03Crest foam — whitecaps on the peaks

<main class="stage">
<div class="cf" id="cf">
  <p class="cf__hint">
    <svg class="cf__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>Only cells whose summed height crosses an amplitude threshold break into cream — stronger wind means more whitecaps</span>
  </p>
  <div class="cf__field" id="field" role="application" aria-label="Crest foam playground" tabindex="0">
    <canvas class="cf__cv" id="cv"></canvas>
    <b class="cf__badge" aria-hidden="true"><span class="cf__badgeNum" id="thrVal">threshold 72%</span><span class="cf__badgeUnit" id="windVal">wind 1.0</span></b>
  </div>
</div>
</main>

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

// 크레스트 거품 — 억제된 바탕 램프 위에 문턱을 넘은 칸만 크림 whitecap. 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.

$cf-base0: mix($subject-blue, $stage-paper, 40%);        // 바탕 마루쪽 — 파랑(억제)
$cf-base1: mix($subject-blue-deep, $stage-paper, 46%);   // 바탕 중간 — 파랑
$cf-deep:  mix($subject-blue-deep, $stage-paper, 62%);   // 가장 깊은 골 — 진한 파랑
$cf-foam:  mix($subject-cream, $stage-paper, 92%);       // whitecap — 크림
$cf-foam2: $subject-cream;                               // whitecap 꼭대기 — 밝은 크림

$duration: $dur-loop;
$easing: $ease-spring;
$color: $cf-foam;

@include stage($stage-paper);

.cf {
  --base0: #{$cf-base0};
  --base1: #{$cf-base1};
  --deep:  #{$cf-deep};
  --foam:  #{$cf-foam};
  --foam2: #{$cf-foam2};

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

.cf__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;
}
.cf__ico { width: 16px; height: 16px; flex: 0 0 auto; color: $subject-blue; }

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

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

.cf__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;
}
.cf__badgeNum { font-size: 13px; color: $stage-yellow; }
.cf__badgeUnit { font-weight: 700; }

@media (max-width: 340px) {
  .cf { gap: $sp-1; }
  .cf__field { height: 124px; }
  .cf__badge { font-size: 10px; padding: 0 $sp-1; }
  .cf__badgeNum { font-size: 11px; }
  .cf__hint { font-size: 10px; }
}

// reduce — 루프가 아예 안 돈다. 거품이 박힌 정적 한 장이 남으므로
// 끌 수 있다는 신호(커서)만 거둔다.
@media (prefers-reduced-motion: reduce) {
  .cf.is-still .cf__field { cursor: default !important; }
  .cf.is-still .cf__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);
}

.cf {
  --base0: rgb(67.3725490196%, 77.0980392157%, 98.5882352941%);
  --base1: rgb(55.8039215686%, 68.2509803922%, 95.8509803922%);
  --deep: rgb(40.431372549%, 57.2078431373%, 94.4078431373%);
  --foam: rgb(100%, 97.1137254902%, 90.9803921569%);
  --foam2: #fff7e6;
  display: flex;
  flex-direction: column;
  gap: 8px;
  width: 100%;
  max-width: 424px;
}

.cf__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;
}

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

.cf__field {
  position: relative;
  height: 208px;
  border-radius: 16px;
  overflow: hidden;
  background: rgb(40.431372549%, 57.2078431373%, 94.4078431373%);
  box-shadow: inset 0 0 0 1px rgba(23, 20, 26, 0.16);
  cursor: crosshair;
  touch-action: none;
}

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

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

.cf__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;
}

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

.cf__badgeUnit {
  font-weight: 700;
}

@media (max-width: 340px) {
  .cf {
    gap: 4px;
  }
  .cf__field {
    height: 124px;
  }
  .cf__badge {
    font-size: 10px;
    padding: 0 4px;
  }
  .cf__badgeNum {
    font-size: 11px;
  }
  .cf__hint {
    font-size: 10px;
  }
}
@media (prefers-reduced-motion: reduce) {
  .cf.is-still .cf__field {
    cursor: default !important;
  }
  .cf.is-still .cf__badge {
    opacity: 0.92 !important;
  }
}

