03Window Droplets Trickling Down and Merging Into Each Other

<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">Each droplet has its weight and drag calculated to reset its trickle speed, and when its path overlaps another droplet the two merge into one, growing larger.</p>
</div>
</main>

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

.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-orange;
}

.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, .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: #ff4d1f;
  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: #ff4d1f;
}

.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, 0.85);
  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: 물방울마다 질량(mass)을 갖고, 매 프레임 무게 대 저항(DROPLET_DRAG_COEF) 비율로
  // 목표 속도를 다시 계산해 그쪽으로 서서히 붙는다(lerp). dropletMassMerge() 가 겹친 두
  // 물방울의 질량을 합쳐 반지름을 키우고 하나로 줄인다 — 무거워질수록 더 빨리 흘러내린다.
  // 하이라이트는 물방울 왼쪽 위에 밝은 점을 얹어 굴절된 빛처럼 보이게 한다.
  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 DROPLET_DRAG_COEF = 3.2;
  var DROPLET_GRAVITY_PX_S = 58;
  var DROPLET_MERGE_DIST_PX = 2.4;
  var DROPLET_SPAWN_PER_S = 2.6;

  function seedRand(seed) {
    var s = seed >>> 0;
    return function () { s = (s * 1664525 + 1013904223) >>> 0; return s / 4294967296; };
  }
  var dRand = seedRand(431031);

  var drops = [];
  var spawnAcc = 0;

  function spawnDroplet() {
    var r = 1.6 + dRand() * 1.6;
    drops.push({
      x: dRand() * CALC_W,
      y: -3,
      r: r,
      mass: r * r,
      vy: 4,
      seed: dRand() * 1000
    });
  }

  function dropletMassMerge(a, b) {
    var mass = a.mass + b.mass;
    var r = Math.sqrt(mass) * 0.92;
    return {
      x: (a.x * a.mass + b.x * b.mass) / mass,
      y: Math.max(a.y, b.y),
      r: r, mass: mass, vy: Math.max(a.vy, b.vy), seed: a.seed
    };
  }

  function stepSim(dtMs, nowMs) {
    var dtS = dtMs / 1000;
    spawnAcc += dtMs * DROPLET_SPAWN_PER_S / 1000;
    while (spawnAcc >= 1 && drops.length < 46) { spawnDroplet(); spawnAcc -= 1; }
    for (var i = 0; i < drops.length; i += 1) {
      var d = drops[i];
      var targetVy = (DROPLET_GRAVITY_PX_S * Math.sqrt(d.mass)) / DROPLET_DRAG_COEF;
      d.vy += (targetVy - d.vy) * Math.min(1, 5 * dtS);
      d.y += d.vy * dtS;
      d.x += Math.sin(nowMs * 0.0012 + d.seed) * 1.6 * dtS;
    }
    // 병합 — 가까운 두 물방울이 겹치면 하나로
    for (var a = 0; a < drops.length; a += 1) {
      for (var b = a + 1; b < drops.length; b += 1) {
        var p1 = drops[a], p2 = drops[b];
        if (!p1 || !p2) continue;
        var dx = p1.x - p2.x, dy = p1.y - p2.y;
        var dist = Math.sqrt(dx * dx + dy * dy);
        if (dist < DROPLET_MERGE_DIST_PX + (p1.r + p2.r) * 0.55) {
          drops[a] = dropletMassMerge(p1, p2);
          drops.splice(b, 1);
          b -= 1;
        }
      }
    }
    var alive = [];
    for (var k = 0; k < drops.length; k += 1) {
      if (drops[k].y - drops[k].r < CALC_H + 6) alive.push(drops[k]);
    }
    drops = alive;
  }

  function draw() {
    var sx = cw / CALC_W, sy = ch / CALC_H;
    var s = (sx + sy) / 2;
    ctx.clearRect(0, 0, cw, ch);
    for (var i = 0; i < drops.length; i += 1) {
      var d = drops[i];
      var dx = d.x * sx, dy = d.y * sy, dr = d.r * s;
      ctx.fillStyle = 'rgba(15,24,52,.42)';
      ctx.beginPath();
      ctx.arc(dx, dy, dr, 0, Math.PI * 2);
      ctx.fill();
      var g = ctx.createRadialGradient(dx - dr * 0.35, dy - dr * 0.4, 0, dx - dr * 0.35, dy - dr * 0.4, dr * 0.7);
      g.addColorStop(0, 'rgba(255,255,255,.85)');
      g.addColorStop(1, 'rgba(255,255,255,0)');
      ctx.fillStyle = g;
      ctx.beginPath();
      ctx.arc(dx - dr * 0.35, dy - dr * 0.4, dr * 0.55, 0, Math.PI * 2);
      ctx.fill();
      ctx.strokeStyle = 'rgba(255,255,255,1)';
      ctx.beginPath();
      ctx.moveTo(dx - dr * 0.1, dy - dr * 1.3);
      ctx.lineTo(dx - dr * 0.1, dy - dr * (1.3 - Math.min(0.7, dr * 0.05)));
      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();
  }

  var reduce = window.matchMedia('(prefers-reduced-motion: reduce)').matches;
  var elapsed = 0;
  if (reduce) root.classList.remove('is-demo');
  var warmSteps = reduce ? 180 : 90;
  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();
    }
    requestAnimationFrame(tick);
  }
  fit();
  window.addEventListener('resize', fit);
  draw();
  requestAnimationFrame(tick);
})();

