图层卷帘(WebGL)

裁剪一个 WebGL 瓦片图层

WebGL 瓦片图层的 prerender 和 postrender 事件可以用来在图层渲染前后操作 WebGL 上下文。在本示例中，使用 gl.scissor() 方法根据 input 滑块位置裁剪最上层图层。

main.js

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();
});

index.html

<!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="./resources/elm-pep.js"></script>
    <script type="module" src="main.js"></script>
  </body>
</html>

package.json

{
  "name": "webgl-layer-swipe",
  "dependencies": {
    "ol": "7.3.0"
  },
  "devDependencies": {
    "vite": "^3.2.3"
  },
  "scripts": {
    "start": "vite",
    "build": "vite build"
  }
}