02Depth attenuation — shallow-water cross-section

<main class="stage">
<div class="da" id="da">
  <p class="da__hint">
    <svg class="da__ico" viewBox="0 0 256 256" aria-hidden="true"><g fill="currentColor"><path d="M16 120c24-32 48-32 72 0s48 32 72 0 48-32 72 0" opacity=".25"/><path d="M16 168c24-32 48-32 72 0s48 32 72 0 48-32 72 0" opacity=".15"/><path d="M228.92 49.09a112 112 0 1 0 14 84.91a8 8 0 0 0-15.5-4a96 96 0 1 1-12-72.91A8 8 0 0 1 216 71.63V40a8 8 0 0 1-8-8h-31.63a8 8 0 0 1-5.65-13.09ZM64 104a12 12 0 1 1 12 12a12 12 0 0 1-12-12m40 64a12 12 0 1 1 12 12a12 12 0 0 1-12-12m72-16a16 16 0 1 1 16 16a16 16 0 0 1-16-16" opacity=".9"/></g></svg>
    <span>Every pixel below the surface attenuates by depth exponentially — a shallow-sea cross-section computed from scratch; move the mouse to stir the water</span>
  </p>
  <div class="da__field" id="field" role="application" aria-label="Depth attenuation cross-section playground" tabindex="0">
    <canvas class="da__cv" id="cv"></canvas>
    <b class="da__badge" aria-hidden="true"><span class="da__badgeNum" id="attenVal">depth x0.06</span><span class="da__badgeUnit" id="windVal">wind 1.0</span></b>
  </div>
</div>
</main>

@use "../../../../site/static/demo-tokens" as *;

// 깊이별 감쇠 — 1D 수면 곡선 아래 픽셀을 수심 지수 감쇠로 물들인다. 움직임은 전부 캔버스 rAF 안에 있다.
// @keyframes 를 두지 않는다(077 방식). 색은 전부 토큰 또는 mix() 로컬 변수(BD26) — 헥스 리터럴 금지.
// 캔버스가 쓰는 색은 커스텀 프로퍼티로 내려보낸다 — 값은 이 파일 한 곳에만 둔다.

$da-c0: mix($subject-mint, $stage-paper, 72%);        // 얕음 — 민트
$da-c1: mix($subject-sky,  $stage-paper, 66%);        // 얕음 보조 — 하늘 민트
$da-t1: mix($subject-blue, $stage-paper, 58%);        // 깊음 보조 — 파랑
$da-t0: mix($subject-blue-deep, $stage-paper, 52%);   // 깊음 — 진한 파랑
$da-sand: mix($subject-cream, $stage-paper, 88%);     // 바닥 모래 — 크림
$da-sky: mix($subject-sky, $stage-paper, 22%);        // 수면 위 배경 — 옅은 하늘

$duration: $dur-loop;
$easing: $ease-spring;
$color: $da-c0;

@include stage($stage-paper);

.da {
  --c0: #{$da-c0};
  --c1: #{$da-c1};
  --t1: #{$da-t1};
  --t0: #{$da-t0};
  --sand: #{$da-sand};

  display: flex; flex-direction: column; gap: $sp-2;
  width: 100%; max-width: 424px;
}

.da__hint {
  display: flex; align-items: center; gap: $sp-1; margin: 0;
  font-weight: 700; font-size: clamp(10px, 2.8vw, 12px); line-height: 1.35;
  color: $subject-ink;
}
.da__ico { width: 16px; height: 16px; flex: 0 0 auto; color: $subject-blue; }

.da__field {
  position: relative; height: 208px;
  border-radius: $r-card; overflow: hidden;
  background: $da-sky;
  box-shadow: inset 0 0 0 1px rgba($subject-ink, .16);
  cursor: crosshair; touch-action: none;
}
.da__field:focus-visible { outline: 2px solid $subject-blue; outline-offset: 2px; }

.da__cv { display: block; width: 100%; height: 100%; }

.da__badge {
  position: absolute; right: $sp-2; top: $sp-2;
  display: flex; align-items: baseline; gap: $sp-1;
  padding: $sp-1 $sp-2; border-radius: $r-pill;
  background: rgba($subject-ink, .78);
  color: $subject-cream; font-weight: 700; font-size: 11px;
}
.da__badgeNum { font-size: 13px; color: $stage-yellow; }
.da__badgeUnit { font-weight: 700; }

