02A background that follows — three parallax layers

<main class="stage">
<div class="bg is-demo" id="bg">
  <div class="bg__scene" role="img" aria-label="A sun rises over hills and grass, its eyes watching the cursor">
    <div class="bg__sky" aria-hidden="true"></div>
    <div class="bg__sun" aria-hidden="true">
      <span class="bg__eye"><span class="bg__pupil"></span></span>
      <span class="bg__eye"><span class="bg__pupil"></span></span>
      <span class="bg__mouth"></span>
    </div>
    <div class="bg__hill bg__hill--far" aria-hidden="true"></div>
    <div class="bg__hill bg__hill--mid" aria-hidden="true"></div>
    <div class="bg__grass" aria-hidden="true">
      <span class="bg__blade"></span><span class="bg__blade"></span><span class="bg__blade"></span>
      <span class="bg__blade"></span><span class="bg__blade"></span><span class="bg__blade"></span>
      <span class="bg__blade"></span><span class="bg__blade"></span><span class="bg__blade"></span>
    </div>
  </div>
  <p class="bg__hint">Move the mouse and the background and grass follow along</p>
</div>
</main>

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

// 따라오는 배경 — 실제 시차는 JS 가 커서 가로 위치(--px, 0~1)를 lerp 로 부드럽게 만들어
// 멀리 언덕은 -10px, 가까운 언덕은 -22px, 풀은 -34px 씩 밀어 서로 다른 속도로 따라오게 한다.
// 풀잎 skewX 는 --px 하나를 공유해 커서 쪽으로 눕고, 해의 눈동자도 같은 값으로 커서를 본다.
// 격자 미리보기(.is-demo)는 커서가 좌우로 지나간 것과 같은 진폭을 @keyframes 로 왕복시킨다.

$duration: $dur-loop;
$easing: $ease-spring;
$color: $subject-ink;

@include stage($stage-yellow);

.bg { display: grid; justify-items: center; gap: $sp-3; width: 100%; }

.bg__scene {
  --px: 0.5;
  position: relative; overflow: hidden;
  width: 100%; height: clamp(110px, 32vh, 160px);
  border: 3px solid $color; border-radius: $r-card;
  background: $stage-yellow;
}

// 하늘 장식 — 반달 모양 구름 두 개 (그라디언트가 아니라 흰 원으로)
.bg__sky::before, .bg__sky::after {
  content: ""; position: absolute; border-radius: 50%;
  background: $stage-paper; opacity: .9;
}
.bg__sky::before { width: 34px; height: 34px; left: 12%; top: 16%; }
.bg__sky::after  { width: 22px; height: 22px; left: 24%; top: 30%; }

// 해 — 눈 두 개가 커서를 쫓는다
.bg__sun {
  position: absolute; right: 10%; top: 12%;
  display: flex; align-items: center; gap: $sp-2;
  width: clamp(44px, 12vw, 56px); height: clamp(44px, 12vw, 56px);
  box-sizing: border-box;
  background: $stage-paper; border: 3px solid $color; border-radius: 50%;
}
.bg__eye {
  position: relative; overflow: hidden;
  width: 10px; height: 10px;
  background: $stage-paper; border: 2px solid $color; border-radius: 50%;
}
.bg__pupil {
  position: absolute; left: 50%; top: 50%;
  width: 4px; height: 4px; margin: -2px 0 0 -2px;
  background: $color; border-radius: 50%;
}
.bg__mouth {
  position: absolute; left: 50%; bottom: 22%;
  width: 14px; height: 6px; margin-left: -7px;
  border-bottom: 3px solid $color; border-radius: 0 0 $r-pill $r-pill;
}

// 언덕 두 겹 — 멀수록 연하게, 밀리는 폭은 JS 가 정한다
.bg__hill { position: absolute; left: -8%; right: -8%; border-radius: $r-pill $r-pill 0 0; }
.bg__hill--far {
  bottom: 26%; height: 34%;
  background: $subject-cream; opacity: .9;
}
.bg__hill--mid {
  bottom: 12%; height: 30%;
  background: $stage-orange;
  border: 3px solid $color; border-bottom: 0;
}

// 풀 — 잎마다 --px 를 공유해 skewX 로 눕는다. 원점은 뿌리(아래)다 (붙는 지점이 있는 요소)
.bg__grass {
  position: absolute; left: 0; right: 0; bottom: 0;
  display: flex; justify-content: space-evenly; align-items: flex-end;
  height: 26%;
}
.bg__blade {
  width: 7px; border-radius: $r-pill $r-pill 0 0;
  background: $color;
  transform-origin: 50% 100%;
  transform: skewX(calc((0.5 - var(--px)) * 26deg));
}
.bg__blade:nth-child(odd)  { height: 100%; }
.bg__blade:nth-child(even) { height: 74%; }

