01양 끝이 투명하게 사라지는 기본 구분선

<main class="stage">
<div class="dv is-demo" id="dvRoot">
  <p class="dv__block">위 단락</p>
  <div class="dv__rail">
    <div class="dv__glow" id="dvGlow"></div>
    <div class="dv__line" id="dvLine" role="separator"></div>
  </div>
  <p class="dv__block">아래 단락</p>
  <label class="dv__ctrl">
    <span>사라지는 구간</span>
    <input type="range" id="dvRange" min="0" max="50" value="30">
  </label>
  <p class="dv__meta"><span class="dv__tag">FADE_PCT</span> <b id="dvPct">30</b>%</p>
</div>
</main>

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

// 핵심: 선 색의 알파를 양 끝 FADE_PCT% 구간에서만 smoothstep 곡선으로 올린다.
// 정지점은 JS 가 계산해 background 에 넣는다 (직선 보간보다 끝이 자연스럽다).

$line-h: 20px;
$line-color: $subject-lilac;

@include stage($stage-ink);

.dv {
  width: min(100%, 440px);
  display: grid;
  gap: $sp-2;
}

.dv__block {
  box-sizing: border-box;
  margin: 0;
  height: 36px;
  padding: 0 $sp-4;
  display: flex;
  align-items: center;
  border-radius: $r-control;
  background: $tick-panel;
  color: $stage-paper;
  font-size: 14px;
  font-weight: 700;
  line-height: 20px;
}

.dv__rail {
  position: relative;
  height: $line-h;
}
.dv__glow {
  position: absolute;
  left: 0;
  right: 0;
  top: -14px;
  bottom: -14px;
  border-radius: $r-pill;
  opacity: .8;
  filter: blur(14px);
}

.dv__line {
  position: relative;
  height: $line-h;
  border-radius: $r-pill;
  background: $line-color;
}

.dv__ctrl {
  display: flex;
  align-items: center;
  gap: $sp-3;
  font-size: 12px;
  line-height: 16px;
  color: rgba($stage-paper, .78);
}
.dv__ctrl input { flex: 1; min-width: 0; accent-color: $line-color; }

.dv__meta {
  margin: 0;
  display: flex;
  align-items: baseline;
  gap: $sp-2;
  font-size: 12px;
  line-height: 16px;
  color: rgba($stage-paper, .78);
}
.dv__tag { font-weight: 700; letter-spacing: .08em; }
.dv__meta b { min-width: 2ch; text-align: right; font-variant-numeric: tabular-nums; color: $stage-paper; }

@media (prefers-reduced-motion: reduce) {
  .dv__line { 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);
}

.dv {
  width: min(100%, 440px);
  display: grid;
  gap: 8px;
}

.dv__block {
  box-sizing: border-box;
  margin: 0;
  height: 36px;
  padding: 0 16px;
  display: flex;
  align-items: center;
  border-radius: 12px;
  background: #231f28;
  color: #ffffff;
  font-size: 14px;
  font-weight: 700;
  line-height: 20px;
}

.dv__rail {
  position: relative;
  height: 20px;
}

.dv__glow {
  position: absolute;
  left: 0;
  right: 0;
  top: -14px;
  bottom: -14px;
  border-radius: 999px;
  opacity: 0.8;
  filter: blur(14px);
}

.dv__line {
  position: relative;
  height: 20px;
  border-radius: 999px;
  background: #b9a0ff;
}

.dv__ctrl {
  display: flex;
  align-items: center;
  gap: 12px;
  font-size: 12px;
  line-height: 16px;
  color: rgba(255, 255, 255, 0.78);
}

.dv__ctrl input {
  flex: 1;
  min-width: 0;
  accent-color: #b9a0ff;
}

.dv__meta {
  margin: 0;
  display: flex;
  align-items: baseline;
  gap: 8px;
  font-size: 12px;
  line-height: 16px;
  color: rgba(255, 255, 255, 0.78);
}

.dv__tag {
  font-weight: 700;
  letter-spacing: 0.08em;
}

.dv__meta b {
  min-width: 2ch;
  text-align: right;
  font-variant-numeric: tabular-nums;
  color: #ffffff;
}

@media (prefers-reduced-motion: reduce) {
  .dv__line {
    transition: none;
  }
}

(function () {
  // 기법: 선 색의 알파를 양 끝 FADE_PCT% 구간에서만 0 에서 1 로 올린다.
  // 직선 보간은 선이 시작하는 곳에 띠가 보여서 smoothstep(3t^2 - 2t^3) 곡선으로 8단계 정지점을 직접 만든다.
  var FADE_PCT = 30;
  var LINE_H_PX = 20;
  var STEPS = 8;
  var CYCLE_MS = 2000;
  var LOW_PCT = 6;
  var HIGH_PCT = 50;
  var RGB = '185,160,255';

  var root = document.getElementById('dvRoot');
  var line = document.getElementById('dvLine');
  var glow = document.getElementById('dvGlow');
  var range = document.getElementById('dvRange');
  var pctEl = document.getElementById('dvPct');
  var reduce = window.matchMedia('(prefers-reduced-motion: reduce)').matches;

