02줄마다 빛이 지나온 길이로 색을 칠하는 대기 산란 하늘

<main class="stage">
<div class="sc is-demo" id="scRoot">
  <canvas class="sc__canvas" id="scCanvas" width="480" height="300" tabindex="0" aria-label="줄마다 빛이 지나온 길이로 색을 칠하는 대기 산란 하늘"></canvas>
  <p class="sc__status" id="scStatus" aria-live="polite">해가 기운다 — 금빛 오후</p>
  <button class="sc__btn" id="scBtn" type="button">멈춤</button>
</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;
}

@media (max-width: 340px) {
  .sc__btn { font-size: 10px; padding: $sp-1 $sp-2; }
  .sc__status { font-size: 10px; max-width: 58%; }
}

@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;
}

@media (max-width: 340px) {
  .sc__btn {
    font-size: 10px;
    padding: 4px 8px;
  }
  .sc__status {
    font-size: 10px;
    max-width: 58%;
  }
}
@media (prefers-reduced-motion: reduce) {
  .sc {
    cursor: default;
  }
}

(function () {
    const W = 480;
    const H = 300;
    const PI2 = Math.PI * 2;
    const HORIZON = 186;
    const SUN_RISE = 150;
    const SUN_X = 312;
    const SUN_ALT = 0.5;
    const ALT_NIGHT = -0.15;
    const PERIOD_MS = 4400;
    const KEY_ALT = 0.05;
    const SKY_BLEND = 1.7;
    // 해 높이(0 = 지평선, 1 = 높이 뜸) 순서대로 고른 색 띠 — 정점 쪽, 지평선 쪽, 해 원반
    const ZENITH_RAMP = [[-0.15, [40, 44, 96]], [0, [92, 92, 156]], [0.12, [128, 126, 182]], [0.3, [150, 160, 214]], [0.6, [150, 192, 238]], [1, [138, 190, 242]]];
    const HORIZON_RAMP = [[-0.15, [110, 104, 160]], [0, [238, 134, 142]], [0.12, [255, 168, 146]], [0.3, [255, 208, 166]], [0.6, [240, 226, 222]], [1, [206, 230, 248]]];
    const SUN_RAMP = [[-0.15, [238, 128, 134]], [0, [255, 150, 126]], [0.15, [255, 190, 140]], [0.4, [255, 226, 176]], [1, [255, 246, 222]]];
    const TXT_HIGH = '해가 높다 — 낮 하늘';
    const TXT_GOLD = '해가 기운다 — 금빛 오후';
    const TXT_GLOW = '해가 낮다 — 노을';
    const TXT_TOUCH = '해가 지평선에 닿는다 — 붉은 띠';
    const TXT_DUSK = '해가 졌다 — 땅거미';
    function labelOf(alt) {
        if (alt > 0.6) {
            return TXT_HIGH;
        }
        if (alt > 0.3) {
            return TXT_GOLD;
        }
        if (alt > 0.08) {
            return TXT_GLOW;
        }
        if (alt > -0.06) {
            return TXT_TOUCH;
        }
        return TXT_DUSK;
    }
    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 mix(a, b, k) { return [lerp(a[0], b[0], k), lerp(a[1], b[1], k), lerp(a[2], b[2], k)]; }
    function css(c, a) {
        return 'rgba(' + Math.round(c[0]) + ',' + Math.round(c[1]) + ',' + Math.round(c[2]) + ',' + a.toFixed(3) + ')';
    }
    function luma(c) { return (0.2126 * c[0] + 0.7152 * c[1] + 0.0722 * c[2]) / 255; }
    // 시드 고정 난수 — 같은 시드면 언제나 같은 수열
    function mulberry32(seed) {
        let s = seed >>> 0;
        return function () {
            s = (s + 0x6D2B79F5) >>> 0;
            let t = s;
            t = Math.imul(t ^ (t >>> 15), t | 1);
            t ^= t + Math.imul(t ^ (t >>> 7), t | 61);
            return ((t ^ (t >>> 14)) >>> 0) / 4294967296;
        };
    }
    function hash2(ix, iy) {
        let h = Math.imul(ix | 0, 374761393) + Math.imul(iy | 0, 668265263);
