-hidden="true"></div>
  </div>
</div>
</main>

@use "../../../../site/static/demo-tokens" as *;

// 변형 3개는 이 위 세 변수만 바꾼다. 계산 겹(값-노이즈 흐름장)은 index.html 의 valueNoise()/step() 에 있다.
$duration: 2s;
$easing:   $ease-out;
$color:    $subject-pink;

@include stage($stage-ink);

.sp {
  --particle-color: #{$color};
  display: flex;
  flex-direction: column;
  align-items: center;
  gap: $sp-2;
  width: min(400px, 92vw);
}

.sp__hint {
  display: flex;
  align-items: center;
  gap: $sp-2;
  margin: 0;
  color: rgba($stage-paper, .78);
  font-size: 12px;
  font-weight: 500;
  line-height: 1.3;
}

.sp__ico {
  width: 16px;
  height: 16px;
  flex: none;
  color: $color;
}

.sp__viewport {
  position: relative;
  width: 100%;
  height: 176px;
  overflow-y: auto;
  overflow-x: hidden;
  border-radius: $r-card;
  background: rgba($stage-paper, .06);
  touch-action: pan-y;
  scrollbar-width: thin;
}

.sp__cv {
  position: sticky;
  top: 0;
  left: 0;
  display: block;
  width: 100%;
  height: 176px;
}

.sp__track {
  height: 320%;
}

.sp__val {
  position: absolute;
  top: $sp-2;
  right: $sp-2;
  padding: $sp-1 $sp-2;
  border-radius: $r-pill;
  background: rgba($stage-paper, .18);
  color: $stage-paper;
  font-size: 11px;
  font-weight: 700;
  line-height: 1.6;
  pointer-events: none;
  transition: opacity $duration $easing;
}

@media (max-width: 340px) {
  .sp__hint { font-size: 11px; }
  .sp__viewport, .sp__cv { height: 120px; }
}

@media (prefers-reduced-motion: reduce) {
  .sp__viewport { scroll-behavior: auto; }
  .sp__val { transition: none; }
}

html, body {
  margin: 0;
  overflow: hidden;
}

.stage {
  box-sizing: border-box;
  min-height: 100vh;
  display: grid;
  place-items: center;
  margin: 0;
  background: #17141a;
  font-family: -apple-system, "Segoe UI", "Noto Sans KR", sans-serif;
  padding: clamp(8px, 4vw, 24px);
}

.sp {
  --particle-color: #ff9ebb;
  display: flex;
  flex-direction: column;
  align-items: center;
  gap: 8px;
  width: min(400px, 92vw);
}

.sp__hint {
  display: flex;
  align-items: center;
  gap: 8px;
  margin: 0;
  color: rgba(255, 255, 255, 0.78);
  font-size: 12px;
  font-weight: 500;
  line-height: 1.3;
}

.sp__ico {
  width: 16px;
  height: 16px;
  flex: none;
  color: #ff9ebb;
}

.sp__viewport {
  position: relative;
  width: 100%;
  height: 176px;
  overflow-y: auto;
  overflow-x: hidden;
  border-radius: 16px;
  background: rgba(255, 255, 255, 0.06);
  touch-action: pan-y;
  scrollbar-width: thin;
}

.sp__cv {
  position: sticky;
  top: 0;
  left: 0;
  display: block;
  width: 100%;
  height: 176px;
}

.sp__track {
  height: 320%;
}

.sp__val {
  position: absolute;
  top: 8px;
  right: 8px;
  padding: 4px 8px;
  border-radius: 999px;
  background: rgba(255, 255, 255, 0.18);
  color: #ffffff;
  font-size: 11px;
  font-weight: 700;
  line-height: 1.6;
  pointer-events: none;
  transition: opacity 2s ease-out;
}

@media (max-width: 340px) {
  .sp__hint {
    font-size: 11px;
  }
  .sp__viewport, .sp__cv {
    height: 120px;
  }
}
@media (prefers-reduced-motion: reduce) {
  .sp__viewport {
    scroll-behavior: auto;
  }
  .sp__val {
    transition: none;
  }
}

