08RGB ヒストグラム

<main class="stage">
<div class="hg is-demo" id="hg">
  <div class="hg__top">
    <div class="hg__frame">
      <canvas class="hg__view" id="view" width="320" height="200" role="img" aria-label="サンプル写真"></canvas>
    </div>
    <div class="hg__chart">
      <p class="hg__cap"><svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 256 256" class="hg__ico" aria-hidden="true"><g fill="currentColor"><path d="M208 40v168h-56V40Z" opacity=".2"/><path d="M224 200h-8V40a8 8 0 0 0-8-8h-56a8 8 0 0 0-8 8v40H96a8 8 0 0 0-8 8v40H48a8 8 0 0 0-8 8v64h-8a8 8 0 0 0 0 16h192a8 8 0 0 0 0-16M160 48h40v152h-40Zm-56 48h40v104h-40Zm-48 48h32v56H56Z"/></g></svg><span>ヒストグラム</span></p>
      <div class="hg__plot">
        <canvas class="hg__hist" id="hist" width="240" height="120" role="img" aria-label="チャンネル別の分布グラフ"></canvas>
        <span class="hg__bars" aria-hidden="true">
          <span class="hg__gb" style="--i:0; --h:.18"></span><span class="hg__gb" style="--i:1; --h:.3"></span><span class="hg__gb" style="--i:2; --h:.46"></span><span class="hg__gb" style="--i:3; --h:.62"></span>
          <span class="hg__gb" style="--i:4; --h:.8"></span><span class="hg__gb" style="--i:5; --h:.95"></span><span class="hg__gb" style="--i:6; --h:.72"></span><span class="hg__gb" style="--i:7; --h:.5"></span>
          <span class="hg__gb" style="--i:8; --h:.38"></span><span class="hg__gb" style="--i:9; --h:.44"></span><span class="hg__gb" style="--i:10; --h:.58"></span><span class="hg__gb" style="--i:11; --h:.7"></span>
          <span class="hg__gb" style="--i:12; --h:.85"></span><span class="hg__gb" style="--i:13; --h:.62"></span><span class="hg__gb" style="--i:14; --h:.4"></span><span class="hg__gb" style="--i:15; --h:.24"></span>
        </span>
      </div>
    </div>
  </div>
  <div class="hg__ctl">
    <span class="hg__lab" id="lab">明るさ</span>
    <span class="hg__track" id="track" role="slider" tabindex="0" aria-labelledby="lab" aria-valuemin="0.6" aria-valuemax="1.6" aria-valuenow="1"><span class="hg__thumb" id="thumb"></span></span>
  </div>
</div>
</main>

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

// RGB 히스토그램 — 실제 막대는 canvas 2D 가 그린다(보정을 먹인 사진을 다시 그린 뒤 getImageData 로 픽셀을 센다).
// 격자 미리보기(.is-demo)는 그 캔버스를 감추고 DOM 막대 열여섯 개를 scaleY 로 물결처럼 오르내리게 한다.
// 사진 쪽은 유령 판이 필요 없다 — 캔버스 자체에 걸린 filter 를 CSS 애니메이션이 그대로 덮어쓴다.

$duration: $dur-loop;
$easing: $ease-out;
$color: $subject-ink;

@include stage($stage-yellow);

.hg { display: flex; flex-direction: column; align-items: center; gap: $sp-2; width: 100%; max-width: 420px; }
.hg__top { display: flex; align-items: stretch; gap: $sp-2; width: 100%; justify-content: center; }
.hg__frame {
  position: relative; flex: 0 0 auto; height: clamp(76px, 36vh, 122px); aspect-ratio: 16 / 10;
  border-radius: $r-sm; overflow: hidden; box-shadow: $shadow-raised;
}
.hg__view { display: block; width: 100%; height: 100%; }
.hg__chart { flex: 1 1 auto; min-width: 0; display: flex; flex-direction: column; gap: $sp-1; }
.hg__cap {
  display: flex; align-items: center; gap: $sp-1; margin: 0;
  font-weight: 800; font-size: clamp(8px, 2.2vw, 10px); color: $color;
}
.hg__ico { width: 12px; height: 12px; flex: 0 0 auto; }
.hg__plot {
  position: relative; flex: 1 1 auto; min-height: 52px;
  border-radius: $r-xs; background: rgba(255, 255, 255, .5);
}
.hg__hist { display: block; width: 100%; height: 100%; }
.hg__bars {
  display: none; position: absolute; inset: 0;
  grid-template-columns: repeat(16, 1fr); align-items: end;
}
.hg__gb {
  width: 72%; height: 100%; justify-self: center;
  // 실제 캔버스는 칸마다 R·G·B 막대 셋을 나란히 그린다 — 유령 막대도 같은 색 차례로 세 쪽을 낸다
  background: linear-gradient(90deg, $chart-1 0 33.33%, $chart-3 33.33% 66.66%, $chart-2 66.66% 100%);
  border-radius: 0;
  transform-origin: bottom; transform: scaleY(var(--h));
}