.bg__hint { margin: 0; color: $color; font-weight: 700; font-size: clamp(9px, 2.4vw, 11px); }

// ── 미리보기 루프: 커서가 왼쪽 끝 → 오른쪽 끝을 지나간 것과 같은 진폭을 겹마다 다르게 왕복한다
.bg.is-demo .bg__hill--far  { animation: bgFar $duration $easing infinite; }
.bg.is-demo .bg__hill--mid  { animation: bgMid $duration $easing infinite; }
.bg.is-demo .bg__grass      { animation: bgNear $duration $easing infinite; }
.bg.is-demo .bg__blade      { animation: bgLean $duration $easing infinite; }
.bg.is-demo .bg__pupil      { animation: bgPeek $duration $easing infinite; }

@keyframes bgFar {
  0%, 4%   { transform: translate3d(5px, 0, 0); }
  34%      { transform: translate3d(-5px, 0, 0); }
  44%, 50% { transform: translate3d(-5px, 0, 0); }
  80%      { transform: translate3d(5px, 0, 0); }
  88%, 100% { transform: translate3d(5px, 0, 0); }
}
@keyframes bgMid {
  0%, 4%   { transform: translate3d(11px, 0, 0); }
  34%      { transform: translate3d(-11px, 0, 0); }
  44%, 50% { transform: translate3d(-11px, 0, 0); }
  80%      { transform: translate3d(11px, 0, 0); }
  88%, 100% { transform: translate3d(11px, 0, 0); }
}
@keyframes bgNear {
  0%, 4%   { transform: translate3d(17px, 0, 0); }
  34%      { transform: translate3d(-17px, 0, 0); }
  44%, 50% { transform: translate3d(-17px, 0, 0); }
  80%      { transform: translate3d(17px, 0, 0); }
  88%, 100% { transform: translate3d(17px, 0, 0); }
}
// 풀잎 눕음 — 커서가 오른쪽(0.5 → 1)에 있으면 왼쪽으로 눕는다
@keyframes bgLean {
  0%, 4%   { transform: skewX(-13deg); }
  34%      { transform: skewX(13deg); }
  44%, 50% { transform: skewX(13deg); }
  80%      { transform: skewX(-13deg); }
  88%, 100% { transform: skewX(-13deg); }
}
@keyframes bgPeek {
  0%, 4%   { transform: translate3d(-3.5px, 0, 0); }
  34%      { transform: translate3d(3.5px, 0, 0); }
  44%, 50% { transform: translate3d(3.5px, 0, 0); }
  80%      { transform: translate3d(-3.5px, 0, 0); }
  88%, 100% { transform: translate3d(-3.5px, 0, 0); }
}

@media (max-width: 340px) {
  .bg { gap: $sp-2; }
  .bg__scene { height: clamp(96px, 28vh, 130px); }
  .bg__sun { right: 6%; }
}

@media (prefers-reduced-motion: reduce) {
  .bg.is-demo .bg__hill--far,
  .bg.is-demo .bg__hill--mid,
  .bg.is-demo .bg__grass,
  .bg.is-demo .bg__blade,
  .bg.is-demo .bg__pupil { animation: none !important; }
  // reduce 에서는 잎이 곧게 선 정경만 보인다
  .bg.is-demo .bg__blade { transform: skewX(0deg) !important; }
}

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

.stage {
  box-sizing: border-box;
  min-height: 100vh;
  display: grid;
  place-items: center;
  margin: 0;
  background: #ffd23f;
  font-family: -apple-system, "Segoe UI", "Noto Sans KR", sans-serif;
  padding: clamp(8px, 4vw, 24px);
}

.bg {
  display: grid;
  justify-items: center;
  gap: 12px;
  width: 100%;
}

.bg__scene {
  --px: 0.5;
  position: relative;
  overflow: hidden;
  width: 100%;
  height: clamp(110px, 32vh, 160px);
  border: 3px solid #17141a;
  border-radius: 16px;
  background: #ffd23f;
}

.bg__sky::before, .bg__sky::after {
  content: "";
  position: absolute;
  border-radius: 50%;
  background: #ffffff;
  opacity: 0.9;
}