(function () {
  var root = document.getElementById('sp');
  var viewport = document.getElementById('viewport');
  var cv = document.getElementById('cv');
  var ctx = cv.getContext('2d');
  var val = document.getElementById('val');
  var reduce = window.matchMedia('(prefers-reduced-motion: reduce)').matches;
  var isDemo = true;

  function makeLcg(seed) {
    var s = seed >>> 0;
    return function () { s = (s * 1664525 + 1013904223) >>> 0; return s / 4294967296; };
  }
  var rng = makeLcg(8675309);

  var css = getComputedStyle(root);
  var PARTICLE_COLOR = css.getPropertyValue('--particle-color').trim() || '#ff9ebb';

  // 계산 겹: 자체 구현 2D 값-노이즈 흐름장(value-noise flow field) — 라이브러리 없이 순열 표를
  // 직접 만들어(makeLcg 로 섞음) 격자 꼭짓점 해시값을 스무스스텝으로 보간한다. 그 값을 각도로 써서
  // 입자마다 흐르는 방향을 정하므로 입자가 "따라 흐르는" 모습이 나온다.
  var PERM_SIZE = 256;
  var perm = (function () {
    var p = [];
    for (var i = 0; i < PERM_SIZE; i++) p.push(i);
    for (var j = PERM_SIZE - 1; j > 0; j--) {
      var k = Math.floor(rng() * (j + 1));
      var t = p[j]; p[j] = p[k]; p[k] = t;
    }
    return p.concat(p);
  })();
  function hash2(ix, iy) {
    var a = perm[(ix & 255)];
    return perm[(a + iy) & 255] / 255;
  }
  function smooth(t) { return t * t * (3 - 2 * t); }
  function valueNoise(x, y) {
    var x0 = Math.floor(x), y0 = Math.floor(y);
    var fx = smooth(x - x0), fy = smooth(y - y0);
    var v00 = hash2(x0, y0), v10 = hash2(x0 + 1, y0);
    var v01 = hash2(x0, y0 + 1), v11 = hash2(x0 + 1, y0 + 1);
    var a = v00 + (v10 - v00) * fx;
    var b = v01 + (v11 - v01) * fx;
    return a + (b - a) * fy;
  }

  var NOISE_SCALE = 0.045;  // 공간 주파수 — 낮을수록 큰 소용돌이 결
  var TIME_GAIN = 0.00014;  // 시간에 따라 장이 흘러가는 속도
  var SCROLL_WARP = 3.4;    // 스크롤 위치가 노이즈 좌표를 미는 양 (위상 이동)
  var BASE_SPEED = 34;      // 기본 흐름 속력 (px/s)
  var FLOW_SPEED = 0.6;     // 흐름 속력 배수 (변형 대상 상수)
  var COUNT = 60;

  var W = 0, H = 0, dpr = 1;
  var dots = [];

  function fit() {
    var r = viewport.getBoundingClientRect();
    dpr = window.devicePixelRatio || 1;
    W = r.width; H = r.height;
    cv.width = Math.round(W * dpr);
    cv.height = Math.round(H * dpr);
    ctx.setTransform(dpr, 0, 0, dpr, 0, 0);
    if (dots.length === 0) initDots();
  }

  function initDots() {
    dots = [];
    for (var i = 0; i < COUNT; i++) {
      dots.push({ x: rng() * W, y: rng() * H, r: 1.2 + rng() * 1.9 });
    }
  }

  var scrollPos = 0, scrollVel = 0, scrollDir = 0, idleMs = 0, lastTop = 0, lastT = 0;
  function sampleScroll(now) {
    var top = viewport.scrollTop;
    var max = Math.max(1, viewport.scrollHeight - viewport.clientHeight);
    scrollPos = top / max;
    var dt = lastT ? Math.max(1, now - lastT) : 16;
    var v = (top - lastTop) / dt * 1000;
    scrollVel += (v - scrollVel) * 0.3;
    if (Math.abs(scrollVel) > 4) { scrollDir = scrollVel > 0 ? 1 : -1; idleMs = 0; } else { idleMs += dt; }
    lastTop = top; lastT = now;
  }
  viewport.addEventListener('scroll', function () {
    sampleScroll(performance.now());
    if (reduce) paintStatic();
  });

  function human() { if (isDemo) { root.classList.remove('is-demo'); isDemo = false; } }
  ['wheel', 'touchstart', 'pointerdown'].forEach(function (evt) {
    viewport.addEventListener(evt, human, { passive: true });
  });

  function fieldAngle(x, y, now) {
    var nx = x * NOISE_SCALE + now * TIME_GAIN + scrollPos * SCROLL_WARP;
    var ny = y * NOISE_SCALE;
    return valueNoise(nx, ny) * Math.PI * 4;
  }