.hg__ctl { display: flex; align-items: center; gap: $sp-2; }
.hg__lab { font-weight: 800; font-size: clamp(8px, 2.2vw, 10px); color: $color; }
.hg__track {
  position: relative; display: block; width: 100px; height: 6px; cursor: pointer;
  border-radius: $r-pill; background: rgba(23, 20, 26, .2);
}
.hg__track:focus-visible { outline: 3px solid $subject-blue; outline-offset: 4px; }
.hg__thumb {
  position: absolute; top: 50%; left: 40%; width: 13px; height: 13px;
  border-radius: 50%; background: $subject-ink; box-shadow: $shadow-press;
  transform: translate(-50%, -50%);
}

// ── 미리보기 루프: 밝기를 1.42배까지 올렸다가 되돌린다. 막대는 음의 지연으로 한 칸씩 어긋나
//    물결처럼 오르내린다 — 지연을 음수로 주면 주기가 깨지지 않아 루프가 이어붙는 지점이 튀지 않는다.
//    복귀는 88% 에 끝난다. 24장 표본의 마지막 장이 2초의 95.8% 지점이라 그 뒤는 안 찍힌다
.hg.is-demo .hg__hist { visibility: hidden; }
.hg.is-demo .hg__bars { display: grid; }
.hg.is-demo .hg__gb {
  animation-name: hgPulse; animation-duration: $duration;
  animation-timing-function: ease-in-out; animation-iteration-count: infinite;
  animation-delay: calc(var(--i) * -125ms);
}
@keyframes hgPulse {
  0%, 100% { transform: scaleY(var(--h)); }
  50% { transform: scaleY(calc(var(--h) * 0.4)); }
}
.hg.is-demo .hg__view {
  animation-name: hgLift; animation-duration: $duration;
  animation-timing-function: $easing; animation-iteration-count: infinite;
}
@keyframes hgLift {
  0%, 6% { filter: brightness(1); }
  40%, 74% { filter: brightness(1.42); }
  88%, 100% { filter: brightness(1); }
}
.hg.is-demo .hg__thumb {
  animation-name: hgThumb; animation-duration: $duration;
  animation-timing-function: $easing; animation-iteration-count: infinite;
}
@keyframes hgThumb {
  0%, 6% { transform: translate(-50%, -50%); }
  40%, 74% { transform: translate(-50%, -50%) translateX(42px); }
  88%, 100% { transform: translate(-50%, -50%); }
}

@media (max-width: 340px) {
  .hg__top { gap: $sp-1; }
  .hg__track { width: 84px; }
}

@media (prefers-reduced-motion: reduce) {
  .hg.is-demo .hg__gb,
  .hg.is-demo .hg__view,
  .hg.is-demo .hg__thumb { animation: none !important; }
  // 움직임 없이 밝기를 올린 상태로 세운다
  .hg.is-demo .hg__view { filter: brightness(1.42) !important; }
  .hg.is-demo .hg__thumb { transform: translate(-50%, -50%) translateX(42px) !important; }
}

html, body {
  margin: 0;
  overflow: hidden;
}

.stage {
  box-sizing: border-box;
  min-height: 100vh;
  display: grid;
  place-items: center;
  margin: 0;
  background: #ffd23f;
  font-family: -apple-system, "Segoe UI", "Noto Sans KR", sans-serif;
  padding: clamp(8px, 4vw, 24px);
}

.hg {
  display: flex;
  flex-direction: column;
  align-items: center;
  gap: 8px;
  width: 100%;
  max-width: 420px;
}

.hg__top {
  display: flex;
  align-items: stretch;
  gap: 8px;
  width: 100%;
  justify-content: center;
}

