05小雨から豪雨まで強さが行き来する場面

<main class="stage">
<div class="rn is-demo" id="rnRoot">
  <canvas class="rn__canvas" id="rnCanvas" width="480" height="300"></canvas>
  <p class="rn__caption">ゆっくりした周期関数ひとつで嵐の強さを上下させ続け、その値に合わせて雨筋の数と風の強さを毎フレーム計算し直し、場面全体が一緒に弱まり強まる。</p>
</div>
</main>

@use "../../../../site/static/demo-tokens" as *;
@include stage($stage-ink);

.rn {
  position: relative;
  width: min(92vw, calc(160vh - 12.8vw));
  aspect-ratio: 480 / 300;
  margin: 0 auto;
  border-radius: $r-card;
  overflow: hidden;
  box-shadow: $shadow-raised;
  background: $stage-ink;
}

.rn__canvas {
  display: block;
  width: 100%;
  height: 100%;
}

.rn__caption {
  position: absolute;
  left: $sp-3;
  bottom: $sp-3;
  max-width: 78%;
  margin: 0;
  padding: $sp-1 $sp-3;
  border-radius: $r-pill;
  background: rgba(255, 247, 230, .85);
  color: $subject-ink;
  font-weight: 500;
  font-size: 12px;
  line-height: 1.4;
  pointer-events: none;
}

@media (max-width: 340px) {
  .rn__caption { font-size: 10px; padding: $sp-1 $sp-2; }
}

@media (prefers-reduced-motion: reduce) {
  .rn { cursor: default; }
}

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);
}

.rn {
  position: relative;
  width: min(92vw, 160vh - 12.8vw);
  aspect-ratio: 480/300;
  margin: 0 auto;
  border-radius: 16px;
  overflow: hidden;
  box-shadow: 0 14px 24px rgba(23, 20, 26, 0.32);
  background: #17141a;
}

.rn__canvas {
  display: block;
  width: 100%;
  height: 100%;
}

.rn__caption {
  position: absolute;
  left: 12px;
  bottom: 12px;
  max-width: 78%;
  margin: 0;
  padding: 4px 12px;
  border-radius: 999px;
  background: rgba(255, 247, 230, 0.85);
  color: #17141a;
  font-weight: 500;
  font-size: 12px;
  line-height: 1.4;
  pointer-events: none;
}

@media (max-width: 340px) {
  .rn__caption {
    font-size: 10px;
    padding: 4px 8px;
  }
}
@media (prefers-reduced-motion: reduce) {
  .rn {
    cursor: default;
  }
}

(function () {
  // 기법: stormIntensity() 하나가 사인 주기(STORM_CYCLE_MS)로 0~1 을 오르내리고, 그 값 하나로
  // 화면에 보이는 빗줄기 수·굵기·기울기·어두운 정도를 매 프레임 다시 계산한다. RAIN_SPAWN_RATE_BASE
  // 는 가장 약한 이슬비일 때도 유지하는 최소 줄기 수 — 그 위로 세기만큼 더 얹는다.
  var root = document.getElementById('rnRoot');
  var canvas = document.getElementById('rnCanvas');
  var ctx = canvas.getContext('2d');
  var wrap = canvas.parentElement;

  var CALC_W = 160, CALC_H = 100;
  var STORM_CYCLE_MS = 9000;
  var RAIN_SPAWN_RATE_BASE = 18;
  var RAIN_POOL_SIZE = 110;

  function seedRand(seed) {
    var s = seed >>> 0;
    return function () { s = (s * 1664525 + 1013904223) >>> 0; return s / 4294967296; };
  }
  var rRand = seedRand(431051);

  function stormIntensity(nowMs) {
    return Math.sin((nowMs / STORM_CYCLE_MS) * Math.PI * 2 - Math.PI / 2) * 0.5 + 0.5;
  }

  var streaks = [];
  for (var i = 0; i < RAIN_POOL_SIZE; i += 1) {
    streaks.push({
      x: rRand() * CALC_W,
      y: rRand() * CALC_H,
      thresh: rRand(),
      speed: 90 + rRand() * 90,
      thick: 0.7 + rRand() * 1.3
    });
  }

