08傘の後ろでは隠れる雨筋

<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">傘型の境界の内側に雨筋の座標が入っているかを毎フレーム判定し、内側に入った部分だけ描かないことで傘が実際に雨を防いでいるように見える。</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: #eef1ef;
}

.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: #eef1ef;
}

.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: umbrellaOcclusion() 이 타원 돔 방정식으로 각 빗줄기 점이 우산 안인지 매 프레임 판정한다.
  // 안에 들면 그 줄기를 지우고 대신 UMBRELLA_BOUNCE_COUNT 개의 튐 방울을 돔 표면에서 바깥으로 쏘아
  // 포물선으로 떨어뜨린다(umbrellaBounce) — 우산 밖 줄기는 그냥 통과해 바닥까지 떨어진다.
  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 UMBRELLA_RADIUS_PX = 34;
  var UMBRELLA_HEIGHT_PX = 16;
  var UMBRELLA_BOUNCE_COUNT = 5;
  var UMBRELLA_SWAY_PX = 6;
  var UMBRELLA_SWAY_PERIOD_MS = 4200;
  var UMBRELLA_BOUNCE_GRAVITY = 240;
  var RAIN_STREAK_COUNT = 80;

  function seedRand(seed) {
    var s = seed >>> 0;
    return function () { s = (s * 1664525 + 1013904223) >>> 0; return s / 4294967296; };
  }
  var rRand = seedRand(431081);

  var umbY = 58, umbBaseX = CALC_W / 2;

  var streaks = [];
  for (var i = 0; i < RAIN_STREAK_COUNT; i += 1) {
    streaks.push({ x: rRand() * CALC_W, y: rRand() * CALC_H, speed: 110 + rRand() * 80, thick: 0.6 + rRand() * 1 });
  }
  var bounces = [];

  function umbrellaOcclusion(x, y, umbX) {
    var dx = (x - umbX) / UMBRELLA_RADIUS_PX;
    if (Math.abs(dx) > 1) return false;
    var edgeY = umbY - UMBRELLA_HEIGHT_PX * Math.sqrt(Math.max(0, 1 - dx * dx));
    return y >= edgeY && y <= umbY;
  }

  function umbrellaBounce(x, y, umbX) {
    var dx = (x - umbX) / UMBRELLA_RADIUS_PX;
    var nx = dx;
    for (var i = 0; i < UMBRELLA_BOUNCE_COUNT; i += 1) {
      var spread = (rRand() - 0.5) * 40;
      bounces.push({
        x: x,
        y: y,
        vx: nx * (40 + rRand() * 30) + spread,
        vy: -(30 + rRand() * 40),
        born: 0,
        life: 380 + rRand() * 200
      });
    }
  }

  function stepSim(dtS, nowMs) {
    var umbX = umbBaseX + Math.sin((nowMs / UMBRELLA_SWAY_PERIOD_MS) * Math.PI * 2) * UMBRELLA_SWAY_PX;
    for (var i = 0; i < streaks.length; i += 1) {
      var s = streaks[i];
      var prevY = s.y;
      s.y += s.speed * dtS;
      if (!(prevY >= umbY) && umbrellaOcclusion(s.x, s.y, umbX)) {
        umbrellaBounce(s.x, umbY - 1, umbX);
        s.y = -4 - rRand() * 10;
        s.x = rRand() * CALC_W;
      }
      if (s.y > CALC_H + 4) { s.y = -4 - rRand() * 6; s.x = rRand() * CALC_W; }
    }
    for (var b = bounces.length - 1; b >= 0; b -= 1) {
      var p = bounces[b];
      p.born += dtS * 1000;
      p.vy += UMBRELLA_BOUNCE_GRAVITY * dtS;
      p.x += p.vx * dtS;
      p.y += p.vy * dtS;
      if (p.born > p.life) bounces.splice(b, 1);
    }
  }

  var cw = 480, ch = 300, elapsedGlobal = 0;
  function draw(nowMs) {
    var sx = cw / CALC_W, sy = ch / CALC_H;
    var umbX = umbBaseX + Math.sin((nowMs / UMBRELLA_SWAY_PERIOD_MS) * Math.PI * 2) * UMBRELLA_SWAY_PX;
    ctx.clearRect(0, 0, cw, ch);
    ctx.fillStyle = '#eef1ef';
    ctx.fillRect(0, 0, cw, ch);

    ctx.strokeStyle = 'rgba(90,110,140,0.5)';
    for (var i = 0; i < streaks.length; i += 1) {
      var s = streaks[i];
      var x = s.x * sx, y = s.y * sy;
      ctx.lineWidth = Math.max(0.6, s.thick * ((sx + sy) / 2));
      ctx.beginPath();
      ctx.moveTo(x, y);
      ctx.lineTo(x, y - s.thick * 4 * sy);
      ctx.stroke();
    }

