03Image Fade-in Where a Scroll-position Value Computes an Intersection Ratio

<main class="stage">
<div class="lfv is-demo" id="lfv">
  <div class="lfv__badge" id="lfvBadge">Intersection ratio 0%</div>
  <div class="lfv__view" id="lfvView">
    <div class="lfv__track">
      <div class="lfv__card" data-card="1"><div class="lfv__placeholder"></div><div class="lfv__img lfv__img--1"></div></div>
      <div class="lfv__card" data-card="2"><div class="lfv__placeholder"></div><div class="lfv__img lfv__img--2"></div></div>
      <div class="lfv__card" data-card="3"><div class="lfv__placeholder"></div><div class="lfv__img lfv__img--3"></div></div>
      <div class="lfv__card" data-card="4"><div class="lfv__placeholder"></div><div class="lfv__img lfv__img--4"></div></div>
    </div>
  </div>
</div>
</main>

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

// 스크롤 위치 값 → 교차 비율(IntersectionObserver). 비율 값으로 블러·스케일을 매번
// 다시 계산해 뷰포트에 걸친 정도만큼 또렷해지고, 기준을 넘으면 로드 완료로 고정한다.

$duration: $dur-base;
$easing: $ease-out;
$color: $subject-blue;

@include stage($stage-ink);

.lfv {
  display: flex;
  flex-direction: column;
  align-items: center;
  gap: $sp-3;
  width: min(100%, 280px);
}

.lfv__badge {
  padding: $sp-1 $sp-3;
  border-radius: $r-pill;
  background: rgba(47, 109, 246, .18);
  color: $subject-blue-light;
  font-weight: 700;
  font-size: 11px;
  font-variant-numeric: tabular-nums;
}

.lfv__view {
  box-sizing: border-box;
  width: 100%;
  height: 200px;
  overflow-y: auto;
  overflow-x: hidden;
  border-radius: $r-card;
  background: rgba(255, 255, 255, .06);
  padding: $sp-3;
  scrollbar-width: thin;
}

.lfv__track {
  display: flex;
  flex-direction: column;
  gap: $sp-3;
}

.lfv__card {
  position: relative;
  box-sizing: border-box;
  height: 120px;
  border-radius: $r-sm;
  overflow: hidden;
  flex: 0 0 auto;
}

.lfv__placeholder {
  position: absolute;
  inset: 0;
  background: rgba(255, 255, 255, .1);
}

.lfv__img {
  position: absolute;
  inset: 0;
  opacity: 0;
  transform-origin: center;
}

.lfv__img--1 { background: linear-gradient(135deg, $subject-blue, $subject-lilac); }
.lfv__img--2 { background: linear-gradient(135deg, $subject-mint, $subject-blue-light); }
.lfv__img--3 { background: linear-gradient(135deg, $subject-pink, $subject-blue); }
.lfv__img--4 { background: linear-gradient(135deg, $subject-sky, $subject-lilac); }

.lfv__card--loaded {
  box-shadow: inset 0 0 0 2px rgba(91, 140, 248, .5);
}

@media (max-width: 340px) {
  .lfv { gap: $sp-2; }
  .lfv__view { height: 108px; }
  .lfv__card { height: 84px; }
}

@media (prefers-reduced-motion: reduce) {
  .lfv__img, .lfv__placeholder { transition: none !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);
}

.lfv {
  display: flex;
  flex-direction: column;
  align-items: center;
  gap: 12px;
  width: min(100%, 280px);
}

.lfv__badge {
  padding: 4px 12px;
  border-radius: 999px;
  background: rgba(47, 109, 246, 0.18);
  color: #5b8cf8;
  font-weight: 700;
  font-size: 11px;
  font-variant-numeric: tabular-nums;
}

.lfv__view {
  box-sizing: border-box;
  width: 100%;
  height: 200px;
  overflow-y: auto;
  overflow-x: hidden;
  border-radius: 16px;
  background: rgba(255, 255, 255, 0.06);
  padding: 12px;
  scrollbar-width: thin;
}

.lfv__track {
  display: flex;
  flex-direction: column;
  gap: 12px;
}

.lfv__card {
  position: relative;
  box-sizing: border-box;
  height: 120px;
  border-radius: 8px;
  overflow: hidden;
  flex: 0 0 auto;
}

.lfv__placeholder {
  position: absolute;
  inset: 0;
  background: rgba(255, 255, 255, 0.1);
}

.lfv__img {
  position: absolute;
  inset: 0;
  opacity: 0;
  transform-origin: center;
}

