09ブラックホールの近くで星の光が輪に曲がる重力レンズ

<main class="stage">
<div class="sc is-demo" id="scRoot">
  <canvas class="sc__canvas" id="scCanvas" width="480" height="300" tabindex="0" aria-label="ブラックホールの近くで星の光が輪に曲がる重力レンズ"></canvas>
  <p class="sc__status" id="scStatus" aria-live="polite">レンズの質量 — 輪が星の光を集める</p>
  <button class="sc__btn" id="scBtn" type="button">一時停止</button>
  <p class="sc__caption">ブラックホールの近くで星の光が輪に曲がる重力レンズ</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;
}

.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;
}

.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 = '一時停止';
    const BTN_PLAY = '再生';
    const BTN_NEXT = '次の場面';
    const BG = [14, 12, 32];
    const STAR_RGBS = [[226, 238, 255], [255, 222, 196], [208, 196, 255], [196, 240, 228], [255, 206, 226]];
    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 rgba(c, a) {
        return 'rgba(' + Math.round(c[0]) + ',' + Math.round(c[1]) + ',' + Math.round(c[2]) + ',' + clamp(a, 0, 1).toFixed(3) + ')';
    }
    function mix(a, b, k) {
        return [lerp(a[0], b[0], k), lerp(a[1], b[1], k), lerp(a[2], b[2], k)];
    }
    // 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;
        };
    }
    function glow(ctx, x, y, r, c, a) {
        const g = ctx.createRadialGradient(x, y, 0, x, y, r);
        g.addColorStop(0, rgba(c, a));
        g.addColorStop(1, rgba(c, 0));
        ctx.fillStyle = g;
        ctx.fillRect(x - r, y - r, r * 2, r * 2);
    }
    function paintBg(ctx, alpha) {
        ctx.fillStyle = rgba(BG, alpha);
        ctx.fillRect(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 LENS_MASS = 2600;
    const RING_RADIUS = Math.sqrt(LENS_MASS);
    const FALLOFF = 1.1;
    const SOFTEN = 5;
    const FIELD_STARS = 260;
    const GALAXY_STARS = 70;
    const BOOST_TICKS = 80;
    const BOOST_GAIN = 1.9;
    const TXT_RING = 'レンズの質量 — 輪が星の光を集める';
    const TXT_HEAVY = '質量が増えた — 輪が大きくなる';
    function makeScene(inp, reduce) {
        const rnd = lcg(450909);
        const field = [];
        for (let i = 0; i < FIELD_STARS; i += 1) {
            field.push({ x: rnd() * W, y: rnd() * H, r: 0.7 + 1.3 * rnd() * rnd(), c: STAR_RGBS[Math.floor(rnd() * STAR_RGBS.length)] });
        }
        // 렌즈 바로 뒤의 작은 은하 — 상대 위치가 0 에 가까워 고리를 만든다
        const galaxy = [];
        for (let i = 0; i < GALAXY_STARS; i += 1) {
            const a = rnd() * PI2;
            const rr = 14 * Math.sqrt(rnd());
            galaxy.push({ x: Math.cos(a) * rr, y: Math.sin(a) * rr, r: 0.9 + 0.8 * rnd(), c: STAR_RGBS[i % 2 === 0 ? 1 : 4] });
        }
        let cx = W / 2;
        let cy = H / 2;
        let t = 0;
        let boost = 0;
        function mass() {
            return LENS_MASS * (1 + (BOOST_GAIN - 1) * smooth(boost / BOOST_TICKS));
        }
        // 점질량 렌즈: 원천이 렌즈 중심에서 b 만큼 떨어져 있으면 상 두 개가 b 방향의 +th 와 반대쪽 th2 에 맺힌다
        function image(ctx, bx, by, r, c, m, alpha) {
            const b = Math.hypot(bx, by);
            const bs = Math.sqrt(b * b + SOFTEN * SOFTEN);
            const root = Math.sqrt(bs * bs + 4 * m);
            const thP = (bs + root) / 2;
            const thM = (root - bs) / 2;
            const u = bs / Math.sqrt(m);
            const mag = (u * u + 2) / (2 * u * Math.sqrt(u * u + 4));
