08Pointer stirring

<main class="stage stage--paper">
<div class="ps" id="ps">
  <p class="ps__hint">
    <svg class="ps__ico" viewBox="0 0 256 256" aria-hidden="true"><g fill="currentColor"><path d="M100.69,180.7,41.34,121.35a16,16,0,0,1,0-22.63L98.72,41.34a16,16,0,0,1,22.63,0l99.31,99.32a16,16,0,0,1,0,22.62l-57.38,57.38a16,16,0,0,1-22.63,0Z" opacity=".2"/><path d="M228.69,133.66,131.32,36.28a24,24,0,0,0-33.94,0L38.34,95.31a24,24,0,0,0,0,34l97.37,97.37a24,24,0,0,0,33.94,0l59-59A24,24,0,0,0,228.69,133.66Zm-16.97,17L152.68,209.7a8,8,0,0,1-11.31,0L44,112.34a8,8,0,0,1,0-11.32L103.06,42a8,8,0,0,1,11.31,0l97.35,97.35A8,8,0,0,1,211.72,150.63Z"/></g></svg>
    <span>The cursor pushes the velocity field by however far it moves — stir faster and the swirl grows bigger</span>
  </p>
  <div class="ps__field" id="field" role="application" aria-label="Pointer stir board" tabindex="0">
    <canvas class="ps__cv" id="cv"></canvas>
    <b class="ps__badge" aria-hidden="true">stir<span class="ps__badgeUnit">splat</span></b>
  </div>
</div>
</main>

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

// 유체 포인터 휘젓기 — 이 편의 인터랙션 중심. 움직임은 전부 캔버스 rAF 루프 안에 있다.
// @keyframes 를 두지 않는다 — 렌더러가 CSS 시계 대신 실시간으로 24장을 찍는다.
// 무대는 $stage-paper(흰 종이). 어두운 파스텔 염료 두 색을 그 위에 직접 잡는다.

$duration: $dur-loop;
$easing: $ease-spring;
$color: $subject-blue;

@include stage($stage-paper);

.ps {
  // 짙은 파스텔 틸(채도 .18)·자두(채도 .20) — 흰 종이 위 대비 8~10 대
  --dye-teal: #33454a;
  --dye-plum: #4a3550;
  --dye-paper: #ffffff;

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

.ps__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;
}
.ps__ico { width: 16px; height: 16px; flex: 0 0 auto; color: $subject-ink; }

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

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

.ps__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, .82);
  color: $subject-cream; font-weight: 700; font-size: 11px;
}
.ps__badgeUnit { font-weight: 700; }

@media (max-width: 340px) {
  .ps { gap: $sp-1; }
  .ps__field { height: 124px; }
  .ps__badge { font-size: 10px; padding: 0 $sp-1; }
}

// reduce — 루프가 멈춘 한 장이 남으므로 커서 신호만 바꾼다.
@media (prefers-reduced-motion: reduce) {
  .ps.is-still .ps__field { cursor: default !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);
}

.ps {
  --dye-teal: #33454a;
  --dye-plum: #4a3550;
  --dye-paper: #ffffff;
  display: flex;
  flex-direction: column;
  gap: 8px;
  width: 100%;
  max-width: 424px;
}

.ps__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;
}

.ps__ico {
  width: 16px;
  height: 16px;
  flex: 0 0 auto;
  color: #17141a;
}

.ps__field {
  position: relative;
  height: 208px;
  border-radius: 16px;
  overflow: hidden;
  background: #ffffff;
  box-shadow: inset 0 0 0 1px rgba(23, 20, 26, 0.16);
  cursor: crosshair;
  touch-action: none;
}

.ps__field:focus-visible {
  outline: 2px solid #17141a;
  outline-offset: 2px;
}

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

.ps__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, 0.82);
  color: #fff7e6;
  font-weight: 700;
  font-size: 11px;
}

.ps__badgeUnit {
  font-weight: 700;
}

@media (max-width: 340px) {
  .ps {
    gap: 4px;
  }
  .ps__field {
    height: 124px;
  }
  .ps__badge {
    font-size: 10px;
    padding: 0 4px;
  }
}
@media (prefers-reduced-motion: reduce) {
  .ps.is-still .ps__field {
    cursor: default !important;
  }
}

