d3 Integration——d3集成

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The example loads TopoJSON geometries and uses d3 (d3.geo.path) to render these geometries to a canvas element that is then used as the image of an OpenLayers image layer.
这个例子加载TopoJSON的几何图形并使用d3（d3.geo.path）来渲染这些几何图形到canvas元素上，然后用作Openlayers image图层的图像。

代码：

<!DOCTYPE html><html>  <head>    <title>d3 Integration</title>    <link rel="stylesheet" href="https://openlayers.org/en/v4.2.0/css/ol.css" type="text/css">    <!-- The line below is only needed for old environments like Internet Explorer and Android 4.x -->    <script src="https://cdn.polyfill.io/v2/polyfill.min.js?features=requestAnimationFrame,Element.prototype.classList,URL"></script>    <script src="https://openlayers.org/en/v4.2.0/build/ol.js"></script>    <script src="https://d3js.org/d3.v3.min.js"></script>    <script src="https://d3js.org/topojson.v1.min.js"></script>  </head>  <body>    <div id="map" class="map"></div>    <script>      var map = new ol.Map({        layers: [          new ol.layer.Tile({            source: new ol.source.Stamen({              layer: 'watercolor'            })          })        ],        target: 'map',        view: new ol.View({          center: ol.proj.fromLonLat([-97, 38]),          zoom: 4        })      });      /**       * Load the topojson data and create an ol.layer.Image for that data.      * 加载topojson数据并为这个数据创建一个ol.layer.Image图层       */      d3.json('https://openlayers.org/en/v4.2.0/examples/data/topojson/us.json', function(error, us) {        var features = topojson.feature(us, us.objects.counties);        /**         * This function uses d3 to render the topojson features to a canvas.         * 这个函数使用d3来把topojson要素渲染到canvas上         * @param {ol.Extent} extent Extent.         * @param {number} resolution Resolution.         * @param {number} pixelRatio Pixel ratio.         * @param {ol.Size} size Size.         * @param {ol.proj.Projection} projection Projection.         * @return {HTMLCanvasElement} A canvas element.         */        var canvasFunction = function(extent, resolution, pixelRatio,            size, projection) {          var canvasWidth = size[0];          var canvasHeight = size[1];          var canvas = d3.select(document.createElement('canvas'));          canvas.attr('width', canvasWidth).attr('height', canvasHeight);          var context = canvas.node().getContext('2d');          var d3Projection = d3.geo.mercator().scale(1).translate([0, 0]);          var d3Path = d3.geo.path().projection(d3Projection);          var pixelBounds = d3Path.bounds(features);          var pixelBoundsWidth = pixelBounds[1][0] - pixelBounds[0][0];          var pixelBoundsHeight = pixelBounds[1][1] - pixelBounds[0][1];          var geoBounds = d3.geo.bounds(features);          var geoBoundsLeftBottom = ol.proj.transform(              geoBounds[0], 'EPSG:4326', projection);          var geoBoundsRightTop = ol.proj.transform(              geoBounds[1], 'EPSG:4326', projection);          var geoBoundsWidth = geoBoundsRightTop[0] - geoBoundsLeftBottom[0];          if (geoBoundsWidth < 0) {            geoBoundsWidth += ol.extent.getWidth(projection.getExtent());          }          var geoBoundsHeight = geoBoundsRightTop[1] - geoBoundsLeftBottom[1];          var widthResolution = geoBoundsWidth / pixelBoundsWidth;          var heightResolution = geoBoundsHeight / pixelBoundsHeight;          var r = Math.max(widthResolution, heightResolution);          var scale = r / (resolution / pixelRatio);          var center = ol.proj.transform(ol.extent.getCenter(extent),              projection, 'EPSG:4326');          d3Projection.scale(scale).center(center)              .translate([canvasWidth / 2, canvasHeight / 2]);          d3Path = d3Path.projection(d3Projection).context(context);          d3Path(features);          context.stroke();          return canvas[0][0];        };        var layer = new ol.layer.Image({          source: new ol.source.ImageCanvas({            canvasFunction: canvasFunction,            projection: 'EPSG:3857'          })        });        map.addLayer(layer);      });    </script>  </body></html>

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