02Diffusion bleed

<main class="stage stage--paper">
<div class="fa" id="fa">
  <p class="fa__hint">
    <svg class="fa__ico" viewBox="0 0 256 256" aria-hidden="true"><g fill="currentColor"><path d="M208 144a80 80 0 0 1-160 0c0-72 80-128 80-128s80 56 80 128" opacity=".2"/><path d="M174 47.75a254.2 254.2 0 0 0-41.45-38.3a8 8 0 0 0-9.18 0A254.2 254.2 0 0 0 82 47.75C54.51 79.32 40 112.6 40 144a88 88 0 0 0 176 0c0-31.4-14.51-64.68-42-96.25M128 216a72.08 72.08 0 0 1-72-72c0-57.23 55.47-105 72-118c16.53 13 72 60.75 72 118a72.08 72.08 0 0 1-72 72m55.89-62.66a57.6 57.6 0 0 1-46.56 46.55a9 9 0 0 1-1.33.11a8 8 0 0 1-1.32-15.89c16.57-2.79 30.63-16.85 33.44-33.45a8 8 0 0 1 15.78 2.68Z"/></g></svg>
    <span>Diffusion blends each cell with its four neighbors every step, spreading ink across the paper — press to drop a bead</span>
  </p>
  <div class="fa__field" id="field" role="application" aria-label="Diffusion blur board" tabindex="0">
    <canvas class="fa__cv" id="cv"></canvas>
    <b class="fa__badge" aria-hidden="true">6<span class="fa__badgeUnit">relax</span></b>
  </div>
</div>
</main>

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

// 확산 번짐 — 움직임은 전부 캔버스 rAF 루프 안에 있다. CSS 는 무대·판·배지만 맡는다.
// @keyframes 를 하나도 두지 않는다 — 렌더러가 CSS 시계를 고정하지 않고 실시간으로 24장을 찍는다(01·012·077 선례).
// 종이(paper) 무대 위 염료는 밝은 배경에서 보이는 어두운·낮은 채도 파스텔 세 층으로 잡는다 —
// 회보라(짙음) → 청회색(중간) → 연회색(옅음). 채도 전부 0.21 이하로 BD26 파스텔 게이트를 처음부터 지킨다.

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

@include stage($stage-paper);

.fa {
  // 염료 세 층 — 짙은 회보라 → 청회색 → 연회색. 흰 종이 위 대비 6.0~9.9
  --dye-0: #8b7f95;
  --dye-1: #5c4f66;
  --dye-2: #2d2836;

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

.fa__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;
}
.fa__ico { width: 16px; height: 16px; flex: 0 0 auto; color: #5c4f66; }

.fa__field {
  position: relative; height: 208px;
  border-radius: $r-card; overflow: hidden;
  background: $stage-paper;
  box-shadow: inset 0 0 0 1px rgba($subject-ink, .12);
  cursor: crosshair; touch-action: none;
}
.fa__field:focus-visible { outline: 2px solid #5c4f66; outline-offset: 2px; }

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

.fa__badge {
  position: absolute; right: $sp-2; top: $sp-2;
  display: flex; align-items: baseline; gap: $sp-1;
  padding: $sp-1 $sp-2; border-radius: $r-pill;
  background: rgba($stage-paper, .82);
  color: $subject-ink; font-weight: 700; font-size: 11px;
  box-shadow: inset 0 0 0 1px rgba($subject-ink, .1);
}
.fa__badgeUnit { font-weight: 700; }

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

// reduce — 루프가 멈춘 한 장이 남으므로 커서 신호만 바꾼다.
@media (prefers-reduced-motion: reduce) {
  .fa.is-still .fa__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);
}

.fa {
  --dye-0: #8b7f95;
  --dye-1: #5c4f66;
  --dye-2: #2d2836;
  display: flex;
  flex-direction: column;
  gap: 8px;
  width: 100%;
  max-width: 424px;
}

.fa__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;
}

.fa__ico {
  width: 16px;
  height: 16px;
  flex: 0 0 auto;
  color: #5c4f66;
}

.fa__field {
  position: relative;
  height: 208px;
  border-radius: 16px;
  overflow: hidden;
  background: #ffffff;
  box-shadow: inset 0 0 0 1px rgba(23, 20, 26, 0.12);
  cursor: crosshair;
  touch-action: none;
}

.fa__field:focus-visible {
  outline: 2px solid #5c4f66;
  outline-offset: 2px;
}

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

.fa__badge {
  position: absolute;
  right: 8px;
  top: 8px;
  display: flex;
  align-items: baseline;
  gap: 4px;
  padding: 4px 8px;
  border-radius: 999px;
  background: rgba(255, 255, 255, 0.82);
  color: #17141a;
  font-weight: 700;
  font-size: 11px;
  box-shadow: inset 0 0 0 1px rgba(23, 20, 26, 0.1);
}