        h = Math.imul(h ^ (h >>> 13), 1274126177);
        return ((h ^ (h >>> 16)) >>> 0) / 4294967296;
    }
    function vnoise(x, y) {
        const ix = Math.floor(x);
        const iy = Math.floor(y);
        const fx = smooth(x - ix);
        const fy = smooth(y - iy);
        return lerp(lerp(hash2(ix, iy), hash2(ix + 1, iy), fx), lerp(hash2(ix, iy + 1), hash2(ix + 1, iy + 1), fx), fy);
    }
    function fbm(x, y) {
        return vnoise(x, y) * 0.55 + vnoise(x * 2.1 + 7.3, y * 2.1 + 3.1) * 0.3 + vnoise(x * 4.3 + 1.7, y * 4.3 + 9.4) * 0.15;
    }
    function ramp(stops, a) {
        if (a <= stops[0][0]) {
            return stops[0][1];
        }
        for (let i = 1; i < stops.length; i += 1) {
            if (a <= stops[i][0]) {
                const p = stops[i - 1];
                const q = stops[i];
                return mix(p[1], q[1], (a - p[0]) / (q[0] - p[0]));
            }
        }
        return stops[stops.length - 1][1];
    }
    // 하늘 한 점의 색 — v 는 0(정점)에서 1(지평선)
    function skyAt(alt, v) {
        return mix(ramp(ZENITH_RAMP, alt), ramp(HORIZON_RAMP, alt), Math.pow(clamp(v, 0, 1), SKY_BLEND));
    }
    function sunY(alt) { return HORIZON - alt * SUN_RISE; }
    function seaDeep(alt) { return mix([84, 128, 170], [36, 48, 98], clamp((0.3 - alt) / 0.4, 0, 1)); }
    function glow(ctx, x, y, r, c, a) {
        const g = ctx.createRadialGradient(x, y, 0, x, y, r);
        g.addColorStop(0, css(c, a));
        g.addColorStop(1, css(c, 0));
        ctx.fillStyle = g;
        ctx.fillRect(x - r, y - r, r * 2, r * 2);
    }
    function skyRows(ctx, alt, y0, y1, step) {
        for (let y = y0; y < y1; y += step) {
            ctx.fillStyle = css(skyAt(alt, (y - y0) / (y1 - y0)), 1);
            ctx.fillRect(0, y, W, step + 1);
        }
    }
    // 바다 바탕 — 지평선 하늘색을 받아 아래로 갈수록 깊어지고, 줄마다 잔물결 밝기가 흔들린다
    function seaBase(ctx, alt, t) {
        const top = mix(skyAt(alt, 1), seaDeep(alt), 0.35);
        const deep = seaDeep(alt);
        for (let y = HORIZON; y < H; y += 3) {
            const k = (y - HORIZON) / (H - HORIZON);
            const c = mix(top, deep, Math.pow(k, 0.7));
            const r = 1 + 0.05 * Math.sin(y * 0.8 + t * 0.0018 + k * 6);
            ctx.fillStyle = css([c[0] * r, c[1] * r, c[2] * r], 1);
            ctx.fillRect(0, y, W, 4);
        }
    }
    function sunDisc(ctx, x, y, rx, ry, c, halo) {
        glow(ctx, x, y, halo, c, 0.5);
        ctx.beginPath();
        ctx.ellipse(x, y, rx, ry, 0, 0, PI2);
        ctx.fillStyle = css(c, 1);
        ctx.fill();
    }
    // 오른쪽 가장자리의 해 높이 눈금 — 입력이 얼마인지 눈에 보이게
    function gauge(ctx, alt, lo, hi) {
        const y0 = 46;
        const y1 = 236;
        const gy = lerp(y1, y0, (alt - lo) / (hi - lo));
        ctx.fillStyle = 'rgba(255,255,255,0.5)';
        ctx.fillRect(W - 14, y0, 2, y1 - y0);
        ctx.beginPath();
        ctx.arc(W - 13, clamp(gy, y0, y1), 5, 0, PI2);
        ctx.fillStyle = 'rgba(255,255,255,0.95)';