  function step(now, dtSec) {
    var speedGain = 1 + Math.min(2.2, Math.abs(scrollVel) / 180);
    for (var i = 0; i < dots.length; i++) {
      var d = dots[i];
      var ang = fieldAngle(d.x, d.y, now);
      var speed = BASE_SPEED * FLOW_SPEED * speedGain;
      d.x += Math.cos(ang) * speed * dtSec;
      d.y += Math.sin(ang) * speed * dtSec;
      if (d.x < 0) d.x += W; else if (d.x > W) d.x -= W;
      if (d.y < 0) d.y += H; else if (d.y > H) d.y -= H;
    }
  }

  function paint() {
    ctx.clearRect(0, 0, W, H);
    ctx.fillStyle = PARTICLE_COLOR;
    for (var i = 0; i < dots.length; i++) {
      var d = dots[i];
      ctx.globalAlpha = 0.78;
      ctx.beginPath(); ctx.arc(d.x, d.y, d.r, 0, Math.PI * 2); ctx.fill();
    }
    ctx.globalAlpha = 1;
  }

  function updateVal() { val.textContent = Math.round(scrollPos * 100) + '%'; }

  function paintStatic() {
    for (var i = 0; i < dots.length; i++) {
      var d = dots[i];
      var ang = fieldAngle(d.x, d.y, 0);
      d.x += Math.cos(ang) * 6;
      d.y += Math.sin(ang) * 6;
      if (d.x < 0) d.x += W; else if (d.x > W) d.x -= W;
      if (d.y < 0) d.y += H; else if (d.y > H) d.y -= H;
    }
    paint();
    updateVal();
  }

  var lastFrameT = 0;
  function frame(now) {
    if (!lastFrameT) lastFrameT = now;
    var dtSec = Math.min(0.05, (now - lastFrameT) / 1000);
    lastFrameT = now;
    step(now, dtSec);
    paint();
    updateVal();
    requestAnimationFrame(frame);
  }

  var DRIVE_PERIOD_MS = 2000;
  var driveStart = 0;
  function triangle(frac) { return frac < 0.5 ? frac * 2 : 2 - frac * 2; }
  function driveTick(now) {
    if (!driveStart) driveStart = now;
    if (isDemo && !reduce) {
      var frac = ((now - driveStart) % DRIVE_PERIOD_MS) / DRIVE_PERIOD_MS;
      var w = triangle(frac);
      var max = Math.max(1, viewport.scrollHeight - viewport.clientHeight);
      viewport.scrollTop = w * max;
    }
    requestAnimationFrame(driveTick);
  }

  window.addEventListener('resize', fit);
  fit();

  if (reduce) {
    root.classList.add('is-still');
    sampleScroll(performance.now());
    paintStatic();
  } else {
    requestAnimationFrame(frame);
    requestAnimationFrame(driveTick);
  }
})();

// NoiseFlow — vanilla 판(run/370/demo/09-scroll-particle-noise-flow/index.html) 의 React 포트.
// 계산 겹: 자체 구현 2D 값-노이즈 흐름장(value-noise flow field) — 라이브러리 없이 순열 표를
// 직접 만들어(makeLcg 로 섞음) 격자 꼭짓점 해시값을 스무스스텝으로 보간한다. 그 값을 각도로 써서
// 입자마다 흐르는 방향을 정하므로 입자가 "따라 흐르는" 모습이 나온다.
import { useEffect, useRef } from "react";
import styles from "./NoiseFlow.module.scss";

type Box<T> = { current: T };
interface Dot { x: number; y: number; r: number; }

export default function NoiseFlow() {
  const rootRef = useRef(null) as unknown as Box<HTMLDivElement>;
  const viewportRef = useRef(null) as unknown as Box<HTMLDivElement>;
  const canvasRef = useRef(null) as unknown as Box<HTMLCanvasElement>;
  const valRef = useRef(null) as unknown as Box<HTMLElement>;
  const rafFrameRef = useRef(0) as unknown as Box<number>;
  const rafDriveRef = useRef(0) as unknown as Box<number>;

  useEffect(() => {
    const root = rootRef.current;
    const viewport = viewportRef.current;
    const canvas = canvasRef.current;
    const val = valRef.current;
    if (!root || !viewport || !canvas || !val) return;
    const ctx = canvas.getContext("2d");
    if (!ctx) return;

