原文链接及内容

原文链接：https://openlayers.org/en/latest/examples/webgl-layer-swipe.html

运行界面

WebGL图层上的preender和postrender事件可以用来在渲染前后操纵WebGL的上下文。在这种情况下，调用gl.scissor()方法来根据滑块的位置裁剪顶部图层。

main.js代码如下：

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import Map from 'ol/Map.js';
import OSM from 'ol/source/OSM.js';
import TileLayer from 'ol/layer/WebGLTile.js';
import View from 'ol/View.js';
import XYZ from 'ol/source/XYZ.js';
import {getRenderPixel} from 'ol/render.js';

const osm = new TileLayer({
  source: new OSM({wrapX: true}),
});

const key = 'Get your own API key at https://www.maptiler.com/cloud/';

const imagery = new TileLayer({
  source: new XYZ({
    url: 'https://api.maptiler.com/tiles/satellite/{z}/{x}/{y}.jpg?key=' + key,
    attributions:
      '<a href="https://www.maptiler.com/copyright/" target="_blank">&copy; MapTiler</a> ' +
      '<a href="https://www.openstreetmap.org/copyright" target="_blank">&copy; OpenStreetMap contributors</a>',
    maxZoom: 20,
  }),
});

const map = new Map({
  layers: [osm, imagery],
  target: 'map',
  view: new View({
    center: [0, 0],
    zoom: 2,
  }),
});

const swipe = document.getElementById('swipe');

imagery.on('prerender', function (event) {
  const gl = event.context;
  gl.enable(gl.SCISSOR_TEST);

  const mapSize = map.getSize(); // [width, height] in CSS pixels

  // get render coordinates and dimensions given CSS coordinates
  const bottomLeft = getRenderPixel(event, [0, mapSize[1]]);
  const topRight = getRenderPixel(event, [mapSize[0], 0]);

  const width = Math.round((topRight[0] - bottomLeft[0]) * (swipe.value / 100));
  const height = topRight[1] - bottomLeft[1];

  gl.scissor(bottomLeft[0], bottomLeft[1], width, height);
});

imagery.on('postrender', function (event) {
  const gl = event.context;
  gl.disable(gl.SCISSOR_TEST);
});

swipe.addEventListener('input', function () {
  map.render();
});

界面布局文件index.html代码如下：

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<!DOCTYPE html>
<html lang="en">
  <head>
    <meta charset="UTF-8">
    <title>Layer Swipe (WebGL)</title>
    <link rel="stylesheet" href="node_modules/ol/ol.css">
    <style>
      .map {
        width: 100%;
        height: 400px;
      }
    </style>
  </head>
  <body>
    <div id="map" class="map"></div>
    <input id="swipe" type="range" style="width: 100%">
    <!-- Pointer events polyfill for old browsers, see https://caniuse.com/#feat=pointer -->
    <script src="https://cdn.jsdelivr.net/npm/elm-pep@1.0.6/dist/elm-pep.js"></script>
    <script type="module" src="main.js"></script>
  </body>
</html>