// 크레스트 foam — 01 과 같은 2D 파도장(정현파 6개 mulberry32(7) + Gerstner 처짐)이지만
// 바탕 램프를 낮게 억제해(거의 단색 파랑 — 거품이 주인공) 물밑에 묻어두고,
// 합성높이 > 실효진폭×threshold 인 칸만 smoothstep 가장자리로 크림색 whitecap 을 얹는다.
// 문턱은 wind 세기에 반비례해 0.82 → 0.55 로 낮아진다 — 바람이 세질수록 거품이 늘어난다.
var root = document.getElementById('cf');
var field = document.getElementById('field');
var cv = document.getElementById('cv');
var thrVal = document.getElementById('thrVal');
var windVal = document.getElementById('windVal');
var ctx = cv.getContext('2d');

var css = getComputedStyle(root);
function v(name) { return css.getPropertyValue(name).trim(); }
var BASE0 = v('--base0'), BASE1 = v('--base1'); // 억제된 바탕 램프 — 파랑 (foam 아닌 물)
var DEEP = v('--deep');                          // 가장 낮은 골 — 진한 파랑
var FOAM = v('--foam');                          // whitecap — 크림
var FOAM2 = v('--foam2');                        // whitecap 꼭대기 — 밝은 크림

var COLS = 110;   // 수면 격자 가로 칸 수(01 과 동일)
var STEP = 3;     // 격자 한 칸이 렌더 버퍼에서 차지하는 픽셀
var NW = 6;       // 겹칠 정현파 개수
var Q = 0.42;     // Gerstner 처짐 계수
var FOAM_EDGE = 0.16; // smoothstep 가장자리 폭(threshold 비율 단위)

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 reduce = window.matchMedia('(prefers-reduced-motion: reduce)').matches;
var W = 0, H = 0, dpr = 1, ROWS = 0;
var hgt = null;
var buf = null, bctx = null, out = null;
var RW = 0, RH = 0;
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 RB0 = hex2rgb(BASE0), RB1 = hex2rgb(BASE1);
var RDEEP = hex2rgb(DEEP);
var RF = hex2rgb(FOAM), RF2 = hex2rgb(FOAM2);

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);
  ROWS = Math.max(40, Math.round(COLS * H / W));
  hgt = new Float32Array(COLS * ROWS);
  RW = COLS * STEP; RH = ROWS * STEP;
  buf = document.createElement('canvas');
  buf.width = RW; buf.height = RH;
  bctx = buf.getContext('2d');
  out = bctx.createImageData(RW, RH);
}

// 파도장 높이 합성 — height(x,y,t) = Σ amp·sin(dot(k·dir,(x,y)) + t·ω + φ) (01 과 같은 뿌리)
function fieldcalc(t, wstr, wang) {
  var i = 0;
  for (var y = 0; y < ROWS; y++) {
    for (var x = 0; x < COLS; x++, i++) {
      var h = 0;
      for (var w2 = 0; w2 < NW; w2++) {
        var WV = WAVES[w2];
        var da = wang * 0.5;
        var dx = WV.dirX * Math.cos(da) - WV.dirY * Math.sin(da);
        var dy = WV.dirX * Math.sin(da) + WV.dirY * Math.cos(da);
        var ph = (x * dx + y * dy) * WV.freq * 0.62 + t * WV.om + WV.ph;
        h += WV.amp * wstr * Math.sin(ph);
      }
      hgt[i] = h;
    }
  }
}

function smoothstep(a, b, x) {
  var t = (x - a) / (b - a);
  if (t < 0) t = 0; else if (t > 1) t = 1;
  return t * t * (3 - 2 * t);
}