.lfv__img--1 {
  background: linear-gradient(135deg, #2f6df6, #b9a0ff);
}

.lfv__img--2 {
  background: linear-gradient(135deg, #4cd4a6, #5b8cf8);
}

.lfv__img--3 {
  background: linear-gradient(135deg, #ff9ebb, #2f6df6);
}

.lfv__img--4 {
  background: linear-gradient(135deg, #6ec8ff, #b9a0ff);
}

.lfv__card--loaded {
  box-shadow: inset 0 0 0 2px rgba(91, 140, 248, 0.5);
}

@media (max-width: 340px) {
  .lfv {
    gap: 8px;
  }
  .lfv__view {
    height: 108px;
  }
  .lfv__card {
    height: 84px;
  }
}
@media (prefers-reduced-motion: reduce) {
  .lfv__img, .lfv__placeholder {
    transition: none !important;
  }
}

(function () {
  var root = document.getElementById('lfv');
  var view = document.getElementById('lfvView');
  var badge = document.getElementById('lfvBadge');
  var cards = Array.prototype.slice.call(document.querySelectorAll('.lfv__card'));

  var INTERSECTION_THRESHOLD = 0.4;
  var STEPS = [];
  for (var s = 0; s <= 20; s += 1) { STEPS.push(s / 20); }

  // 계산 겹: Intersection ratio 값으로 블러 반경·스케일을 매 관찰마다 다시 계산해
  // 뷰포트에 걸친 정도만큼 이미지가 또렷해진다. 비율이 기준을 넘으면 로드 완료로 고정.
  function applyRatio(card, ratio) {
    var img = card.querySelector('.lfv__img');
    var placeholder = card.querySelector('.lfv__placeholder');
    var blur = 10 * (1 - ratio);
    var scale = 0.92 + 0.08 * ratio;
    img.style.opacity = String(ratio);
    img.style.filter = 'blur(' + blur.toFixed(1) + 'px)';
    img.style.transform = 'scale(' + scale.toFixed(3) + ')';
    placeholder.style.opacity = String(Math.max(0, 1 - ratio * 1.4));
    if (ratio >= INTERSECTION_THRESHOLD) {
      card.classList.add('lfv__card--loaded');
    }
    if (card === cards[Math.min(cards.length - 1, Math.round(lastMaxIndex))]) {
      badge.textContent = 'Intersection ratio ' + Math.round(ratio * 100) + '%';
    }
  }

  var lastMaxIndex = 0;
  var io = new IntersectionObserver(function (entries) {
    var maxRatio = 0, maxIdx = 0;
    entries.forEach(function (entry) {
      var idx = cards.indexOf(entry.target);
      applyRatio(entry.target, entry.intersectionRatio);
      if (entry.intersectionRatio >= maxRatio) { maxRatio = entry.intersectionRatio; maxIdx = idx; }
    });
    lastMaxIndex = maxIdx;
    badge.textContent = 'Intersection ratio ' + Math.round(maxRatio * 100) + '%';
  }, { root: view, threshold: STEPS });

  cards.forEach(function (c) { io.observe(c); });

  var dir = 1;
  var autoplayTimer = null;
  function autoplayTick() {
    var max = view.scrollHeight - view.clientHeight;
    view.scrollTop += dir * 2.2;
    if (view.scrollTop >= max - 1) { dir = -1; }
    if (view.scrollTop <= 1) { dir = 1; }
  }

  function userTakesOver() {
    if (!root.classList.contains('is-demo')) { return; }
    root.classList.remove('is-demo');
    if (autoplayTimer) { clearInterval(autoplayTimer); autoplayTimer = null; }
  }
  view.addEventListener('wheel', userTakesOver, { passive: true });
  view.addEventListener('touchstart', userTakesOver, { passive: true });
  view.addEventListener('scroll', function () {
    // 스크롤 위치 값 자체가 IntersectionObserver 재계산의 입력이 된다 (root 스크롤).
  });

  var reduce = window.matchMedia('(prefers-reduced-motion: reduce)').matches;
  if (reduce) {
    root.classList.remove('is-demo');
    cards.forEach(function (c) { applyRatio(c, 1); });
    badge.textContent = 'Intersection ratio 100%';
  } else {
    autoplayTimer = setInterval(autoplayTick, 45);
  }
})();

import { useEffect, useRef, useState } from "react";
import styles from "./ImageFadeInViewport.module.scss";

