06회전 입체의 면 밝기를 점 밀도로 바꾸는 3D 디더 큐브

<main class="stage">
<div class="sc is-demo" id="scRoot">
  <canvas class="sc__canvas" id="scCanvas" width="480" height="300" tabindex="0" aria-label="회전 입체의 면 밝기를 점 밀도로 바꾸는 3D 디더 큐브"></canvas>
  <p class="sc__status" id="scStatus" aria-live="polite">면의 밝기가 점 밀도로 바뀐다 — 큐브가 돈다</p>
  <button class="sc__btn" id="scBtn" type="button">멈춤</button>
  <p class="sc__caption">회전 입체의 면 밝기를 점 밀도로 바꾸는 3D 디더 큐브</p>
</div>
</main>

@use "../../../../site/static/demo-tokens" as *;
@include stage($stage-ink);

.sc {
  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: $subject-ink;
}

.sc__canvas {
  display: block;
  width: 100%;
  height: 100%;
  touch-action: none;
  image-rendering: pixelated;
}

.sc__canvas:focus-visible {
  outline: 2px solid rgba(255, 255, 255, .85);
  outline-offset: -6px;
}

.sc__status {
  position: absolute;
  left: $sp-3;
  top: $sp-3;
  max-width: 62%;
  margin: 0;
  padding: $sp-1 $sp-3;
  border-radius: $r-pill;
  background: rgba(255, 255, 255, .85);
  color: $subject-ink;
  font-weight: 500;
  font-size: 11px;
  line-height: 1.4;
  pointer-events: none;
}

.sc__btn {
  position: absolute;
  right: $sp-3;
  bottom: $sp-3;
  margin: 0;
  padding: $sp-1 $sp-3;
  border: 0;
  border-radius: $r-pill;
  background: rgba(255, 255, 255, .85);
  color: $subject-ink;
  font: inherit;
  font-weight: 500;
  font-size: 12px;
  line-height: 1.4;
  cursor: pointer;
}

.sc__btn:focus-visible {
  outline: 2px solid #ffffff;
  outline-offset: 2px;
}

.sc__caption {
  position: absolute;
  left: $sp-3;
  bottom: $sp-3;
  max-width: 68%;
  margin: 0;
  padding: $sp-1 $sp-3;
  border-radius: $r-pill;
  background: rgba(255, 255, 255, .85);
  color: $subject-ink;
  font-weight: 500;
  font-size: 12px;
  line-height: 1.4;
  pointer-events: none;
}

@media (max-width: 340px) {
  .sc__caption, .sc__btn { font-size: 10px; padding: $sp-1 $sp-2; }
  .sc__status { font-size: 10px; }
}

@media (prefers-reduced-motion: reduce) {
  .sc { 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);
}

.sc {
  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;
}

.sc__canvas {
  display: block;
  width: 100%;
  height: 100%;
  touch-action: none;
  image-rendering: pixelated;
}

.sc__canvas:focus-visible {
  outline: 2px solid rgba(255, 255, 255, 0.85);
  outline-offset: -6px;
}

.sc__status {
  position: absolute;
  left: 12px;
  top: 12px;
  max-width: 62%;
  margin: 0;
  padding: 4px 12px;
  border-radius: 999px;
  background: rgba(255, 255, 255, 0.85);
  color: #17141a;
  font-weight: 500;
  font-size: 11px;
  line-height: 1.4;
  pointer-events: none;
}

.sc__btn {
  position: absolute;
  right: 12px;
  bottom: 12px;
  margin: 0;
  padding: 4px 12px;
  border: 0;
  border-radius: 999px;
  background: rgba(255, 255, 255, 0.85);
  color: #17141a;
  font: inherit;
  font-weight: 500;
  font-size: 12px;
  line-height: 1.4;
  cursor: pointer;
}

.sc__btn:focus-visible {
  outline: 2px solid #ffffff;
  outline-offset: 2px;
}

.sc__caption {
  position: absolute;
  left: 12px;
  bottom: 12px;
  max-width: 68%;
  margin: 0;
  padding: 4px 12px;
  border-radius: 999px;
  background: rgba(255, 255, 255, 0.85);
  color: #17141a;
  font-weight: 500;
  font-size: 12px;
  line-height: 1.4;
  pointer-events: none;
}

@media (max-width: 340px) {
  .sc__caption, .sc__btn {
    font-size: 10px;
    padding: 4px 8px;
  }
  .sc__status {
    font-size: 10px;
  }
}
@media (prefers-reduced-motion: reduce) {
  .sc {
    cursor: default;
  }
}