        ctx.fill();
        ctx.font = '600 11px -apple-system, "Segoe UI", sans-serif';
        ctx.textAlign = 'right';
        ctx.fillStyle = 'rgba(255,255,255,0.88)';
        ctx.fillRect(W - 76, 8, 68, 17);
        ctx.fillStyle = 'rgba(40,36,64,0.85)';
        ctx.fillText('alt ' + alt.toFixed(2), W - 14, 20);
    }
    function main(rootEl, canvas, statusEl, btnEl, scene) {
        const ctx0 = canvas.getContext('2d');
        if (!ctx0) {
            return function () { return; };
        }
        const ctx = ctx0;
        canvas.width = W;
        canvas.height = H;
        const reduce = window.matchMedia('(prefers-reduced-motion: reduce)').matches;
        if (reduce) {
            rootEl.classList.remove('is-demo');
        }
        const lo = Math.min(scene.lo, scene.hi);
        const hi = Math.max(scene.lo, scene.hi);
        let t = 0;
        let last = null;
        let paused = false;
        let manual = null;
        let preset = 0;
        let raf = 0;
        let pressed = false;
        let lastLabel = '';
        const cleaners = [];
        function listen(target, type, fn) {
            target.addEventListener(type, fn);
            cleaners.push(function () { target.removeEventListener(type, fn); });
        }
        function currentAlt() {
            if (manual !== null) {
                return manual;
            }
            if (reduce) {
                return scene.from;
            }
            return lerp(scene.from, scene.to, 0.5 - 0.5 * Math.cos(PI2 * t / PERIOD_MS));
        }
        function show() {
            const a = currentAlt();
            scene.draw(ctx, a, t);
            gauge(ctx, a, lo, hi);
            const s = labelOf(a);
            if (s !== lastLabel) {
                lastLabel = s;
                statusEl.textContent = s;
            }
        }
        function setButton() {
            btnEl.textContent = reduce ? '다음 장면' : (paused ? '재생' : '멈춤');
        }
        function takeOver(a) {
            manual = clamp(a, lo, hi);
            if (!reduce) {
                paused = true;
                rootEl.classList.remove('is-demo');
                setButton();
            }
            show();
        }
        function onButton() {
            if (reduce) {
                preset = (preset + 1) % 4;
                manual = lerp(scene.from, scene.to, preset / 3);
                show();
                return;
            }
            paused = !paused;
            if (paused) {
                rootEl.classList.remove('is-demo');
            }
            else {
                rootEl.classList.add('is-demo');
                manual = null;
            }
            setButton();
        }
        function altAtY(e) {
            const r = canvas.getBoundingClientRect();
            const py = clamp((e.clientY - r.top) * H / r.height, 0, H);
            return lerp(hi, lo, py / H);
        }
        function tick(ts) {
            if (last === null) {
                last = ts;
            }
