原文链接及内容

原文链接：https://openlayers.org/en/latest/examples/canvas-gradient-pattern.html

运行界面

这个例子创建了一个CanvasGradient。矢量数据从文件中加载，并给各个要素填充了渐变色。同样的技术也可以用于CanvasPattern。

main.js代码如下：

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import KML from 'ol/format/KML.js';
import Map from 'ol/Map.js';
import VectorLayer from 'ol/layer/Vector.js';
import VectorSource from 'ol/source/Vector.js';
import View from 'ol/View.js';
import {DEVICE_PIXEL_RATIO} from 'ol/has.js';
import {fromLonLat} from 'ol/proj.js';

// Gradient and pattern are in canvas pixel space, so we adjust for the
// renderer's pixel ratio
// 渐变和图案都存在于Canvas的像素空间，因此我们只需调整渲染器的像素比例
const pixelRatio = DEVICE_PIXEL_RATIO;

// Generate a rainbow gradient
// 生成一个彩虹风格的渐变
const canvas = document.createElement('canvas');
const context = canvas.getContext('2d');
const gradient = context.createLinearGradient(0, 0, 1024 * pixelRatio, 0);
gradient.addColorStop(0, 'red');
gradient.addColorStop(1 / 6, 'orange');
gradient.addColorStop(2 / 6, 'yellow');
gradient.addColorStop(3 / 6, 'green');
gradient.addColorStop(4 / 6, 'aqua');
gradient.addColorStop(5 / 6, 'blue');
gradient.addColorStop(1, 'purple');

const vectorLayer = new VectorLayer({
  background: 'white',
  source: new VectorSource({
    url: 'data/kml/states.kml',
    format: new KML({extractStyles: false}),
  }),
  style: {
    'fill-color': gradient,
    'stroke-width': 1,
    'stroke-color': '#333',
  },
});

const map = new Map({
  layers: [vectorLayer],
  target: 'map',
  view: new View({
    center: fromLonLat([-100, 38.5]),
    zoom: 4,
  }),
});

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>Styling feature with CanvasGradient or CanvasPattern</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>