(function () {
    const W = 480;
    const H = 300;
    const PI2 = Math.PI * 2;
    const FIXED_DT = 16;
    const REDUCE_STEP_MS = 900;
    const KEY_STEP = 14;
    const IDLE_TICKS = 150;
    const BTN_PAUSE = '멈춤';
    const BTN_PLAY = '재생';
    const BTN_NEXT = '다음 장면';
    // 점을 찍는 작은 격자 — 화면에는 3배로 확대해 올린다
    const GW = 160;
    const GH = 100;
    function clamp(v, lo, hi) { return v < lo ? lo : (v > hi ? hi : v); }
    function lerp(a, b, k) { return a + (b - a) * k; }
    function smooth(x) { const c = clamp(x, 0, 1); return c * c * (3 - 2 * c); }
    function frac(x) { return x - Math.floor(x); }
    // 시드 고정 난수 — 같은 시드면 언제나 같은 수열 (선형 합동 방식)
    function lcg(seed) {
        let s = seed >>> 0;
        return function () {
            s = (Math.imul(s, 1664525) + 1013904223) >>> 0;
            return s / 4294967296;
        };
    }
    // 좌표 두 개와 시드로 0 이상 1 미만의 값을 만드는 정수 해시 (난수 함수 없이도 같은 좌표는 늘 같은 값)
    function hash2(x, y, seed) {
        let h = (Math.imul(x | 0, 374761393) + Math.imul(y | 0, 668265263) + Math.imul(seed | 0, 1442695041)) | 0;
        h = Math.imul(h ^ (h >>> 13), 1274126177);
        h ^= h >>> 16;
        return (h >>> 0) / 4294967296;
    }
    // 격자 네 점의 해시를 smooth 로 이어 붙인 값 노이즈
    function vnoise(x, y, seed) {
        const xi = Math.floor(x);
        const yi = Math.floor(y);
        const fx = smooth(x - xi);
        const fy = smooth(y - yi);
        const a = lerp(hash2(xi, yi, seed), hash2(xi + 1, yi, seed), fx);
        const b = lerp(hash2(xi, yi + 1, seed), hash2(xi + 1, yi + 1, seed), fx);
        return lerp(a, b, fy);
    }
    // 베이어 임계 행렬 — n 은 2 의 거듭제곱. 값은 0 초과 1 미만으로 맞춘다
    function makeBayer(n) {
        let size = 2;
        let m = [0, 2, 3, 1];
        const quad = [0, 2, 3, 1];
        while (size < n) {
            const s2 = size * 2;
            const next = [];
            for (let y = 0; y < s2; y += 1) {
                for (let x = 0; x < s2; x += 1) {
                    const q = (y >= size ? 2 : 0) + (x >= size ? 1 : 0);
                    next.push(4 * m[(y % size) * size + (x % size)] + quad[q]);
                }
            }
            m = next;
            size = s2;
        }
        const out = new Float32Array(n * n);
        for (let i = 0; i < out.length; i += 1) {
            out[i] = (m[i] + 0.5) / (n * n);
        }
        return out;
    }
    // 색 하나를 캔버스 픽셀 한 개(리틀 엔디언 RGBA)로 묶는다
    function pack(c) {
        return ((255 << 24) | (Math.round(c[2]) << 16) | (Math.round(c[1]) << 8) | Math.round(c[0])) >>> 0;
    }
    function mixPack(a, b, k) {
        return pack([lerp(a[0], b[0], k), lerp(a[1], b[1], k), lerp(a[2], b[2], k)]);
    }
    function makeBuf() {
        const off = document.createElement('canvas');
        off.width = GW;
        off.height = GH;
        const octx = off.getContext('2d');
        if (!octx) {
            throw new Error('2d context unavailable');
        }
        const img = octx.createImageData(GW, GH);
        return { off: off, octx: octx, img: img, px: new Uint32Array(img.data.buffer) };
    }
    // 작은 격자를 큰 캔버스에 보간 없이 확대해 올린다
    function flush(ctx, b) {
        b.octx.putImageData(b.img, 0, 0);
        ctx.imageSmoothingEnabled = false;
        ctx.drawImage(b.off, 0, 0, W, H);
    }
    function start(root, canvas, statusEl, btnEl, make) {
        const ctx0 = canvas.getContext('2d');
        if (!ctx0) {
            return function () { return; };
        }
        const ctx = ctx0;
        const dpr = Math.min(window.devicePixelRatio || 1, 2);
        canvas.width = Math.round(W * dpr);
        canvas.height = Math.round(H * dpr);
        ctx.setTransform(dpr, 0, 0, dpr, 0, 0);
        const reduce = window.matchMedia('(prefers-reduced-motion: reduce)').matches;
        const inp = { x: W / 2, y: H / 2, idle: 99999 };
        const scene = make(inp, reduce);
        let paused = false;
        let rafId = 0;
        let lastTs = -1;
        let accMs = 0;
        let lastLabel = '';
        const cleaners = [];
        function listen(target, type, fn) {
            target.addEventListener(type, fn);
            cleaners.push(function () { target.removeEventListener(type, fn); });
        }
        function show() {
            scene.draw(ctx);
            const t = scene.label();
            if (t !== lastLabel) {
                lastLabel = t;
                statusEl.textContent = t;
            }
        }
        function advance(ms) {
            const n = Math.round(ms / FIXED_DT);
            for (let i = 0; i < n; i += 1) {
                inp.idle += 1;
