02Rain Streaks Bending as the Wind Direction Shifts Over Time

<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">A slowly oscillating wave value keeps recalculating wind strength and angle, and every streak's tilt and fall speed leans together with that same wind value.</p>
</div>
</main>

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

.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-yellow;
}

.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, 255, 255, .82);
  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: #ffd23f;
  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: #ffd23f;
}

.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, 255, 255, 0.82);
  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: 두 사인파를 합성한 windGustAngle() 값 하나가 바람의 각도를 정하고, 모든 빗줄기가
  // 매 프레임 그 같은 값을 읽어 기울기·낙하 속도를 다시 계산한다. WIND_GUST_MAX_DEG 는 그
  // 각도의 최댓값 — 값을 늘리면 약한 바람에도 크게 기울고, 줄이면 강한 바람에도 거의 수직이다.
  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 WIND_GUST_MAX_DEG = 8;
  var RAIN_WIND_CYCLE_MS = 5200;
  var RAIN_STREAK_COUNT = 70;
  var RAIN_FALL_SPEED_PX_S = 130;

  function seedRand(seed) {
    var s = seed >>> 0;
    return function () { s = (s * 1664525 + 1013904223) >>> 0; return s / 4294967296; };
  }
  var rRand = seedRand(431021);

  function windGustAngle(nowMs) {
    var a = Math.sin(nowMs / RAIN_WIND_CYCLE_MS * Math.PI * 2) * 0.62;
    var b = Math.sin(nowMs / (RAIN_WIND_CYCLE_MS * 1.73) * Math.PI * 2 + 1.3) * 0.38;
    return (a + b) * WIND_GUST_MAX_DEG;
  }

  var streaks = [];
  for (var i = 0; i < RAIN_STREAK_COUNT; i += 1) {
    streaks.push({
      x: rRand() * CALC_W,
      y: rRand() * CALC_H,
      speed: RAIN_FALL_SPEED_PX_S * (0.6 + rRand() * 0.7),
      thick: 0.8 + rRand() * 1.4
    });
  }

  var elapsed = 0;
  function stepSim(dtS, nowMs) {
    var deg = windGustAngle(nowMs);
    var rad = deg * Math.PI / 180;
    var sinA = Math.sin(rad), cosA = Math.cos(rad);
    for (var i = 0; i < streaks.length; i += 1) {
      var s = streaks[i];
      s.y += s.speed * cosA * dtS;
      s.x += s.speed * sinA * 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 deg = windGustAngle(nowMs);
    var rad = deg * Math.PI / 180;
    var lx = Math.sin(rad), ly = Math.cos(rad);
    ctx.clearRect(0, 0, cw, ch);
    for (var i = 0; i < streaks.length; i += 1) {
      var s = streaks[i];
      var len = 5 + s.speed * 0.03;
      var x0 = s.x * sx, y0 = s.y * sy;
      var x1 = (s.x - lx * len) * sx, y1 = (s.y - ly * len) * sy;
      ctx.strokeStyle = 'rgba(20,26,54,.7)';
      ctx.lineWidth = Math.max(0.6, s.thick * ((sx + sy) / 2));
      ctx.beginPath();
      ctx.moveTo(x0, y0);
      ctx.lineTo(x1, 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;
  if (reduce) root.classList.remove('is-demo');
  var warmSteps = reduce ? 130 : 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 "./RainEffectCanvasWindGustDrift.module.scss";

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

export default function RainEffectCanvasWindGustDrift() {
  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 WIND_GUST_MAX_DEG = 8;
    const RAIN_WIND_CYCLE_MS = 5200;
    const RAIN_STREAK_COUNT = 70;
    const RAIN_FALL_SPEED_PX_S = 130;

    function seedRand(seed: number) {
      let s = seed >>> 0;
      return function () { s = (s * 1664525 + 1013904223) >>> 0; return s / 4294967296; };
    }
    const rRand = seedRand(431022);

    function windGustAngle(nowMs: number) {
      const a = Math.sin((nowMs / RAIN_WIND_CYCLE_MS) * Math.PI * 2) * 0.62;
      const b = Math.sin((nowMs / (RAIN_WIND_CYCLE_MS * 1.73)) * Math.PI * 2 + 1.3) * 0.38;
      return (a + b) * WIND_GUST_MAX_DEG;
    }

    const streaks: Streak[] = [];
    for (let i = 0; i < RAIN_STREAK_COUNT; i += 1) {
      streaks.push({
        x: rRand() * CALC_W,
        y: rRand() * CALC_H,
        speed: RAIN_FALL_SPEED_PX_S * (0.6 + rRand() * 0.7),
        thick: 0.8 + rRand() * 1.4
      });
    }

    let elapsed = 0;
    function stepSim(dtS: number, nowMs: number) {
      const deg = windGustAngle(nowMs);
      const rad = (deg * Math.PI) / 180;
      const sinA = Math.sin(rad), cosA = Math.cos(rad);
      for (let i = 0; i < streaks.length; i += 1) {
        const s = streaks[i];
        s.y += s.speed * cosA * dtS;
        s.x += s.speed * sinA * 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 deg = windGustAngle(nowMs);
      const rad = (deg * Math.PI) / 180;
      const lx = Math.sin(rad), ly = Math.cos(rad);
      ctx!.clearRect(0, 0, cw, ch);
      for (let i = 0; i < streaks.length; i += 1) {
        const s = streaks[i];
        const len = 5 + s.speed * 0.03;
        const x0 = s.x * sx, y0 = s.y * sy;
        const x1 = (s.x - lx * len) * sx, y1 = (s.y - ly * len) * sy;
        ctx!.strokeStyle = "rgba(20,26,54,.7)";
        ctx!.lineWidth = Math.max(0.6, s.thick * ((sx + sy) / 2));
        ctx!.beginPath();
        ctx!.moveTo(x0, y0);
        ctx!.lineTo(x1, 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;
    if (reduce) root.classList.remove("is-demo");
    const warmSteps = reduce ? 130 : 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}>느리게 흔들리는 파형 값으로 바람의 세기와 각도를 계속 다시 계산해, 빗줄기 전체의 기울기와 낙하 속도가 같은 바람 값을 따라 함께 기운다</p>
      </div>
    </main>
  );
}