04Splash Bursts Where Each Raindrop Lands

<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">The instant a raindrop hits the ground, it splits into several splash particles with randomized direction and speed, and gravity is re-added to each one every frame so they scatter in arcs.</p>
</div>
</main>

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

.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-paper;
}

.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(23, 20, 26, .78);
  color: $subject-cream;
  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: #ffffff;
  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: #ffffff;
}

.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(23, 20, 26, 0.78);
  color: #fff7e6;
  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: 빗줄기가 바닥선(GROUND_Y)에 닿는 순간 splashBurst() 가 SPLASH_PARTICLE_COUNT 개
  // 입자를 방향·속도를 나눠 흩뿌리고, 그 입자마다 SPLASH_GRAVITY_PX_S2 를 매 프레임 다시 더해
  // 포물선을 그리며 옆으로 흩어지다 옅어진다. 속성 트윈이 아니라 매 프레임 중력 적분이다.
  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 GROUND_Y = 88;
  var SPLASH_PARTICLE_COUNT = 6;
  var SPLASH_GRAVITY_PX_S2 = 260;
  var SPLASH_LIFE_MS = 420;
  var RAIN_STREAK_COUNT = 46;

  function seedRand(seed) {
    var s = seed >>> 0;
    return function () { s = (s * 1664525 + 1013904223) >>> 0; return s / 4294967296; };
  }
  var rRand = seedRand(431041);
  var sRand = seedRand(431042);

  var streaks = [];
  for (var i = 0; i < RAIN_STREAK_COUNT; i += 1) {
    streaks.push({
      x: rRand() * CALC_W,
      y: rRand() * GROUND_Y,
      speed: 120 + rRand() * 60,
      thick: 0.8 + rRand() * 1.2
    });
  }

  var splashes = [];
  function splashBurst(x, nowMs) {
    for (var i = 0; i < SPLASH_PARTICLE_COUNT; i += 1) {
      var ang = Math.PI + sRand() * Math.PI; // 위쪽 반원으로 퍼진다
      var speed = 16 + sRand() * 26;
      splashes.push({
        x: x, y: GROUND_Y,
        vx: Math.cos(ang) * speed,
        vy: Math.sin(ang) * speed,
        born: nowMs
      });
    }
  }

  function stepSim(dtMs, nowMs) {
    var dtS = dtMs / 1000;
    for (var i = 0; i < streaks.length; i += 1) {
      var s = streaks[i];
      s.y += s.speed * dtS;
      if (s.y >= GROUND_Y) {
        splashBurst(s.x, nowMs);
        s.y = -4 - rRand() * 10;
        s.x = rRand() * CALC_W;
      }
    }
    var alive = [];
    for (var j = 0; j < splashes.length; j += 1) {
      var p = splashes[j];
      p.vy += SPLASH_GRAVITY_PX_S2 * dtS;
      p.x += p.vx * dtS;
      p.y += p.vy * dtS;
      if (nowMs - p.born < SPLASH_LIFE_MS) alive.push(p);
    }
    splashes = alive;
  }

  function draw(nowMs) {
    var sx = cw / CALC_W, sy = ch / CALC_H;
    var sc = (sx + sy) / 2;
    ctx.clearRect(0, 0, cw, ch);
    ctx.strokeStyle = 'rgba(20,26,54,.22)';
    ctx.lineWidth = Math.max(1, 1.4 * sc);
    ctx.beginPath();
    ctx.moveTo(0, GROUND_Y * sy);
    ctx.lineTo(cw, GROUND_Y * sy);
    ctx.stroke();
    for (var i = 0; i < streaks.length; i += 1) {
      var s = streaks[i];
      if (s.y < 0 || s.y > GROUND_Y) continue;
      var x = s.x * sx, y0 = s.y * sy, y1 = (s.y - s.thick * 5) * sy;
      ctx.strokeStyle = 'rgba(20,26,54,.62)';
      ctx.lineWidth = Math.max(0.6, s.thick * sc);
      ctx.beginPath();
      ctx.moveTo(x, y0);
      ctx.lineTo(x, y1);
      ctx.stroke();
    }
    for (var k = 0; k < splashes.length; k += 1) {
      var p = splashes[k];
      var f = Math.max(0, Math.min(1, (nowMs - p.born) / SPLASH_LIFE_MS));
      var alpha = (1 - f) * 0.75;
      var dx = p.x * sx, dy = p.y * sy, dr = (0.7 + (1 - f) * 0.7) * sc;
      ctx.fillStyle = 'rgba(20,26,54,' + alpha + ')';
      ctx.beginPath();
      ctx.arc(dx, dy, dr, 0, Math.PI * 2);
      ctx.fill();
    }
  }