                scene.step();
            }
        }
        function refresh() {
            if (reduce) {
                advance(FIXED_DT * 6);
                show();
            }
        }
        function toLocal(e) {
            const r = canvas.getBoundingClientRect();
            inp.x = clamp((e.clientX - r.left) * W / r.width, 0, W);
            inp.y = clamp((e.clientY - r.top) * H / r.height, 0, H);
            inp.idle = 0;
        }
        function tick(ts) {
            if (lastTs < 0) {
                lastTs = ts;
            }
            const frameDt = Math.min(ts - lastTs, 160);
            lastTs = ts;
            if (!paused) {
                accMs += frameDt;
                while (accMs >= FIXED_DT) {
                    inp.idle += 1;
                    scene.step();
                    accMs -= FIXED_DT;
                }
                show();
            }
            rafId = requestAnimationFrame(tick);
        }
        btnEl.textContent = reduce ? BTN_NEXT : BTN_PAUSE;
        if (reduce) {
            root.classList.remove('is-demo');
            advance(1400);
            show();
        }
        listen(btnEl, 'click', function () {
            if (reduce) {
                advance(REDUCE_STEP_MS);
                show();
                return;
            }
            paused = !paused;
            if (paused) {
                root.classList.remove('is-demo');
            }
            else {
                root.classList.add('is-demo');
            }
            btnEl.textContent = paused ? BTN_PLAY : BTN_PAUSE;
        });
        listen(canvas, 'pointermove', function (e) {
            toLocal(e);
            refresh();
        });
        listen(canvas, 'pointerdown', function (e) {
            toLocal(e);
            scene.tap();
            refresh();
        });
        listen(canvas, 'keydown', function (e) {
            const k = e.key;
            let dx = 0;
            let dy = 0;
            if (k === 'Enter' || k === ' ') {
                e.preventDefault();
                scene.tap();
                refresh();
                return;
            }
            if (k === 'ArrowLeft') {
                dx = -KEY_STEP;
            }
            else if (k === 'ArrowRight') {
                dx = KEY_STEP;
            }
            else if (k === 'ArrowUp') {
                dy = -KEY_STEP;
            }
            else if (k === 'ArrowDown') {
                dy = KEY_STEP;
            }
            else {
                return;
            }
            e.preventDefault();
            inp.x = clamp(inp.x + dx, 0, W);
            inp.y = clamp(inp.y + dy, 0, H);
            inp.idle = 0;
            refresh();
        });
        if (!reduce) {
            rafId = requestAnimationFrame(tick);
        }
        return function () {
            cancelAnimationFrame(rafId);
            for (let i = 0; i < cleaners.length; i += 1) {
                cleaners[i]();
            }
        };
    }
    const ROT_SPEED = 0.03;
    const CAM_DIST = 4.2;
    const FOCAL = 112;
    const SIDE = 0.8;
    const LIGHT_DIR = [-0.45, 0.75, 0.5];
    const FACE_SHADE = 0.2;
    const CUBE_DARK = [35, 30, 52];
    const CUBE_LIGHT = [255, 238, 206];
    const EDGE = [255, 170, 200];
    const BG_LIGHT = [98, 82, 138];
    const TXT_SPIN = '면의 밝기가 점 밀도로 바뀐다 — 큐브가 돈다';
    const TXT_LIGHT = '포인터가 빛의 방향을 정한다';
    function makeScene(inp, reduce) {
        const buf = makeBuf();
        const m8 = makeBayer(8);
        const m4 = makeBayer(4);
        const dark = pack(CUBE_DARK);
        const light = pack(CUBE_LIGHT);
        const edge = pack(EDGE);
        const bgLight = pack(BG_LIGHT);
        const faces = [
            { n: [1, 0, 0], u: [0, 1, 0], v: [0, 0, 1] },
            { n: [-1, 0, 0], u: [0, 1, 0], v: [0, 0, 1] },
            { n: [0, 1, 0], u: [1, 0, 0], v: [0, 0, 1] },
            { n: [0, -1, 0], u: [1, 0, 0], v: [0, 0, 1] },
            { n: [0, 0, 1], u: [1, 0, 0], v: [0, 1, 0] },
            { n: [0, 0, -1], u: [1, 0, 0], v: [0, 1, 0] }
        ];
        let t = 0;
        let dir = 1;
        let lightX = LIGHT_DIR[0];
        let lightY = LIGHT_DIR[1];
        let angle = 0.4;
        function rot(p, a, b) {
            const x1 = p[0] * Math.cos(a) + p[2] * Math.sin(a);
            const z1 = -p[0] * Math.sin(a) + p[2] * Math.cos(a);
            const y2 = p[1] * Math.cos(b) - z1 * Math.sin(b);
            const z2 = p[1] * Math.sin(b) + z1 * Math.cos(b);
            return [x1, y2, z2];
        }
        function drawFace(f, a, b, lx, ly, lz) {