.bg__sky::before {
  width: 34px;
  height: 34px;
  left: 12%;
  top: 16%;
}

.bg__sky::after {
  width: 22px;
  height: 22px;
  left: 24%;
  top: 30%;
}

.bg__sun {
  position: absolute;
  right: 10%;
  top: 12%;
  display: flex;
  align-items: center;
  gap: 8px;
  width: clamp(44px, 12vw, 56px);
  height: clamp(44px, 12vw, 56px);
  box-sizing: border-box;
  background: #ffffff;
  border: 3px solid #17141a;
  border-radius: 50%;
}

.bg__eye {
  position: relative;
  overflow: hidden;
  width: 10px;
  height: 10px;
  background: #ffffff;
  border: 2px solid #17141a;
  border-radius: 50%;
}

.bg__pupil {
  position: absolute;
  left: 50%;
  top: 50%;
  width: 4px;
  height: 4px;
  margin: -2px 0 0 -2px;
  background: #17141a;
  border-radius: 50%;
}

.bg__mouth {
  position: absolute;
  left: 50%;
  bottom: 22%;
  width: 14px;
  height: 6px;
  margin-left: -7px;
  border-bottom: 3px solid #17141a;
  border-radius: 0 0 999px 999px;
}

.bg__hill {
  position: absolute;
  left: -8%;
  right: -8%;
  border-radius: 999px 999px 0 0;
}

.bg__hill--far {
  bottom: 26%;
  height: 34%;
  background: #fff7e6;
  opacity: 0.9;
}

.bg__hill--mid {
  bottom: 12%;
  height: 30%;
  background: #ff4d1f;
  border: 3px solid #17141a;
  border-bottom: 0;
}

.bg__grass {
  position: absolute;
  left: 0;
  right: 0;
  bottom: 0;
  display: flex;
  justify-content: space-evenly;
  align-items: flex-end;
  height: 26%;
}

.bg__blade {
  width: 7px;
  border-radius: 999px 999px 0 0;
  background: #17141a;
  transform-origin: 50% 100%;
  transform: skewX(calc((0.5 - var(--px)) * 26deg));
}

.bg__blade:nth-child(odd) {
  height: 100%;
}

.bg__blade:nth-child(even) {
  height: 74%;
}

.bg__hint {
  margin: 0;
  color: #17141a;
  font-weight: 700;
  font-size: clamp(9px, 2.4vw, 11px);
}

.bg.is-demo .bg__hill--far {
  animation: bgFar 2s cubic-bezier(0.2, 0.8, 0.2, 1) infinite;
}

.bg.is-demo .bg__hill--mid {
  animation: bgMid 2s cubic-bezier(0.2, 0.8, 0.2, 1) infinite;
}

.bg.is-demo .bg__grass {
  animation: bgNear 2s cubic-bezier(0.2, 0.8, 0.2, 1) infinite;
}

.bg.is-demo .bg__blade {
  animation: bgLean 2s cubic-bezier(0.2, 0.8, 0.2, 1) infinite;
}

.bg.is-demo .bg__pupil {
  animation: bgPeek 2s cubic-bezier(0.2, 0.8, 0.2, 1) infinite;
}

@keyframes bgFar {
  0%, 4% {
    transform: translate3d(5px, 0, 0);
  }
  34% {
    transform: translate3d(-5px, 0, 0);
  }
  44%, 50% {
    transform: translate3d(-5px, 0, 0);
  }
  80% {
    transform: translate3d(5px, 0, 0);
  }
  88%, 100% {
    transform: translate3d(5px, 0, 0);
  }
}
@keyframes bgMid {
  0%, 4% {
    transform: translate3d(11px, 0, 0);
  }
  34% {
    transform: translate3d(-11px, 0, 0);
  }
  44%, 50% {
    transform: translate3d(-11px, 0, 0);
  }
  80% {
    transform: translate3d(11px, 0, 0);
  }
  88%, 100% {
    transform: translate3d(11px, 0, 0);
  }
}
@keyframes bgNear {
  0%, 4% {
    transform: translate3d(17px, 0, 0);
  }
  34% {
    transform: translate3d(-17px, 0, 0);
  }
  44%, 50% {
    transform: translate3d(-17px, 0, 0);
  }
  80% {
    transform: translate3d(17px, 0, 0);
  }
  88%, 100% {
    transform: translate3d(17px, 0, 0);
  }
}
@keyframes bgLean {
  0%, 4% {
    transform: skewX(-13deg);
  }
  34% {
    transform: skewX(13deg);
  }
  44%, 50% {
    transform: skewX(13deg);
  }
  80% {
    transform: skewX(-13deg);
  }
  88%, 100% {
    transform: skewX(-13deg);
  }
}
@keyframes bgPeek {
  0%, 4% {
    transform: translate3d(-3.5px, 0, 0);
  }
  34% {
    transform: translate3d(3.5px, 0, 0);
  }
  44%, 50% {
    transform: translate3d(3.5px, 0, 0);
  }
  80% {
    transform: translate3d(-3.5px, 0, 0);
  }
  88%, 100% {
    transform: translate3d(-3.5px, 0, 0);
  }
}
@media (max-width: 340px) {
  .bg {
    gap: 8px;
  }
  .bg__scene {
    height: clamp(96px, 28vh, 130px);
  }
  .bg__sun {
    right: 6%;
  }
}
@media (prefers-reduced-motion: reduce) {
  .bg.is-demo .bg__hill--far,
  .bg.is-demo .bg__hill--mid,
  .bg.is-demo .bg__grass,
  .bg.is-demo .bg__blade,
  .bg.is-demo .bg__pupil {
    animation: none !important;
  }
  .bg.is-demo .bg__blade {
    transform: skewX(0deg) !important;
  }
}