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

type Droplet = { x: number; y: number; r: number; mass: number; vy: number; seed: number };

export default function RainEffectCanvasWindowDropletTrickle() {
  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 DROPLET_DRAG_COEF = 3.2;
    const DROPLET_GRAVITY_PX_S = 58;
    const DROPLET_MERGE_DIST_PX = 2.4;
    const DROPLET_SPAWN_PER_S = 2.6;

    function seedRand(seed: number) {
      let s = seed >>> 0;
      return function () { s = (s * 1664525 + 1013904223) >>> 0; return s / 4294967296; };
    }
    const dRand = seedRand(431032);

    let drops: Droplet[] = [];
    let spawnAcc = 0;

    function spawnDroplet() {
      const r = 1.6 + dRand() * 1.6;
      drops.push({ x: dRand() * CALC_W, y: -3, r, mass: r * r, vy: 4, seed: dRand() * 1000 });
    }

    function dropletMassMerge(a: Droplet, b: Droplet): Droplet {
      const mass = a.mass + b.mass;
      const r = Math.sqrt(mass) * 0.92;
      return {
        x: (a.x * a.mass + b.x * b.mass) / mass,
        y: Math.max(a.y, b.y),
        r, mass, vy: Math.max(a.vy, b.vy), seed: a.seed
      };
    }

    function stepSim(dtMs: number, nowMs: number) {
      const dtS = dtMs / 1000;
      spawnAcc += (dtMs * DROPLET_SPAWN_PER_S) / 1000;
      while (spawnAcc >= 1 && drops.length < 46) { spawnDroplet(); spawnAcc -= 1; }
      for (let i = 0; i < drops.length; i += 1) {
        const d = drops[i];
        const targetVy = (DROPLET_GRAVITY_PX_S * Math.sqrt(d.mass)) / DROPLET_DRAG_COEF;
        d.vy += (targetVy - d.vy) * Math.min(1, 5 * dtS);
        d.y += d.vy * dtS;
        d.x += Math.sin(nowMs * 0.0012 + d.seed) * 1.6 * dtS;
      }
      for (let a = 0; a < drops.length; a += 1) {
        for (let b = a + 1; b < drops.length; b += 1) {
          const p1 = drops[a], p2 = drops[b];
          if (!p1 || !p2) continue;
          const dx = p1.x - p2.x, dy = p1.y - p2.y;
          const dist = Math.sqrt(dx * dx + dy * dy);
          if (dist < DROPLET_MERGE_DIST_PX + (p1.r + p2.r) * 0.55) {
            drops[a] = dropletMassMerge(p1, p2);
            drops.splice(b, 1);
            b -= 1;
          }
        }
      }
      drops = drops.filter((d) => d.y - d.r < CALC_H + 6);
    }

    let cw = 480, ch = 300;
    function draw() {
      const sx = cw / CALC_W, sy = ch / CALC_H;
      const s = (sx + sy) / 2;
      ctx!.clearRect(0, 0, cw, ch);
      for (let i = 0; i < drops.length; i += 1) {
        const d = drops[i];
        const dx = d.x * sx, dy = d.y * sy, dr = d.r * s;
        ctx!.fillStyle = "rgba(15,24,52,.42)";
        ctx!.beginPath();
        ctx!.arc(dx, dy, dr, 0, Math.PI * 2);
        ctx!.fill();
        const g = ctx!.createRadialGradient(dx - dr * 0.35, dy - dr * 0.4, 0, dx - dr * 0.35, dy - dr * 0.4, dr * 0.7);
        g.addColorStop(0, "rgba(255,255,255,.85)");
        g.addColorStop(1, "rgba(255,255,255,0)");
        ctx!.fillStyle = g;
        ctx!.beginPath();
        ctx!.arc(dx - dr * 0.35, dy - dr * 0.4, dr * 0.55, 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();
    }

    const reduce = window.matchMedia("(prefers-reduced-motion: reduce)").matches;
    let elapsed = 0;
    if (reduce) root.classList.remove("is-demo");
    const warmSteps = reduce ? 180 : 90;
    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();
      }
      raf = requestAnimationFrame(tick);
    }
    fit();
    window.addEventListener("resize", fit);
    draw();
    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}>물방울마다 무게와 저항을 계산해 흘러내리는 속도를 다시 정하고, 지나가는 길에 다른 물방울과 겹치면 둘을 하나로 합쳐 크기를 키운다</p>
      </div>
    </main>
  );
}