@media (max-width: 340px) {
  .da { gap: $sp-1; }
  .da__field { height: 124px; }
  .da__badge { font-size: 10px; padding: 0 $sp-1; }
  .da__badgeNum { font-size: 11px; }
  .da__hint { font-size: 10px; }
}

// reduce — 루프가 아예 안 돈다. 파도가 살아 있는 정적 한 장이 남으므로
// 끌 수 있다는 신호(커서)만 거둔다.
@media (prefers-reduced-motion: reduce) {
  .da.is-still .da__field { cursor: default !important; }
  .da.is-still .da__badge { opacity: .92 !important; }
}

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);
}

.da {
  --c0: rgb(49.4588235294%, 87.8588235294%, 74.8705882353%);
  --c1: rgb(62.4705882353%, 85.7647058824%, 100%);
  --t1: rgb(52.6901960784%, 66.7921568627%, 97.9529411765%);
  --t0: rgb(50.0392156863%, 64.1098039216%, 95.3098039216%);
  --sand: rgb(100%, 97.2392156863%, 91.3725490196%);
  display: flex;
  flex-direction: column;
  gap: 8px;
  width: 100%;
  max-width: 424px;
}

.da__hint {
  display: flex;
  align-items: center;
  gap: 4px;
  margin: 0;
  font-weight: 700;
  font-size: clamp(10px, 2.8vw, 12px);
  line-height: 1.35;
  color: #17141a;
}

.da__ico {
  width: 16px;
  height: 16px;
  flex: 0 0 auto;
  color: #2f6df6;
}

.da__field {
  position: relative;
  height: 208px;
  border-radius: 16px;
  overflow: hidden;
  background: rgb(87.4901960784%, 95.2549019608%, 100%);
  box-shadow: inset 0 0 0 1px rgba(23, 20, 26, 0.16);
  cursor: crosshair;
  touch-action: none;
}

.da__field:focus-visible {
  outline: 2px solid #2f6df6;
  outline-offset: 2px;
}

.da__cv {
  display: block;
  width: 100%;
  height: 100%;
}

.da__badge {
  position: absolute;
  right: 8px;
  top: 8px;
  display: flex;
  align-items: baseline;
  gap: 4px;
  padding: 4px 8px;
  border-radius: 999px;
  background: rgba(23, 20, 26, 0.78);
  color: #fff7e6;
  font-weight: 700;
  font-size: 11px;
}

.da__badgeNum {
  font-size: 13px;
  color: #ffd23f;
}

.da__badgeUnit {
  font-weight: 700;
}

@media (max-width: 340px) {
  .da {
    gap: 4px;
  }
  .da__field {
    height: 124px;
  }
  .da__badge {
    font-size: 10px;
    padding: 0 4px;
  }
  .da__badgeNum {
    font-size: 11px;
  }
  .da__hint {
    font-size: 10px;
  }
}
@media (prefers-reduced-motion: reduce) {
  .da.is-still .da__field {
    cursor: default !important;
  }
  .da.is-still .da__badge {
    opacity: 0.92 !important;
  }
}

// 깊이별 감쇠 — 얕은 물빛 단면. 01 과 같은 뿌리(정현파 6개 mulberry32(7))를 1D 로 읽어
// 수면선 높이를 구하고, 그 선 아래 픽셀마다 수심 d = y - surfaceY 에 exp(-감쇠계수*d) 를
// 적용해 민트(얕음)→라일락 하늘빛→파랑→진한 파랑(깊음) 4단 램프로 섞는다. 바닥 20px 은 크림 모래.
// 감쇠계수 자체도 바람이 세질수록 커져(물이 흐려져) 얕은 색이 더 빨리 사라진다.
var root = document.getElementById('da');
var field = document.getElementById('field');
var cv = document.getElementById('cv');
var attenVal = document.getElementById('attenVal');
var windVal = document.getElementById('windVal');
var ctx = cv.getContext('2d');