// 스크롤 위치 값 → 교차 비율(IntersectionObserver). 비율 값으로 블러·스케일을 매번
// 다시 계산해 뷰포트에 걸친 정도만큼 또렷해지고, 기준을 넘으면 로드 완료로 고정한다.
const INTERSECTION_THRESHOLD = 0.4;
const STEPS: number[] = [];
for (let s = 0; s <= 20; s += 1) { STEPS.push(s / 20); }
const CARD_KEYS = [1, 2, 3, 4];

type Box<T> = { current: T };
function useBox<T>(initial: T): Box<T> {
  return useRef(initial) as unknown as Box<T>;
}

export default function ImageFadeInViewport() {
  const rootRef = useRef<HTMLDivElement>(null);
  const viewRef = useRef<HTMLDivElement>(null);
  const cardRefs = useBox<Array<HTMLDivElement | null>>([null, null, null, null]);
  const imgRefs = useBox<Array<HTMLDivElement | null>>([null, null, null, null]);
  const placeholderRefs = useBox<Array<HTMLDivElement | null>>([null, null, null, null]);
  const loadedRefs = useBox<boolean[]>([false, false, false, false]);
  const [badge, setBadge] = useState("교차 비율 0%");
  const [demo, setDemo] = useState(true);
  const [, forceRender] = useState(0);

  function applyRatio(i: number, ratio: number) {
    const img = imgRefs.current[i];
    const placeholder = placeholderRefs.current[i];
    if (!img || !placeholder) { return; }
    const blur = 10 * (1 - ratio);
    const scale = 0.92 + 0.08 * ratio;
    img.style.opacity = String(ratio);
    img.style.filter = "blur(" + blur.toFixed(1) + "px)";
    img.style.transform = "scale(" + scale.toFixed(3) + ")";
    placeholder.style.opacity = String(Math.max(0, 1 - ratio * 1.4));
    if (ratio >= INTERSECTION_THRESHOLD && !loadedRefs.current[i]) {
      loadedRefs.current[i] = true;
      forceRender((n) => n + 1);
    }
  }

  useEffect(() => {
    const view = viewRef.current;
    const root = rootRef.current;
    if (!view || !root) { return; }

    const io = new IntersectionObserver((entries) => {
      let maxRatio = 0;
      entries.forEach((entry) => {
        const idx = cardRefs.current.indexOf(entry.target as HTMLDivElement);
        if (idx === -1) { return; }
        applyRatio(idx, entry.intersectionRatio);
        if (entry.intersectionRatio >= maxRatio) { maxRatio = entry.intersectionRatio; }
      });
      setBadge("교차 비율 " + Math.round(maxRatio * 100) + "%");
    }, { root: view, threshold: STEPS });

    cardRefs.current.forEach((c) => { if (c) { io.observe(c); } });

    function userTakesOver() {
      if (!root!.classList.contains(styles["is-demo"])) { return; }
      setDemo(false);
      if (autoplayTimer !== null) { window.clearInterval(autoplayTimer); }
    }
    view.addEventListener("wheel", userTakesOver, { passive: true });
    view.addEventListener("touchstart", userTakesOver, { passive: true });

    let dir = 1;
    let autoplayTimer: number | null = null;
    const reduce = window.matchMedia("(prefers-reduced-motion: reduce)").matches;
    if (reduce) {
      setDemo(false);
      CARD_KEYS.forEach((_, i) => applyRatio(i, 1));
      setBadge("교차 비율 100%");
    } else {
      autoplayTimer = window.setInterval(() => {
        const max = view.scrollHeight - view.clientHeight;
        view.scrollTop += dir * 2.2;
        if (view.scrollTop >= max - 1) { dir = -1; }
        if (view.scrollTop <= 1) { dir = 1; }
      }, 45);
    }

    return () => {
      io.disconnect();
      view.removeEventListener("wheel", userTakesOver);
      view.removeEventListener("touchstart", userTakesOver);
      if (autoplayTimer !== null) { window.clearInterval(autoplayTimer); }
    };
    // eslint-disable-next-line react-hooks/exhaustive-deps
  }, []);

  return (
    <main className={styles.stage}>
      <div ref={rootRef} className={`${styles.lfv} ${demo ? styles["is-demo"] : ""}`}>
        <div className={styles["lfv__badge"]}>{badge}</div>
        <div ref={viewRef} className={styles["lfv__view"]}>
          <div className={styles["lfv__track"]}>
            {CARD_KEYS.map((k, i) => (
              <div
                key={k}
                ref={(el: HTMLDivElement | null) => { cardRefs.current[i] = el; }}
                className={`${styles["lfv__card"]} ${loadedRefs.current[i] ? styles["lfv__card--loaded"] : ""}`}
              >
                <div ref={(el: HTMLDivElement | null) => { placeholderRefs.current[i] = el; }} className={styles["lfv__placeholder"]} />
                <div ref={(el: HTMLDivElement | null) => { imgRefs.current[i] = el; }} className={`${styles["lfv__img"]} ${styles["lfv__img--" + k]}`} />
              </div>
            ))}
          </div>
        </div>
      </div>
    </main>
  );
}