// 유체 포인터 휘젓기 — 이 편 9종의 인터랙션 중심. 실제 포인터든 결정적 미리보기든
// 프레임 간 위치 차이(델타)를 같은 splat() 한 함수로 속도장에 꽂아 넣는다.
// 01 보다 반경·세기를 키워 "휘젓는 느낌"을 화면 전체로 퍼뜨린다.
var root = document.getElementById('ps');
var field = document.getElementById('field');
var cv = document.getElementById('cv');
var ctx = cv.getContext('2d', { willReadFrequently: true });

var css = getComputedStyle(root);
var TEAL = css.getPropertyValue('--dye-teal').trim();
var PLUM = css.getPropertyValue('--dye-plum').trim();
var PAPER = css.getPropertyValue('--dye-paper').trim();
function hex(c) { return [parseInt(c.slice(1, 3), 16), parseInt(c.slice(3, 5), 16), parseInt(c.slice(5, 7), 16)]; }
var CR = hex(PAPER), K1 = hex(TEAL), K2 = hex(PLUM);

var N = 40;
var SIZE = (N + 2) * (N + 2);
var IX = function (x, y) { return x + (N + 2) * y; };
var u = new Float32Array(SIZE), v = new Float32Array(SIZE);
var u0 = new Float32Array(SIZE), v0 = new Float32Array(SIZE);
var d = new Float32Array(SIZE), d0 = new Float32Array(SIZE);

var reduce = window.matchMedia('(prefers-reduced-motion: reduce)').matches;
var W = 0, H = 0, raf = 0, seedT = 0;

function fit() {
  var rect = field.getBoundingClientRect();
  W = rect.width; H = rect.height;
  cv.width = Math.round(W); cv.height = Math.round(H);
}

function advect(b, dt) {
  var src = b === 0 ? u0 : b === 1 ? v0 : d0;
  var dst = b === 0 ? u : b === 1 ? v : d;
  for (var j = 1; j <= N; j++) {
    for (var i = 1; i <= N; i++) {
      var x = i - dt * N * u0[IX(i, j)];
      var y = j - dt * N * v0[IX(i, j)];
      if (x < 0.5) x = 0.5; else if (x > N + 0.5) x = N + 0.5;
      var i0 = x | 0, i1 = i0 + 1;
      if (y < 0.5) y = 0.5; else if (y > N + 0.5) y = N + 0.5;
      var j0 = y | 0, j1 = j0 + 1;
      var s1 = x - i0, s0 = 1 - s1, t1 = y - j0, t0 = 1 - t1;
      dst[IX(i, j)] = s0 * (t0 * src[IX(i0, j0)] + t1 * src[IX(i0, j1)]) +
                      s1 * (t0 * src[IX(i1, j0)] + t1 * src[IX(i1, j1)]);
    }
  }
}

function bounds() {
  for (var i = 1; i <= N; i++) {
    d[IX(i, 0)] = d[IX(i, 1)] * 0.5; d[IX(i, N + 1)] = d[IX(i, N)] * 0.5;
    u[IX(i, 0)] = u[IX(i, 1)] * 0.5; u[IX(i, N + 1)] = u[IX(i, N)] * 0.5;
    v[IX(i, 0)] = v[IX(i, 1)] * 0.5; v[IX(i, N + 1)] = v[IX(i, N)] * 0.5;
  }
  for (var j = 1; j <= N; j++) {
    d[IX(0, j)] = d[IX(1, j)] * 0.5; d[IX(N + 1, j)] = d[IX(N, j)] * 0.5;
    u[IX(0, j)] = u[IX(1, j)] * 0.5; u[IX(N + 1, j)] = u[IX(N, j)] * 0.5;
    v[IX(0, j)] = v[IX(1, j)] * 0.5; v[IX(N + 1, j)] = v[IX(N, j)] * 0.5;
  }
}

function step(dt) {
  u0.set(u); v0.set(v); d0.set(d);
  advect(0, dt); u0.set(u);
  advect(1, dt); v0.set(v);
  advect(2, dt);
  bounds();
  for (var k = 0; k < SIZE; k++) { u[k] *= 0.992; v[k] *= 0.992; d[k] *= 0.986; }
  seedT += dt;
}