    for (var b = 0; b < bounces.length; b += 1) {
      var p = bounces[b];
      var f = 1 - p.born / p.life;
      ctx.fillStyle = 'rgba(90,110,140,' + Math.max(0, f * 0.8).toFixed(3) + ')';
      ctx.beginPath();
      ctx.arc(p.x * sx, p.y * sy, Math.max(0.8, 1.6 * f * ((sx + sy) / 2)), 0, Math.PI * 2);
      ctx.fill();
    }

    var cx = umbX * sx, cy = umbY * sy, rx = UMBRELLA_RADIUS_PX * sx, ry = UMBRELLA_HEIGHT_PX * sy;
    ctx.fillStyle = '#2c2430';
    ctx.beginPath();
    ctx.ellipse(cx, cy, rx, ry, 0, Math.PI, Math.PI * 2);
    ctx.lineTo(cx + rx, cy);
    ctx.closePath();
    ctx.fill();

    ctx.strokeStyle = 'rgba(255,255,255,0.18)';
    ctx.lineWidth = 1;
    for (var k = -2; k <= 2; k += 1) {
      var t = k / 2.4;
      var ex = cx + t * rx;
      var ey = cy - Math.sqrt(Math.max(0, 1 - t * t)) * ry;
      ctx.beginPath();
      ctx.moveTo(cx, cy - ry * 0.2);
      ctx.lineTo(ex, ey);
      ctx.stroke();
    }

    ctx.strokeStyle = '#2c2430';
    ctx.lineWidth = Math.max(1.2, 1.4 * ((sx + sy) / 2));
    ctx.beginPath();
    ctx.moveTo(cx, cy);
    ctx.lineTo(cx, cy + ry * 3.2);
    ctx.quadraticCurveTo(cx, cy + ry * 4.2, cx + rx * 0.22, cy + ry * 4);
    ctx.stroke();
  }

  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(elapsedGlobal);
  }

  var reduce = window.matchMedia('(prefers-reduced-motion: reduce)').matches;
  var elapsed = 0;
  if (reduce) root.classList.remove('is-demo');
  var warmSteps = reduce ? 30 : 45;
  for (var w = 0; w < warmSteps; w += 1) { elapsed += 20; stepSim(0.02, elapsed); }
  elapsedGlobal = 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;
      elapsedGlobal = elapsed;
      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 "./RainEffectCanvasUmbrellaOcclusion.module.scss";

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

export default function RainEffectCanvasUmbrellaOcclusion() {
  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 UMBRELLA_RADIUS_PX = 34;
    const UMBRELLA_HEIGHT_PX = 16;
    const UMBRELLA_BOUNCE_COUNT = 5;
    const UMBRELLA_SWAY_PX = 6;
    const UMBRELLA_SWAY_PERIOD_MS = 4200;
    const UMBRELLA_BOUNCE_GRAVITY = 240;
    const RAIN_STREAK_COUNT = 80;

    function seedRand(seed: number) {
      let s = seed >>> 0;
      return function () { s = (s * 1664525 + 1013904223) >>> 0; return s / 4294967296; };
    }
    const rRand = seedRand(431082);

    const umbY = 58, umbBaseX = CALC_W / 2;

    const streaks: Streak[] = [];
    for (let i = 0; i < RAIN_STREAK_COUNT; i += 1) {
      streaks.push({ x: rRand() * CALC_W, y: rRand() * CALC_H, speed: 110 + rRand() * 80, thick: 0.6 + rRand() * 1 });
    }
    const bounces: Bounce[] = [];

    function umbrellaOcclusion(x: number, y: number, umbX: number) {
      const dx = (x - umbX) / UMBRELLA_RADIUS_PX;
      if (Math.abs(dx) > 1) return false;
      const edgeY = umbY - UMBRELLA_HEIGHT_PX * Math.sqrt(Math.max(0, 1 - dx * dx));
      return y >= edgeY && y <= umbY;
    }

    function umbrellaBounce(x: number, y: number, umbX: number) {
      const dx = (x - umbX) / UMBRELLA_RADIUS_PX;
      const nx = dx;
      for (let i = 0; i < UMBRELLA_BOUNCE_COUNT; i += 1) {
        const spread = (rRand() - 0.5) * 40;
        bounces.push({
          x, y,
          vx: nx * (40 + rRand() * 30) + spread,
          vy: -(30 + rRand() * 40),
          born: 0,
          life: 380 + rRand() * 200,
        });
      }
    }