    const reduce = window.matchMedia("(prefers-reduced-motion: reduce)").matches;
    let isDemo = true;

    function makeLcg(seed: number) {
      let s = seed >>> 0;
      return function () { s = (s * 1664525 + 1013904223) >>> 0; return s / 4294967296; };
    }
    const rng = makeLcg(8675309);

    const css = getComputedStyle(root);
    const PARTICLE_COLOR = css.getPropertyValue("--particle-color").trim() || "#ff9ebb";

    const PERM_SIZE = 256;
    const permBase: number[] = [];
    for (let i = 0; i < PERM_SIZE; i++) permBase.push(i);
    for (let j = PERM_SIZE - 1; j > 0; j--) {
      const k = Math.floor(rng() * (j + 1));
      const t = permBase[j]; permBase[j] = permBase[k]; permBase[k] = t;
    }
    const perm = permBase.concat(permBase);

    function hash2(ix: number, iy: number) {
      const a = perm[ix & 255];
      return perm[(a + iy) & 255] / 255;
    }
    function smooth(t: number) { return t * t * (3 - 2 * t); }
    function valueNoise(x: number, y: number) {
      const x0 = Math.floor(x), y0 = Math.floor(y);
      const fx = smooth(x - x0), fy = smooth(y - y0);
      const v00 = hash2(x0, y0), v10 = hash2(x0 + 1, y0);
      const v01 = hash2(x0, y0 + 1), v11 = hash2(x0 + 1, y0 + 1);
      const a = v00 + (v10 - v00) * fx;
      const b = v01 + (v11 - v01) * fx;
      return a + (b - a) * fy;
    }

    const NOISE_SCALE = 0.045;
    const TIME_GAIN = 0.00014;
    const SCROLL_WARP = 3.4;
    const BASE_SPEED = 34;
    const FLOW_SPEED = 0.6;
    const COUNT = 60;

    let W = 0, H = 0, dpr = 1;
    let dots: Dot[] = [];

    function initDots() {
      dots = [];
      for (let i = 0; i < COUNT; i++) {
        dots.push({ x: rng() * W, y: rng() * H, r: 1.2 + rng() * 1.9 });
      }
    }

    function fit() {
      const r = viewport!.getBoundingClientRect();
      dpr = window.devicePixelRatio || 1;
      W = r.width; H = r.height;
      canvas!.width = Math.round(W * dpr);
      canvas!.height = Math.round(H * dpr);
      ctx!.setTransform(dpr, 0, 0, dpr, 0, 0);
      if (dots.length === 0) initDots();
    }

    let scrollPos = 0, scrollVel = 0, scrollDir = 0, idleMs = 0, lastTop = 0, lastT = 0;
    function sampleScroll(now: number) {
      const top = viewport!.scrollTop;
      const max = Math.max(1, viewport!.scrollHeight - viewport!.clientHeight);
      scrollPos = top / max;
      const dt = lastT ? Math.max(1, now - lastT) : 16;
      const v = (top - lastTop) / dt * 1000;
      scrollVel += (v - scrollVel) * 0.3;
      if (Math.abs(scrollVel) > 4) { scrollDir = scrollVel > 0 ? 1 : -1; idleMs = 0; } else { idleMs += dt; }
      lastTop = top; lastT = now;
    }

    function onScroll() {
      sampleScroll(performance.now());
      if (reduce) paintStatic();
    }
    viewport.addEventListener("scroll", onScroll);

    function human() { if (isDemo) { isDemo = false; } }
    viewport.addEventListener("wheel", human, { passive: true });
    viewport.addEventListener("touchstart", human, { passive: true });
    viewport.addEventListener("pointerdown", human, { passive: true });

    function fieldAngle(x: number, y: number, now: number) {
      const nx = x * NOISE_SCALE + now * TIME_GAIN + scrollPos * SCROLL_WARP;
      const ny = y * NOISE_SCALE;
      return valueNoise(nx, ny) * Math.PI * 4;
    }