var css = getComputedStyle(root);
function v(name) { return css.getPropertyValue(name).trim(); }
var C0 = v('--c0'), C1 = v('--c1'); // 얕음 — 민트 → 하늘
var T1 = v('--t1'), T0 = v('--t0'); // 깊음 — 파랑 → 진한 파랑
var SAND = v('--sand');

var COLS = 160;   // 1D 수면 곡선 표본 수 — 가로 해상도
var STEP = 3;     // 격자 한 칸이 렌더 버퍼에서 차지하는 픽셀 → RW = 480
var NW = 6;       // 겹칠 정현파 개수(공통 물리)
var SAND_PX = 20; // 바닥 모래 두께(버퍼 기준 px)

function mulberry32(a) {
  return function () {
    a |= 0; a = (a + 0x6D2B79F5) | 0;
    var t = Math.imul(a ^ (a >>> 15), 1 | a);
    var u = (t + Math.imul(t ^ (t >>> 7), 61 | t)) ^ t;
    return ((u ^ (u >>> 14)) >>> 0) / 4294967296;
  };
}
var rng = mulberry32(7);

var WAVES = [];
(function () {
  var lam = 34;
  for (var i = 0; i < NW; i++) {
    var ang = rng() * Math.PI * 2;
    WAVES.push({
      lam: lam,
      freq: (Math.PI * 2) / lam,
      dirX: Math.cos(ang), dirY: Math.sin(ang),
      amp: 0.58 / (i * 0.62 + 1),
      spd: 0.55 + rng() * 0.25,
      ph: rng() * Math.PI * 2
    });
    lam *= 0.55;
  }
})();
for (var w = 0; w < NW; w++) {
  WAVES[w].om = WAVES[w].spd * 3.6 * Math.sqrt(1 / WAVES[w].lam) * 6;
}
var AMP_SUM = 0;
for (var w2 = 0; w2 < NW; w2++) AMP_SUM += WAVES[w2].amp;

var reduce = window.matchMedia('(prefers-reduced-motion: reduce)').matches;
var W = 0, H = 0, dpr = 1;
var surf = null;              // 1D 수면 높이(버퍼 px, Float32Array)
var buf = null, bctx = null, out = null;
var RW = 480, RH = 160;
var raf = 0, auto = true, nowSec = 0;
var wind = 1.0, windT = 1.0;
var windAng = 0.5, windAngT = 0.5;
var lastPX = -1, lastPY = -1, lastPT = 0;

function hex2rgb(s) {
  var pm = /^\s*rgb\(\s*([\d.]+)%\s*,\s*([\d.]+)%\s*,\s*([\d.]+)%\s*\)\s*$/.exec(s);
  if (pm) return [Math.round(pm[1] * 2.55), Math.round(pm[2] * 2.55), Math.round(pm[3] * 2.55)];
  s = s.replace('#', '');
  if (s.length === 3) s = s[0] + s[0] + s[1] + s[1] + s[2] + s[2];
  var n = parseInt(s, 16);
  return [(n >> 16) & 255, (n >> 8) & 255, n & 255];
}
var RC0 = hex2rgb(C0), RC1 = hex2rgb(C1);
var RT1 = hex2rgb(T1), RT0 = hex2rgb(T0);
var RSAND = hex2rgb(SAND);

function fit() {
  var r = field.getBoundingClientRect();
  dpr = window.devicePixelRatio || 1;
  W = r.width; H = r.height;
  cv.width = Math.round(W * dpr);
  cv.height = Math.round(H * dpr);
  ctx.setTransform(dpr, 0, 0, dpr, 0, 0);
  surf = new Float32Array(COLS);
  buf = document.createElement('canvas');
  buf.width = RW; buf.height = RH;
  bctx = buf.getContext('2d');
  out = bctx.createImageData(RW, RH);
}

// 1D 수면 곡선 — height(x,t) = Σ amp·sin(x·dirX·freq·0.62 + t·ω + φ). y 항은 0 고정(단면이라 깊이 축이 없다)
function surfcalc(t, wstr, wang) {
  for (var x = 0; x < COLS; x++) {
    var h = 0;
    for (var w3 = 0; w3 < NW; w3++) {
      var WV = WAVES[w3];
      var da = wang * 0.5;
      var dx = WV.dirX * Math.cos(da) - WV.dirY * Math.sin(da);
      var ph = (x * dx) * WV.freq * 0.62 + t * WV.om + WV.ph;
      h += WV.amp * wstr * Math.sin(ph);
    }
    surf[x] = h;
  }
}