// splat — 인터랙션의 유일한 진입점. 실제 포인터도, 아래 lissajous 미리보기도 이 함수 하나만 부른다.
// 01(반경 3, amt 계수 3)보다 반경을 두 배 이상 키우고 완만한 지수 감쇠를 써 소용돌이가 넓게 번지게 한다.
function splat(gx, gy, fx, fy, amt) {
  var rad = 7;
  for (var j = -rad; j <= rad; j++) {
    for (var i = -rad; i <= rad; i++) {
      var q = (i * i + j * j) / (rad * rad);
      if (q > 1) continue;
      var gi = gx + i, gj = gy + j;
      if (gi < 1 || gi > N || gj < 1 || gj > N) continue;
      var f = Math.exp(-q * 1.8) * amt;
      var idx = IX(gi, gj);
      u[idx] += fx * f; v[idx] += fy * f;
      d[idx] = Math.min(2.4, d[idx] + f * 0.62);
    }
  }
}

// 결정적 미리보기 — 리사주 곡선 위를 도는 가짜 포인터. 이전 위치와의 델타를 실제 포인터와
// 똑같이 계산해 splat() 에 넘긴다. seedT 로 고정되므로 gif·poster 는 항상 같은 궤적을 그린다.
var lastLX = null, lastLY = null;
function inject(t) {
  var lx = N * 0.5 + Math.sin(t * 1.3) * N * 0.36;
  var ly = N * 0.5 + Math.sin(t * 2.1 + 1.2) * N * 0.30;
  var gx = Math.round(lx), gy = Math.round(ly);
  if (lastLX !== null) {
    var fx = (lx - lastLX) * 5.2, fy = (ly - lastLY) * 5.2;
    splat(gx, gy, fx, fy, Math.min(2.6, Math.hypot(fx, fy) * 0.9 + 0.4));
  } else {
    splat(gx, gy, 0, 0, 1.1);
  }
  lastLX = lx; lastLY = ly;
}

function render() {
  var img = ctx.createImageData(cv.width, cv.height);
  var px = img.data;
  var cellW = cv.width / N, cellH = cv.height / N;
  for (var py = 0; py < cv.height; py++) {
    var gy = Math.min(N - 1, (py / cellH) | 0);
    for (var pxx = 0; pxx < cv.width; pxx++) {
      var gx = Math.min(N - 1, (pxx / cellW) | 0);
      var q = d[IX(gx + 1, gy + 1)];
      var o = (py * cv.width + pxx) * 4;
      if (q <= 0.02) { px[o] = CR[0]; px[o + 1] = CR[1]; px[o + 2] = CR[2]; px[o + 3] = 255; continue; }
      var a = Math.min(1, q);
      var band = q > 1.1 ? K2 : K1;
      px[o]     = CR[0] + (band[0] - CR[0]) * a;
      px[o + 1] = CR[1] + (band[1] - CR[1]) * a;
      px[o + 2] = CR[2] + (band[2] - CR[2]) * a;
      px[o + 3] = 255;
    }
  }
  ctx.putImageData(img, 0, 0);
}

function frame() {
  inject(seedT);
  step(1 / 30);
  render();
  raf = requestAnimationFrame(frame);
}

var last = null;
field.addEventListener('pointermove', function (e) {
  var rect = field.getBoundingClientRect();
  var gx = 1 + Math.round((e.clientX - rect.left) / rect.width * (N - 1));
  var gy = 1 + Math.round((e.clientY - rect.top) / rect.height * (N - 1));
  if (last) {
    var fx = (e.clientX - last.x) * 0.42, fy = (e.clientY - last.y) * 0.42;
    splat(gx, gy, fx, fy, Math.min(2.6, Math.hypot(fx, fy)));
  } else {
    splat(gx, gy, 0, 0, 1.1);
  }
  last = { x: e.clientX, y: e.clientY };
  if (reduce) render();
});
field.addEventListener('pointerleave', function () { last = null; });
field.addEventListener('keydown', function (e) {
  var dir = { ArrowLeft: [-3, 0], ArrowRight: [3, 0], ArrowUp: [0, -3], ArrowDown: [0, 3] }[e.key];
  if (!dir) return;
  e.preventDefault();
  splat(N >> 1, N >> 1, dir[0], dir[1], 1.8);
  if (reduce) render();
});
window.addEventListener('resize', function () { fit(); if (reduce) render(); });

fit();
if (reduce) {
  // reduce — 리사주 미리보기를 46 걸음 앞으로 감아 소용돌이가 자리 잡은 한 장을 그린다
  for (var k = 0; k < 46; k++) { inject(seedT); step(1 / 30); }
  render();
} else {
  raf = requestAnimationFrame(frame);
}

