原文链接及内容

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

运行界面

图层渲染可以在prerender和postrender事件监听器中进行，这些监听器会获得一个带有对Canvas渲染上下文引用的事件。在这个例子中，prerender在鼠标所在位置周围设置了一个裁剪遮罩层——类似望远镜的效果，可以在一个图层上查看另一个图层。在地图上移动鼠标看看效果，可以使用↑和↓方向键来调整望远镜的大小。

main.js代码如下：

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import Map from 'ol/Map.js';
import TileLayer from 'ol/layer/Tile.js';
import View from 'ol/View.js';
import XYZ from 'ol/source/XYZ.js';
import {fromLonLat} from 'ol/proj.js';
import {getRenderPixel} from 'ol/render.js';

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

const roads = new TileLayer({
  source: new XYZ({
    attributions: attributions,
    url: 'https://api.maptiler.com/maps/streets/{z}/{x}/{y}.png?key=' + key,
    tileSize: 512,
    maxZoom: 22,
  }),
});

const imagery = new TileLayer({
  source: new XYZ({
    attributions: attributions,
    url: 'https://api.maptiler.com/tiles/satellite/{z}/{x}/{y}.jpg?key=' + key,
    maxZoom: 20,
  }),
});

const container = document.getElementById('map');

const map = new Map({
  layers: [roads, imagery],
  target: container,
  view: new View({
    center: fromLonLat([-109, 46.5]),
    zoom: 6,
  }),
});

let radius = 75;
document.addEventListener('keydown', function (evt) {
  if (evt.which === 38) {
    radius = Math.min(radius + 5, 150);
    map.render();
    evt.preventDefault();
  } else if (evt.which === 40) {
    radius = Math.max(radius - 5, 25);
    map.render();
    evt.preventDefault();
  }
});

// get the pixel position with every move
let mousePosition = null;

container.addEventListener('mousemove', function (event) {
  mousePosition = map.getEventPixel(event);
  map.render();
});

container.addEventListener('mouseout', function () {
  mousePosition = null;
  map.render();
});

// before rendering the layer, do some clipping
imagery.on('prerender', function (event) {
  const ctx = event.context;
  ctx.save();
  ctx.beginPath();
  if (mousePosition) {
    // only show a circle around the mouse
    const pixel = getRenderPixel(event, mousePosition);
    const offset = getRenderPixel(event, [
      mousePosition[0] + radius,
      mousePosition[1],
    ]);
    const canvasRadius = Math.sqrt(
      Math.pow(offset[0] - pixel[0], 2) + Math.pow(offset[1] - pixel[1], 2)
    );
    ctx.arc(pixel[0], pixel[1], canvasRadius, 0, 2 * Math.PI);
    ctx.lineWidth = (5 * canvasRadius) / radius;
    ctx.strokeStyle = 'rgba(0,0,0,0.5)';
    ctx.stroke();
  }
  ctx.clip();
});

// after rendering the layer, restore the canvas context
imagery.on('postrender', function (event) {
  const ctx = event.context;
  ctx.restore();
});

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 Spy</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>
    <!-- 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>