// 렌더 — 수면선 위는 투명(카드 배경이 하늘), 아래는 수심 d 에 지수 감쇠로 4단 램프.
// attenK 는 바람이 세질수록 커진다(흐려진 물이 색을 더 빨리 삼킨다).
function render(attenK) {
  var o = out.data;
  var baseline = RH * 0.4;   // 수면 중심선(버퍼 px)
  var ampPx = RH * 0.34;     // 수면 진폭(버퍼 px) — surf 값(대략 -0.75..0.75) 스케일
  for (var cx = 0; cx < COLS; cx++) {
    var surfaceY = baseline + surf[cx] * ampPx;
    if (surfaceY < 0) surfaceY = 0; else if (surfaceY > RH - 2) surfaceY = RH - 2;
    for (var sx = 0; sx < STEP; sx++) {
      var px = cx * STEP + sx;
      for (var y = 0; y < RH; y++) {
        var base = (y * RW + px) * 4;
        if (y < surfaceY) {
          o[base] = 0; o[base + 1] = 0; o[base + 2] = 0; o[base + 3] = 0;   // 물 위 — 투명(카드 배경이 하늘)
          continue;
        }
        if (y >= RH - SAND_PX) {
          o[base] = RSAND[0]; o[base + 1] = RSAND[1]; o[base + 2] = RSAND[2]; o[base + 3] = 255;
          continue;
        }
        var d = y - surfaceY;
        var u = 1 - Math.exp(-attenK * d);   // 0(얕음)..1(깊음), 클램프
        if (u < 0) u = 0; else if (u > 1) u = 1;
        // 4단 램프 — C0(민트,얕음) → C1(하늘) → T1(파랑) → T0(진한 파랑,깊음). 전 구간을 다 쓴다.
        var r, g, b;
        if (u < 1 / 3) {
          var k = u * 3;
          r = RC0[0] + (RC1[0] - RC0[0]) * k; g = RC0[1] + (RC1[1] - RC0[1]) * k; b = RC0[2] + (RC1[2] - RC0[2]) * k;
        } else if (u < 2 / 3) {
          var k2 = (u - 1 / 3) * 3;
          r = RC1[0] + (RT1[0] - RC1[0]) * k2; g = RC1[1] + (RT1[1] - RC1[1]) * k2; b = RC1[2] + (RT1[2] - RC1[2]) * k2;
        } else {
          var k3 = (u - 2 / 3) * 3;
          r = RT1[0] + (RT0[0] - RT1[0]) * k3; g = RT1[1] + (RT0[1] - RT1[1]) * k3; b = RT1[2] + (RT0[2] - RT1[2]) * k3;
        }
        o[base] = r; o[base + 1] = g; o[base + 2] = b; o[base + 3] = 255;
      }
    }
  }
  bctx.putImageData(out, 0, 0);
  ctx.clearRect(0, 0, W, H);
  ctx.drawImage(buf, 0, 0, W, H);
}

function frame(now) {
  nowSec = now / 1000;
  if (auto) {
    var th = nowSec * 0.31;
    windT = 1.05 + Math.sin(th * 0.9 + 0.4) * 0.55 + Math.sin(th * 2.3) * 0.12;
    windAngT = 0.5 + Math.sin(th * 0.42) * 0.85;
  }
  wind += (windT - wind) * 0.05;
  windAng += (windAngT - windAng) * 0.05;
  surfcalc(nowSec, wind, windAng);
  var attenK = 0.045 + (wind / (AMP_SUM * 3)) * 0.03;   // 바람이 세질수록 감쇠가 커진다
  render(attenK);
  attenVal.textContent = 'depth x' + attenK.toFixed(3);
  windVal.textContent = 'wind ' + wind.toFixed(2);
  raf = requestAnimationFrame(frame);
}

function still() {
  surfcalc(7.3, 1.35, 0.5);
  render(0.06);
}

function localXY(e) {
  var r = field.getBoundingClientRect();
  return [e.clientX - r.left, e.clientY - r.top];
}