var root = document.getElementById('bg');
var scene = root.querySelector('.bg__scene');
var far = root.querySelector('.bg__hill--far');
var mid = root.querySelector('.bg__hill--mid');
var grass = root.querySelector('.bg__grass');
var pupils = Array.prototype.slice.call(root.querySelectorAll('.bg__pupil'));

var reduce = window.matchMedia('(prefers-reduced-motion: reduce)').matches;

function lerp(a, b, k) { return a + (b - a) * k; }

// --px 는 0~1 사이 커서 가로 위치. 풀잎 skewX 가 이 값 하나를 공유해 같이 눕는다
var px = 0.5;
var pxNow = 0.5;

function tick() {
  pxNow = lerp(pxNow, px, 0.08);
  scene.style.setProperty('--px', pxNow.toFixed(4));
  var dx = pxNow - 0.5;
  // 멀수록 조금, 가까울수록 많이 밀린다 — 시차
  far.style.transform  = 'translate3d(' + (dx * -10).toFixed(1) + 'px,0,0)';
  mid.style.transform  = 'translate3d(' + (dx * -22).toFixed(1) + 'px,0,0)';
  grass.style.transform = 'translate3d(' + (dx * -34).toFixed(1) + 'px,0,0)';
  for (var i = 0; i < pupils.length; i++) {
    pupils[i].style.transform = 'translate3d(' + (dx * 7).toFixed(1) + 'px,0,0)';
  }
  requestAnimationFrame(tick);
}

function setFromPointer(cx) {
  var r = scene.getBoundingClientRect();
  px = Math.min(1, Math.max(0, (cx - r.left) / r.width));
}

window.addEventListener('pointermove', function (e) {
  root.classList.remove('is-demo');
  setFromPointer(e.clientX);
});
window.addEventListener('pointerdown', function (e) {
  root.classList.remove('is-demo');
  setFromPointer(e.clientX);
});

if (reduce) {
  // reduce 에서는 시차 이동 없이 풀잎 눕음만 즉시 값으로 반영한다
  window.addEventListener('pointermove', function (e) {
    setFromPointer(e.clientX);
    scene.style.setProperty('--px', px.toFixed(4));
  });
} else {
  requestAnimationFrame(tick);
}

// BackgroundParallax — vanilla 판(run/079/demo/02-follow-background-parallax/index.html) 의 React 포트.
// 커서 가로 위치(--px, 0~1)를 lerp(0.08)로 부드럽게 만들어 멀리 언덕은 -10px · 가까운 언덕은 -22px ·
// 풀은 -34px 씩 밀어 서로 다른 속도로 따라오게 한다. 풀잎 skewX 는 --px 하나를 공유해 커서 쪽으로 눕고
// 해의 눈동자도 같은 값으로 커서를 본다. 값·수치는 vanilla 와 같다.
import { useEffect, useRef, useState } from "react";
import type { CSSProperties } from "react";
import styles from "./BackgroundParallax.module.scss";

export type BackgroundParallaxProps = {
  duration?: number;
  easing?: string;
  color?: string;
};

type Vars = CSSProperties & { "--duration"?: string; "--easing"?: string; "--color"?: string };
// useRef 는 T | null 을 돌려준다. 처음부터 값이 있고 절대 비지 않는 상자는 이렇게 좁혀 쓴다
type Box<T> = { current: T };
// 전역 DOM 타입에 기대지 않는다 — 포인터 이벤트에서 실제로 읽는 것만 적는다
type PtrEvt = { clientX: number };

const SCENE_LABEL = "언덕과 풀 위에 해가 뜨고 해의 눈이 커서를 바라보는 그림";
const HINT = "마우스를 움직이면 배경과 풀이 따라옵니다";
const BLADE_COUNT = 9;