            const dt = Math.min(ts - last, 160);
            last = ts;
            if (!paused) {
                t += dt;
                show();
            }
            raf = requestAnimationFrame(tick);
        }
        listen(btnEl, 'click', function () { onButton(); });
        listen(canvas, 'pointerdown', function (e) { pressed = true; takeOver(altAtY(e)); });
        listen(canvas, 'pointermove', function (e) { if (pressed) {
            takeOver(altAtY(e));
        } });
        listen(canvas, 'pointerup', function () { pressed = false; });
        listen(canvas, 'pointercancel', function () { pressed = false; });
        listen(canvas, 'keydown', function (e) {
            const k = e.key;
            if (k === 'Enter' || k === ' ') {
                e.preventDefault();
                onButton();
                return;
            }
            let d = 0;
            if (k === 'ArrowUp' || k === 'ArrowRight') {
                d = KEY_ALT;
            }
            else if (k === 'ArrowDown' || k === 'ArrowLeft') {
                d = -KEY_ALT;
            }
            else {
                return;
            }
            e.preventDefault();
            takeOver(currentAlt() + d);
        });
        setButton();
        show();
        if (!reduce) {
            raf = requestAnimationFrame(tick);
        }
        return function () {
            cancelAnimationFrame(raf);
            for (let i = 0; i < cleaners.length; i += 1) {
                cleaners[i]();
            }
        };
    }
    const AIR_DENSITY = 1.0;
    const PATH_GAIN = 0.22;
    const RED_KEEP = 0.55;
    const ROW_STEP = 2;
    const EXPOSURE = 6.8;
    const SCATTER = [0.28, 0.7, 1.7];
    const NIGHT_FLOOR = [0.03, 0.04, 0.11];
    // 한 줄의 색 — el 은 그 줄을 보는 시선의 높이(1 = 정점, 0 = 지평선). 낮을수록 대기를 지나는 길이 길어진다
    function rowColor(alt, el) {
        const sunPath = 1 / (0.9 * Math.max(alt, 0) + 0.1);
        const loss = [0.05 * (1 - RED_KEEP), 0.24, 0.42];
        const day = smooth((alt + 0.16) / 0.34);
        const rowPath = 1 / (el + 0.1);
        const out = [0, 0, 0];
        for (let c = 0; c < 3; c += 1) {
            const direct = Math.exp(-loss[c] * PATH_GAIN * sunPath * 4);
            const seen = 1 - Math.exp(-SCATTER[c] * AIR_DENSITY * rowPath * 0.18);
            const rad = direct * seen * (0.1 + 0.9 * day) + NIGHT_FLOOR[c] * (1 - day);
            out[c] = 255 * (0.2 + 0.8 * (1 - Math.exp(-EXPOSURE * rad)));
        }
        return out;
    }
    const scene = {
        from: SUN_ALT,
        to: -0.12,
        hi: 1,
        lo: ALT_NIGHT,
        draw: function (ctx, alt, t) {
            for (let y = 0; y < HORIZON; y += ROW_STEP) {
                ctx.fillStyle = css(rowColor(alt, 1 - y / HORIZON), 1);
                ctx.fillRect(0, y, W, ROW_STEP + 1);
            }
            // 왼쪽 막대 — 줄마다 시선이 지나는 길이 (지평선에 가까울수록 길다)
            for (let y = 52; y < HORIZON; y += 6) {
                const path = 1 / ((1 - y / HORIZON) + 0.1);