field.addEventListener('pointermove', function (e) {
  var p = localXY(e);
  var t = performance.now();
  if (lastPX >= 0 && t > lastPT) {
    var vx = p[0] - lastPX, vy = p[1] - lastPY;
    var sp2 = Math.sqrt(vx * vx + vy * vy);
    if (sp2 > 0.5) {
      windT = Math.max(0.35, Math.min(2.1, 0.9 + sp2 * 0.02));
      windAngT = Math.atan2(vy, vx) * 0.16;
    }
  }
  lastPX = p[0]; lastPY = p[1]; lastPT = t;
  auto = false;
});
field.addEventListener('pointerleave', function () {
  auto = true;
  lastPX = -1;
});
field.addEventListener('keydown', function (e) {
  var d = { ArrowLeft: [-1, 0], ArrowRight: [1, 0], ArrowUp: [0, -1], ArrowDown: [0, 1] }[e.key];
  if (!d) return;
  e.preventDefault();
  auto = false;
  windT = Math.max(0.35, Math.min(2.1, windT + d[0] * 0.18));
  windAngT = windAngT + d[1] * 0.1;
  if (reduce) {
    surfcalc(nowSec || 3.1, windT, windAngT);
    render(0.045 + (windT / (AMP_SUM * 3)) * 0.03);
  }
});
window.addEventListener('resize', function () { fit(); if (reduce) still(); });

fit();
if (reduce) {
  root.classList.add('is-still');
  still();
} else {
  raf = requestAnimationFrame(frame);
}

// DepthAttenuate — vanilla 판(run/313/demo/02-ocean-depth-attenuate/index.html) 의 React 포트.
// 1D 수면 곡선(정현파 6개, 시드 7)을 시간 팬으로 읽고 그 아래 픽셀마다 수심 d 에
// exp(-감쇠계수·d) 를 적용해 민트(얕음)→진한 파랑(깊음) 4단 램프로 섞는다. 바닥은 크림 모래.
// 바람은 자동 리사주 순환 + 포인터 실측. 움직임은 캔버스 rAF 루프가 전부이므로 @keyframes 는 없다.
import { useEffect, useRef } from "react";
import type { CSSProperties } from "react";
import styles from "./DepthAttenuate.module.scss";

export type DepthAttenuateProps = {
  duration?: string;
  easing?: string;
  color?: string;
};

type Vars = CSSProperties & { "--duration"?: string; "--easing"?: string; "--color"?: string };
type Box<T> = { current: T };
type PointerMotionEvent = { clientX: number; clientY: number };

const COLS = 160;    // 1D 수면 곡선 표본 수 — 가로 해상도
const STEP = 3;      // 격자 한 칸이 렌더 버퍼에서 차지하는 픽셀 → RW = 480
const NW = 6;        // 겹칠 정현파 개수
const RW = 480;
const RH = 160;
const SAND_PX = 20;  // 바닥 모래 두께(버퍼 기준 px)

// 결정적 난수 mulberry32(7)
function mulberry32(a: number): () => number {
  return function () {
    a |= 0; a = (a + 0x6D2B79F5) | 0;
    const t = Math.imul(a ^ (a >>> 15), 1 | a);
    const u = (t + Math.imul(t ^ (t >>> 7), 61 | t)) ^ t;
    return ((u ^ (u >>> 14)) >>> 0) / 4294967296;
  };
}

type Wave = {
  lam: number; freq: number; dirX: number; dirY: number;
  amp: number; spd: number; ph: number; om: number;
};

function buildWaves(): Wave[] {
  const rng = mulberry32(7);
  const out: Wave[] = [];
  let lam = 34;
  for (let i = 0; i < NW; i++) {
    const ang = rng() * Math.PI * 2;
    const wv: Wave = {
      lam,
      freq: (Math.PI * 2) / lam,
      dirX: Math.cos(ang),
      dirY: Math.sin(ang),
      amp: 0.58 / (i * 0.62 + 1),
      spd: 0.55 + rng() * 0.25,
      ph: rng() * Math.PI * 2,
      om: 0,
    };
    wv.om = wv.spd * 3.6 * Math.sqrt(1 / wv.lam) * 6;
    out.push(wv);
    lam *= 0.55;
  }
  return out;
}