            const muP = clamp(mag + 0.5, 1, 9);
            const muM = clamp(mag - 0.5, 0, 9);
            const ux = b > 0.001 ? bx / b : 1;
            const uy = b > 0.001 ? by / b : 0;
            const ang = Math.atan2(uy, ux) + Math.PI / 2;
            const stretchP = clamp(thP / bs, 1, 6);
            ctx.fillStyle = rgba(c, alpha * clamp(Math.pow(muP, FALLOFF) * 0.8, 0.3, 1));
            ctx.beginPath();
            ctx.ellipse(cx + ux * thP, cy + uy * thP, r * stretchP, r * 0.8, ang, 0, PI2);
            ctx.fill();
            if (muM > 0.04) {
                const stretchM = clamp(thM / bs, 0.4, 6);
                ctx.fillStyle = rgba(c, alpha * clamp(Math.pow(muM, FALLOFF) * 0.8, 0, 0.9));
                ctx.beginPath();
                ctx.ellipse(cx - ux * thM, cy - uy * thM, r * Math.max(stretchM, 0.8), r * 0.7, ang, 0, PI2);
                ctx.fill();
            }
        }
        const scene = {
            step() {
                t += 1;
                if (boost > 0) {
                    boost -= 1;
                }
                let tx = W / 2 + Math.sin(t * 0.012) * 110;
                let ty = H / 2 + Math.sin(t * 0.017) * 50;
                if (inp.idle < IDLE_TICKS) {
                    tx = inp.x;
                    ty = inp.y;
                }
                cx = lerp(cx, tx, 0.1);
                cy = lerp(cy, ty, 0.1);
                for (let i = 0; i < field.length; i += 1) {
                    field[i].x -= 0.7;
                    if (field[i].x < -8) {
                        field[i].x += W + 16;
                    }
                }
            },
            draw(ctx) {
                paintBg(ctx, 1);
                const m = mass();
                for (let i = 0; i < field.length; i += 1) {
                    const s = field[i];
                    image(ctx, s.x - cx, s.y - cy, s.r, s.c, m, 0.9);
                }
                const gx = Math.sin(t * 0.02) * 7;
                const gy = Math.cos(t * 0.027) * 5;
                for (let i = 0; i < galaxy.length; i += 1) {
                    const s = galaxy[i];
                    image(ctx, s.x + gx, s.y + gy, s.r, s.c, m, 0.8);
                }
                const ring = Math.sqrt(m);
                ctx.strokeStyle = rgba([208, 196, 255], 0.14);
                ctx.lineWidth = 1;
                ctx.beginPath();
                ctx.arc(cx, cy, ring, 0, PI2);
                ctx.stroke();
                glow(ctx, cx, cy, ring * 1.1, [186, 150, 255], 0.12);
                const hole = ctx.createRadialGradient(cx, cy, 0, cx, cy, RING_RADIUS * 0.3);
                hole.addColorStop(0, rgba(BG, 1));
                hole.addColorStop(0.7, rgba(BG, 1));
                hole.addColorStop(1, rgba(BG, 0));
                ctx.fillStyle = hole;
                ctx.fillRect(cx - RING_RADIUS, cy - RING_RADIUS, RING_RADIUS * 2, RING_RADIUS * 2);
            },
            label() {
                return boost > 0 ? TXT_HEAVY : TXT_RING;
            },
            tap() {
                boost = BOOST_TICKS;
            }
        };
        if (reduce) {
            for (let i = 0; i < 40; 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 "./SfGravityLensField.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';