    function stepSim(dtS: number, nowMs: number) {
      const umbX = umbBaseX + Math.sin((nowMs / UMBRELLA_SWAY_PERIOD_MS) * Math.PI * 2) * UMBRELLA_SWAY_PX;
      for (let i = 0; i < streaks.length; i += 1) {
        const s = streaks[i];
        const prevY = s.y;
        s.y += s.speed * dtS;
        if (!(prevY >= umbY) && umbrellaOcclusion(s.x, s.y, umbX)) {
          umbrellaBounce(s.x, umbY - 1, umbX);
          s.y = -4 - rRand() * 10;
          s.x = rRand() * CALC_W;
        }
        if (s.y > CALC_H + 4) { s.y = -4 - rRand() * 6; s.x = rRand() * CALC_W; }
      }
      for (let b = bounces.length - 1; b >= 0; b -= 1) {
        const p = bounces[b];
        p.born += dtS * 1000;
        p.vy += UMBRELLA_BOUNCE_GRAVITY * dtS;
        p.x += p.vx * dtS;
        p.y += p.vy * dtS;
        if (p.born > p.life) bounces.splice(b, 1);
      }
    }

    let cw = 480, ch = 300, elapsedGlobal = 0;
    function draw(nowMs: number) {
      const sx = cw / CALC_W, sy = ch / CALC_H;
      const umbX = umbBaseX + Math.sin((nowMs / UMBRELLA_SWAY_PERIOD_MS) * Math.PI * 2) * UMBRELLA_SWAY_PX;
      ctx!.clearRect(0, 0, cw, ch);
      ctx!.fillStyle = "#eef1ef";
      ctx!.fillRect(0, 0, cw, ch);

      ctx!.strokeStyle = "rgba(90,110,140,0.5)";
      for (let i = 0; i < streaks.length; i += 1) {
        const s = streaks[i];
        const x = s.x * sx, y = s.y * sy;
        ctx!.lineWidth = Math.max(0.6, s.thick * ((sx + sy) / 2));
        ctx!.beginPath();
        ctx!.moveTo(x, y);
        ctx!.lineTo(x, y - s.thick * 4 * sy);
        ctx!.stroke();
      }

      for (let b = 0; b < bounces.length; b += 1) {
        const p = bounces[b];
        const f = 1 - p.born / p.life;
        ctx!.fillStyle = "rgba(90,110,140," + Math.max(0, f * 0.8).toFixed(3) + ")";
        ctx!.beginPath();
        ctx!.arc(p.x * sx, p.y * sy, Math.max(0.8, 1.6 * f * ((sx + sy) / 2)), 0, Math.PI * 2);
        ctx!.fill();
      }

      const cx = umbX * sx, cy = umbY * sy, rx = UMBRELLA_RADIUS_PX * sx, ry = UMBRELLA_HEIGHT_PX * sy;
      ctx!.fillStyle = "#2c2430";
      ctx!.beginPath();
      ctx!.ellipse(cx, cy, rx, ry, 0, Math.PI, Math.PI * 2);
      ctx!.lineTo(cx + rx, cy);
      ctx!.closePath();
      ctx!.fill();

      ctx!.strokeStyle = "rgba(255,255,255,0.18)";
      ctx!.lineWidth = 1;
      for (let k = -2; k <= 2; k += 1) {
        const t = k / 2.4;
        const ex = cx + t * rx;
        const ey = cy - Math.sqrt(Math.max(0, 1 - t * t)) * ry;
        ctx!.beginPath();
        ctx!.moveTo(cx, cy - ry * 0.2);
        ctx!.lineTo(ex, ey);
        ctx!.stroke();
      }

      ctx!.strokeStyle = "#2c2430";
      ctx!.lineWidth = Math.max(1.2, 1.4 * ((sx + sy) / 2));
      ctx!.beginPath();
      ctx!.moveTo(cx, cy);
      ctx!.lineTo(cx, cy + ry * 3.2);
      ctx!.quadraticCurveTo(cx, cy + ry * 4.2, cx + rx * 0.22, cy + ry * 4);
      ctx!.stroke();
      elapsedGlobal = nowMs;
    }

    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(elapsedGlobal);
    }

    const reduce = window.matchMedia("(prefers-reduced-motion: reduce)").matches;
    let elapsed = 0;
    if (reduce) root.classList.remove("is-demo");
    const warmSteps = reduce ? 30 : 45;
    for (let w = 0; w < warmSteps; w += 1) { elapsed += 20; stepSim(0.02, elapsed); }
    elapsedGlobal = 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;
        elapsedGlobal = elapsed;
        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}>우산 모양의 경계 안에 빗줄기 좌표가 들어오는지 매 프레임 판정해, 경계 안쪽에 들어온 부분만 그리지 않아 우산이 실제로 비를 막는 것처럼 보인다</p>
      </div>
    </main>
  );
}