function hex2rgb(s: string): [number, number, number] {
  // sass 가 mix() 값을 rgb(r%, g%, b%) 로 내보낸다 — 헥스와 둘 다 읽는다
  const pm = /^\s*rgb\(\s*([\d.]+)%\s*,\s*([\d.]+)%\s*,\s*([\d.]+)%\s*\)\s*$/.exec(s);
  if (pm) return [Math.round(+pm[1] * 2.55), Math.round(+pm[2] * 2.55), Math.round(+pm[3] * 2.55)];
  let t = s.replace("#", "");
  if (t.length === 3) t = t[0] + t[0] + t[1] + t[1] + t[2] + t[2];
  const n = parseInt(t, 16);
  return [(n >> 16) & 255, (n >> 8) & 255, n & 255];
}

export default function DepthAttenuate({
  duration = "2s",                       // vanilla $duration: $dur-loop
  easing = "cubic-bezier(.2, .8, .2, 1)", // vanilla $easing: $ease-spring
  color = "#8fdac2",                     // vanilla $color: $da-c0 (근사값)
}: DepthAttenuateProps) {
  const rootRef = useRef<HTMLDivElement | null>(null);
  const fieldRef = useRef<HTMLDivElement | null>(null);
  const cvRef = useRef<HTMLCanvasElement | null>(null);
  const ctxRef = useRef<CanvasRenderingContext2D | null>(null);
  const attenValRef = useRef<HTMLSpanElement | null>(null);
  const windValRef = useRef<HTMLSpanElement | null>(null);
  const wavesRef = useRef<Wave[] | null>(null);
  const sizeRef = useRef({ w: 0, h: 0 }) as Box<{ w: number; h: number }>;
  const rafRef = useRef(0) as Box<number>;
  const stillRef = useRef(false) as Box<boolean>;
  const surfRef = useRef(new Float32Array(COLS)) as Box<Float32Array>;
  type St = {
    buf: HTMLCanvasElement | null;
    bctx: CanvasRenderingContext2D | null;
    out: ImageData | null;
    ampSum: number;
    auto: boolean;
    wind: number; windT: number;
    windAng: number; windAngT: number;
    lastPX: number; lastPY: number; lastPT: number;
    c0: [number, number, number] | null;
    c1: [number, number, number] | null;
    t0: [number, number, number] | null;
    t1: [number, number, number] | null;
    sand: [number, number, number] | null;
  };
  const stRef = useRef({
    buf: null,
    bctx: null,
    out: null,
    ampSum: 0,
    auto: true,
    wind: 1.0, windT: 1.0,
    windAng: 0.5, windAngT: 0.5,
    lastPX: -1, lastPY: -1, lastPT: 0,
    c0: null, c1: null, t0: null, t1: null, sand: null,
  } as St) as Box<St>;
  const vars: Vars = { "--duration": duration, "--easing": easing, "--color": color };

  function reduced(): boolean {
    return window.matchMedia("(prefers-reduced-motion: reduce)").matches;
  }

  function fit(): void {
    const field = fieldRef.current, cv = cvRef.current;
    if (!field || !cv) return;
    const r = field.getBoundingClientRect();
    const dpr = window.devicePixelRatio || 1;
    sizeRef.current.w = r.width;
    sizeRef.current.h = r.height;
    cv.width = Math.round(r.width * dpr);
    cv.height = Math.round(r.height * dpr);
    const ctx = ctxRef.current;
    if (ctx) ctx.setTransform(dpr, 0, 0, dpr, 0, 0);
    const st = stRef.current;
    const buf = document.createElement("canvas");
    buf.width = RW; buf.height = RH;
    st.buf = buf;
    st.bctx = buf.getContext("2d");
    if (st.bctx) st.out = st.bctx.createImageData(RW, RH);
  }

