03Light that follows — a spotlight

<main class="stage">
<div class="sl is-demo" id="sl">
  <div class="sl__scene" role="img" aria-label="A blob of light wanders the dark stage late, lighting words and stars">
    <p class="sl__word">LIGHT FOLLOWS</p>
    <span class="sl__star" style="left:8%; top:18%" aria-hidden="true"></span>
    <span class="sl__star" style="left:16%; top:64%" aria-hidden="true"></span>
    <span class="sl__star" style="left:83%; top:22%" aria-hidden="true"></span>
    <span class="sl__star" style="left:90%; top:66%" aria-hidden="true"></span>
    <span class="sl__star" style="left:70%; top:80%" aria-hidden="true"></span>
    <span class="sl__star" style="left:28%; top:84%" aria-hidden="true"></span>
    <span class="sl__light" id="slLight" aria-hidden="true"></span>
  </div>
  <p class="sl__hint">Move the mouse and the light chases it a beat behind</p>
</div>
</main>

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

// 따라다니는 빛 — radial-gradient 빛 덩어리가 mix-blend-mode: screen 으로 어두운 무대를 덮으며
// 커서를 lerp(0.09)로 늦게 쫓는다. screen 은 밝은 쪽만 더 밝게 하므로 지나온 글자와 별이 드러난다.
// 격자 미리보기(.is-demo)는 빛이 문장을 왼쪽 끝에서 오른쪽 끝까지 훑고 돌아오는 길을 @keyframes 로 움직인다.

$duration: $dur-loop;
$easing: $ease-out;
$color: $subject-cream;

@include stage($stage-ink);

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

.sl__scene {
  position: relative; overflow: hidden;
  width: 100%; height: clamp(110px, 32vh, 160px);
  border: 3px solid $color; border-radius: $r-card;
  background: #2a272c;
}

.sl__word {
  position: absolute; left: 50%; top: 50%;
  width: max-content; margin: 0;
  transform: translate(-50%, -50%);
  color: rgba($color, .78);
  font-weight: 800; font-size: clamp(20px, 6vw, 30px); line-height: 1.1;
  letter-spacing: .04em;
}

.sl__star {
  position: absolute;
  width: 6px; height: 6px;
  background: rgba($color, .55);
  transform: rotate(45deg);
}

// 빛 덩어리 — 크고 부드러운 원형 그라디언트. z 순서는 글자 위
.sl__light {
  position: absolute; left: 0; top: 0;
  width: 220px; height: 220px;
  background: radial-gradient(circle, rgba($subject-cream, .95) 0%, rgba($stage-yellow, .38) 38%, rgba($stage-yellow, 0) 68%);
  mix-blend-mode: screen;
  border-radius: 50%;
  will-change: transform;
}

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

// ── 미리보기 루프: 빛이 문장을 왼쪽에서 오른쪽으로 훑고 돌아온다
.sl.is-demo .sl__light { animation: slSweep $duration $easing infinite; }

@keyframes slSweep {
  0%, 6%   { transform: translate3d(-96px, -46px, 0); }
  44%, 54% { transform: translate3d(190px, -30px, 0); }
  88%, 100% { transform: translate3d(-96px, -46px, 0); }
}

@media (max-width: 340px) {
  .sl { gap: $sp-2; }
  .sl__scene { height: clamp(96px, 28vh, 130px); }
  .sl__word { font-size: clamp(16px, 5vw, 22px); }
  .sl.is-demo .sl__light { animation-name: slSweepNarrow; }
}

@keyframes slSweepNarrow {
  0%, 6%   { transform: translate3d(-100px, -44px, 0); }
  44%, 54% { transform: translate3d(160px, -30px, 0); }
  88%, 100% { transform: translate3d(-100px, -44px, 0); }
}

@media (prefers-reduced-motion: reduce) {
  .sl.is-demo .sl__light { animation: none !important; }
  // reduce 에서는 빛이 문장 가운데에 고정되어 한 번에 다 보인다
  .sl.is-demo .sl__light { transform: translate3d(60px, -40px, 0) !important; }
}

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);
}

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

.sl__scene {
  position: relative;
  overflow: hidden;
  width: 100%;
  height: clamp(110px, 32vh, 160px);
  border: 3px solid #fff7e6;
  border-radius: 16px;
  background: #2a272c;
}

.sl__word {
  position: absolute;
  left: 50%;
  top: 50%;
  width: max-content;
  margin: 0;
  transform: translate(-50%, -50%);
  color: rgba(255, 247, 230, 0.78);
  font-weight: 800;
  font-size: clamp(20px, 6vw, 30px);
  line-height: 1.1;
  letter-spacing: 0.04em;
}

.sl__star {
  position: absolute;
  width: 6px;
  height: 6px;
  background: rgba(255, 247, 230, 0.55);
  transform: rotate(45deg);
}

.sl__light {
  position: absolute;
  left: 0;
  top: 0;
  width: 220px;
  height: 220px;
  background: radial-gradient(circle, rgba(255, 247, 230, 0.95) 0%, rgba(255, 210, 63, 0.38) 38%, rgba(255, 210, 63, 0) 68%);
  mix-blend-mode: screen;
  border-radius: 50%;
  will-change: transform;
}

.sl__hint {
  margin: 0;
  color: #fff7e6;
  font-weight: 700;
  font-size: clamp(9px, 2.4vw, 11px);
}