// PointerStir — vanilla 판(run/308/demo/08-fluid-pointer-stir/index.html) 의 React 포트.
// 이 편 9종의 인터랙션 중심. 실제 포인터든 결정적 리사주 미리보기든 프레임 간 델타를
// 같은 splat() 한 함수로 속도장에 꽂아 넣는다.
import { useEffect, useRef } from "react";
import type { CSSProperties } from "react";
import styles from "./PointerStir.module.scss";

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

type Vars = CSSProperties & { "--duration"?: string; "--easing"?: string; "--color"?: string };
type Box<T> = { current: T };
type KeyEvt = { key: string; preventDefault(): void };
type PtrEvt = { clientX: number; clientY: number };

const N = 40;
const SIZE = (N + 2) * (N + 2);
function IX(x: number, y: number): number { return x + (N + 2) * y; }

export default function PointerStir({
  duration = "2s",
  easing = "cubic-bezier(.2, .8, .2, 1)",
  color = "#2f6df6",
}: PointerStirProps) {
  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 palRef = useRef({ cr: [255, 255, 255], k1: [51, 69, 74], k2: [74, 53, 80] }) as Box<{ cr: number[]; k1: number[]; k2: number[] }>;

  const u = useRef(new Float32Array(SIZE)) as Box<Float32Array>;
  const v = useRef(new Float32Array(SIZE)) as Box<Float32Array>;
  const u0 = useRef(new Float32Array(SIZE)) as Box<Float32Array>;
  const v0 = useRef(new Float32Array(SIZE)) as Box<Float32Array>;
  const d = useRef(new Float32Array(SIZE)) as Box<Float32Array>;
  const d0 = useRef(new Float32Array(SIZE)) as Box<Float32Array>;

  const seedTRef = useRef(0) as Box<number>;
  const lastRef = useRef<{ x: number; y: number } | null>(null) as Box<{ x: number; y: number } | null>;
  const lastLRef = useRef<{ x: number; y: number } | null>(null) as Box<{ x: number; y: number } | null>;
  const rafRef = useRef(0) as Box<number>;

  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;
    const cv = cvRef.current;
    if (!field || !cv) return;
    const rect = field.getBoundingClientRect();
    cv.width = Math.round(rect.width);
    cv.height = Math.round(rect.height);
  }

  function advect(b: 0 | 1 | 2, dt: number): void {
    const src = b === 0 ? u0.current : b === 1 ? v0.current : d0.current;
    const dst = b === 0 ? u.current : b === 1 ? v.current : d.current;
    const uu = u0.current, vv = v0.current;
    for (let j = 1; j <= N; j++) {
      for (let i = 1; i <= N; i++) {
        let x = i - dt * N * uu[IX(i, j)];
        let y = j - dt * N * vv[IX(i, j)];
        if (x < 0.5) x = 0.5; else if (x > N + 0.5) x = N + 0.5;
        const i0 = x | 0, i1 = i0 + 1;
        if (y < 0.5) y = 0.5; else if (y > N + 0.5) y = N + 0.5;
        const j0 = y | 0, j1 = j0 + 1;
        const s1 = x - i0, s0 = 1 - s1, t1 = y - j0, t0 = 1 - t1;
        dst[IX(i, j)] = s0 * (t0 * src[IX(i0, j0)] + t1 * src[IX(i0, j1)]) +
                        s1 * (t0 * src[IX(i1, j0)] + t1 * src[IX(i1, j1)]);
      }
    }
  }