    function step(now: number, dtSec: number) {
      const speedGain = 1 + Math.min(2.2, Math.abs(scrollVel) / 180);
      for (let i = 0; i < dots.length; i++) {
        const d = dots[i];
        const ang = fieldAngle(d.x, d.y, now);
        const speed = BASE_SPEED * FLOW_SPEED * speedGain;
        d.x += Math.cos(ang) * speed * dtSec;
        d.y += Math.sin(ang) * speed * dtSec;
        if (d.x < 0) d.x += W; else if (d.x > W) d.x -= W;
        if (d.y < 0) d.y += H; else if (d.y > H) d.y -= H;
      }
    }

    function paint() {
      ctx!.clearRect(0, 0, W, H);
      ctx!.fillStyle = PARTICLE_COLOR;
      for (let i = 0; i < dots.length; i++) {
        const d = dots[i];
        ctx!.globalAlpha = 0.78;
        ctx!.beginPath(); ctx!.arc(d.x, d.y, d.r, 0, Math.PI * 2); ctx!.fill();
      }
      ctx!.globalAlpha = 1;
    }

    function updateVal() { val!.textContent = Math.round(scrollPos * 100) + "%"; }

    function paintStatic() {
      for (let i = 0; i < dots.length; i++) {
        const d = dots[i];
        const ang = fieldAngle(d.x, d.y, 0);
        d.x += Math.cos(ang) * 6;
        d.y += Math.sin(ang) * 6;
        if (d.x < 0) d.x += W; else if (d.x > W) d.x -= W;
        if (d.y < 0) d.y += H; else if (d.y > H) d.y -= H;
      }
      paint();
      updateVal();
    }

    let lastFrameT = 0;
    function frame(now: number) {
      if (!lastFrameT) lastFrameT = now;
      const dtSec = Math.min(0.05, (now - lastFrameT) / 1000);
      lastFrameT = now;
      step(now, dtSec);
      paint();
      updateVal();
      rafFrameRef.current = requestAnimationFrame(frame);
    }

    const DRIVE_PERIOD_MS = 2000;
    let driveStart = 0;
    function triangle(frac: number) { return frac < 0.5 ? frac * 2 : 2 - frac * 2; }
    function driveTick(now: number) {
      if (!driveStart) driveStart = now;
      if (isDemo && !reduce) {
        const frac = ((now - driveStart) % DRIVE_PERIOD_MS) / DRIVE_PERIOD_MS;
        const w = triangle(frac);
        const max = Math.max(1, viewport!.scrollHeight - viewport!.clientHeight);
        viewport!.scrollTop = w * max;
      }
      rafDriveRef.current = requestAnimationFrame(driveTick);
    }

    function onResize() { fit(); }
    window.addEventListener("resize", onResize);
    fit();

    if (reduce) {
      sampleScroll(performance.now());
      paintStatic();
    } else {
      rafFrameRef.current = requestAnimationFrame(frame);
      rafDriveRef.current = requestAnimationFrame(driveTick);
    }

    return () => {
      window.removeEventListener("resize", onResize);
      viewport.removeEventListener("scroll", onScroll);
      viewport.removeEventListener("wheel", human);
      viewport.removeEventListener("touchstart", human);
      viewport.removeEventListener("pointerdown", human);
      cancelAnimationFrame(rafFrameRef.current);
      cancelAnimationFrame(rafDriveRef.current);
    };
  }, []);

  return (
    <main className={styles.stage}>
      <div className={styles.root} ref={rootRef}>
        <p className={styles.hint}>
          <svg className={styles.icon} viewBox="0 0 256 256" aria-hidden="true"><g fill="currentColor"><path d="M224 128a96 96 0 1 1-96-96a96 96 0 0 1 96 96" opacity=".2" /><path d="M232 128a104 104 0 0 1-208 0c0-41 23.81-78.36 60.66-95.27a8 8 0 0 1 6.68 14.54C60.15 61.59 40 93.27 40 128a88 88 0 0 0 176 0c0-34.73-20.15-66.41-51.34-80.73a8 8 0 0 1 6.68-14.54C208.19 49.64 232 87 232 128" /></g></svg>
          <span>스크롤 속도가 흐름장의 빠르기를, 위치가 물결의 위상을 바꿉니다</span>
        </p>
        <div className={styles.viewport} ref={viewportRef} tabIndex={0} role="region" aria-label="입자 노이즈 흐름 스크롤 영역">
          <canvas className={styles.canvas} ref={canvasRef} aria-hidden="true" />
          <b className={styles.val} ref={valRef} aria-hidden="true">0%</b>
          <div className={styles.track} aria-hidden="true" />
        </div>
      </div>
    </main>
  );
}