            const n = rot(f.n, a, b);
            const c = rot([f.n[0] * SIDE, f.n[1] * SIDE, f.n[2] * SIDE], a, b);
            // 카메라(0, 0, -CAM_DIST)를 향한 면만 그린다
            const toCam = n[0] * (0 - c[0]) + n[1] * (0 - c[1]) + n[2] * (-CAM_DIST - c[2]);
            if (toCam <= 0) {
                return;
            }
            const signs = [[-1, -1], [1, -1], [1, 1], [-1, 1]];
            const pts = [];
            for (let k = 0; k < 4; k += 1) {
                const s = signs[k];
                const corner = rot([
                    (f.n[0] + s[0] * f.u[0] + s[1] * f.v[0]) * SIDE,
                    (f.n[1] + s[0] * f.u[1] + s[1] * f.v[1]) * SIDE,
                    (f.n[2] + s[0] * f.u[2] + s[1] * f.v[2]) * SIDE
                ], a, b);
                const k2 = FOCAL / (corner[2] + CAM_DIST);
                pts.push([GW / 2 + corner[0] * k2, GH / 2 - corner[1] * k2]);
            }
            const lam = Math.max(0, n[0] * lx + n[1] * ly + n[2] * lz);
            const shade = FACE_SHADE + (1 - FACE_SHADE) * lam;
            let x0 = GW;
            let x1 = 0;
            let y0 = GH;
            let y1 = 0;
            for (let k = 0; k < 4; k += 1) {
                x0 = Math.min(x0, pts[k][0]);
                x1 = Math.max(x1, pts[k][0]);
                y0 = Math.min(y0, pts[k][1]);
                y1 = Math.max(y1, pts[k][1]);
            }
            for (let y = Math.max(0, Math.floor(y0)); y <= Math.min(GH - 1, Math.ceil(y1)); y += 1) {
                for (let x = Math.max(0, Math.floor(x0)); x <= Math.min(GW - 1, Math.ceil(x1)); x += 1) {
                    const px = x + 0.5;
                    const py = y + 0.5;
                    let neg = 0;
                    let pos = 0;
                    let nearEdge = 99;
                    for (let k = 0; k < 4; k += 1) {
                        const p = pts[k];
                        const q = pts[(k + 1) % 4];
                        const ex = q[0] - p[0];
                        const ey = q[1] - p[1];
                        const cross = ex * (py - p[1]) - ey * (px - p[0]);
                        if (cross < 0) {
                            neg += 1;
                        }
                        else {
                            pos += 1;
                        }
                        nearEdge = Math.min(nearEdge, Math.abs(cross) / Math.hypot(ex, ey));
                    }
                    if (neg > 0 && pos > 0) {
                        continue;
                    }
                    const i = y * GW + x;
                    if (nearEdge < 0.8) {
                        buf.px[i] = edge;
                    }
                    else {
                        buf.px[i] = shade > m8[(y & 7) * 8 + (x & 7)] ? light : dark;
                    }
                }
            }
        }
        const scene = {
            step() {
                t += 1;
                angle += ROT_SPEED * dir;
                const tx = inp.idle < IDLE_TICKS ? (inp.x / W - 0.5) * 2 : LIGHT_DIR[0];
                const ty = inp.idle < IDLE_TICKS ? -(inp.y / H - 0.5) * 2 : LIGHT_DIR[1];
                lightX = lerp(lightX, tx, 0.1);
                lightY = lerp(lightY, ty, 0.1);
            },
            draw(ctx) {
                // 배경 — 느리게 흐르는 대각선 그라디언트를 4×4 행렬로 찍은 흐린 점무늬
                for (let y = 0; y < GH; y += 1) {
                    for (let x = 0; x < GW; x += 1) {
                        const v = 0.2 + 0.2 * (0.5 + 0.5 * Math.sin(((x * 0.5 + y * 0.8) / GH) * PI2 * 1.3 - t * 0.07));
                        buf.px[y * GW + x] = v > m4[(y & 3) * 4 + (x & 3)] ? bgLight : dark;
                    }
                }
                const a = angle;
                const b = 0.55 + 0.35 * Math.sin(angle * 0.7);
                const ln = Math.hypot(lightX, lightY, LIGHT_DIR[2]);
                for (let k = 0; k < faces.length; k += 1) {
                    drawFace(faces[k], a, b, lightX / ln, lightY / ln, LIGHT_DIR[2] / ln);
                }
                flush(ctx, buf);
            },
            label() {
                return inp.idle < IDLE_TICKS ? TXT_LIGHT : TXT_SPIN;
            },
            tap() {
                dir = -dir;
            }
        };
        if (reduce) {
            for (let i = 0; i < 20; i += 1) {
                scene.step();
            }
        }
        return scene;
    }
    start(document.getElementById('scRoot'), document.getElementById('scCanvas'), document.getElementById('scStatus'), document.getElementById('scBtn'), makeScene);
})();