  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 = 0;
  if (reduce) root.classList.remove('is-demo');
  var warmSteps = reduce ? 60 : 20;
  for (var w = 0; w < warmSteps; w += 1) { elapsed += 20; stepSim(20, 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, elapsed);
      draw(elapsed);
    }
    requestAnimationFrame(tick);
  }
  fit();
  window.addEventListener('resize', fit);
  draw(elapsed);
  requestAnimationFrame(tick);
})();

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

type Streak = { x: number; y: number; speed: number; thick: number };
type Splash = { x: number; y: number; vx: number; vy: number; born: number };

export default function RainEffectCanvasGroundSplashPhysics() {
  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 GROUND_Y = 88;
    const SPLASH_PARTICLE_COUNT = 6;
    const SPLASH_GRAVITY_PX_S2 = 260;
    const SPLASH_LIFE_MS = 420;
    const RAIN_STREAK_COUNT = 46;

    function seedRand(seed: number) {
      let s = seed >>> 0;
      return function () { s = (s * 1664525 + 1013904223) >>> 0; return s / 4294967296; };
    }
    const rRand = seedRand(431043);
    const sRand = seedRand(431044);

    const streaks: Streak[] = [];
    for (let i = 0; i < RAIN_STREAK_COUNT; i += 1) {
      streaks.push({ x: rRand() * CALC_W, y: rRand() * GROUND_Y, speed: 120 + rRand() * 60, thick: 0.8 + rRand() * 1.2 });
    }

    let splashes: Splash[] = [];
    function splashBurst(x: number, nowMs: number) {
      for (let i = 0; i < SPLASH_PARTICLE_COUNT; i += 1) {
        const ang = Math.PI + sRand() * Math.PI;
        const speed = 16 + sRand() * 26;
        splashes.push({ x, y: GROUND_Y, vx: Math.cos(ang) * speed, vy: Math.sin(ang) * speed, born: nowMs });
      }
    }

    function stepSim(dtMs: number, nowMs: number) {
      const dtS = dtMs / 1000;
      for (let i = 0; i < streaks.length; i += 1) {
        const s = streaks[i];
        s.y += s.speed * dtS;
        if (s.y >= GROUND_Y) {
          splashBurst(s.x, nowMs);
          s.y = -4 - rRand() * 10;
          s.x = rRand() * CALC_W;
        }
      }
      const alive: Splash[] = [];
      for (let j = 0; j < splashes.length; j += 1) {
        const p = splashes[j];
        p.vy += SPLASH_GRAVITY_PX_S2 * dtS;
        p.x += p.vx * dtS;
        p.y += p.vy * dtS;
        if (nowMs - p.born < SPLASH_LIFE_MS) alive.push(p);
      }
      splashes = alive;
    }

    let cw = 480, ch = 300;
    function draw(nowMs: number) {
      const sx = cw / CALC_W, sy = ch / CALC_H;
      const sc = (sx + sy) / 2;
      ctx!.clearRect(0, 0, cw, ch);
      ctx!.strokeStyle = "rgba(20,26,54,.22)";
      ctx!.lineWidth = Math.max(1, 1.4 * sc);
      ctx!.beginPath();
      ctx!.moveTo(0, GROUND_Y * sy);
      ctx!.lineTo(cw, GROUND_Y * sy);
      ctx!.stroke();
      for (let i = 0; i < streaks.length; i += 1) {
        const s = streaks[i];
        if (s.y < 0 || s.y > GROUND_Y) continue;
        const x = s.x * sx, y0 = s.y * sy, y1 = (s.y - s.thick * 5) * sy;
        ctx!.strokeStyle = "rgba(20,26,54,.62)";
        ctx!.lineWidth = Math.max(0.6, s.thick * sc);
        ctx!.beginPath();
        ctx!.moveTo(x, y0);
        ctx!.lineTo(x, y1);
        ctx!.stroke();
      }
      for (let k = 0; k < splashes.length; k += 1) {
        const p = splashes[k];
        const f = Math.max(0, Math.min(1, (nowMs - p.born) / SPLASH_LIFE_MS));
        const alpha = (1 - f) * 0.75;
        const dx = p.x * sx, dy = p.y * sy, dr = (0.7 + (1 - f) * 0.7) * sc;
        ctx!.fillStyle = "rgba(20,26,54," + alpha + ")";
        ctx!.beginPath();
        ctx!.arc(dx, dy, dr, 0, Math.PI * 2);
        ctx!.fill();
      }
    }

    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 = 0;
    if (reduce) root.classList.remove("is-demo");
    const warmSteps = reduce ? 60 : 20;
    for (let w = 0; w < warmSteps; w += 1) { elapsed += 20; stepSim(20, 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, 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}>빗방울이 바닥에 닿는 순간 방향과 속도를 무작위로 나눠 튀는 입자 여러 개를 만들고, 그 입자마다 중력을 매 프레임 다시 더해 포물선을 그리며 흩어진다</p>
      </div>
    </main>
  );
}