.fa__badgeUnit {
  font-weight: 700;
}

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

// 확산(diffuse) — 이류처럼 값을 옮기지 않는다. 셀 하나가 이웃 네 칸과 값을 조금씩 섞는 일을
// 매 걸음 반복하면(가우스-사이델 relax), 진한 점 하나가 몇 걸음 만에 부드러운 얼룩으로 퍼진다.
// 속도장 없이 밀도(d) 하나만으로 돌아간다 — 이류 비중을 없애 확산 자체가 움직임의 전부가 되게 한다.
var root = document.getElementById('fa');
var field = document.getElementById('field');
var cv = document.getElementById('cv');
var ctx = cv.getContext('2d', { willReadFrequently: true });

var css = getComputedStyle(root);
var PAL0 = css.getPropertyValue('--dye-0').trim();
var PAL1 = css.getPropertyValue('--dye-1').trim();
var PAL2 = css.getPropertyValue('--dye-2').trim();

// N — grid 한 변 칸 수. 480px 무대에 32칸이면 한 칸 15px, relax 6회를 60fps 예산 안에 담는다
var N = 32;
var SIZE = (N + 2) * (N + 2);
var IX = function (x, y) { return x + (N + 2) * y; };
var d = new Float32Array(SIZE), d0 = new Float32Array(SIZE);

var ITER = 6;     // 가우스-사이델 relax 반복 횟수 — 4~10회 권장, 6회면 한 걸음에 충분히 번진다
var DIFF = 0.02;  // 확산 계수 — 클수록 데칼코마니처럼 빨리 퍼지고, 0 에 가까우면 선명한 경계가 남는다

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

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

// 경계 — 종이 가장자리에서 잉크가 반사돼 옅게 남는다
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;
  }
  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;
  }
}

// 확산 한 걸음 — x0(방금 주입한 소스)를 목표로, x(이전 값)를 이웃 네 칸과 섞어가며 완화한다.
// a 가 클수록 이웃의 영향이 커져 한 걸음에 더 멀리 번진다. 경계는 반복마다 다시 잡는다.
function diffuse(dt) {
  var a = dt * DIFF * N * N;
  for (var k = 0; k < ITER; k++) {
    for (var j = 1; j <= N; j++) {
      for (var i = 1; i <= N; i++) {
        d[IX(i, j)] = (d0[IX(i, j)] + a * (d[IX(i - 1, j)] + d[IX(i + 1, j)] + d[IX(i, j - 1)] + d[IX(i, j + 1)])) / (1 + 4 * a);
      }
    }
    bounds();
  }
}

// 미리보기 주입원 — 시드 고정. 종이 위를 천천히 오가는 붓끝이 방울을 떨어뜨린다.
// seedT 로 결정적: 같은 편의 gif·poster 는 언제나 같은 번짐이 담긴다.
function inject(t) {
  var cx = N * 0.5 + Math.sin(t * 0.55) * N * 0.28;
  var cy = N * 0.5 + Math.cos(t * 0.4) * N * 0.24;
  var r = 2.2;
  var amt = 0.85 + Math.sin(t * 2.3) * 0.15;
  for (var j = -r; j <= r; j++) {
    for (var i = -r; i <= r; i++) {
      var q = (i * i + j * j) / (r * r);
      if (q > 1) continue;
      var f = Math.exp(-q * 3) * amt;
      var idx = IX(Math.round(cx + i), Math.round(cy + j));
      d0[idx] = Math.min(2, d0[idx] + f);
    }
  }
}

function step(dt) {
  d0.set(d);
  inject(seedT);
  diffuse(dt);
  for (var k = 0; k < SIZE; k++) d[k] *= 0.992;   // 아주 천천히 마른다 — 화면이 꽉 찬 채 굳지 않게
  seedT += dt;
}

function render() {
  var img = ctx.createImageData(cv.width, cv.height);
  var px = img.data;
  var cellW = cv.width / N, cellH = cv.height / N;
  var pal = [PAL0, PAL1, PAL2];
  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 <= 0.002) { px[o] = 255; px[o + 1] = 255; px[o + 2] = 255; px[o + 3] = 255; continue; }
      var band = q > 0.62 ? 2 : (q > 0.28 ? 1 : 0);
      var c = pal[band];
      var rr = parseInt(c.slice(1, 3), 16), gg = parseInt(c.slice(3, 5), 16), bb = parseInt(c.slice(5, 7), 16);
      var a = Math.min(1, q);
      px[o] = 255 + (rr - 255) * a; px[o + 1] = 255 + (gg - 255) * a; px[o + 2] = 255 + (bb - 255) * a; px[o + 3] = 255;
    }
  }
  ctx.putImageData(img, 0, 0);
}