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

const W = 480;
const H = 300;
const PI2 = Math.PI * 2;
const FIXED_DT = 16;
const REDUCE_STEP_MS = 900;
const KEY_STEP = 14;
const IDLE_TICKS = 150;
const BTN_PAUSE = '멈춤';
const BTN_PLAY = '재생';
const BTN_NEXT = '다음 장면';
// 점을 찍는 작은 격자 — 화면에는 3배로 확대해 올린다
const GW = 160;
const GH = 100;

type RGB = [number, number, number];
interface Input { x: number; y: number; idle: number; }
interface Scene {
    step(): void;
    draw(ctx: CanvasRenderingContext2D): void;
    label(): string;
    tap(): void;
}
type Maker = (inp: Input, reduce: boolean) => Scene;
interface Buf {
    off: HTMLCanvasElement;
    octx: CanvasRenderingContext2D;
    img: ImageData;
    px: Uint32Array;
}

function clamp(v: number, lo: number, hi: number): number { return v < lo ? lo : (v > hi ? hi : v); }
function lerp(a: number, b: number, k: number): number { return a + (b - a) * k; }
function smooth(x: number): number { const c = clamp(x, 0, 1); return c * c * (3 - 2 * c); }
function frac(x: number): number { return x - Math.floor(x); }
// 시드 고정 난수 — 같은 시드면 언제나 같은 수열 (선형 합동 방식)
function lcg(seed: number): () => number {
    let s = seed >>> 0;
    return function (): number {
        s = (Math.imul(s, 1664525) + 1013904223) >>> 0;
        return s / 4294967296;
    };
}
// 좌표 두 개와 시드로 0 이상 1 미만의 값을 만드는 정수 해시 (난수 함수 없이도 같은 좌표는 늘 같은 값)
function hash2(x: number, y: number, seed: number): number {
    let h = (Math.imul(x | 0, 374761393) + Math.imul(y | 0, 668265263) + Math.imul(seed | 0, 1442695041)) | 0;
    h = Math.imul(h ^ (h >>> 13), 1274126177);
    h ^= h >>> 16;
    return (h >>> 0) / 4294967296;
}
// 격자 네 점의 해시를 smooth 로 이어 붙인 값 노이즈
function vnoise(x: number, y: number, seed: number): number {
    const xi = Math.floor(x);
    const yi = Math.floor(y);
    const fx = smooth(x - xi);
    const fy = smooth(y - yi);
    const a = lerp(hash2(xi, yi, seed), hash2(xi + 1, yi, seed), fx);
    const b = lerp(hash2(xi, yi + 1, seed), hash2(xi + 1, yi + 1, seed), fx);
    return lerp(a, b, fy);
}
// 베이어 임계 행렬 — n 은 2 의 거듭제곱. 값은 0 초과 1 미만으로 맞춘다
function makeBayer(n: number): Float32Array {
    let size = 2;
    let m: number[] = [0, 2, 3, 1];
    const quad = [0, 2, 3, 1];
    while (size < n) {
        const s2 = size * 2;
        const next: number[] = [];
        for (let y = 0; y < s2; y += 1) {
            for (let x = 0; x < s2; x += 1) {
                const q = (y >= size ? 2 : 0) + (x >= size ? 1 : 0);
                next.push(4 * m[(y % size) * size + (x % size)] + quad[q]);
            }
        }
        m = next;
        size = s2;
    }
    const out = new Float32Array(n * n);
    for (let i = 0; i < out.length; i += 1) {
        out[i] = (m[i] + 0.5) / (n * n);
    }
    return out;
}
// 색 하나를 캔버스 픽셀 한 개(리틀 엔디언 RGBA)로 묶는다
function pack(c: RGB): number {
    return ((255 << 24) | (Math.round(c[2]) << 16) | (Math.round(c[1]) << 8) | Math.round(c[0])) >>> 0;
}
function mixPack(a: RGB, b: RGB, k: number): number {
    return pack([lerp(a[0], b[0], k), lerp(a[1], b[1], k), lerp(a[2], b[2], k)]);
}
function makeBuf(): Buf {
    const off = document.createElement('canvas');
    off.width = GW;
    off.height = GH;
    const octx = off.getContext('2d');
    if (!octx) {
        throw new Error('2d context unavailable');
    }
    const img = octx.createImageData(GW, GH);
    return { off: off, octx: octx, img: img, px: new Uint32Array(img.data.buffer) };
}
// 작은 격자를 큰 캔버스에 보간 없이 확대해 올린다
function flush(ctx: CanvasRenderingContext2D, b: Buf): void {
    b.octx.putImageData(b.img, 0, 0);
    ctx.imageSmoothingEnabled = false;
    ctx.drawImage(b.off, 0, 0, W, H);
}