// lerp: 매 프레임 남은 거리의 일부만 더 가는 선형 보간
function lerp(a: number, b: number, k: number): number {
  return a + (b - a) * k;
}

export default function BackgroundParallax({
  duration = 2000,
  easing = "cubic-bezier(.2, .8, .2, 1)",
  color = "#17141a",
}: BackgroundParallaxProps) {
  const sceneRef = useRef<HTMLDivElement | null>(null) as Box<HTMLDivElement>;
  const farRef = useRef<HTMLDivElement | null>(null);
  const midRef = useRef<HTMLDivElement | null>(null);
  const grassRef = useRef<HTMLDivElement | null>(null);
  const pupilRef = useRef<(HTMLSpanElement | null)[]>(new Array(2).fill(null)) as Box<(HTMLSpanElement | null)[]>;
  const [demo, setDemo] = useState(true);
  const vars: Vars = { "--duration": `${duration}ms`, "--easing": easing, "--color": color };

  function setPupil(index: number) {
    return (el: HTMLSpanElement | null): void => {
      pupilRef.current[index] = el;
    };
  }

  useEffect(() => {
    const scene = sceneRef.current;
    if (!scene) return;
    const reduce = window.matchMedia("(prefers-reduced-motion: reduce)").matches;

    // --px 는 0~1 사이 커서 가로 위치. 풀잎 skewX 가 이 값 하나를 공유해 같이 눕는다
    let px = 0.5;
    let pxNow = 0.5;

    function tick(): void {
      pxNow = lerp(pxNow, px, 0.08);
      scene.style.setProperty("--px", pxNow.toFixed(4));
      const dx = pxNow - 0.5;
      // 멀수록 조금, 가까울수록 많이 밀린다 — 시차
      const far = farRef.current;
      const mid = midRef.current;
      const grass = grassRef.current;
      if (far) far.style.transform = "translate3d(" + (dx * -10).toFixed(1) + "px,0,0)";
      if (mid) mid.style.transform = "translate3d(" + (dx * -22).toFixed(1) + "px,0,0)";
      if (grass) grass.style.transform = "translate3d(" + (dx * -34).toFixed(1) + "px,0,0)";
      for (let i = 0; i < pupilRef.current.length; i++) {
        const pupil = pupilRef.current[i];
        if (pupil) pupil.style.transform = "translate3d(" + (dx * 7).toFixed(1) + "px,0,0)";
      }
      raf = requestAnimationFrame(tick);
    }

    function setFromPointer(cx: number): void {
      const r = scene.getBoundingClientRect();
      px = Math.min(1, Math.max(0, (cx - r.left) / r.width));
    }

    function onMove(event: PtrEvt): void {
      setDemo(false);
      setFromPointer(event.clientX);
      if (reduce) {
        // reduce 에서는 시차 이동 없이 풀잎 눕음만 즉시 값으로 반영한다
        scene.style.setProperty("--px", px.toFixed(4));
      }
    }
    function onDown(event: PtrEvt): void {
      setDemo(false);
      setFromPointer(event.clientX);
    }

    window.addEventListener("pointermove", onMove);
    window.addEventListener("pointerdown", onDown);

    let raf = 0;
    if (!reduce) raf = requestAnimationFrame(tick);
    return () => {
      window.removeEventListener("pointermove", onMove);
      window.removeEventListener("pointerdown", onDown);
      cancelAnimationFrame(raf);
    };
  }, []);

  return (
    <main className={styles.stage}>
      <div className={styles.root + (demo ? " " + styles.isDemo : "")} style={vars}>
        <div className={styles.scene} ref={sceneRef} role="img" aria-label={SCENE_LABEL}>
          <div className={styles.sky} aria-hidden="true"></div>
          <div className={styles.sun} aria-hidden="true">
            <span className={styles.eye}><span className={styles.pupil} ref={setPupil(0)}></span></span>
            <span className={styles.eye}><span className={styles.pupil} ref={setPupil(1)}></span></span>
            <span className={styles.mouth}></span>
          </div>
          <div className={styles.hill + " " + styles.hillFar} ref={farRef} aria-hidden="true"></div>
          <div className={styles.hill + " " + styles.hillMid} ref={midRef} aria-hidden="true"></div>
          <div className={styles.grass} ref={grassRef} aria-hidden="true">
            {Array.from({ length: BLADE_COUNT }, (_, i) => (
              <span className={styles.blade} key={i}></span>
            ))}
          </div>
        </div>
        <p className={styles.hint}>{HINT}</p>
      </div>
    </main>
  );
}