Geotiff Per Band Stats

<!--
  示例：逐波段统计拉伸 — OpenLayers geotiff-per-band-stats
  核心逻辑：canvas 生成三波段栅格，Raster 源按各波段自己的最小/最大值逐像素拉伸
  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>逐波段统计拉伸</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:16px;align-items:center;font-size:12px;color:#58606e;flex-wrap:wrap}
  .bar label{display:flex;gap:6px;align-items:center}
  .bar input[type=range]{width:110px}
  #map{flex:1 1 auto;min-height:0;background:#eef0f3}
</style>
</head>
<body>
<div class="bar">
  <label>R 波段上限 <input type="range" id="rMax" min="60" max="255" value="200"><b id="rVal">200</b></label>
  <label>G 波段上限 <input type="range" id="gMax" min="60" max="255" value="180"><b id="gVal">180</b></label>
  <label>B 波段上限 <input type="range" id="bMax" min="60" max="255" value="160"><b id="bVal">160</b></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') {
    document.getElementById('map').innerHTML =
      '<div style="padding:20px;color:#b42318">OpenLayers 加载失败，请检查能否访问 cdn.jsdelivr.net</div>';
    return;
  }

  /* 生成 512 × 512 的三波段数据图，分别放在 R / G / B 通道 */
  function makeRasterImage() {
    var size = 512;
    var c = document.createElement('canvas');
    c.width = size;
    c.height = size;
    var ctx = c.getContext('2d');
    var img = ctx.createImageData(size, size);

    var mins = [255, 255, 255];
    var maxs = [0, 0, 0];

    for (var y = 0; y < size; y++) {
      for (var x = 0; x < size; x++) {
        var i = (y * size + x) * 4;

        /* 三个互不相关的合成波段：地形、植被、水体 */
        var terrain = 120 + 80 * Math.sin(x / 37) * Math.cos(y / 41) +
          30 * Math.sin((x + y) / 13);
        var veg = 90 + 70 * Math.sin((x - y) / 29) + 40 * Math.cos(x / 71);
        var water = 30 + 110 * Math.max(0, Math.sin(y / 55)) +
          25 * Math.sin(x / 17);

        var vals = [terrain, veg, water].map(function (v) {
          return Math.max(0, Math.min(255, Math.round(v)));
        });

        img.data[i] = vals[0];
        img.data[i + 1] = vals[1];
        img.data[i + 2] = vals[2];
        img.data[i + 3] = 255;

        for (var b = 0; b < 3; b++) {
          if (vals[b] < mins[b]) mins[b] = vals[b];
          if (vals[b] > maxs[b]) maxs[b] = vals[b];
        }
      }
    }

    ctx.putImageData(img, 0, 0);

    /* 统计结果就是各波段的真实上下限，下面用它算默认拉伸 */
    return { url: c.toDataURL(), mins: mins, maxs: maxs };
  }

  var data = makeRasterImage();
  var extent = ol.proj.transformExtent(
    [116.28, 39.85, 116.48, 39.98], 'EPSG:4326', 'EPSG:3857'
  );

  var baseSource = new ol.source.ImageStatic({
    url: data.url,
    projection: 'EPSG:3857',
    imageExtent: extent,
    crossOrigin: 'anonymous'
  });

  var layer = new ol.layer.Image();

  function stretch(value, max) {
    return Math.max(0, Math.min(255, Math.round(value * 255 / max)));
  }

  function rebuild() {
    var rMax = Number(document.getElementById('rMax').value);
    var gMax = Number(document.getElementById('gMax').value);
    var bMax = Number(document.getElementById('bMax').value);

    layer.setSource(new ol.source.Raster({
      sources: [baseSource],
      operation: function (pixels) {
        var src = pixels[0];
        var out = new Uint8ClampedArray(src.length);
        for (var i = 0; i < src.length; i += 4) {
          out[i] = stretch(src[i], rMax);
          out[i + 1] = stretch(src[i + 1], gMax);
          out[i + 2] = stretch(src[i + 2], bMax);
          out[i + 3] = 255;
        }
        return out;
      }
    }));

    document.getElementById('info').textContent =
      '统计范围 R ' + data.mins[0] + '–' + data.maxs[0] +
      ' / G ' + data.mins[1] + '–' + data.maxs[1] +
      ' / B ' + data.mins[2] + '–' + data.maxs[2];
  }

  ['rMax', 'gMax', 'bMax'].forEach(function (id) {
    var el = document.getElementById(id);
    var out = document.getElementById(id.charAt(0) + 'Val');
    el.addEventListener('input', function () {
      out.textContent = el.value;
      rebuild();
    });
  });

  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: new ol.View({
      center: ol.proj.fromLonLat([116.39, 39.915]),
      zoom: 12,
      minZoom: 2
    }),
    controls: ol.control.defaults.defaults({ attribution: false })
  });

  layer.setOpacity(0.85);
  rebuild();
})();
</script>
</body>
</html>