function start(root: HTMLElement, canvas: HTMLCanvasElement, statusEl: HTMLElement, btnEl: HTMLButtonElement, make: Maker): () => void {
    const ctx0 = canvas.getContext('2d');
    if (!ctx0) {
        return function (): void { return; };
    }
    const ctx: CanvasRenderingContext2D = ctx0;
    const dpr = Math.min(window.devicePixelRatio || 1, 2);
    canvas.width = Math.round(W * dpr);
    canvas.height = Math.round(H * dpr);
    ctx.setTransform(dpr, 0, 0, dpr, 0, 0);
    const reduce = window.matchMedia('(prefers-reduced-motion: reduce)').matches;
    const inp: Input = { x: W / 2, y: H / 2, idle: 99999 };
    const scene = make(inp, reduce);
    let paused = false;
    let rafId = 0;
    let lastTs = -1;
    let accMs = 0;
    let lastLabel = '';
    const cleaners: Array<() => void> = [];
    function listen(target: EventTarget, type: string, fn: (e: Event) => void): void {
        target.addEventListener(type, fn);
        cleaners.push(function (): void { target.removeEventListener(type, fn); });
    }
    function show(): void {
        scene.draw(ctx);
        const t = scene.label();
        if (t !== lastLabel) {
            lastLabel = t;
            statusEl.textContent = t;
        }
    }
    function advance(ms: number): void {
        const n = Math.round(ms / FIXED_DT);
        for (let i = 0; i < n; i += 1) {
            inp.idle += 1;
            scene.step();
        }
    }
    function refresh(): void {
        if (reduce) {
            advance(FIXED_DT * 6);
            show();
        }
    }
    function toLocal(e: PointerEvent): void {
        const r = canvas.getBoundingClientRect();
        inp.x = clamp((e.clientX - r.left) * W / r.width, 0, W);
        inp.y = clamp((e.clientY - r.top) * H / r.height, 0, H);
        inp.idle = 0;
    }
    function tick(ts: number): void {
        if (lastTs < 0) {
            lastTs = ts;
        }
        const frameDt = Math.min(ts - lastTs, 160);
        lastTs = ts;
        if (!paused) {
            accMs += frameDt;
            while (accMs >= FIXED_DT) {
                inp.idle += 1;
                scene.step();
                accMs -= FIXED_DT;
            }
            show();
        }
        rafId = requestAnimationFrame(tick);
    }
    btnEl.textContent = reduce ? BTN_NEXT : BTN_PAUSE;
    if (reduce) {
        root.classList.remove('is-demo');
        advance(1400);
        show();
    }
    listen(btnEl, 'click', function (): void {
        if (reduce) {
            advance(REDUCE_STEP_MS);
            show();
            return;
        }
        paused = !paused;
        if (paused) {
            root.classList.remove('is-demo');
        } else {
            root.classList.add('is-demo');
        }
        btnEl.textContent = paused ? BTN_PLAY : BTN_PAUSE;
    });
    listen(canvas, 'pointermove', function (e: Event): void {
        toLocal(e as PointerEvent);
        refresh();
    });
    listen(canvas, 'pointerdown', function (e: Event): void {
        toLocal(e as PointerEvent);
        scene.tap();
        refresh();
    });
    listen(canvas, 'keydown', function (e: Event): void {
        const k = (e as KeyboardEvent).key;
        let dx = 0;
        let dy = 0;
        if (k === 'Enter' || k === ' ') {
            e.preventDefault();
            scene.tap();
            refresh();
            return;
        }
        if (k === 'ArrowLeft') { dx = -KEY_STEP; }
        else if (k === 'ArrowRight') { dx = KEY_STEP; }
        else if (k === 'ArrowUp') { dy = -KEY_STEP; }
        else if (k === 'ArrowDown') { dy = KEY_STEP; }
        else { return; }
        e.preventDefault();
        inp.x = clamp(inp.x + dx, 0, W);
        inp.y = clamp(inp.y + dy, 0, H);
        inp.idle = 0;
        refresh();
    });
    if (!reduce) {
        rafId = requestAnimationFrame(tick);
    }
    return function (): void {
        cancelAnimationFrame(rafId);
        for (let i = 0; i < cleaners.length; i += 1) {
            cleaners[i]();
        }
    };
}