.hg__frame {
  position: relative;
  flex: 0 0 auto;
  height: clamp(76px, 36vh, 122px);
  aspect-ratio: 16/10;
  border-radius: 8px;
  overflow: hidden;
  box-shadow: 0 14px 24px rgba(23, 20, 26, 0.32);
}

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

.hg__chart {
  flex: 1 1 auto;
  min-width: 0;
  display: flex;
  flex-direction: column;
  gap: 4px;
}

.hg__cap {
  display: flex;
  align-items: center;
  gap: 4px;
  margin: 0;
  font-weight: 800;
  font-size: clamp(8px, 2.2vw, 10px);
  color: #17141a;
}

.hg__ico {
  width: 12px;
  height: 12px;
  flex: 0 0 auto;
}

.hg__plot {
  position: relative;
  flex: 1 1 auto;
  min-height: 52px;
  border-radius: 4px;
  background: rgba(255, 255, 255, 0.5);
}

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

.hg__bars {
  display: none;
  position: absolute;
  inset: 0;
  grid-template-columns: repeat(16, 1fr);
  align-items: end;
}

.hg__gb {
  width: 72%;
  height: 100%;
  justify-self: center;
  background: linear-gradient(90deg, #2f6df6 0 33.33%, #2b9d74 33.33% 66.66%, #c2361a 66.66% 100%);
  border-radius: 0;
  transform-origin: bottom;
  transform: scaleY(var(--h));
}

.hg__ctl {
  display: flex;
  align-items: center;
  gap: 8px;
}

.hg__lab {
  font-weight: 800;
  font-size: clamp(8px, 2.2vw, 10px);
  color: #17141a;
}

.hg__track {
  position: relative;
  display: block;
  width: 100px;
  height: 6px;
  cursor: pointer;
  border-radius: 999px;
  background: rgba(23, 20, 26, 0.2);
}

.hg__track:focus-visible {
  outline: 3px solid #2f6df6;
  outline-offset: 4px;
}

.hg__thumb {
  position: absolute;
  top: 50%;
  left: 40%;
  width: 13px;
  height: 13px;
  border-radius: 50%;
  background: #17141a;
  box-shadow: 0 6px 12px rgba(23, 20, 26, 0.32);
  transform: translate(-50%, -50%);
}

.hg.is-demo .hg__hist {
  visibility: hidden;
}

.hg.is-demo .hg__bars {
  display: grid;
}

.hg.is-demo .hg__gb {
  animation-name: hgPulse;
  animation-duration: 2s;
  animation-timing-function: ease-in-out;
  animation-iteration-count: infinite;
  animation-delay: calc(var(--i) * -125ms);
}

@keyframes hgPulse {
  0%, 100% {
    transform: scaleY(var(--h));
  }
  50% {
    transform: scaleY(calc(var(--h) * 0.4));
  }
}
.hg.is-demo .hg__view {
  animation-name: hgLift;
  animation-duration: 2s;
  animation-timing-function: ease-out;
  animation-iteration-count: infinite;
}

@keyframes hgLift {
  0%, 6% {
    filter: brightness(1);
  }
  40%, 74% {
    filter: brightness(1.42);
  }
  88%, 100% {
    filter: brightness(1);
  }
}
.hg.is-demo .hg__thumb {
  animation-name: hgThumb;
  animation-duration: 2s;
  animation-timing-function: ease-out;
  animation-iteration-count: infinite;
}

@keyframes hgThumb {
  0%, 6% {
    transform: translate(-50%, -50%);
  }
  40%, 74% {
    transform: translate(-50%, -50%) translateX(42px);
  }
  88%, 100% {
    transform: translate(-50%, -50%);
  }
}
@media (max-width: 340px) {
  .hg__top {
    gap: 4px;
  }
  .hg__track {
    width: 84px;
  }
}
@media (prefers-reduced-motion: reduce) {
  .hg.is-demo .hg__gb,
  .hg.is-demo .hg__view,
  .hg.is-demo .hg__thumb {
    animation: none !important;
  }
  .hg.is-demo .hg__view {
    filter: brightness(1.42) !important;
  }
  .hg.is-demo .hg__thumb {
    transform: translate(-50%, -50%) translateX(42px) !important;
  }
}

var hg = document.getElementById('hg');
var view = document.getElementById('view');
var hist = document.getElementById('hist');
var track = document.getElementById('track');
var thumb = document.getElementById('thumb');
var vctx = view.getContext('2d');
var hctx = hist.getContext('2d');
var work = document.createElement('canvas');
var wctx = work.getContext('2d', { willReadFrequently: true });
var BINS = 16;
var CHANNEL = ['rgba(47, 109, 246, .85)', 'rgba(43, 157, 116, .85)', 'rgba(194, 54, 26, .85)'];
var level = 1, queued = 0;
var VOICE = '%v 倍';