  // 1D 수면 곡선 — height(x,t) = Σ amp·sin(x·dirX·freq·0.62 + t·ω + φ)
  function surfcalc(t: number, wstr: number, wang: number): void {
    const WV = wavesRef.current;
    const surf = surfRef.current;
    if (!WV) return;
    for (let x = 0; x < COLS; x++) {
      let h = 0;
      for (let k = 0; k < NW; k++) {
        const wv = WV[k];
        const da = wang * 0.5;
        const dx = wv.dirX * Math.cos(da) - wv.dirY * Math.sin(da);
        const ph = (x * dx) * wv.freq * 0.62 + t * wv.om + wv.ph;
        h += wv.amp * wstr * Math.sin(ph);
      }
      surf[x] = h;
    }
  }

  // 렌더 — 수면선 위는 투명, 아래는 수심 지수 감쇠 4단 램프, 바닥 20px 모래
  function render(attenK: number): void {
    const st = stRef.current;
    const ctx = ctxRef.current;
    const surf = surfRef.current;
    if (!st.out || !st.bctx || !st.buf || !ctx || !st.c0 || !st.c1 || !st.t0 || !st.t1 || !st.sand) return;
    const o = st.out.data;
    const RC0 = st.c0, RC1 = st.c1, RT0 = st.t0, RT1 = st.t1, RSAND = st.sand;
    const baseline = RH * 0.4;
    const ampPx = RH * 0.34;
    for (let cx = 0; cx < COLS; cx++) {
      let surfaceY = baseline + surf[cx] * ampPx;
      if (surfaceY < 0) surfaceY = 0; else if (surfaceY > RH - 2) surfaceY = RH - 2;
      for (let sx = 0; sx < STEP; sx++) {
        const px = cx * STEP + sx;
        for (let y = 0; y < RH; y++) {
          const base = (y * RW + px) * 4;
          if (y < surfaceY) {
            o[base] = 0; o[base + 1] = 0; o[base + 2] = 0; o[base + 3] = 0;
            continue;
          }
          if (y >= RH - SAND_PX) {
            o[base] = RSAND[0]; o[base + 1] = RSAND[1]; o[base + 2] = RSAND[2]; o[base + 3] = 255;
            continue;
          }
          const d = y - surfaceY;
          let u = 1 - Math.exp(-attenK * d);
          if (u < 0) u = 0; else if (u > 1) u = 1;
          let r: number, g: number, b: number;
          if (u < 1 / 3) {
            const k = u * 3;
            r = RC0[0] + (RC1[0] - RC0[0]) * k; g = RC0[1] + (RC1[1] - RC0[1]) * k; b = RC0[2] + (RC1[2] - RC0[2]) * k;
          } else if (u < 2 / 3) {
            const k2 = (u - 1 / 3) * 3;
            r = RC1[0] + (RT1[0] - RC1[0]) * k2; g = RC1[1] + (RT1[1] - RC1[1]) * k2; b = RC1[2] + (RT1[2] - RC1[2]) * k2;
          } else {
            const k3 = (u - 2 / 3) * 3;
            r = RT1[0] + (RT0[0] - RT1[0]) * k3; g = RT1[1] + (RT0[1] - RT1[1]) * k3; b = RT1[2] + (RT0[2] - RT1[2]) * k3;
          }
          o[base] = r; o[base + 1] = g; o[base + 2] = b; o[base + 3] = 255;
        }
      }
    }
    st.bctx.putImageData(st.out, 0, 0);
    ctx.clearRect(0, 0, sizeRef.current.w, sizeRef.current.h);
    ctx.drawImage(st.buf, 0, 0, sizeRef.current.w, sizeRef.current.h);
  }

  function frame(now: number): void {
    const st = stRef.current;
    const sec = now / 1000;
    if (st.auto) {
      const th = sec * 0.31;
      st.windT = 1.05 + Math.sin(th * 0.9 + 0.4) * 0.55 + Math.sin(th * 2.3) * 0.12;
      st.windAngT = 0.5 + Math.sin(th * 0.42) * 0.85;
    }
    st.wind += (st.windT - st.wind) * 0.05;
    st.windAng += (st.windAngT - st.windAng) * 0.05;
    surfcalc(sec, st.wind, st.windAng);
    const attenK = 0.045 + (st.wind / (st.ampSum * 3)) * 0.03;
    render(attenK);
    if (attenValRef.current) attenValRef.current.textContent = "수심 x" + attenK.toFixed(3);
    if (windValRef.current) windValRef.current.textContent = "바람 " + st.wind.toFixed(2);
    rafRef.current = requestAnimationFrame(frame);
  }