                ctx.fillStyle = 'rgba(255,255,255,0.7)';
                ctx.fillRect(14, y, 3 + path * 2.6, 3);
            }
            const sc = ramp(SUN_RAMP, alt);
            ctx.save();
            ctx.beginPath();
            ctx.rect(0, 0, W, HORIZON);
            ctx.clip();
            glow(ctx, SUN_X, HORIZON, 140, rowColor(alt, 0), 0.4);
            sunDisc(ctx, SUN_X, sunY(alt), 14, 14, sc, 70);
            ctx.restore();
            seaBase(ctx, alt, t);
            ctx.save();
            ctx.translate(SUN_X, HORIZON + 36);
            ctx.scale(1, 2.2);
            glow(ctx, 0, 0, 44, rowColor(alt, 0), 0.28 * smooth((alt + 0.1) / 0.3));
            ctx.restore();
        }
    };
    main(document.getElementById('scRoot'), document.getElementById('scCanvas'), document.getElementById('scStatus'), document.getElementById('scBtn'), scene);
})();

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

type RGB = [number, number, number];
type Stop = [number, RGB];
type Ctx = CanvasRenderingContext2D;
const W = 480;
const H = 300;
const PI2 = Math.PI * 2;
const HORIZON = 186;
const SUN_RISE = 150;
const SUN_X = 312;
const SUN_ALT = 0.5;
const ALT_NIGHT = -0.15;
const PERIOD_MS = 4400;
const KEY_ALT = 0.05;
const SKY_BLEND = 1.7;
// 해 높이(0 = 지평선, 1 = 높이 뜸) 순서대로 고른 색 띠 — 정점 쪽, 지평선 쪽, 해 원반
const ZENITH_RAMP: Stop[] = [[-0.15, [40, 44, 96]], [0, [92, 92, 156]], [0.12, [128, 126, 182]], [0.3, [150, 160, 214]], [0.6, [150, 192, 238]], [1, [138, 190, 242]]];
const HORIZON_RAMP: Stop[] = [[-0.15, [110, 104, 160]], [0, [238, 134, 142]], [0.12, [255, 168, 146]], [0.3, [255, 208, 166]], [0.6, [240, 226, 222]], [1, [206, 230, 248]]];
const SUN_RAMP: Stop[] = [[-0.15, [238, 128, 134]], [0, [255, 150, 126]], [0.15, [255, 190, 140]], [0.4, [255, 226, 176]], [1, [255, 246, 222]]];
const TXT_HIGH = '해가 높다 — 낮 하늘';
const TXT_GOLD = '해가 기운다 — 금빛 오후';
const TXT_GLOW = '해가 낮다 — 노을';
const TXT_TOUCH = '해가 지평선에 닿는다 — 붉은 띠';
const TXT_DUSK = '해가 졌다 — 땅거미';
function labelOf(alt: number): string {
  if (alt > 0.6) { return TXT_HIGH; }
  if (alt > 0.3) { return TXT_GOLD; }
  if (alt > 0.08) { return TXT_GLOW; }
  if (alt > -0.06) { return TXT_TOUCH; }
  return TXT_DUSK;
}
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 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)]; }
function css(c: RGB, a: number): string {
  return 'rgba(' + Math.round(c[0]) + ',' + Math.round(c[1]) + ',' + Math.round(c[2]) + ',' + a.toFixed(3) + ')';
}
function luma(c: RGB): number { return (0.2126 * c[0] + 0.7152 * c[1] + 0.0722 * c[2]) / 255; }
// 시드 고정 난수 — 같은 시드면 언제나 같은 수열
function mulberry32(seed: number): () => number {
  let s = seed >>> 0;
  return function (): number {
    s = (s + 0x6D2B79F5) >>> 0;
    let t = s;
    t = Math.imul(t ^ (t >>> 15), t | 1);
    t ^= t + Math.imul(t ^ (t >>> 7), t | 61);
    return ((t ^ (t >>> 14)) >>> 0) / 4294967296;
  };
}
function hash2(ix: number, iy: number): number {
  let h = Math.imul(ix | 0, 374761393) + Math.imul(iy | 0, 668265263);