// 가짜 사진 — 외부 파일 없이 그라디언트와 도형으로 합성한다. 필터·히스토그램이 보이도록 대비를 세게 준다
function paintPhoto(c, w, h) {
  var sky = c.createLinearGradient(0, 0, 0, h);
  sky.addColorStop(0, '#2f6df6'); sky.addColorStop(0.55, '#ff9ebb'); sky.addColorStop(1, '#ffd23f');
  c.fillStyle = sky; c.fillRect(0, 0, w, h);
  c.fillStyle = '#fff7e6';
  c.beginPath(); c.arc(w * 0.74, h * 0.28, h * 0.13, 0, Math.PI * 2); c.fill();
  // 곧은 수평선 하나 — 기울기 보정이 눈에 보이려면 비교할 직선이 있어야 한다
  c.fillStyle = 'rgba(23, 20, 26, 0.5)'; c.fillRect(0, Math.round(h * 0.66), w, 2);
  c.fillStyle = '#17141a';
  c.beginPath(); c.moveTo(0, h); c.lineTo(w * 0.34, h * 0.36); c.lineTo(w * 0.62, h); c.closePath(); c.fill();
  c.fillStyle = '#c2361a';
  c.beginPath(); c.moveTo(w * 0.46, h); c.lineTo(w * 0.78, h * 0.52); c.lineTo(w, h); c.closePath(); c.fill();
  for (var i = 0; i < 6; i++) { c.fillStyle = i % 2 ? '#fff7e6' : '#17141a'; c.fillRect(w * (0.08 + i * 0.07), h * (0.14 + (i % 3) * 0.06), 7, 7); }
}

// 보정을 먹인 사진을 작은 판에 다시 그린 다음 픽셀을 통째로 읽어 칸마다 센다.
// 한 칸은 明るさ 16단계 — 256 단계를 そのまま 세면 막대가 너무 잘아서 모양이 안 읽힌다.
// ctx.filter 는 Safari 에 아직 없다(caniuse 81.08%). 없으면 읽은 값에 직접 곱해 같은 결과를 만든다
var CAN_FILTER = typeof wctx.filter === 'string';

function countBins() {
  if (CAN_FILTER) { wctx.filter = 'brightness(' + level.toFixed(2) + ')'; }
  wctx.clearRect(0, 0, work.width, work.height);
  wctx.drawImage(view, 0, 0, work.width, work.height);
  var data = wctx.getImageData(0, 0, work.width, work.height).data;
  var bins = [], i;
  for (i = 0; i < BINS * 3; i++) { bins.push(0); }
  for (var p = 0; p < data.length; p += 4) {
    for (var ch = 0; ch < 3; ch++) {
      var v = CAN_FILTER ? data[p + ch] : Math.min(255, Math.round(data[p + ch] * level));
      bins[Math.min(BINS - 1, v >> 4) * 3 + ch] += 1;
    }
  }
  return bins;
}

function drawHist(bins) {
  var w = hist.width, h = hist.height, top = 1, i;
  for (i = 0; i < bins.length; i++) { if (bins[i] > top) { top = bins[i]; } }
  hctx.clearRect(0, 0, w, h);
  var slot = w / BINS, bar = slot / 3;
  for (i = 0; i < BINS; i++) {
    for (var ch = 0; ch < 3; ch++) {
      var tall = bins[i * 3 + ch] / top * h;
      hctx.fillStyle = CHANNEL[ch];
      hctx.fillRect(i * slot + ch * bar, h - tall, bar - 1, tall);
    }
  }
}

function refresh() {
  view.style.filter = 'brightness(' + level.toFixed(2) + ')';
  thumb.style.left = ((level - 0.6) * 100) + '%';
  track.setAttribute('aria-valuenow', level.toFixed(2));
  track.setAttribute('aria-valuetext', VOICE.replace('%v', level.toFixed(2)));
  // 픽셀 세기는 무거우니 프레임마다 한 번으로 묶는다 — 슬라이더를 빠르게 끌어도 밀리지 않는다
  if (queued) { return; }
  queued = requestAnimationFrame(function () { queued = 0; drawHist(countBins()); });
}