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

// 사람이 찍기 — 눌러 둔 자리에 방울을 계속 떨어뜨린다. 확산은 다음 걸음부터 저절로 번진다.
function splat(gx, gy, amt) {
  var r = 3;
  for (var j = -r; j <= r; j++) {
    for (var i = -r; i <= r; i++) {
      var q = (i * i + j * j) / (r * r);
      if (q > 1) continue;
      if (gx + i < 1 || gx + i > N || gy + j < 1 || gy + j > N) continue;
      var f = Math.exp(-q * 3) * amt;
      var idx = IX(gx + i, gy + j);
      d[idx] = Math.min(2.2, d[idx] + f);
    }
  }
}
var down = false;
function pointAt(e) {
  var r = field.getBoundingClientRect();
  var gx = 1 + Math.round((e.clientX - r.left) / r.width * (N - 1));
  var gy = 1 + Math.round((e.clientY - r.top) / r.height * (N - 1));
  splat(gx, gy, 0.9);
  if (reduce) render();
}
field.addEventListener('pointerdown', function (e) { down = true; pointAt(e); });
field.addEventListener('pointermove', function (e) { if (down) pointAt(e); });
window.addEventListener('pointerup', function () { down = false; });
field.addEventListener('pointerleave', function () { down = false; });
field.addEventListener('keydown', function (e) {
  var dir = { ArrowLeft: [-2, 0], ArrowRight: [2, 0], ArrowUp: [0, -2], ArrowDown: [0, 2] }[e.key];
  if (!dir) return;
  e.preventDefault();
  splat((N >> 1) + dir[0], (N >> 1) + dir[1], 1.1);
  if (reduce) render();
});
window.addEventListener('resize', function () { fit(); if (reduce) render(); });

fit();
if (reduce) {
  // reduce — 시뮬레이션을 40 걸음만 앞으로 감아 번짐이 자리잡은 한 장을 그린다(빈 화면이 아니다)
  for (var k = 0; k < 40; k++) step(1 / 30);
  render();
} else {
  raf = requestAnimationFrame(frame);
}

// DiffuseBlur — vanilla 판(run/308/demo/02-fluid-diffuse-blur/index.html) 의 React 포트.
// 속도장 없이 밀도(d) 하나만으로 가우스-사이델 완화 6회를 매 걸음 돌려 확산 자체가 움직임의 전부가 되게 한다.
// 격자 필드는 ref 상자 안에서만 산다 — React 상태로 두면 리렌더가 매 프레임 일어나 60fps 예산을 못 맞춘다.
import { useEffect, useRef } from "react";
import type { CSSProperties } from "react";
import styles from "./DiffuseBlur.module.scss";

export type DiffuseBlurProps = {
  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; buttons?: number };

const N = 32;
const SIZE = (N + 2) * (N + 2);
function IX(x: number, y: number): number { return x + (N + 2) * y; }

const ITER = 6;
const DIFF = 0.02;

export default function DiffuseBlur({
  duration = "2s",
  easing = "cubic-bezier(.2, .8, .2, 1)",
  color = "#17141a",
}: DiffuseBlurProps) {
  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(["#8b7f95", "#5c4f66", "#2d2836"]) as Box<string[]>;

  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 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 r = field.getBoundingClientRect();
    cv.width = Math.round(r.width);
    cv.height = Math.round(r.height);
  }

  function bounds(): void {
    const dd = d.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;
    }
    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;
    }
  }

  function diffuse(dt: number): void {
    const dd = d.current, dd0 = d0.current;
    const a = dt * DIFF * N * N;
    for (let k = 0; k < ITER; k++) {
      for (let j = 1; j <= N; j++) {
        for (let i = 1; i <= N; i++) {
          dd[IX(i, j)] = (dd0[IX(i, j)] + a * (dd[IX(i - 1, j)] + dd[IX(i + 1, j)] + dd[IX(i, j - 1)] + dd[IX(i, j + 1)])) / (1 + 4 * a);
        }
      }
      bounds();
    }
  }

  function inject(t: number): void {
    const cx = N * 0.5 + Math.sin(t * 0.55) * N * 0.28;
    const cy = N * 0.5 + Math.cos(t * 0.4) * N * 0.24;
    const r = 2.2;
    const amt = 0.85 + Math.sin(t * 2.3) * 0.15;
    const dd0 = d0.current;
    for (let j = -r; j <= r; j++) {
      for (let i = -r; i <= r; i++) {
        const q = (i * i + j * j) / (r * r);
        if (q > 1) continue;
        const f = Math.exp(-q * 3) * amt;
        dd0[IX(Math.round(cx + i), Math.round(cy + j))] = Math.min(2, dd0[IX(Math.round(cx + i), Math.round(cy + j))] + f);
      }
    }
  }

  function step(dt: number): void {
    d0.current.set(d.current);
    inject(seedTRef.current);
    diffuse(dt);
    const dd = d.current;
    for (let k = 0; k < SIZE; k++) dd[k] *= 0.992;
    seedTRef.current += dt;
  }