  h = Math.imul(h ^ (h >>> 13), 1274126177);
  return ((h ^ (h >>> 16)) >>> 0) / 4294967296;
}
function vnoise(x: number, y: number): number {
  const ix = Math.floor(x);
  const iy = Math.floor(y);
  const fx = smooth(x - ix);
  const fy = smooth(y - iy);
  return lerp(lerp(hash2(ix, iy), hash2(ix + 1, iy), fx), lerp(hash2(ix, iy + 1), hash2(ix + 1, iy + 1), fx), fy);
}
function fbm(x: number, y: number): number {
  return vnoise(x, y) * 0.55 + vnoise(x * 2.1 + 7.3, y * 2.1 + 3.1) * 0.3 + vnoise(x * 4.3 + 1.7, y * 4.3 + 9.4) * 0.15;
}
function ramp(stops: Stop[], a: number): RGB {
  if (a <= stops[0][0]) { return stops[0][1]; }
  for (let i = 1; i < stops.length; i += 1) {
    if (a <= stops[i][0]) {
      const p = stops[i - 1];
      const q = stops[i];
      return mix(p[1], q[1], (a - p[0]) / (q[0] - p[0]));
    }
  }
  return stops[stops.length - 1][1];
}
// 하늘 한 점의 색 — v 는 0(정점)에서 1(지평선)
function skyAt(alt: number, v: number): RGB {
  return mix(ramp(ZENITH_RAMP, alt), ramp(HORIZON_RAMP, alt), Math.pow(clamp(v, 0, 1), SKY_BLEND));
}
function sunY(alt: number): number { return HORIZON - alt * SUN_RISE; }
function seaDeep(alt: number): RGB { return mix([84, 128, 170], [36, 48, 98], clamp((0.3 - alt) / 0.4, 0, 1)); }
function glow(ctx: Ctx, x: number, y: number, r: number, c: RGB, a: number): void {
  const g = ctx.createRadialGradient(x, y, 0, x, y, r);
  g.addColorStop(0, css(c, a));
  g.addColorStop(1, css(c, 0));
  ctx.fillStyle = g;
  ctx.fillRect(x - r, y - r, r * 2, r * 2);
}
function skyRows(ctx: Ctx, alt: number, y0: number, y1: number, step: number): void {
  for (let y = y0; y < y1; y += step) {
    ctx.fillStyle = css(skyAt(alt, (y - y0) / (y1 - y0)), 1);
    ctx.fillRect(0, y, W, step + 1);
  }
}
// 바다 바탕 — 지평선 하늘색을 받아 아래로 갈수록 깊어지고, 줄마다 잔물결 밝기가 흔들린다
function seaBase(ctx: Ctx, alt: number, t: number): void {
  const top = mix(skyAt(alt, 1), seaDeep(alt), 0.35);
  const deep = seaDeep(alt);
  for (let y = HORIZON; y < H; y += 3) {
    const k = (y - HORIZON) / (H - HORIZON);
    const c = mix(top, deep, Math.pow(k, 0.7));
    const r = 1 + 0.05 * Math.sin(y * 0.8 + t * 0.0018 + k * 6);
    ctx.fillStyle = css([c[0] * r, c[1] * r, c[2] * r], 1);
    ctx.fillRect(0, y, W, 4);
  }
}
function sunDisc(ctx: Ctx, x: number, y: number, rx: number, ry: number, c: RGB, halo: number): void {
  glow(ctx, x, y, halo, c, 0.5);
  ctx.beginPath();
  ctx.ellipse(x, y, rx, ry, 0, 0, PI2);
  ctx.fillStyle = css(c, 1);
  ctx.fill();
}
// 오른쪽 가장자리의 해 높이 눈금 — 입력이 얼마인지 눈에 보이게
function gauge(ctx: Ctx, alt: number, lo: number, hi: number): void {
  const y0 = 46;
  const y1 = 236;
  const gy = lerp(y1, y0, (alt - lo) / (hi - lo));
  ctx.fillStyle = 'rgba(255,255,255,0.5)';
  ctx.fillRect(W - 14, y0, 2, y1 - y0);
  ctx.beginPath();
  ctx.arc(W - 13, clamp(gy, y0, y1), 5, 0, PI2);
  ctx.fillStyle = 'rgba(255,255,255,0.95)';
  ctx.fill();
  ctx.font = '600 11px -apple-system, "Segoe UI", sans-serif';
  ctx.textAlign = 'right';
  ctx.fillStyle = 'rgba(255,255,255,0.88)';
