Cog Stretch

<!--
  说明：波段对比度拉伸的中文示例，数据在页面内生成，可独立运行。
  核心逻辑：(v - min) / (max - min) 拉伸 + canvas 重绘 + ImageStatic 更新
  依赖：OpenLayers 10.3.1 · jsDelivr CDN
-->
<!DOCTYPE html>
<html lang="zh-CN">
<head>
<meta name="robots" content="noindex,follow">
<meta charset="utf-8">
<meta name="viewport" content="width=device-width, initial-scale=1">
<title>波段对比度拉伸</title>
<link rel="stylesheet" href="https://cdn.jsdelivr.net/npm/ol@10.3.1/ol.css">
<style>
  html,body{height:100%;margin:0}
  body{display:flex;flex-direction:column;overflow:hidden;
       font:13px/1.6 -apple-system,"Segoe UI","PingFang SC",sans-serif}
  .bar{flex:0 0 auto;padding:8px 12px;background:#f6f7f9;border-bottom:1px solid #e3e6ea;
       display:flex;gap:10px;align-items:center;flex-wrap:wrap;font-size:12px;color:#58606e}
  .bar label{display:flex;gap:5px;align-items:center;cursor:pointer}
  .bar button{font:inherit;padding:3px 9px;border:1px solid #c9ced6;background:#fff;
       border-radius:5px;cursor:pointer;color:#333}
  .bar button:hover{background:#eef2f8}
  .bar input[type=range]{width:110px;vertical-align:middle}
  .bar input[type=text]{font:inherit;padding:2px 6px;border:1px solid #c9ced6;border-radius:4px}
  #map{flex:1 1 auto;min-height:0;background:#eef0f3;position:relative}
  .panel{flex:0 0 auto;max-height:34%;overflow:auto;padding:8px 12px;font-size:12px;
       background:#fff;border-top:1px solid #e3e6ea;color:#3d4451}
  .panel pre{margin:0;font-family:Consolas,"Courier New",monospace;font-size:11px;
       white-space:pre-wrap;word-break:break-all;line-height:1.5}
  .ol-attribution{font-size:11px}

</style>
</head>
<body>
<div class="bar">
<label>低值 <input type="range" id="lo" min="0" max="100" value="20"></label>
<label>高值 <input type="range" id="hi" min="0" max="100" value="80"></label>
<span id="info">—</span>
</div>
<div id="map"></div>

<script src="https://cdn.jsdelivr.net/npm/ol@10.3.1/dist/ol.js"></script>
<script>
(function () {
  'use strict';

  if (typeof ol === 'undefined') {
    var errBox = document.getElementById('map');
    if (errBox) {
      errBox.innerHTML = '<div style="padding:20px;color:#b42318">OpenLayers 加载失败，请检查能否访问 cdn.jsdelivr.net</div>';
    }
    return;
  }

  var fromLonLat = ol.proj.fromLonLat;
  var N = 512;
  var band = new Float32Array(N * N);
  var vmin = 1, vmax = 0;

  for (var y = 0; y < N; y++) {
    for (var x = 0; x < N; x++) {
      var v = 0.5 + 0.28 * Math.sin(x / 38) * Math.cos(y / 52) +
              0.16 * Math.sin((x + y) / 23) + 0.06 * Math.cos(x / 7);
      v = Math.min(1, Math.max(0, v));
      band[y * N + x] = v;
      if (v < vmin) { vmin = v; }
      if (v > vmax) { vmax = v; }
    }
  }

  var canvas = document.createElement('canvas');
  canvas.width = canvas.height = N;
  var ctx = canvas.getContext('2d');
  var image = ctx.createImageData(N, N);

  var extent = ol.proj.transformExtent([115.8, 39.4, 117.0, 40.4], 'EPSG:4326', 'EPSG:3857');

  var layer = new ol.layer.Image({
    source: new ol.source.ImageStatic({
      url: canvas.toDataURL('image/png'),
      imageExtent: extent,
      projection: 'EPSG:3857'
    })
  });

  var view = new ol.View({center: fromLonLat([116.4, 39.9]), zoom: 11});
  var map = new ol.Map({
    target: 'map',
    layers: [new ol.layer.Tile({source: new ol.source.XYZ({url: 'https://server.arcgisonline.com/ArcGIS/rest/services/World_Street_Map/MapServer/tile/{z}/{y}/{x}', attributions: 'Tiles © Esri'})}), layer],
    view: view,
    controls: ol.control.defaults.defaults({attribution: false, zoom: true, rotate: false})
  });

  var info = document.getElementById('info');
  var lo = document.getElementById('lo');
  var hi = document.getElementById('hi');

  function render() {
    var a = +lo.value / 100;
    var b = +hi.value / 100;
    if (b - a < 0.02) { b = a + 0.02; }
    for (var i = 0; i < N * N; i++) {
      var t = (band[i] - a) / (b - a);
      t = t < 0 ? 0 : (t > 1 ? 1 : t);
      var g = Math.round(t * 255);
      image.data[i * 4] = g;
      image.data[i * 4 + 1] = g;
      image.data[i * 4 + 2] = g;
      image.data[i * 4 + 3] = 255;
    }
    ctx.putImageData(image, 0, 0);
    layer.setSource(new ol.source.ImageStatic({
      url: canvas.toDataURL('image/png'),
      imageExtent: extent,
      projection: 'EPSG:3857'
    }));
    info.textContent = '拉伸区间 [' + a.toFixed(2) + ', ' + b.toFixed(2) + '] · 原始值域 [' +
      vmin.toFixed(2) + ', ' + vmax.toFixed(2) + ']';
  }

  lo.addEventListener('input', render);
  hi.addEventListener('input', render);
  render();
})();
</script>
</body>
</html>