// 렌더 — 억제된 바탕(골 DEEP → BASE0 → BASE1, 마루 직전까지만) 위에
// 문턱을 넘은 칸을 크림 whitecap 으로. 가장자리는 smoothstep 로 부드럽게, 꼭대기는 FOAM2 로 더 밝게.
function render(thr) {
  var o = out.data;
  // 실효 진폭 — 현재 바람까지 반영한 이 파도장의 실제 스케일. 문턱은 이 배율로 작동한다.
  var eff = 0;
  for (var i = 0; i < NW; i++) eff += WAVES[i].amp;
  eff *= wind;
  var cut = eff * thr;
  for (var y = 0; y < ROWS; y++) {
    var row = y * COLS;
    for (var x = 0; x < COLS; x++) {
      var idx = row + x;
      var h = hgt[idx];
      // 억제된 바탕 램프 — h 를 0.55 배로 눌러 물색이 거의 단색에 가깝게(거품이 주인공)
      var u = h * 0.55 * 1.24 + 0.5;
      if (u < 0) u = 0; else if (u > 1) u = 1;
      var r, g, b;
      if (u > 0.5) {
        var k = (u - 0.5) * 2;
        r = RB1[0] * (1 - k) + RB0[0] * k;
        g = RB1[1] * (1 - k) + RB0[1] * k;
        b = RB1[2] * (1 - k) + RB0[2] * k;
      } else {
        var k2 = u * 2;
        r = RDEEP[0] * (1 - k2) + RB0[0] * k2;
        g = RDEEP[1] * (1 - k2) + RB0[1] * k2;
        b = RDEEP[2] * (1 - k2) + RB0[2] * k2;
      }
      // whitecap — h 가 cut 을 넘으면 크림. 가장자리 smoothstep(폭 FOAM_EDGE·eff), 꼭대기는 FOAM2 로 두 단
      var f = smoothstep(cut - FOAM_EDGE * eff, cut + FOAM_EDGE * eff, h);
      if (f > 0) {
        var top = smoothstep(cut + FOAM_EDGE * eff * 2.2, cut + eff * 0.42, h);
        var fr = RF[0] * (1 - top) + RF2[0] * top;
        var fg = RF[1] * (1 - top) + RF2[1] * top;
        var fb = RF[2] * (1 - top) + RF2[2] * top;
        r = r * (1 - f) + fr * f;
        g = g * (1 - f) + fg * f;
        b = b * (1 - f) + fb * f;
      }
      for (var sy = 0; sy < STEP; sy++) {
        var base = ((y * STEP + sy) * RW + x * STEP) * 4;
        for (var sx = 0; sx < STEP; sx++, base += 4) {
          o[base] = r;
          o[base + 1] = g;
          o[base + 2] = b;
          o[base + 3] = 255;
        }
      }
    }
  }
  bctx.putImageData(out, 0, 0);
  ctx.drawImage(buf, 0, 0, W, H);
}

// 문턱은 바람에 반비례 — 0.82(약한 wind) → 0.55(강한 wind)
function thresholdFor(w) {
  return 0.82 - 0.27 * Math.max(0, Math.min(1, (w - 0.35) / (2.1 - 0.35)));
}

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;
  fieldcalc(nowSec, wind, windAng);
  var thr = thresholdFor(wind);
  render(thr);
  thrVal.textContent = 'threshold ' + Math.round(thr * 100) + '%';
  windVal.textContent = 'wind ' + wind.toFixed(2);
  raf = requestAnimationFrame(frame);
}

function still() {
  fieldcalc(7.3, 1.35, 0.5);
  render(thresholdFor(1.35));
}

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) {
    fieldcalc(nowSec || 3.1, windT, windAngT);
    render(thresholdFor(windT));
  }
});
window.addEventListener('resize', function () { fit(); if (reduce) still(); });