.sl.is-demo .sl__light {
  animation: slSweep 2s ease-out infinite;
}

@keyframes slSweep {
  0%, 6% {
    transform: translate3d(-96px, -46px, 0);
  }
  44%, 54% {
    transform: translate3d(190px, -30px, 0);
  }
  88%, 100% {
    transform: translate3d(-96px, -46px, 0);
  }
}
@media (max-width: 340px) {
  .sl {
    gap: 8px;
  }
  .sl__scene {
    height: clamp(96px, 28vh, 130px);
  }
  .sl__word {
    font-size: clamp(16px, 5vw, 22px);
  }
  .sl.is-demo .sl__light {
    animation-name: slSweepNarrow;
  }
}
@keyframes slSweepNarrow {
  0%, 6% {
    transform: translate3d(-100px, -44px, 0);
  }
  44%, 54% {
    transform: translate3d(160px, -30px, 0);
  }
  88%, 100% {
    transform: translate3d(-100px, -44px, 0);
  }
}
@media (prefers-reduced-motion: reduce) {
  .sl.is-demo .sl__light {
    animation: none !important;
  }
  .sl.is-demo .sl__light {
    transform: translate3d(60px, -40px, 0) !important;
  }
}

var root = document.getElementById('sl');
var scene = root.querySelector('.sl__scene');
var light = document.getElementById('slLight');

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

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

var target = { x: 0, y: 0 };
var pos = { x: 0, y: 0 };

function setTarget(cx, cy) {
  var r = scene.getBoundingClientRect();
  target.x = cx - r.left;
  target.y = cy - r.top;
}

function draw() {
  // 빛 덩어리는 자기 중심이 좌표에 오도록 절반만큼 뺀다
  light.style.transform = 'translate3d(' + (pos.x - 110) + 'px,' + (pos.y - 110) + 'px,0)';
}

function tick() {
  pos.x = lerp(pos.x, target.x, 0.09);
  pos.y = lerp(pos.y, target.y, 0.09);
  draw();
  requestAnimationFrame(tick);
}

function start() {
  var r = scene.getBoundingClientRect();
  target = { x: r.width / 2, y: r.height / 2 };
  pos = { x: r.width / 2, y: r.height / 2 };
}

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

start();
if (reduce) {
  // reduce 에서는 빛이 한가운데에 머문다
  window.addEventListener('pointermove', function (e) { setTarget(e.clientX, e.clientY); pos = { x: target.x, y: target.y }; draw(); });
} else {
  requestAnimationFrame(tick);
}

// SpotlightLight — vanilla 판(run/079/demo/03-follow-spotlight-light/index.html) 의 React 포트.
// radial-gradient 빛 덩어리가 mix-blend-mode: screen 으로 어두운 무대를 덮으며 커서를 lerp(0.09)로 늦게 쫓는다.
// screen 은 밝은 쪽만 더 밝게 하므로 지나온 글자와 별이 드러난다. 값·수치는 vanilla 와 같다.
import { useEffect, useRef, useState } from "react";
import type { CSSProperties } from "react";
import styles from "./SpotlightLight.module.scss";

export type SpotlightLightProps = {
  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; clientY: number };

const SCENE_LABEL = "어두운 무대를 빛 덩어리가 늦게 따라다니며 글자와 별을 밝히는 그림";
const WORD = "빛이 따라온다";
const HINT = "마우스를 움직이면 빛이 한 박자 늦게 따라옵니다";
// 별 여섯 개의 자리 — vanilla index.html 의 인라인 스타일과 같은 값
const STARS: { left: string; top: string }[] = [
  { left: "8%", top: "18%" },
  { left: "16%", top: "64%" },
  { left: "83%", top: "22%" },
  { left: "90%", top: "66%" },
  { left: "70%", top: "80%" },
  { left: "28%", top: "84%" },
];

// 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 SpotlightLight({
  duration = 2000,
  easing = "ease-out",
  color = "#fff7e6",
}: SpotlightLightProps) {
  const sceneRef = useRef<HTMLDivElement | null>(null) as Box<HTMLDivElement>;
  const lightRef = useRef<HTMLSpanElement | null>(null);
  const [demo, setDemo] = useState(true);
  const vars: Vars = { "--duration": `${duration}ms`, "--easing": easing, "--color": color };

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

    const target = { x: 0, y: 0 };
    const pos = { x: 0, y: 0 };

    function setTarget(cx: number, cy: number): void {
      const r = scene.getBoundingClientRect();
      target.x = cx - r.left;
      target.y = cy - r.top;
    }

    function draw(): void {
      const light = lightRef.current;
      // 빛 덩어리는 자기 중심이 좌표에 오도록 절반만큼 뺀다
      if (light) light.style.transform = "translate3d(" + (pos.x - 110) + "px," + (pos.y - 110) + "px,0)";
    }

    function tick(): void {
      pos.x = lerp(pos.x, target.x, 0.09);
      pos.y = lerp(pos.y, target.y, 0.09);
      draw();
      raf = requestAnimationFrame(tick);
    }

    function start(): void {
      const r = scene.getBoundingClientRect();
      target.x = r.width / 2;
      target.y = r.height / 2;
      pos.x = r.width / 2;
      pos.y = r.height / 2;
    }

    function onMove(event: PtrEvt): void {
      setDemo(false);
      setTarget(event.clientX, event.clientY);
      if (reduce) {
        // reduce 에서는 빛이 한가운데에 머문다
        pos.x = target.x;
        pos.y = target.y;
        draw();
      }
    }
    function onDown(event: PtrEvt): void {
      setDemo(false);
      setTarget(event.clientX, event.clientY);
    }

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

    let raf = 0;
    start();
    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}>
          <p className={styles.word}>{WORD}</p>
          {STARS.map((star, i) => (
            <span className={styles.star} style={{ left: star.left, top: star.top }} aria-hidden="true" key={i}></span>
          ))}
          <span className={styles.light} ref={lightRef} aria-hidden="true"></span>
        </div>
        <p className={styles.hint}>{HINT}</p>
      </div>
    </main>
  );
}