const ROT_SPEED = 0.03;
const CAM_DIST = 4.2;
const FOCAL = 112;
const SIDE = 0.8;
const LIGHT_DIR: [number, number, number] = [-0.45, 0.75, 0.5];
const FACE_SHADE = 0.2;
const CUBE_DARK: RGB = [35, 30, 52];
const CUBE_LIGHT: RGB = [255, 238, 206];
const EDGE: RGB = [255, 170, 200];
const BG_LIGHT: RGB = [98, 82, 138];
const TXT_SPIN = '면의 밝기가 점 밀도로 바뀐다 — 큐브가 돈다';
const TXT_LIGHT = '포인터가 빛의 방향을 정한다';

interface Face { n: [number, number, number]; u: [number, number, number]; v: [number, number, number]; }

function makeScene(inp: Input, reduce: boolean): Scene {
    const buf = makeBuf();
    const m8 = makeBayer(8);
    const m4 = makeBayer(4);
    const dark = pack(CUBE_DARK);
    const light = pack(CUBE_LIGHT);
    const edge = pack(EDGE);
    const bgLight = pack(BG_LIGHT);
    const faces: Face[] = [
        { n: [1, 0, 0], u: [0, 1, 0], v: [0, 0, 1] },
        { n: [-1, 0, 0], u: [0, 1, 0], v: [0, 0, 1] },
        { n: [0, 1, 0], u: [1, 0, 0], v: [0, 0, 1] },
        { n: [0, -1, 0], u: [1, 0, 0], v: [0, 0, 1] },
        { n: [0, 0, 1], u: [1, 0, 0], v: [0, 1, 0] },
        { n: [0, 0, -1], u: [1, 0, 0], v: [0, 1, 0] }
    ];
    let t = 0;
    let dir = 1;
    let lightX = LIGHT_DIR[0];
    let lightY = LIGHT_DIR[1];
    let angle = 0.4;
    function rot(p: [number, number, number], a: number, b: number): [number, number, number] {
        const x1 = p[0] * Math.cos(a) + p[2] * Math.sin(a);
        const z1 = -p[0] * Math.sin(a) + p[2] * Math.cos(a);
        const y2 = p[1] * Math.cos(b) - z1 * Math.sin(b);
        const z2 = p[1] * Math.sin(b) + z1 * Math.cos(b);
        return [x1, y2, z2];
    }
    function drawFace(f: Face, a: number, b: number, lx: number, ly: number, lz: number): void {
        const n = rot(f.n, a, b);
        const c = rot([f.n[0] * SIDE, f.n[1] * SIDE, f.n[2] * SIDE], a, b);
        // 카메라(0, 0, -CAM_DIST)를 향한 면만 그린다
        const toCam = n[0] * (0 - c[0]) + n[1] * (0 - c[1]) + n[2] * (-CAM_DIST - c[2]);
        if (toCam <= 0) {
            return;
        }
        const signs: Array<[number, number]> = [[-1, -1], [1, -1], [1, 1], [-1, 1]];
        const pts: Array<[number, number]> = [];
        for (let k = 0; k < 4; k += 1) {
            const s = signs[k];
            const corner = rot([
                (f.n[0] + s[0] * f.u[0] + s[1] * f.v[0]) * SIDE,
                (f.n[1] + s[0] * f.u[1] + s[1] * f.v[1]) * SIDE,
                (f.n[2] + s[0] * f.u[2] + s[1] * f.v[2]) * SIDE
            ], a, b);
            const k2 = FOCAL / (corner[2] + CAM_DIST);
            pts.push([GW / 2 + corner[0] * k2, GH / 2 - corner[1] * k2]);
        }
        const lam = Math.max(0, n[0] * lx + n[1] * ly + n[2] * lz);
        const shade = FACE_SHADE + (1 - FACE_SHADE) * lam;
        let x0 = GW;
        let x1 = 0;
        let y0 = GH;
        let y1 = 0;
        for (let k = 0; k < 4; k += 1) {
            x0 = Math.min(x0, pts[k][0]);
            x1 = Math.max(x1, pts[k][0]);
            y0 = Math.min(y0, pts[k][1]);
            y1 = Math.max(y1, pts[k][1]);
        }
        for (let y = Math.max(0, Math.floor(y0)); y <= Math.min(GH - 1, Math.ceil(y1)); y += 1) {
            for (let x = Math.max(0, Math.floor(x0)); x <= Math.min(GW - 1, Math.ceil(x1)); x += 1) {
                const px = x + 0.5;
                const py = y + 0.5;
                let neg = 0;
                let pos = 0;
                let nearEdge = 99;
                for (let k = 0; k < 4; k += 1) {