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

// CrestFoam — vanilla 판(run/313/demo/03-ocean-crest-foam/index.html) 의 React 포트.
// 01 과 같은 2D 파도장(정현파 6개, 시드 7)을 억제된 파랑 바탕 위에 그리고,
// 합성높이 > 실효진폭×문턱 인 칸만 smoothstep 가장자리로 크림 whitecap 을 얹는다.
// 문턱은 바람 세기에 반비례(0.82→0.55). 움직임은 캔버스 rAF 루프가 전부이므로 @keyframes 는 없다.
import { useEffect, useRef } from "react";
import type { CSSProperties } from "react";
import styles from "./CrestFoam.module.scss";

export type CrestFoamProps = {
  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 };
type RGB = [number, number, number];

const COLS = 110;      // 수면 격자 가로 칸 수
const STEP = 3;        // 격자 한 칸이 렌더 버퍼에서 차지하는 픽셀
const NW = 6;          // 겹칠 정현파 개수
const FOAM_EDGE = 0.16; // smoothstep 가장자리 폭(문턱 비율 단위)

// 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): RGB {
  // 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];
}

function smoothstep(a: number, b: number, x: number): number {
  let t = (x - a) / (b - a);
  if (t < 0) t = 0; else if (t > 1) t = 1;
  return t * t * (3 - 2 * t);
}

// 문턱은 바람에 반비례 — 0.82(약한 바람) → 0.55(강한 바람)
function thresholdFor(w: number): number {
  return 0.82 - 0.27 * Math.max(0, Math.min(1, (w - 0.35) / (2.1 - 0.35)));
}

type St = {
  rows: number; rw: number; rh: number;
  hgt: Float32Array | null;
  out: ImageData | null;
  bctx: CanvasRenderingContext2D | null;
  buf: HTMLCanvasElement | null;
  ampSum: number;
  auto: boolean;
  wind: number; windT: number;
  windAng: number; windAngT: number;
  lastPX: number; lastPY: number; lastPT: number;
  rb0: RGB | null; rb1: RGB | null; rdeep: RGB | null;
  rf: RGB | null; rf2: RGB | null;
};

export default function CrestFoam({
  duration = "2s",                       // vanilla $duration: $dur-loop
  easing = "cubic-bezier(.2, .8, .2, 1)", // vanilla $easing: $ease-spring
  color = "#fff7e6",                     // vanilla $color: $cf-foam (근사값)
}: CrestFoamProps) {
  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 thrValRef = 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 stRef = useRef({
    rows: 0, rw: 0, rh: 0,
    hgt: null,
    out: null,
    bctx: null,
    buf: null,
    ampSum: 0,
    auto: true,
    wind: 1.0, windT: 1.0,
    windAng: 0.5, windAngT: 0.5,
    lastPX: -1, lastPY: -1, lastPT: 0,
    rb0: null, rb1: null, rdeep: null, rf: null, rf2: 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;
    st.rows = Math.max(40, Math.round(COLS * r.height / r.width));
    st.hgt = new Float32Array(COLS * st.rows);
    st.rw = COLS * STEP;
    st.rh = st.rows * STEP;
    const buf = document.createElement("canvas");
    buf.width = st.rw; buf.height = st.rh;
    st.buf = buf;
    st.bctx = buf.getContext("2d");
    if (st.bctx) st.out = st.bctx.createImageData(st.rw, st.rh);
  }

  // 파도장 높이 합성 — height(x,y,t) = Σ amp·sin(dot(k·dir,(x,y)) + t·ω + φ)
  function fieldcalc(t: number, wstr: number, wang: number): void {
    const st = stRef.current;
    const WV = wavesRef.current;
    if (!st.hgt || !WV) return;
    let i = 0;
    for (let y = 0; y < st.rows; y++) {
      for (let x = 0; x < COLS; x++, i++) {
        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 dy = wv.dirX * Math.sin(da) + wv.dirY * Math.cos(da);
          const ph = (x * dx + y * dy) * wv.freq * 0.62 + t * wv.om + wv.ph;
          h += wv.amp * wstr * Math.sin(ph);
        }
        st.hgt[i] = h;
      }
    }
  }