  ctx.fillRect(W - 76, 8, 68, 17);
  ctx.fillStyle = 'rgba(40,36,64,0.85)';
  ctx.fillText('alt ' + alt.toFixed(2), W - 14, 20);
}

interface Scene {
  from: number;
  to: number;
  hi: number;
  lo: number;
  draw(ctx: Ctx, alt: number, t: number): void;
}
function main(rootEl: HTMLElement, canvas: HTMLCanvasElement, statusEl: HTMLElement, btnEl: HTMLElement, scene: Scene): () => void {
  const ctx0 = canvas.getContext('2d');
  if (!ctx0) { return function (): void { return; }; }
  const ctx: Ctx = ctx0;
  canvas.width = W;
  canvas.height = H;
  const reduce = window.matchMedia('(prefers-reduced-motion: reduce)').matches;
  if (reduce) { rootEl.classList.remove('is-demo'); }
  const lo = Math.min(scene.lo, scene.hi);
  const hi = Math.max(scene.lo, scene.hi);
  let t = 0;
  let last: number | null = null;
  let paused = false;
  let manual: number | null = null;
  let preset = 0;
  let raf = 0;
  let pressed = false;
  let lastLabel = '';
  const cleaners: Array<() => void> = [];
  function listen(target: HTMLElement, type: string, fn: (e: Event) => void): void {
    target.addEventListener(type, fn);
    cleaners.push(function (): void { target.removeEventListener(type, fn); });
  }
  function currentAlt(): number {
    if (manual !== null) { return manual; }
    if (reduce) { return scene.from; }
    return lerp(scene.from, scene.to, 0.5 - 0.5 * Math.cos(PI2 * t / PERIOD_MS));
  }
  function show(): void {
    const a = currentAlt();
    scene.draw(ctx, a, t);
    gauge(ctx, a, lo, hi);
    const s = labelOf(a);
    if (s !== lastLabel) { lastLabel = s; statusEl.textContent = s; }
  }
  function setButton(): void {
    btnEl.textContent = reduce ? '다음 장면' : (paused ? '재생' : '멈춤');
  }
  function takeOver(a: number): void {
    manual = clamp(a, lo, hi);
    if (!reduce) { paused = true; rootEl.classList.remove('is-demo'); setButton(); }
    show();
  }
  function onButton(): void {
    if (reduce) {
      preset = (preset + 1) % 4;
      manual = lerp(scene.from, scene.to, preset / 3);
      show();
      return;
    }
    paused = !paused;
    if (paused) { rootEl.classList.remove('is-demo'); }
    else { rootEl.classList.add('is-demo'); manual = null; }
    setButton();
  }
  function altAtY(e: PointerEvent): number {
    const r = canvas.getBoundingClientRect();
    const py = clamp((e.clientY - r.top) * H / r.height, 0, H);
    return lerp(hi, lo, py / H);
  }
  function tick(ts: number): void {
    if (last === null) { last = ts; }
    const dt = Math.min(ts - last, 160);
    last = ts;
    if (!paused) { t += dt; show(); }
    raf = requestAnimationFrame(tick);
  }
  listen(btnEl, 'click', function (): void { onButton(); });
  listen(canvas, 'pointerdown', function (e: Event): void { pressed = true; takeOver(altAtY(e as PointerEvent)); });
  listen(canvas, 'pointermove', function (e: Event): void { if (pressed) { takeOver(altAtY(e as PointerEvent)); } });
  listen(canvas, 'pointerup', function (): void { pressed = false; });