  function stepSim(dtS, nowMs) {
    var inten = stormIntensity(nowMs);
    var wind = (inten - 0.5) * 20;
    for (var i = 0; i < streaks.length; i += 1) {
      var s = streaks[i];
      var visible = inten >= s.thresh * (1 - RAIN_SPAWN_RATE_BASE / 100);
      if (!visible) continue;
      s.y += s.speed * (0.6 + inten * 0.7) * dtS;
      s.x += wind * dtS;
      if (s.y > CALC_H + 6) { s.y = -6 - rRand() * 8; s.x = rRand() * CALC_W; }
      if (s.x < -6) s.x = CALC_W + 6;
      if (s.x > CALC_W + 6) s.x = -6;
    }
  }

  function draw(nowMs) {
    var sx = cw / CALC_W, sy = ch / CALC_H;
    var inten = stormIntensity(nowMs);
    ctx.clearRect(0, 0, cw, ch);
    ctx.fillStyle = 'rgba(6,8,20,' + (inten * 0.4).toFixed(3) + ')';
    ctx.fillRect(0, 0, cw, ch);
    for (var i = 0; i < streaks.length; i += 1) {
      var s = streaks[i];
      var visible = inten >= s.thresh * (1 - RAIN_SPAWN_RATE_BASE / 100);
      if (!visible) continue;
      var alpha = 0.35 + inten * 0.55;
      var x = s.x * sx, y0 = s.y * sy, y1 = (s.y - s.thick * 5) * sy;
      ctx.strokeStyle = 'rgba(214,228,255,' + alpha + ')';
      ctx.lineWidth = Math.max(0.6, s.thick * ((sx + sy) / 2));
      ctx.beginPath();
      ctx.moveTo(x, y0);
      ctx.lineTo(x, y1);
      ctx.stroke();
    }
  }

  var cw = 480, ch = 300;
  function fit() {
    var r = wrap.getBoundingClientRect();
    cw = Math.max(1, Math.round(r.width));
    ch = Math.max(1, Math.round(r.height));
    canvas.width = cw; canvas.height = ch;
    draw(elapsed);
  }

  var reduce = window.matchMedia('(prefers-reduced-motion: reduce)').matches;
  var elapsed = 2200;
  if (reduce) root.classList.remove('is-demo');
  var warmSteps = reduce ? 0 : 40;
  for (var w = 0; w < warmSteps; w += 1) { elapsed += 20; stepSim(0.02, elapsed); }

  var lastTs = null;
  function tick(ts) {
    if (lastTs === null) lastTs = ts;
    var dt = Math.min(ts - lastTs, 48);
    lastTs = ts;
    if (!reduce) {
      elapsed += dt;
      stepSim(dt / 1000, elapsed);
      draw(elapsed);
    }
    requestAnimationFrame(tick);
  }
  fit();
  window.addEventListener('resize', fit);
  draw(elapsed);
  requestAnimationFrame(tick);
})();

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

type Streak = { x: number; y: number; thresh: number; speed: number; thick: number };

export default function RainEffectCanvasStormIntensityCycle() {
  const rootRef = useRef<HTMLDivElement>(null);
  const canvasRef = useRef<HTMLCanvasElement>(null);

  useEffect(() => {
    const root = rootRef.current;
    const canvas = canvasRef.current;
    if (!root || !canvas) return;
    const ctx = canvas.getContext("2d");
    if (!ctx) return;
    const wrap = canvas.parentElement as HTMLElement;

    const CALC_W = 160, CALC_H = 100;
    const STORM_CYCLE_MS = 9000;
    const RAIN_SPAWN_RATE_BASE = 18;
    const RAIN_POOL_SIZE = 110;

    function seedRand(seed: number) {
      let s = seed >>> 0;
      return function () { s = (s * 1664525 + 1013904223) >>> 0; return s / 4294967296; };
    }
    const rRand = seedRand(431052);

    function stormIntensity(nowMs: number) {
      return Math.sin((nowMs / STORM_CYCLE_MS) * Math.PI * 2 - Math.PI / 2) * 0.5 + 0.5;
    }