  // 렌더 — 억제된 바탕 램프 위에 문턱을 넘은 칸만 크림 whitecap(smoothstep 가장자리)
  function render(thr: number): void {
    const st = stRef.current;
    const ctx = ctxRef.current;
    if (!st.out || !st.bctx || !st.buf || !ctx || !st.hgt) return;
    if (!st.rb0 || !st.rb1 || !st.rdeep || !st.rf || !st.rf2) return;
    const o = st.out.data;
    const RB0 = st.rb0, RB1 = st.rb1, RDEEP = st.rdeep, RF = st.rf, RF2 = st.rf2;
    const eff = st.ampSum * st.wind;   // 실효 진폭 — 현재 바람까지 반영한 파도장 스케일
    const cut = eff * thr;
    for (let y = 0; y < st.rows; y++) {
      const row = y * COLS;
      for (let x = 0; x < COLS; x++) {
        const idx = row + x;
        const h = st.hgt[idx];
        let u = h * 0.55 * 1.24 + 0.5;
        if (u < 0) u = 0; else if (u > 1) u = 1;
        let r: number, g: number, b: number;
        if (u > 0.5) {
          const k = (u - 0.5) * 2;
          r = RB1[0] * (1 - k) + RB0[0] * k;
          g = RB1[1] * (1 - k) + RB0[1] * k;
          b = RB1[2] * (1 - k) + RB0[2] * k;
        } else {
          const k2 = u * 2;
          r = RDEEP[0] * (1 - k2) + RB0[0] * k2;
          g = RDEEP[1] * (1 - k2) + RB0[1] * k2;
          b = RDEEP[2] * (1 - k2) + RB0[2] * k2;
        }
        const f = smoothstep(cut - FOAM_EDGE * eff, cut + FOAM_EDGE * eff, h);
        if (f > 0) {
          const top = smoothstep(cut + FOAM_EDGE * eff * 2.2, cut + eff * 0.42, h);
          const fr = RF[0] * (1 - top) + RF2[0] * top;
          const fg = RF[1] * (1 - top) + RF2[1] * top;
          const fb = RF[2] * (1 - top) + RF2[2] * top;
          r = r * (1 - f) + fr * f;
          g = g * (1 - f) + fg * f;
          b = b * (1 - f) + fb * f;
        }
        for (let sy = 0; sy < STEP; sy++) {
          let base = ((y * STEP + sy) * st.rw + x * STEP) * 4;
          for (let sx = 0; sx < STEP; sx++, base += 4) {
            o[base] = r;
            o[base + 1] = g;
            o[base + 2] = b;
            o[base + 3] = 255;
          }
        }
      }
    }
    st.bctx.putImageData(st.out, 0, 0);
    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;
    fieldcalc(sec, st.wind, st.windAng);
    const thr = thresholdFor(st.wind);
    render(thr);
    if (thrValRef.current) thrValRef.current.textContent = "문턱 " + Math.round(thr * 100) + "%";
    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()) {
      fieldcalc(7.3, 1.35, 0.5);
      render(thresholdFor(1.35));
    }
  }

  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.rb0 = hex2rgb(tok("--base0"));
    st.rb1 = hex2rgb(tok("--base1"));
    st.rdeep = hex2rgb(tok("--deep"));
    st.rf = hex2rgb(tok("--foam"));
    st.rf2 = hex2rgb(tok("--foam2"));
    fit();
    window.addEventListener("resize", onResize);
    if (reduced()) {
      stillRef.current = true;
      fieldcalc(7.3, 1.35, 0.5);
      render(thresholdFor(1.35));
    } 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>합성 높이가 진폭의 문턱을 넘는 칸만 크림색으로 부서뜨립니다 — 바람이 세질수록 흰 꼭대기가 늘어요</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={thrValRef}>문턱 72%</span><span className={styles.badgeUnit} ref={windValRef}>바람 1.0</span></b>
        </div>
      </div>
    </main>
  );
}