  function bounds(): void {
    const dd = d.current, uu = u.current, vv = v.current;
    for (let i = 1; i <= N; i++) {
      dd[IX(i, 0)] = dd[IX(i, 1)] * 0.5; dd[IX(i, N + 1)] = dd[IX(i, N)] * 0.5;
      uu[IX(i, 0)] = uu[IX(i, 1)] * 0.5; uu[IX(i, N + 1)] = uu[IX(i, N)] * 0.5;
      vv[IX(i, 0)] = vv[IX(i, 1)] * 0.5; vv[IX(i, N + 1)] = vv[IX(i, N)] * 0.5;
    }
    for (let j = 1; j <= N; j++) {
      dd[IX(0, j)] = dd[IX(1, j)] * 0.5; dd[IX(N + 1, j)] = dd[IX(N, j)] * 0.5;
      uu[IX(0, j)] = uu[IX(1, j)] * 0.5; uu[IX(N + 1, j)] = uu[IX(N, j)] * 0.5;
      vv[IX(0, j)] = vv[IX(1, j)] * 0.5; vv[IX(N + 1, j)] = vv[IX(N, j)] * 0.5;
    }
  }

  function step(dt: number): void {
    u0.current.set(u.current); v0.current.set(v.current); d0.current.set(d.current);
    advect(0, dt); u0.current.set(u.current);
    advect(1, dt); v0.current.set(v.current);
    advect(2, dt);
    bounds();
    const uu = u.current, vv = v.current, dd = d.current;
    for (let k = 0; k < SIZE; k++) { uu[k] *= 0.992; vv[k] *= 0.992; dd[k] *= 0.986; }
    seedTRef.current += dt;
  }

  function splat(gx: number, gy: number, fx: number, fy: number, amt: number): void {
    const rad = 7;
    const uu = u.current, vv = v.current, dd = d.current;
    for (let j = -rad; j <= rad; j++) {
      for (let i = -rad; i <= rad; i++) {
        const q = (i * i + j * j) / (rad * rad);
        if (q > 1) continue;
        const gi = gx + i, gj = gy + j;
        if (gi < 1 || gi > N || gj < 1 || gj > N) continue;
        const f = Math.exp(-q * 1.8) * amt;
        const idx = IX(gi, gj);
        uu[idx] += fx * f; vv[idx] += fy * f;
        dd[idx] = Math.min(2.4, dd[idx] + f * 0.62);
      }
    }
  }

  function inject(t: number): void {
    const lx = N * 0.5 + Math.sin(t * 1.3) * N * 0.36;
    const ly = N * 0.5 + Math.sin(t * 2.1 + 1.2) * N * 0.30;
    const gx = Math.round(lx), gy = Math.round(ly);
    const last = lastLRef.current;
    if (last) {
      const fx = (lx - last.x) * 5.2, fy = (ly - last.y) * 5.2;
      splat(gx, gy, fx, fy, Math.min(2.6, Math.hypot(fx, fy) * 0.9 + 0.4));
    } else {
      splat(gx, gy, 0, 0, 1.1);
    }
    lastLRef.current = { x: lx, y: ly };
  }

  function render(): void {
    const ctx = ctxRef.current, cv = cvRef.current;
    if (!ctx || !cv) return;
    const img = ctx.createImageData(cv.width, cv.height);
    const px = img.data;
    const cellW = cv.width / N, cellH = cv.height / N;
    const pal = palRef.current;
    const dd = d.current;
    for (let py = 0; py < cv.height; py++) {
      const gy = Math.min(N - 1, (py / cellH) | 0);
      for (let pxx = 0; pxx < cv.width; pxx++) {
        const gx = Math.min(N - 1, (pxx / cellW) | 0);
        const q = dd[IX(gx + 1, gy + 1)];
        const o = (py * cv.width + pxx) * 4;
        if (q <= 0.02) { px[o] = pal.cr[0]; px[o + 1] = pal.cr[1]; px[o + 2] = pal.cr[2]; px[o + 3] = 255; continue; }
        const a = Math.min(1, q);
        const band = q > 1.1 ? pal.k2 : pal.k1;
        px[o]     = pal.cr[0] + (band[0] - pal.cr[0]) * a;
        px[o + 1] = pal.cr[1] + (band[1] - pal.cr[1]) * a;
        px[o + 2] = pal.cr[2] + (band[2] - pal.cr[2]) * a;
        px[o + 3] = 255;
      }
    }
    ctx.putImageData(img, 0, 0);
  }

  function frame(): void {
    inject(seedTRef.current);
    step(1 / 30);
    render();
    rafRef.current = requestAnimationFrame(frame);
  }