function setLevel(next) {
  level = Math.max(0.6, Math.min(1.6, Math.round(next * 100) / 100));
  hg.classList.remove('is-demo');
  refresh();
}

function fromPointer(e) {
  var r = track.getBoundingClientRect();
  setLevel((e.clientX - r.left) / r.width + 0.6);
}
track.addEventListener('pointerdown', function (e) { track.setPointerCapture(e.pointerId); fromPointer(e); });
track.addEventListener('pointermove', function (e) { if (e.buttons) { fromPointer(e); } });
// 화살표 0.02, Shift 와 함께면 0.1
track.addEventListener('keydown', function (e) {
  var step = e.shiftKey ? 0.1 : 0.02;
  if (e.key === 'ArrowLeft' || e.key === 'ArrowDown') { e.preventDefault(); setLevel(level - step); }
  else if (e.key === 'ArrowRight' || e.key === 'ArrowUp') { e.preventDefault(); setLevel(level + step); }
});

work.width = 80; work.height = 50;
paintPhoto(vctx, view.width, view.height);
refresh();

// RgbHistogram — vanilla 판(run/295/demo/08-image-edit-histogram-rgb/index.html) 의 React 포트.
// 보정을 먹인 사진을 작은 판에 다시 그린 뒤 getImageData 로 픽셀을 통째로 읽어 16칸 3채널을 센다.
// 격자 미리보기(.isDemo)는 그 캔버스를 감추고 DOM 막대 열여섯 개를 scaleY 로 오르내리게 한다.
// 값·수치는 vanilla 와 같다.
import { useCallback, useEffect, useRef, useState } from "react";
import type { CSSProperties, PointerEvent as ReactPointerEvent } from "react";
import styles from "./RgbHistogram.module.scss";

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

type Vars = CSSProperties & { "--duration"?: string; "--easing"?: string; "--color"?: string };
type BarVars = CSSProperties & { "--i"?: string; "--h"?: string };
// ambient 타입에 KeyboardEvent 가 없다 — 화살표 조작에 쓰는 것만 지역 타입으로 선언한다
type KeyEvt = { key: string; shiftKey: boolean; preventDefault(): void };
// useRef 는 T | null 을 돌려준다. 처음부터 값이 있고 절대 비지 않는 상자는 이렇게 좁혀 쓴다
type Box<T> = { current: T };

const BINS = 16;
const CHANNEL = ["rgba(47, 109, 246, .85)", "rgba(43, 157, 116, .85)", "rgba(194, 54, 26, .85)"];
const VOICE = "%v배";
// 미리보기 막대의 키 — 실제로 센 값이 아니라 모양만 보여주는 유령 판이다
const GHOST = [0.18, 0.3, 0.46, 0.62, 0.8, 0.95, 0.72, 0.5, 0.38, 0.44, 0.58, 0.7, 0.85, 0.62, 0.4, 0.24];

// 가짜 사진 — 외부 파일 없이 그라디언트와 도형으로 합성한다. 필터·히스토그램이 보이도록 대비를 세게 준다
function paintPhoto(c: CanvasRenderingContext2D, w: number, h: number): void {
  const sky = c.createLinearGradient(0, 0, 0, h);
  sky.addColorStop(0, "#2f6df6");
  sky.addColorStop(0.55, "#ff9ebb");
  sky.addColorStop(1, "#ffd23f");
  c.fillStyle = sky;
  c.fillRect(0, 0, w, h);
  c.fillStyle = "#fff7e6";
  c.beginPath();
  c.arc(w * 0.74, h * 0.28, h * 0.13, 0, Math.PI * 2);
  c.fill();
  // 곧은 수평선 하나 — 기울기 보정이 눈에 보이려면 비교할 직선이 있어야 한다
  c.fillStyle = "rgba(23, 20, 26, 0.5)";
  c.fillRect(0, Math.round(h * 0.66), w, 2);
  c.fillStyle = "#17141a";
  c.beginPath();
  c.moveTo(0, h);
  c.lineTo(w * 0.34, h * 0.36);
  c.lineTo(w * 0.62, h);
  c.closePath();
  c.fill();
  c.fillStyle = "#c2361a";
  c.beginPath();
  c.moveTo(w * 0.46, h);
  c.lineTo(w * 0.78, h * 0.52);
  c.lineTo(w, h);
  c.closePath();
  c.fill();
  for (let i = 0; i < 6; i++) {
    c.fillStyle = i % 2 ? "#fff7e6" : "#17141a";
    c.fillRect(w * (0.08 + i * 0.07), h * (0.14 + (i % 3) * 0.06), 7, 7);
  }
}