    const streaks: Streak[] = [];
    for (let i = 0; i < RAIN_POOL_SIZE; i += 1) {
      streaks.push({ x: rRand() * CALC_W, y: rRand() * CALC_H, thresh: rRand(), speed: 90 + rRand() * 90, thick: 0.7 + rRand() * 1.3 });
    }

    function stepSim(dtS: number, nowMs: number) {
      const inten = stormIntensity(nowMs);
      const wind = (inten - 0.5) * 20;
      for (let i = 0; i < streaks.length; i += 1) {
        const s = streaks[i];
        const visible = inten >= s.thresh * (1 - RAIN_SPAWN_RATE_BASE / 100);
        if (!visible) continue;
        s.y += s.speed * (0.6 + inten * 0.7) * dtS;
        s.x += wind * dtS;
        if (s.y > CALC_H + 6) { s.y = -6 - rRand() * 8; s.x = rRand() * CALC_W; }
        if (s.x < -6) s.x = CALC_W + 6;
        if (s.x > CALC_W + 6) s.x = -6;
      }
    }

    let cw = 480, ch = 300;
    function draw(nowMs: number) {
      const sx = cw / CALC_W, sy = ch / CALC_H;
      const inten = stormIntensity(nowMs);
      ctx!.clearRect(0, 0, cw, ch);
      ctx!.fillStyle = "rgba(6,8,20," + (inten * 0.4).toFixed(3) + ")";
      ctx!.fillRect(0, 0, cw, ch);
      for (let i = 0; i < streaks.length; i += 1) {
        const s = streaks[i];
        const visible = inten >= s.thresh * (1 - RAIN_SPAWN_RATE_BASE / 100);
        if (!visible) continue;
        const alpha = 0.35 + inten * 0.55;
        const x = s.x * sx, y0 = s.y * sy, y1 = (s.y - s.thick * 5) * sy;
        ctx!.strokeStyle = "rgba(214,228,255," + alpha + ")";
        ctx!.lineWidth = Math.max(0.6, s.thick * ((sx + sy) / 2));
        ctx!.beginPath();
        ctx!.moveTo(x, y0);
        ctx!.lineTo(x, y1);
        ctx!.stroke();
      }
    }

    function fit() {
      const r = wrap.getBoundingClientRect();
      cw = Math.max(1, Math.round(r.width));
      ch = Math.max(1, Math.round(r.height));
      canvas!.width = cw; canvas!.height = ch;
      draw(elapsed);
    }

    const reduce = window.matchMedia("(prefers-reduced-motion: reduce)").matches;
    let elapsed = 2200;
    if (reduce) root.classList.remove("is-demo");
    const warmSteps = reduce ? 0 : 40;
    for (let w = 0; w < warmSteps; w += 1) { elapsed += 20; stepSim(0.02, elapsed); }

    let lastTs: number | null = null;
    let raf = 0;
    function tick(ts: number) {
      if (lastTs === null) lastTs = ts;
      const dt = Math.min(ts - lastTs, 48);
      lastTs = ts;
      if (!reduce) {
        elapsed += dt;
        stepSim(dt / 1000, elapsed);
        draw(elapsed);
      }
      raf = requestAnimationFrame(tick);
    }
    fit();
    window.addEventListener("resize", fit);
    draw(elapsed);
    raf = requestAnimationFrame(tick);

    return () => {
      cancelAnimationFrame(raf);
      window.removeEventListener("resize", fit);
    };
  }, []);

  return (
    <main className={styles.stage}>
      <div className={`${styles.rn} is-demo`} ref={rootRef}>
        <canvas className={styles.rn__canvas} ref={canvasRef} width={480} height={300} />
        <p className={styles.rn__caption}>느린 주기 함수 하나로 폭풍의 세기를 계속 오르내리게 하고, 그 값에 맞춰 빗줄기 개수와 바람 세기를 매 프레임 다시 계산해 장면 전체가 함께 약해지고 세진다</p>
      </div>
    </main>
  );
}