                    const p = pts[k];
                    const q = pts[(k + 1) % 4];
                    const ex = q[0] - p[0];
                    const ey = q[1] - p[1];
                    const cross = ex * (py - p[1]) - ey * (px - p[0]);
                    if (cross < 0) {
                        neg += 1;
                    } else {
                        pos += 1;
                    }
                    nearEdge = Math.min(nearEdge, Math.abs(cross) / Math.hypot(ex, ey));
                }
                if (neg > 0 && pos > 0) {
                    continue;
                }
                const i = y * GW + x;
                if (nearEdge < 0.8) {
                    buf.px[i] = edge;
                } else {
                    buf.px[i] = shade > m8[(y & 7) * 8 + (x & 7)] ? light : dark;
                }
            }
        }
    }
    const scene: Scene = {
        step(): void {
            t += 1;
            angle += ROT_SPEED * dir;
            const tx = inp.idle < IDLE_TICKS ? (inp.x / W - 0.5) * 2 : LIGHT_DIR[0];
            const ty = inp.idle < IDLE_TICKS ? -(inp.y / H - 0.5) * 2 : LIGHT_DIR[1];
            lightX = lerp(lightX, tx, 0.1);
            lightY = lerp(lightY, ty, 0.1);
        },
        draw(ctx: CanvasRenderingContext2D): void {
            // 배경 — 느리게 흐르는 대각선 그라디언트를 4×4 행렬로 찍은 흐린 점무늬
            for (let y = 0; y < GH; y += 1) {
                for (let x = 0; x < GW; x += 1) {
                    const v = 0.2 + 0.2 * (0.5 + 0.5 * Math.sin(((x * 0.5 + y * 0.8) / GH) * PI2 * 1.3 - t * 0.07));
                    buf.px[y * GW + x] = v > m4[(y & 3) * 4 + (x & 3)] ? bgLight : dark;
                }
            }
            const a = angle;
            const b = 0.55 + 0.35 * Math.sin(angle * 0.7);
            const ln = Math.hypot(lightX, lightY, LIGHT_DIR[2]);
            for (let k = 0; k < faces.length; k += 1) {
                drawFace(faces[k], a, b, lightX / ln, lightY / ln, LIGHT_DIR[2] / ln);
            }
            flush(ctx, buf);
        },
        label(): string {
            return inp.idle < IDLE_TICKS ? TXT_LIGHT : TXT_SPIN;
        },
        tap(): void {
            dir = -dir;
        }
    };
    if (reduce) {
        for (let i = 0; i < 20; i += 1) {
            scene.step();
        }
    }
    return scene;
}

export default function DtRotatingCubeFlatShade() {
  const rootRef = useRef<HTMLDivElement | null>(null);
  const canvasRef = useRef<HTMLCanvasElement | null>(null);
  const statusRef = useRef<HTMLParagraphElement | null>(null);
  const btnRef = useRef<HTMLButtonElement | null>(null);

  useEffect(() => {
    const root = rootRef.current;
    const canvas = canvasRef.current;
    const statusEl = statusRef.current;
    const btnEl = btnRef.current;
    if (!root || !canvas || !statusEl || !btnEl) return;
    return start(root, canvas, statusEl, btnEl, makeScene);
  }, []);

  return (
    <main className={styles.stage}>
      <div className={`${styles.sc} is-demo`} ref={rootRef}>
        <canvas className={styles.sc__canvas} ref={canvasRef} width={480} height={300} tabIndex={0} aria-label="회전 입체의 면 밝기를 점 밀도로 바꾸는 3D 디더 큐브" />
        <p className={styles.sc__status} ref={statusRef} aria-live="polite">면의 밝기가 점 밀도로 바뀐다 — 큐브가 돈다</p>
        <button className={styles.sc__btn} ref={btnRef} type="button">멈춤</button>
        <p className={styles.sc__caption}>회전 입체의 면 밝기를 점 밀도로 바꾸는 3D 디더 큐브</p>
      </div>
    </main>
  );
}