// 보정을 먹인 사진을 작은 판에 다시 그린 다음 픽셀을 통째로 읽어 칸마다 센다.
// 한 칸은 밝기 16단계 — 256 단계를 그대로 세면 막대가 너무 잘아서 모양이 안 읽힌다
// ctx.filter 는 Safari 에 아직 없다(caniuse 81.08%). 없으면 읽은 값에 직접 곱해 같은 결과를 만든다
function countBins(wctx: CanvasRenderingContext2D, work: HTMLCanvasElement, view: HTMLCanvasElement, level: number): number[] {
  const canFilter = typeof wctx.filter === "string";
  if (canFilter) wctx.filter = "brightness(" + level.toFixed(2) + ")";
  wctx.clearRect(0, 0, work.width, work.height);
  wctx.drawImage(view, 0, 0, work.width, work.height);
  const data = wctx.getImageData(0, 0, work.width, work.height).data;
  const bins: number[] = [];
  for (let i = 0; i < BINS * 3; i++) bins.push(0);
  for (let p = 0; p < data.length; p += 4) {
    for (let ch = 0; ch < 3; ch++) {
      const v = canFilter ? data[p + ch] : Math.min(255, Math.round(data[p + ch] * level));
      bins[Math.min(BINS - 1, v >> 4) * 3 + ch] += 1;
    }
  }
  return bins;
}

function drawHist(hctx: CanvasRenderingContext2D, hist: HTMLCanvasElement, bins: number[]): void {
  const w = hist.width;
  const h = hist.height;
  let top = 1;
  for (let i = 0; i < bins.length; i++) {
    if (bins[i] > top) top = bins[i];
  }
  hctx.clearRect(0, 0, w, h);
  const slot = w / BINS;
  const bar = slot / 3;
  for (let i = 0; i < BINS; i++) {
    for (let ch = 0; ch < 3; ch++) {
      const tall = (bins[i * 3 + ch] / top) * h;
      hctx.fillStyle = CHANNEL[ch];
      hctx.fillRect(i * slot + ch * bar, h - tall, bar - 1, tall);
    }
  }
}

export default function RgbHistogram({
  duration = 2000,
  easing = "ease-out",
  color = "#17141a",
}: RgbHistogramProps) {
  const viewRef = useRef<HTMLCanvasElement | null>(null);
  const histRef = useRef<HTMLCanvasElement | null>(null);
  const trackRef = useRef<HTMLSpanElement | null>(null);
  const workRef = useRef<HTMLCanvasElement | null>(null);
  const wctxRef = useRef<CanvasRenderingContext2D | null>(null);
  const queuedRef = useRef<number>(0) as Box<number>;
  const levelRef = useRef<number>(1) as Box<number>;
  const [demo, setDemo] = useState(true);
  const [level, setLevel] = useState(1);
  const vars: Vars = { "--duration": `${duration}ms`, "--easing": easing, "--color": color };

  // 픽셀을 읽는 판은 한 번만 만든다 — willReadFrequently 는 만들 때 한 번만 먹는 옵션이다
  const ensureWork = useCallback((): HTMLCanvasElement | null => {
    if (!workRef.current) {
      const work = document.createElement("canvas");
      work.width = 80;
      work.height = 50;
      const wctx = work.getContext("2d", { willReadFrequently: true });
      if (!wctx) return null;
      workRef.current = work;
      wctxRef.current = wctx;
    }
    return workRef.current;
  }, []);

  useEffect(() => {
    const view = viewRef.current;
    if (!view) return;
    const vctx = view.getContext("2d");
    if (vctx) paintPhoto(vctx, view.width, view.height);
  }, []);