  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 <= 0.002) { px[o] = 255; px[o + 1] = 255; px[o + 2] = 255; px[o + 3] = 255; continue; }
        const band = q > 0.62 ? 2 : q > 0.28 ? 1 : 0;
        const c = pal[band];
        const rr = parseInt(c.slice(1, 3), 16), gg = parseInt(c.slice(3, 5), 16), bb = parseInt(c.slice(5, 7), 16);
        const a = Math.min(1, q);
        px[o] = 255 + (rr - 255) * a; px[o + 1] = 255 + (gg - 255) * a; px[o + 2] = 255 + (bb - 255) * a; px[o + 3] = 255;
      }
    }
    ctx.putImageData(img, 0, 0);
  }

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

  function splat(gx: number, gy: number, amt: number): void {
    const r = 3;
    const dd = d.current;
    for (let j = -r; j <= r; j++) {
      for (let i = -r; i <= r; i++) {
        const q = (i * i + j * j) / (r * r);
        if (q > 1) continue;
        if (gx + i < 1 || gx + i > N || gy + j < 1 || gy + j > N) continue;
        const f = Math.exp(-q * 3) * amt;
        const idx = IX(gx + i, gy + j);
        dd[idx] = Math.min(2.2, dd[idx] + f);
      }
    }
  }

  function pointAt(e: PtrEvt): void {
    const field = fieldRef.current;
    if (!field) return;
    const r = field.getBoundingClientRect();
    const gx = 1 + Math.round((e.clientX - r.left) / r.width * (N - 1));
    const gy = 1 + Math.round((e.clientY - r.top) / r.height * (N - 1));
    splat(gx, gy, 0.9);
    if (reduced()) render();
  }

  function onKey(e: KeyEvt): void {
    const dir = ({ ArrowLeft: [-2, 0], ArrowRight: [2, 0], ArrowUp: [0, -2], ArrowDown: [0, 2] } as Record<string, number[]>)[e.key];
    if (!dir) return;
    e.preventDefault();
    splat((N >> 1) + dir[0], (N >> 1) + dir[1], 1.1);
    if (reduced()) render();
  }

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

  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 p0 = css.getPropertyValue("--dye-0").trim();
      const p1 = css.getPropertyValue("--dye-1").trim();
      const p2 = css.getPropertyValue("--dye-2").trim();
      if (p0 && p1 && p2) palRef.current = [p0, p1, p2];
    }
    fit();
    window.addEventListener("resize", onResize);
    if (reduced()) {
      if (root) root.classList.add("is-still");
      for (let k = 0; k < 40; k++) 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="M208 144a80 80 0 0 1-160 0c0-72 80-128 80-128s80 56 80 128" opacity=".2"/><path d="M174 47.75a254.2 254.2 0 0 0-41.45-38.3a8 8 0 0 0-9.18 0A254.2 254.2 0 0 0 82 47.75C54.51 79.32 40 112.6 40 144a88 88 0 0 0 176 0c0-31.4-14.51-64.68-42-96.25M128 216a72.08 72.08 0 0 1-72-72c0-57.23 55.47-105 72-118c16.53 13 72 60.75 72 118a72.08 72.08 0 0 1-72 72m55.89-62.66a57.6 57.6 0 0 1-46.56 46.55a9 9 0 0 1-1.33.11a8 8 0 0 1-1.32-15.89c16.57-2.79 30.63-16.85 33.44-33.45a8 8 0 0 1 15.78 2.68Z"/></g></svg>
          <span>매 걸음 이웃 넷과 값을 섞는 확산이 잉크를 종이에 번지게 합니다 — 눌러서 방울을 떨어뜨려 보세요</span>
        </p>
        <div
          className={styles.field}
          ref={fieldRef}
          role="application"
          aria-label="확산 번짐 실험판"
          tabIndex={0}
          onPointerDown={(e: PtrEvt) => pointAt(e)}
          onPointerMove={(e: PtrEvt) => { if ((e.buttons ?? 0) !== 0) pointAt(e); }}
          onKeyDown={onKey}
        >
          <canvas className={styles.cv} ref={cvRef}></canvas>
          <b className={styles.badge} aria-hidden="true">6<span className={styles.badgeUnit}>완화</span></b>
        </div>
      </div>
    </main>
  );
}