  function gradientOf(pct) {
    var stops = [];
    var i;
    for (i = 0; i <= STEPS; i += 1) {
      var t = i / STEPS;
      var a = t * t * (3 - 2 * t);
      stops.push('rgba(' + RGB + ',' + a.toFixed(3) + ') ' + (pct * t).toFixed(2) + '%');
    }
    for (i = STEPS; i >= 0; i -= 1) {
      var u = i / STEPS;
      var b = u * u * (3 - 2 * u);
      stops.push('rgba(' + RGB + ',' + b.toFixed(3) + ') ' + (100 - pct * u).toFixed(2) + '%');
    }
    return 'linear-gradient(90deg,' + stops.join(',') + ')';
  }
  function paint(pct) {
    var g = gradientOf(pct);
    line.style.background = g;
    glow.style.background = g;
    pctEl.textContent = String(Math.round(pct));
    range.value = String(Math.round(pct));
  }
  function stopDemo() { root.classList.remove('is-demo'); }

  range.addEventListener('input', function () {
    stopDemo();
    FADE_PCT = Number(range.value);
    paint(FADE_PCT);
  });

  var t0 = null;
  function tick(ts) {
    if (t0 === null) t0 = ts;
    if (root.classList.contains('is-demo')) {
      var phase = ((ts - t0) % CYCLE_MS) / CYCLE_MS;
      var wave = 0.5 - 0.5 * Math.cos(phase * Math.PI * 2);
      paint(LOW_PCT + (HIGH_PCT - LOW_PCT) * wave);
    }
    requestAnimationFrame(tick);
  }

  line.style.height = LINE_H_PX + 'px';
  paint(FADE_PCT);
  if (reduce) {
    stopDemo();
  } else {
    requestAnimationFrame(tick);
  }
})();

// FadeDivider — vanilla 판(01-divl-gradient-fade-ends-line) 의 React 포트.
// 선 색의 알파를 양 끝 FADE_PCT% 구간에서만 smoothstep 곡선으로 올린다. 정지점은 JS 가 계산한다.
import { useEffect, useRef } from "react";
import styles from "./FadeDivider.module.scss";

type Box<T> = { current: T };

const LINE_H_PX = 20;
const STEPS = 8;
const CYCLE_MS = 2000;
const LOW_PCT = 6;
const HIGH_PCT = 50;
const RGB = "185,160,255";

function gradientOf(pct: number): string {
  const stops: string[] = [];
  for (let i = 0; i <= STEPS; i += 1) {
    const t = i / STEPS;
    const a = t * t * (3 - 2 * t);
    stops.push("rgba(" + RGB + "," + a.toFixed(3) + ") " + (pct * t).toFixed(2) + "%");
  }
  for (let i = STEPS; i >= 0; i -= 1) {
    const u = i / STEPS;
    const b = u * u * (3 - 2 * u);
    stops.push("rgba(" + RGB + "," + b.toFixed(3) + ") " + (100 - pct * u).toFixed(2) + "%");
  }
  return "linear-gradient(90deg," + stops.join(",") + ")";
}

export default function FadeDivider() {
  const lineRef = useRef<HTMLDivElement | null>(null) as Box<HTMLDivElement | null>;
  const glowRef = useRef<HTMLDivElement | null>(null) as Box<HTMLDivElement | null>;
  const rangeRef = useRef<HTMLInputElement | null>(null) as Box<HTMLInputElement | null>;
  const pctRef = useRef<HTMLElement | null>(null) as Box<HTMLElement | null>;

  useEffect(() => {
    const line = lineRef.current;
    const glow = glowRef.current;
    const range = rangeRef.current;
    const pctEl = pctRef.current;
    if (!line || !glow || !range || !pctEl) return;
    const reduce = window.matchMedia("(prefers-reduced-motion: reduce)").matches;
    let demo = !reduce;
    let t0: number | null = null;
    let raf = 0;

    const paint = (pct: number): void => {
      const g = gradientOf(pct);
      line.style.background = g;
      glow.style.background = g;
      pctEl.textContent = String(Math.round(pct));
      range.value = String(Math.round(pct));
    };
    const onInput = (): void => {
      demo = false;
      paint(Number(range.value));
    };
    range.addEventListener("input", onInput);

    function tick(ts: number): void {
      if (t0 === null) t0 = ts;
      if (demo) {
        const phase = ((ts - t0) % CYCLE_MS) / CYCLE_MS;
        const wave = 0.5 - 0.5 * Math.cos(phase * Math.PI * 2);
        paint(LOW_PCT + (HIGH_PCT - LOW_PCT) * wave);
      }
      raf = requestAnimationFrame(tick);
    }

    line.style.height = LINE_H_PX + "px";
    paint(30);
    if (!reduce) raf = requestAnimationFrame(tick);
    return () => {
      cancelAnimationFrame(raf);
      range.removeEventListener("input", onInput);
    };
  }, []);

  return (
    <div className={styles.dv}>
      <p className={styles.block}>위 단락</p>
      <div className={styles.rail}>
        <div className={styles.glow} ref={glowRef} />
        <div className={styles.line} ref={lineRef} role="separator" />
      </div>
      <p className={styles.block}>아래 단락</p>
      <label className={styles.ctrl}>
        <span>사라지는 구간</span>
        <input type="range" min={0} max={50} defaultValue={30} ref={rangeRef} />
      </label>
      <p className={styles.meta}>
        <span className={styles.tag}>FADE_PCT</span> <b ref={pctRef}>30</b>%
      </p>
    </div>
  );
}