04Depth parallax layer — each particle carries its own depth value, so the same scroll distance applies at a different distance per depth value

<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>The scroll position is the value that pushes the camera's depth.</span>
  </p>
  <div class="sp__viewport" id="viewport" tabindex="0" role="region" aria-label="Particle depth 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(원근 투영·화가 알고리즘)은 index.html 의 paint() 에 있다.
$duration: 2s;
$easing:   $ease-out;
$color:    $stage-ink;

@include stage($stage-paper);

.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-ink, .78);
  font-size: 12px;
  font-weight: 500;
  line-height: 1.3;
}

.sp__ico {
  width: 16px;
  height: 16px;
  flex: none;
  color: $subject-blue-deep;
}

.sp__viewport {
  position: relative;
  width: 100%;
  height: 176px;
  overflow-y: auto;
  overflow-x: hidden;
  border-radius: $r-card;
  background: rgba($stage-ink, .04);
  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-ink, .78);
  color: $subject-cream;
  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: #ffffff;
  font-family: -apple-system, "Segoe UI", "Noto Sans KR", sans-serif;
  padding: clamp(8px, 4vw, 24px);
}

.sp {
  --particle-color: #17141a;
  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(23, 20, 26, 0.78);
  font-size: 12px;
  font-weight: 500;
  line-height: 1.3;
}

.sp__ico {
  width: 16px;
  height: 16px;
  flex: none;
  color: #0a4fe8;
}

.sp__viewport {
  position: relative;
  width: 100%;
  height: 176px;
  overflow-y: auto;
  overflow-x: hidden;
  border-radius: 16px;
  background: rgba(23, 20, 26, 0.04);
  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(23, 20, 26, 0.78);
  color: #fff7e6;
  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(15485863);

  var css = getComputedStyle(root);
  var PARTICLE_COLOR = css.getPropertyValue('--particle-color').trim() || '#17141a';

  // 계산 겹: 원근 투영(perspective projection) + 화가 알고리즘(painter's algorithm) —
  // 입자마다 3D 깊이(z)를 갖고, 스크롤 위치가 카메라 깊이(cameraZ)를 정한다.
  // 화면 좌표는 FOV/(FOV+상대깊이) 배율로 투영하고, 매 프레임 깊이가 먼 것부터 가까운 순으로 다시 그린다.
  var FOV = 130;          // 시야 거리 상수 — 클수록 투영 왜곡이 완만하다
  var DOLLY_RANGE = 520;  // 스크롤 0→100% 가 카메라를 밀어 넣는 깊이 범위 (world 단위)
  var Z_FAR = 560;
  var NEAR = 6;
  var COUNT = 70;

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

  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);
  }

  function spawnAt(p, first) {
    p.wx = (rng() - 0.5) * 260;
    p.wy = (rng() - 0.5) * 160;
    p.z = first ? rng() * Z_FAR : Z_FAR - 4 + rng() * 40;
    p.r = 1.1 + rng() * 1.6;
  }

  function initParticles() {
    particles = [];
    for (var i = 0; i < COUNT; i++) { var p = {}; spawnAt(p, true); particles.push(p); }
  }

  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 cameraZ() { return scrollPos * DOLLY_RANGE; }

  function project(p, camZ) {
    var depth = p.z - camZ;
    var scale = FOV / (FOV + Math.max(depth, NEAR));
    return { sx: W / 2 + p.wx * scale, sy: H / 2 + p.wy * scale, scale: scale, depth: depth };
  }

  function step(dtSec) {
    var camZ = cameraZ();
    for (var i = 0; i < particles.length; i++) {
      var p = particles[i];
      if (p.z - camZ < NEAR) spawnAt(p, false); // 카메라를 지나친 입자는 가장 먼 자리로 재배치한다
    }
  }

  function paint() {
    var camZ = cameraZ();
    ctx.clearRect(0, 0, W, H);
    // 화가 알고리즘: 깊이가 먼 것부터 그린다 — 가까운(큰) 입자가 항상 먼 입자를 덮는다
    var order = particles.slice().sort(function (a, b) { return (b.z - camZ) - (a.z - camZ); });
    for (var i = 0; i < order.length; i++) {
      var p = order[i];
      var pr = project(p, camZ);
      var a = Math.max(0, Math.min(1, pr.scale * 1.1));
      ctx.globalAlpha = a;
      ctx.fillStyle = PARTICLE_COLOR;
      ctx.beginPath(); ctx.arc(pr.sx, pr.sy, Math.max(0.4, p.r * pr.scale * 5), 0, Math.PI * 2); ctx.fill();
    }
    ctx.globalAlpha = 1;
  }

