3D Tiles Terrain Classification

<!--
  示例：地形分级设色
  说明：Canvas 逐像素生成五级高程色带并叠加差分阴影，作为矩形范围的地面材质
  核心逻辑：程序化高程函数 → 分带取色 + 差分阴影 → ImageMaterialProperty 贴到矩形
  依赖：CesiumJS 1.121 · 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>地形分级设色</title>
<link rel="stylesheet" href="https://cdn.jsdelivr.net/npm/cesium@1.121.0/Build/Cesium/Widgets/widgets.css">
<style>
  html,body{height:100%;margin:0}
  body{background:#0e1116;color:#e6e8ec;overflow:hidden;
       font:13px/1.6 -apple-system,"Segoe UI","PingFang SC",sans-serif}
  #c{height:100%}
  .tip{position:absolute;left:12px;top:12px;z-index:5;
       background:rgba(23,28,35,.9);border:1px solid #2a313b;border-radius:6px;
       padding:8px 12px;font-size:12px;color:#a2a9b6;max-width:320px;line-height:1.6}
  .cesium-viewer-bottom .cesium-widget-credits,.cesium-viewer-animationContainer,
  .cesium-viewer-timelineContainer{display:none!important}

</style>
</head>
<body>
<div id="c"></div>
<div class="tip">五级色带对应五段高程，亮度差异来自左上方打来的模拟光照。</div>

<script src="https://cdn.jsdelivr.net/npm/cesium@1.121.0/Build/Cesium/Cesium.js"></script>
<script>
(function () {
  'use strict';

  var viewer = new Cesium.Viewer('c', {
    baseLayer: false,
    baseLayerPicker: false, geocoder: false, homeButton: false,
    sceneModePicker: false, navigationHelpButton: false,
    animation: false, timeline: false, fullscreenButton: false,
    infoBox: false, selectionIndicator: false
  });

  var scene = viewer.scene;
  scene.globe.baseColor = Cesium.Color.fromCssColorString('#1c2733');
  scene.backgroundColor = Cesium.Color.fromCssColorString('#0e1116');
  scene.skyBox.show = false;
  scene.sun.show = false;
  scene.moon.show = false;
  if (scene.skyAtmosphere) scene.skyAtmosphere.show = false;

  /* Canvas 画一张地形分类图：按高程分带 + 简单山体阴影 */
  var N = 256;
  var cv = document.createElement('canvas');
  cv.width = cv.height = N;
  var ctx = cv.getContext('2d');
  var img = ctx.createImageData(N, N);

  function elev(x, y) {
    return 0.5 + 0.28 * Math.sin(x * 0.07) * Math.cos(y * 0.05) +
           0.18 * Math.sin(x * 0.13 + 1.7) * Math.cos(y * 0.09 + 0.6);
  }

  var bands = [
    [0.00, [0x1f, 0x33, 0x2a]],   /* 低洼 */
    [0.35, [0x2f, 0x5a, 0x3c]],   /* 草地 */
    [0.52, [0x6d, 0x6a, 0x3a]],   /* 丘陵 */
    [0.68, [0x8a, 0x6a, 0x4a]],   /* 裸岩 */
    [0.82, [0xd8, 0xdb, 0xdf]]    /* 雪线 */
  ];

  function bandColor(t) {
    var c = bands[0][1];
    for (var i = 0; i < bands.length; i++) if (t >= bands[i][0]) c = bands[i][1];
    return c;
  }

  for (var y = 0; y < N; y++) {
    for (var x = 0; x < N; x++) {
      var t = elev(x, y);
      var c = bandColor(t);
      /* 用左上方差分做阴影 */
      var dz = elev(x + 1, y + 1) - elev(x, y);
      var shade = 1 + dz * 2.4;
      var i = (y * N + x) * 4;
      img.data[i]     = Math.min(255, c[0] * shade);
      img.data[i + 1] = Math.min(255, c[1] * shade);
      img.data[i + 2] = Math.min(255, c[2] * shade);
      img.data[i + 3] = 255;
    }
  }
  ctx.putImageData(img, 0, 0);

  viewer.entities.add({
    rectangle: {
      coordinates: Cesium.Rectangle.fromDegrees(116.372, 39.892, 116.412, 39.922),
      height: 0,
      material: new Cesium.ImageMaterialProperty({ image: cv })
    }
  });

  viewer.camera.flyTo({
    destination: Cesium.Cartesian3.fromDegrees(116.372, 39.884, 3600),
    orientation: { heading: 0, pitch: Cesium.Math.toRadians(-52), roll: 0 },
    duration: 0
  });
})();
</script>
</body>
</html>