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;

const BG: RGB = [14, 12, 32];
const STAR_RGBS: RGB[] = [[226, 238, 255], [255, 222, 196], [208, 196, 255], [196, 240, 228], [255, 206, 226]];

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 rgba(c: RGB, a: number): string {
    return 'rgba(' + Math.round(c[0]) + ',' + Math.round(c[1]) + ',' + Math.round(c[2]) + ',' + clamp(a, 0, 1).toFixed(3) + ')';
}
function mix(a: RGB, b: RGB, k: number): RGB {
    return [lerp(a[0], b[0], k), lerp(a[1], b[1], k), lerp(a[2], b[2], k)];
}
// 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;
    };
}
function glow(ctx: CanvasRenderingContext2D, x: number, y: number, r: number, c: RGB, a: number): void {
    const g = ctx.createRadialGradient(x, y, 0, x, y, r);
    g.addColorStop(0, rgba(c, a));
    g.addColorStop(1, rgba(c, 0));
    ctx.fillStyle = g;
    ctx.fillRect(x - r, y - r, r * 2, r * 2);
}
function paintBg(ctx: CanvasRenderingContext2D, alpha: number): void {
    ctx.fillStyle = rgba(BG, alpha);
    ctx.fillRect(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 LENS_MASS = 2600;
const RING_RADIUS = Math.sqrt(LENS_MASS);
const FALLOFF = 1.1;
const SOFTEN = 5;
const FIELD_STARS = 260;
const GALAXY_STARS = 70;
const BOOST_TICKS = 80;
const BOOST_GAIN = 1.9;
const TXT_RING = '렌즈 질량 — 고리가 별빛을 모은다';
const TXT_HEAVY = '질량 증가 — 고리가 커진다';

