01Rain Streaks Parallaxing at a Different Thickness and Speed Per Depth Layer

<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 rain streak gets a virtual distance that sets its thickness and fall speed inversely — near streaks fall thick and fast, far ones thin and slow.</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: 빗줄기마다 층(RAIN_LAYER_COUNT)을 매겨 가상의 거리(depth z, 0=멀다 1=가깝다)를 부여한다.
  // 굵기·속도·불투명도를 모두 z 에 비례해 정하므로 가까운 줄기는 굵고 빠르고 진하게, 먼 줄기는
  // 가늘고 느리고 옅게 떨어진다 — 거리 하나가 세 가지 값을 동시에 결정하는 시차(parallax) 계산.
  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 RAIN_LAYER_COUNT = 4;
  var RAIN_NEAR_SPEED_PX_S = 150;
  var RAIN_FAR_SPEED_PX_S = 40;

  function seedRand(seed) {
    var s = seed >>> 0;
    return function () { s = (s * 1664525 + 1013904223) >>> 0; return s / 4294967296; };
  }
  var rRand = seedRand(431011);

  function streakDepthZ(layerIndex) {
    return RAIN_LAYER_COUNT <= 1 ? 1 : layerIndex / (RAIN_LAYER_COUNT - 1);
  }

  var streaks = [];
  for (var li = 0; li < RAIN_LAYER_COUNT; li += 1) {
    var z = streakDepthZ(li);
    var count = Math.round(32 - z * 18);
    var speed = RAIN_FAR_SPEED_PX_S + (RAIN_NEAR_SPEED_PX_S - RAIN_FAR_SPEED_PX_S) * z;
    var thick = 0.5 + z * 2.1;
    var alpha = 0.28 + z * 0.62;
    for (var i = 0; i < count; i += 1) {
      streaks.push({
        x: rRand() * CALC_W,
        y: rRand() * CALC_H,
        z: z, speed: speed, thick: thick, alpha: alpha,
        len: 4 + z * 12
      });
    }
  }
  streaks.sort(function (a, b) { return a.z - b.z; });

  function stepSim(dtS) {
    for (var i = 0; i < streaks.length; i += 1) {
      var s = streaks[i];
      s.y += s.speed * dtS;
      if (s.y - s.len > CALC_H + 4) {
        s.y = -4 - rRand() * 10;
        s.x = rRand() * CALC_W;
      }
    }
  }

  function draw() {
    var sx = cw / CALC_W, sy = ch / CALC_H;
    ctx.clearRect(0, 0, cw, ch);
    for (var i = 0; i < streaks.length; i += 1) {
      var s = streaks[i];
      var x = s.x * sx;
      var y0 = s.y * sy, y1 = (s.y - s.len) * sy;
      ctx.strokeStyle = 'rgba(214,232,255,' + s.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();
  }

  var reduce = window.matchMedia('(prefers-reduced-motion: reduce)').matches;
  if (reduce) root.classList.remove('is-demo');
  var warmSteps = reduce ? 90 : 40;
  for (var w = 0; w < warmSteps; w += 1) stepSim(0.02);

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

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

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

export default function RainEffectCanvasStreakDepthParallax() {
  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 RAIN_LAYER_COUNT = 4;
    const RAIN_NEAR_SPEED_PX_S = 150;
    const RAIN_FAR_SPEED_PX_S = 40;

    function seedRand(seed: number) {
      let s = seed >>> 0;
      return function () { s = (s * 1664525 + 1013904223) >>> 0; return s / 4294967296; };
    }
    const rRand = seedRand(431012);

    function streakDepthZ(layerIndex: number) {
      return RAIN_LAYER_COUNT <= 1 ? 1 : layerIndex / (RAIN_LAYER_COUNT - 1);
    }

    const streaks: Streak[] = [];
    for (let li = 0; li < RAIN_LAYER_COUNT; li += 1) {
      const z = streakDepthZ(li);
      const count = Math.round(32 - z * 18);
      const speed = RAIN_FAR_SPEED_PX_S + (RAIN_NEAR_SPEED_PX_S - RAIN_FAR_SPEED_PX_S) * z;
      const thick = 0.5 + z * 2.1;
      const alpha = 0.28 + z * 0.62;
      for (let i = 0; i < count; i += 1) {
        streaks.push({
          x: rRand() * CALC_W,
          y: rRand() * CALC_H,
          z, speed, thick, alpha,
          len: 4 + z * 12
        });
      }
    }
    streaks.sort((a, b) => a.z - b.z);

    function stepSim(dtS: number) {
      for (let i = 0; i < streaks.length; i += 1) {
        const s = streaks[i];
        s.y += s.speed * dtS;
        if (s.y - s.len > CALC_H + 4) {
          s.y = -4 - rRand() * 10;
          s.x = rRand() * CALC_W;
        }
      }
    }

    let cw = 480, ch = 300;
    function draw() {
      const sx = cw / CALC_W, sy = ch / CALC_H;
      ctx!.clearRect(0, 0, cw, ch);
      for (let i = 0; i < streaks.length; i += 1) {
        const s = streaks[i];
        const x = s.x * sx;
        const y0 = s.y * sy, y1 = (s.y - s.len) * sy;
        ctx!.strokeStyle = "rgba(214,232,255," + s.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();
    }

    const reduce = window.matchMedia("(prefers-reduced-motion: reduce)").matches;
    if (reduce) root.classList.remove("is-demo");
    const warmSteps = reduce ? 90 : 40;
    for (let w = 0; w < warmSteps; w += 1) stepSim(0.02);

    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) {
        stepSim(dt / 1000);
        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}>빗줄기마다 가상의 거리를 부여해 굵기와 떨어지는 속도를 거리에 반비례하게 정하고, 가까운 줄기는 굵고 빠르게, 먼 줄기는 가늘고 느리게 떨어진다</p>
      </div>
    </main>
  );
}