  function onMove(e: PtrEvt): void {
    const field = fieldRef.current;
    if (!field) return;
    const rect = field.getBoundingClientRect();
    const gx = 1 + Math.round((e.clientX - rect.left) / rect.width * (N - 1));
    const gy = 1 + Math.round((e.clientY - rect.top) / rect.height * (N - 1));
    const last = lastRef.current;
    if (last) {
      const fx = (e.clientX - last.x) * 0.42, fy = (e.clientY - last.y) * 0.42;
      splat(gx, gy, fx, fy, Math.min(2.6, Math.hypot(fx, fy)));
    } else {
      splat(gx, gy, 0, 0, 1.1);
    }
    lastRef.current = { x: e.clientX, y: e.clientY };
    if (reduced()) render();
  }

  function onLeave(): void { lastRef.current = null; }

  function onKey(e: KeyEvt): void {
    const dir: Record<string, number[]> = { ArrowLeft: [-3, 0], ArrowRight: [3, 0], ArrowUp: [0, -3], ArrowDown: [0, 3] };
    const dv = dir[e.key];
    if (!dv) return;
    e.preventDefault();
    splat(N >> 1, N >> 1, dv[0], dv[1], 1.8);
    if (reduced()) render();
  }

  function onResize(): void { fit(); if (reduced()) render(); }

  function hexToRgb(c: string): number[] {
    return [parseInt(c.slice(1, 3), 16), parseInt(c.slice(3, 5), 16), parseInt(c.slice(5, 7), 16)];
  }

  useEffect(() => {
    const cv = cvRef.current;
    if (!cv) return;
    ctxRef.current = cv.getContext("2d", { willReadFrequently: true });
    if (!ctxRef.current) return;
    const root = rootRef.current;
    if (root) {
      const css = getComputedStyle(root);
      const pt = css.getPropertyValue("--dye-teal").trim();
      const pp = css.getPropertyValue("--dye-plum").trim();
      const pw = css.getPropertyValue("--dye-paper").trim();
      if (pt && pp && pw) {
        palRef.current = { cr: hexToRgb(pw), k1: hexToRgb(pt), k2: hexToRgb(pp) };
      }
    }
    fit();
    window.addEventListener("resize", onResize);
    if (reduced()) {
      if (root) root.classList.add("is-still");
      for (let k = 0; k < 46; k++) { inject(seedTRef.current); step(1 / 30); }
      render();
    } else {
      rafRef.current = requestAnimationFrame(frame);
    }
    return () => {
      window.removeEventListener("resize", onResize);
      cancelAnimationFrame(rafRef.current);
    };
    // eslint-disable-next-line react-hooks/exhaustive-deps
  }, []);

  return (
    <main className={styles.stage}>
      <div className={styles.root} 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="M100.69,180.7,41.34,121.35a16,16,0,0,1,0-22.63L98.72,41.34a16,16,0,0,1,22.63,0l99.31,99.32a16,16,0,0,1,0,22.62l-57.38,57.38a16,16,0,0,1-22.63,0Z" opacity=".2"/><path d="M228.69,133.66,131.32,36.28a24,24,0,0,0-33.94,0L38.34,95.31a24,24,0,0,0,0,34l97.37,97.37a24,24,0,0,0,33.94,0l59-59A24,24,0,0,0,228.69,133.66Zm-16.97,17L152.68,209.7a8,8,0,0,1-11.31,0L44,112.34a8,8,0,0,1,0-11.32L103.06,42a8,8,0,0,1,11.31,0l97.35,97.35A8,8,0,0,1,211.72,150.63Z"/></g></svg>
          <span>커서가 움직인 만큼 속도장을 밀어냅니다 — 빠르게 휘저을수록 소용돌이가 커집니다</span>
        </p>
        <div
          className={styles.field}
          ref={fieldRef}
          role="application"
          aria-label="포인터 휘젓기 실험판"
          tabIndex={0}
          onPointerMove={onMove}
          onPointerLeave={onLeave}
          onKeyDown={onKey}
        >
          <canvas className={styles.cv} ref={cvRef}></canvas>
          <b className={styles.badge} aria-hidden="true">stir<span className={styles.badgeUnit}>splat</span></b>
        </div>
      </div>
    </main>
  );
}