  listen(canvas, 'pointercancel', function (): void { pressed = false; });
  listen(canvas, 'keydown', function (e: Event): void {
    const k = (e as KeyboardEvent).key;
    if (k === 'Enter' || k === ' ') { e.preventDefault(); onButton(); return; }
    let d = 0;
    if (k === 'ArrowUp' || k === 'ArrowRight') { d = KEY_ALT; }
    else if (k === 'ArrowDown' || k === 'ArrowLeft') { d = -KEY_ALT; }
    else { return; }
    e.preventDefault();
    takeOver(currentAlt() + d);
  });
  setButton();
  show();
  if (!reduce) { raf = requestAnimationFrame(tick); }
  return function (): void {
    cancelAnimationFrame(raf);
    for (let i = 0; i < cleaners.length; i += 1) { cleaners[i](); }
  };
}

const AIR_DENSITY = 1.0;
const PATH_GAIN = 0.22;
const RED_KEEP = 0.55;
const ROW_STEP = 2;
const EXPOSURE = 6.8;
const SCATTER: RGB = [0.28, 0.7, 1.7];
const NIGHT_FLOOR: RGB = [0.03, 0.04, 0.11];
// 한 줄의 색 — el 은 그 줄을 보는 시선의 높이(1 = 정점, 0 = 지평선). 낮을수록 대기를 지나는 길이 길어진다
function rowColor(alt: number, el: number): RGB {
  const sunPath = 1 / (0.9 * Math.max(alt, 0) + 0.1);
  const loss: RGB = [0.05 * (1 - RED_KEEP), 0.24, 0.42];
  const day = smooth((alt + 0.16) / 0.34);
  const rowPath = 1 / (el + 0.1);
  const out: RGB = [0, 0, 0];
  for (let c = 0; c < 3; c += 1) {
    const direct = Math.exp(-loss[c] * PATH_GAIN * sunPath * 4);
    const seen = 1 - Math.exp(-SCATTER[c] * AIR_DENSITY * rowPath * 0.18);
    const rad = direct * seen * (0.1 + 0.9 * day) + NIGHT_FLOOR[c] * (1 - day);
    out[c] = 255 * (0.2 + 0.8 * (1 - Math.exp(-EXPOSURE * rad)));
  }
  return out;
}
const scene: Scene = {
  from: SUN_ALT,
  to: -0.12,
  hi: 1,
  lo: ALT_NIGHT,
  draw: function (ctx: Ctx, alt: number, t: number): void {
    for (let y = 0; y < HORIZON; y += ROW_STEP) {
      ctx.fillStyle = css(rowColor(alt, 1 - y / HORIZON), 1);
      ctx.fillRect(0, y, W, ROW_STEP + 1);
    }
    // 왼쪽 막대 — 줄마다 시선이 지나는 길이 (지평선에 가까울수록 길다)
    for (let y = 52; y < HORIZON; y += 6) {
      const path = 1 / ((1 - y / HORIZON) + 0.1);
      ctx.fillStyle = 'rgba(255,255,255,0.7)';
      ctx.fillRect(14, y, 3 + path * 2.6, 3);
    }
    const sc = ramp(SUN_RAMP, alt);
    ctx.save();
    ctx.beginPath();
    ctx.rect(0, 0, W, HORIZON);
    ctx.clip();
    glow(ctx, SUN_X, HORIZON, 140, rowColor(alt, 0), 0.4);
    sunDisc(ctx, SUN_X, sunY(alt), 14, 14, sc, 70);
    ctx.restore();
    seaBase(ctx, alt, t);
    ctx.save();
    ctx.translate(SUN_X, HORIZON + 36);
    ctx.scale(1, 2.2);
    glow(ctx, 0, 0, 44, rowColor(alt, 0), 0.28 * smooth((alt + 0.1) / 0.3));
    ctx.restore();
  }
};

export default function SsPathLengthScatterRows() {
  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 main(root, canvas, statusEl, btnEl, scene);
  }, []);

  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="줄마다 빛이 지나온 길이로 색을 칠하는 대기 산란 하늘" />
        <p className={styles.sc__status} ref={statusRef} aria-live="polite">해가 기운다 — 금빛 오후</p>
        <button className={styles.sc__btn} ref={btnRef} type="button">멈춤</button>
      </div>
    </main>
  );
}