  function onPointerMove(e: PointerMotionEvent): void {
    const field = fieldRef.current;
    if (!field) return;
    const r = field.getBoundingClientRect();
    const px = e.clientX - r.left, py = e.clientY - r.top;
    const t = performance.now();
    const st = stRef.current;
    if (st.lastPX >= 0 && t > st.lastPT) {
      const vx = px - st.lastPX, vy = py - st.lastPY;
      const sp = Math.sqrt(vx * vx + vy * vy);
      if (sp > 0.5) {
        st.windT = Math.max(0.35, Math.min(2.1, 0.9 + sp * 0.02));
        st.windAngT = Math.atan2(vy, vx) * 0.16;
      }
    }
    st.lastPX = px; st.lastPY = py; st.lastPT = t;
    st.auto = false;
  }

  function onPointerLeave(): void {
    stRef.current.auto = true;
    stRef.current.lastPX = -1;
  }

  function onResize(): void {
    fit();
    if (reduced()) {
      surfcalc(7.3, 1.35, 0.5);
      render(0.06);
    }
  }

  useEffect(() => {
    const cv = cvRef.current;
    const root = rootRef.current;
    if (!cv || !root) return;
    ctxRef.current = cv.getContext("2d");
    if (!ctxRef.current) return;
    wavesRef.current = buildWaves();
    let sum = 0;
    for (let i = 0; i < NW; i++) sum += wavesRef.current[i].amp;
    stRef.current.ampSum = sum;
    const cs = getComputedStyle(root);
    const tok = (name: string) => cs.getPropertyValue(name).trim();
    const st = stRef.current;
    st.c0 = hex2rgb(tok("--c0"));
    st.c1 = hex2rgb(tok("--c1"));
    st.t0 = hex2rgb(tok("--t0"));
    st.t1 = hex2rgb(tok("--t1"));
    st.sand = hex2rgb(tok("--sand"));
    fit();
    window.addEventListener("resize", onResize);
    if (reduced()) {
      stillRef.current = true;
      surfcalc(7.3, 1.35, 0.5);
      render(0.06);
    } else {
      rafRef.current = requestAnimationFrame(frame);
    }
    return () => {
      window.removeEventListener("resize", onResize);
      cancelAnimationFrame(rafRef.current);
    };
  }, []);

  const cls = [styles.root, stillRef.current ? styles.isStill : ""].filter(Boolean).join(" ");
  return (
    <main className={styles.stage}>
      <div className={cls} ref={rootRef} style={vars}>
        <p className={styles.hint}>
          <svg className={styles.ico} viewBox="0 0 256 256" aria-hidden="true"><g fill="currentColor"><path d="M16 120c24-32 48-32 72 0s48 32 72 0 48-32 72 0" opacity=".25"/><path d="M16 168c24-32 48-32 72 0s48 32 72 0 48-32 72 0" opacity=".15"/><path d="M228.92 49.09a112 112 0 1 0 14 84.91a8 8 0 0 0-15.5-4a96 96 0 1 1-12-72.91A8 8 0 0 1 216 71.63V40a8 8 0 0 1-8-8h-31.63a8 8 0 0 1-5.65-13.09ZM64 104a12 12 0 1 1 12 12a12 12 0 0 1-12-12m40 64a12 12 0 1 1 12 12a12 12 0 0 1-12-12m72-16a16 16 0 1 1 16 16a16 16 0 0 1-16-16" opacity=".9"/></g></svg>
          <span>수면 아래 픽셀마다 수심을 지수로 감쇠시켜 얕은 바다 단면을 계산합니다 — 마우스로 물빛을 흔드세요</span>
        </p>
        <div className={styles.field} ref={fieldRef} onPointerMove={onPointerMove} onPointerLeave={onPointerLeave}>
          <canvas className={styles.cv} ref={cvRef}></canvas>
          <b className={styles.badge} aria-hidden="true"><span className={styles.badgeNum} ref={attenValRef}>수심 x0.060</span><span className={styles.badgeUnit} ref={windValRef}>바람 1.0</span></b>
        </div>
      </div>
    </main>
  );
}