Imagery Color to Alpha

<!--
  示例：指定颜色转透明
  说明：把影像里的深色海面变成透明，下层底图透出来
  核心逻辑：layer.colorToAlpha = Color → colorToAlphaThreshold 控制容差
  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>指定颜色转透明</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">上半部分海面被转成透明，下面的蓝色底图透出来；阈值越小转得越干净。</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;

  function makeImage(withSea) {
    var c = document.createElement('canvas');
    c.width = 512; c.height = 512;
    var ctx = c.getContext('2d');
    ctx.fillStyle = '#2c3a2a';
    ctx.fillRect(0, 0, 512, 512);
    if (withSea) {
      ctx.fillStyle = '#000410';       /* 深色海面，稍后抠成透明 */
      ctx.fillRect(0, 0, 512, 210);
    }
    ctx.fillStyle = '#8a9a6a';
    for (var i = 0; i < 80; i++) {
      ctx.fillRect((i * 61) % 500, 230 + (i * 37) % 270, 30, 24);
    }
    return c.toDataURL('image/png');
  }

  var rect = Cesium.Rectangle.fromDegrees(115.9, 39.5, 116.9, 40.2);

  /* 下层：纯蓝底 */
  viewer.imageryLayers.addImageryProvider(
    new Cesium.SingleTileImageryProvider({
      url: (function () {
        var c = document.createElement('canvas');
        c.width = c.height = 8;
        var ctx = c.getContext('2d');
        ctx.fillStyle = '#1c4f78';
        ctx.fillRect(0, 0, 8, 8);
        return c.toDataURL('image/png');
      })(),
      rectangle: rect
    })
  );

  /* 上层：海面用 colorToAlpha 抠掉 */
  var top = viewer.imageryLayers.addImageryProvider(
    new Cesium.SingleTileImageryProvider({ url: makeImage(true), rectangle: rect })
  );
  top.colorToAlpha = new Cesium.Color(0.0, 0.016, 0.06, 1.0);
  top.colorToAlphaThreshold = 0.12;
  viewer.camera.flyTo({
    destination: Cesium.Rectangle.fromDegrees(115.9, 39.5, 116.9, 40.2),
    duration: 0
  });
})();
</script>
</body>
</html>