  // 픽셀 세기는 무거우니 프레임마다 한 번으로 묶는다 — 슬라이더를 빠르게 끌어도 밀리지 않는다
  useEffect(() => {
    if (queuedRef.current) return;
    queuedRef.current = requestAnimationFrame(() => {
      queuedRef.current = 0;
      const view = viewRef.current;
      const hist = histRef.current;
      const work = ensureWork();
      const wctx = wctxRef.current;
      if (!view || !hist || !work || !wctx) return;
      const hctx = hist.getContext("2d");
      if (!hctx) return;
      drawHist(hctx, hist, countBins(wctx, work, view, levelRef.current));
    });
  }, [level, ensureWork]);

  useEffect(() => {
    return () => {
      if (queuedRef.current) cancelAnimationFrame(queuedRef.current);
    };
  }, []);

  const applyLevel = useCallback((next: number) => {
    const value = Math.max(0.6, Math.min(1.6, Math.round(next * 100) / 100));
    levelRef.current = value;
    setLevel(value);
    setDemo(false);
  }, []);

  function fromPointer(event: ReactPointerEvent<HTMLSpanElement>): void {
    const track = trackRef.current;
    if (!track) return;
    const r = track.getBoundingClientRect();
    applyLevel((event.clientX - r.left) / r.width + 0.6);
  }

  function onTrackDown(event: ReactPointerEvent<HTMLSpanElement>): void {
    const track = trackRef.current;
    if (!track) return;
    track.setPointerCapture(event.pointerId);
    fromPointer(event);
  }

  function onTrackMove(event: ReactPointerEvent<HTMLSpanElement>): void {
    if (event.buttons) fromPointer(event);
  }

  // 화살표 0.02, Shift 와 함께면 0.1
  function onTrackKey(event: KeyEvt): void {
    const step = event.shiftKey ? 0.1 : 0.02;
    if (event.key === "ArrowLeft" || event.key === "ArrowDown") {
      event.preventDefault();
      applyLevel(level - step);
    } else if (event.key === "ArrowRight" || event.key === "ArrowUp") {
      event.preventDefault();
      applyLevel(level + step);
    }
  }

  const cls = [styles.root, demo ? styles.isDemo : ""].filter(Boolean).join(" ");
  return (
    <main className={styles.stage}>
      <div className={cls} style={vars}>
        <div className={styles.top}>
          <div className={styles.frame}>
            <canvas
              className={styles.view}
              ref={viewRef}
              width={320}
              height={200}
              role="img"
              aria-label="샘플 사진"
              style={{ filter: `brightness(${level.toFixed(2)})` }}
            ></canvas>
          </div>
          <div className={styles.chart}>
            <p className={styles.cap}>
              <svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 256 256" className={styles.ico} aria-hidden="true"><g fill="currentColor"><path d="M208 40v168h-56V40Z" opacity=".2"/><path d="M224 200h-8V40a8 8 0 0 0-8-8h-56a8 8 0 0 0-8 8v40H96a8 8 0 0 0-8 8v40H48a8 8 0 0 0-8 8v64h-8a8 8 0 0 0 0 16h192a8 8 0 0 0 0-16M160 48h40v152h-40Zm-56 48h40v104h-40Zm-48 48h32v56H56Z"/></g></svg>
              <span>히스토그램</span>
            </p>
            <div className={styles.plot}>
              <canvas className={styles.hist} ref={histRef} width={240} height={120} role="img" aria-label="채널별 분포 그래프"></canvas>
              <span className={styles.bars} aria-hidden="true">
                {GHOST.map((h, i) => {
                  const barVars: BarVars = { "--i": String(i), "--h": String(h) };
                  return <span key={i} className={styles.gb} style={barVars}></span>;
                })}
              </span>
            </div>
          </div>
        </div>
        <div className={styles.ctl}>
          <span className={styles.lab} id="levelLab">밝기</span>
          <span
            className={styles.track}
            ref={trackRef}
            role="slider"
            tabIndex={0}
            aria-labelledby="levelLab"
            aria-valuemin={0.6}
            aria-valuemax={1.6}
            aria-valuenow={level.toFixed(2)}
            aria-valuetext={VOICE.replace("%v", level.toFixed(2))}
            onPointerDown={onTrackDown}
            onPointerMove={onTrackMove}
            onKeyDown={onTrackKey}
          >
            <span className={styles.thumb} style={{ left: `${(level - 0.6) * 100}%` }}></span>
          </span>
        </div>
      </div>
    </main>
  );
}