interface Src { x: number; y: number; r: number; c: RGB; }

function makeScene(inp: Input, reduce: boolean): Scene {
    const rnd = lcg(450909);
    const field: Src[] = [];
    for (let i = 0; i < FIELD_STARS; i += 1) {
        field.push({ x: rnd() * W, y: rnd() * H, r: 0.7 + 1.3 * rnd() * rnd(), c: STAR_RGBS[Math.floor(rnd() * STAR_RGBS.length)] });
    }
    // 렌즈 바로 뒤의 작은 은하 — 상대 위치가 0 에 가까워 고리를 만든다
    const galaxy: Src[] = [];
    for (let i = 0; i < GALAXY_STARS; i += 1) {
        const a = rnd() * PI2;
        const rr = 14 * Math.sqrt(rnd());
        galaxy.push({ x: Math.cos(a) * rr, y: Math.sin(a) * rr, r: 0.9 + 0.8 * rnd(), c: STAR_RGBS[i % 2 === 0 ? 1 : 4] });
    }
    let cx = W / 2;
    let cy = H / 2;
    let t = 0;
    let boost = 0;
    function mass(): number {
        return LENS_MASS * (1 + (BOOST_GAIN - 1) * smooth(boost / BOOST_TICKS));
    }
    // 점질량 렌즈: 원천이 렌즈 중심에서 b 만큼 떨어져 있으면 상 두 개가 b 방향의 +th 와 반대쪽 th2 에 맺힌다
    function image(ctx: CanvasRenderingContext2D, bx: number, by: number, r: number, c: RGB, m: number, alpha: number): void {
        const b = Math.hypot(bx, by);
        const bs = Math.sqrt(b * b + SOFTEN * SOFTEN);
        const root = Math.sqrt(bs * bs + 4 * m);
        const thP = (bs + root) / 2;
        const thM = (root - bs) / 2;
        const u = bs / Math.sqrt(m);
        const mag = (u * u + 2) / (2 * u * Math.sqrt(u * u + 4));
        const muP = clamp(mag + 0.5, 1, 9);
        const muM = clamp(mag - 0.5, 0, 9);
        const ux = b > 0.001 ? bx / b : 1;
        const uy = b > 0.001 ? by / b : 0;
        const ang = Math.atan2(uy, ux) + Math.PI / 2;
        const stretchP = clamp(thP / bs, 1, 6);
        ctx.fillStyle = rgba(c, alpha * clamp(Math.pow(muP, FALLOFF) * 0.8, 0.3, 1));
        ctx.beginPath();
        ctx.ellipse(cx + ux * thP, cy + uy * thP, r * stretchP, r * 0.8, ang, 0, PI2);
        ctx.fill();
        if (muM > 0.04) {
            const stretchM = clamp(thM / bs, 0.4, 6);
            ctx.fillStyle = rgba(c, alpha * clamp(Math.pow(muM, FALLOFF) * 0.8, 0, 0.9));
            ctx.beginPath();
            ctx.ellipse(cx - ux * thM, cy - uy * thM, r * Math.max(stretchM, 0.8), r * 0.7, ang, 0, PI2);
            ctx.fill();
        }
    }
    const scene: Scene = {
        step(): void {
            t += 1;
            if (boost > 0) {
                boost -= 1;
            }
            let tx = W / 2 + Math.sin(t * 0.012) * 110;
            let ty = H / 2 + Math.sin(t * 0.017) * 50;
            if (inp.idle < IDLE_TICKS) {
                tx = inp.x;
                ty = inp.y;
            }
            cx = lerp(cx, tx, 0.1);
            cy = lerp(cy, ty, 0.1);
            for (let i = 0; i < field.length; i += 1) {
                field[i].x -= 0.7;
                if (field[i].x < -8) {
                    field[i].x += W + 16;
                }
            }
        },
        draw(ctx: CanvasRenderingContext2D): void {
            paintBg(ctx, 1);
            const m = mass();
            for (let i = 0; i < field.length; i += 1) {
                const s = field[i];
                image(ctx, s.x - cx, s.y - cy, s.r, s.c, m, 0.9);
            }
            const gx = Math.sin(t * 0.02) * 7;
            const gy = Math.cos(t * 0.027) * 5;
            for (let i = 0; i < galaxy.length; i += 1) {
                const s = galaxy[i];
                image(ctx, s.x + gx, s.y + gy, s.r, s.c, m, 0.8);
            }
            const ring = Math.sqrt(m);
            ctx.strokeStyle = rgba([208, 196, 255], 0.14);
            ctx.lineWidth = 1;
            ctx.beginPath();
            ctx.arc(cx, cy, ring, 0, PI2);
            ctx.stroke();
            glow(ctx, cx, cy, ring * 1.1, [186, 150, 255], 0.12);
            const hole = ctx.createRadialGradient(cx, cy, 0, cx, cy, RING_RADIUS * 0.3);
            hole.addColorStop(0, rgba(BG, 1));
            hole.addColorStop(0.7, rgba(BG, 1));
            hole.addColorStop(1, rgba(BG, 0));
            ctx.fillStyle = hole;
            ctx.fillRect(cx - RING_RADIUS, cy - RING_RADIUS, RING_RADIUS * 2, RING_RADIUS * 2);
        },
        label(): string {
            return boost > 0 ? TXT_HEAVY : TXT_RING;
        },
        tap(): void {
            boost = BOOST_TICKS;
        }
    };
    if (reduce) {
        for (let i = 0; i < 40; i += 1) {
            scene.step();
        }
    }
    return scene;
}

export default function SfGravityLensField() {
  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="블랙홀 가까이에서 별빛이 고리로 휘는 중력 렌즈" />
        <p className={styles.sc__status} ref={statusRef} aria-live="polite">렌즈 질량 — 고리가 별빛을 모은다</p>
        <button className={styles.sc__btn} ref={btnRef} type="button">멈춤</button>
        <p className={styles.sc__caption}>블랙홀 가까이에서 별빛이 고리로 휘는 중력 렌즈</p>
      </div>
    </main>
  );
}