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

  function paintStatic() {
    var camZ = cameraZ();
    ctx.clearRect(0, 0, W, H);
    var order = particles.slice().sort(function (a, b) { return (b.z - camZ) - (a.z - camZ); });
    for (var i = 0; i < order.length; i++) {
      var p = order[i];
      var pr = project(p, camZ);
      var a = Math.max(0, Math.min(1, pr.scale * 1.1));
      ctx.globalAlpha = a;
      ctx.fillStyle = PARTICLE_COLOR;
      ctx.beginPath(); ctx.arc(pr.sx, pr.sy, Math.max(0.4, p.r * pr.scale * 5), 0, Math.PI * 2); ctx.fill();
    }
    ctx.globalAlpha = 1;
    updateVal();
  }

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

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

// DepthParallax — vanilla 판(run/370/demo/04-scroll-particle-depth-parallax/index.html) 의 React 포트.
// 계산 겹: 원근 투영(perspective projection) + 화가 알고리즘(painter's algorithm) —
// 입자마다 3D 깊이(z)를 갖고, 스크롤 위치가 카메라 깊이(cameraZ)를 정한다.
// 화면 좌표는 FOV/(FOV+상대깊이) 배율로 투영하고, 매 프레임 깊이가 먼 것부터 가까운 순으로 다시 그린다.
import { useEffect, useRef } from "react";
import styles from "./DepthParallax.module.scss";

type Box<T> = { current: T };
interface Particle { wx: number; wy: number; z: number; r: number; }

export default function DepthParallax() {
  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(15485863);

    const css = getComputedStyle(root);
    const PARTICLE_COLOR = css.getPropertyValue("--particle-color").trim() || "#17141a";

    const FOV = 130;
    const DOLLY_RANGE = 520;
    const Z_FAR = 560;
    const NEAR = 6;
    const COUNT = 70;

    let W = 0, H = 0, dpr = 1;
    let particles: Particle[] = [];

    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);
    }

    function spawnAt(p: Particle, first: boolean) {
      p.wx = (rng() - 0.5) * 260;
      p.wy = (rng() - 0.5) * 160;
      p.z = first ? rng() * Z_FAR : Z_FAR - 4 + rng() * 40;
      p.r = 1.1 + rng() * 1.6;
    }

    function initParticles() {
      particles = [];
      for (let i = 0; i < COUNT; i++) {
        const p: Particle = { wx: 0, wy: 0, z: 0, r: 0 };
        spawnAt(p, true);
        particles.push(p);
      }
    }

    let scrollPos = 0, scrollVel = 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) { 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 cameraZ() { return scrollPos * DOLLY_RANGE; }

    function project(p: Particle, camZ: number) {
      const depth = p.z - camZ;
      const scale = FOV / (FOV + Math.max(depth, NEAR));
      return { sx: W / 2 + p.wx * scale, sy: H / 2 + p.wy * scale, scale, depth };
    }

    function step(_dtSec: number) {
      const camZ = cameraZ();
      for (let i = 0; i < particles.length; i++) {
        const p = particles[i];
        if (p.z - camZ < NEAR) spawnAt(p, false);
      }
    }

    function paint() {
      const camZ = cameraZ();
      ctx!.clearRect(0, 0, W, H);
      const order = particles.slice().sort((a, b) => (b.z - camZ) - (a.z - camZ));
      for (let i = 0; i < order.length; i++) {
        const p = order[i];
        const pr = project(p, camZ);
        const a = Math.max(0, Math.min(1, pr.scale * 1.1));
        ctx!.globalAlpha = a;
        ctx!.fillStyle = PARTICLE_COLOR;
        ctx!.beginPath(); ctx!.arc(pr.sx, pr.sy, Math.max(0.4, p.r * pr.scale * 5), 0, Math.PI * 2); ctx!.fill();
      }
      ctx!.globalAlpha = 1;
    }

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

    function paintStatic() {
      const camZ = cameraZ();
      ctx!.clearRect(0, 0, W, H);
      const order = particles.slice().sort((a, b) => (b.z - camZ) - (a.z - camZ));
      for (let i = 0; i < order.length; i++) {
        const p = order[i];
        const pr = project(p, camZ);
        const a = Math.max(0, Math.min(1, pr.scale * 1.1));
        ctx!.globalAlpha = a;
        ctx!.fillStyle = PARTICLE_COLOR;
        ctx!.beginPath(); ctx!.arc(pr.sx, pr.sy, Math.max(0.4, p.r * pr.scale * 5), 0, Math.PI * 2); ctx!.fill();
      }
      ctx!.globalAlpha = 1;
      updateVal();
    }

    let lastFrameT = 0;
    function frame(now: number) {
      if (!lastFrameT) lastFrameT = now;
      const dtSec = Math.min(0.05, (now - lastFrameT) / 1000);
      lastFrameT = now;
      step(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();
    initParticles();

    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 위치가 카메라 깊이를 밀어 넣는 값입니다</span>
        </p>
        <div className={styles.viewport} ref={viewportRef} tabIndex={0} role="region" aria-label="입자 깊이 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>
  );
}