openlayers3 聚合分析专题图 cluster

 在地图上查询结果通常以标记点的形式展现，但是如果标记点较多，不仅会大大增加客户端的渲染时间，让客户端变得很卡，而且会让人产生密集恐惧症，为了解决这一问题，我们需要一种手段能在用户有限的可视区域范围内，利用最小的区域展示出最全面的信息，而又不产生重叠覆盖。具体算法探讨：点击打开链接

openlayers3提供api具体实例使用方法:

  一.简单实例：点击打开链接

1.创建feature，添加到ol.source.Cluster 中；设置聚合半径distance红色标注

var count = 20000;

 var features = new Array(count); var e = 4500000; for (var i = 0; i < count; ++i) { var coordinates = [2 * e * Math.random() - e, 2 * e * Math.random() - e]; features[i] = new ol.Feature(new ol.geom.Point(coordinates)); } var source = new ol.source.Vector({ features: features }); var clusterSource = new ol.source.Cluster({ distance: parseInt(distance.value, 10), source: source });

2.添加到ol.layer.Vector图层中，并根据feature属性设置对应style

var styleCache = {}; var clusters = new ol.layer.Vector({ source: clusterSource, style: function(feature) { var size = feature.get('features').length; var style = styleCache[size]; if (!style) { style = new ol.style.Style({ image: new ol.style.Circle({ radius: 10, stroke: new ol.style.Stroke({ color: '#fff' }), fill: new ol.style.Fill({ color: '#3399CC' }) }), text: new ol.style.Text({ text: size.toString(), fill: new ol.style.Fill({ color: '#fff' }) }) }); styleCache[size] = style; } return style; } });

3.添加到map中即可

 var map = new ol.Map({ layers: [raster, clusters], target: 'map', view: new ol.View({ center: [0, 0], zoom: 2 }) });

二.实际应用 EarthQuake

1.初始化聚合图层，设置distance，绑定缩放事件

 vector = new ol.layer.Vector({
        source: new ol.source.Cluster({
          distance: 40,
          source: new ol.source.Vector({
            url: 'https://openlayers.org/en/v4.2.0/examples/data/kml/2012_Earthquakes_Mag5.kml',
            format: new ol.format.KML({
              extractStyles: false
            })
          })
        }),
        style: styleFunction
      });




2.根据当前缩放resolution，及feature属性设置渲染style

var currentResolution; function styleFunction(feature, resolution) { if (resolution != currentResolution) { calculateClusterInfo(resolution); currentResolution = resolution; } var style; var size = feature.get('features').length; if (size > 1) { style = new ol.style.Style({ image: new ol.style.Circle({ radius: feature.get('radius'), fill: new ol.style.Fill({ color: [255, 153, 0, Math.min(0.8, 0.4 + (size / maxFeatureCount))] }) }), text: new ol.style.Text({ text: size.toString(), fill: textFill, stroke: textStroke }) }); } else { var originalFeature = feature.get('features')[0]; style = createEarthquakeStyle(originalFeature); } return style; }

3.根据点击聚合图形，设置渲染style

function selectStyleFunction(feature) { var styles = [new ol.style.Style({ image: new ol.style.Circle({ radius: feature.get('radius'), fill: invisibleFill }) })]; var originalFeatures = feature.get('features'); var originalFeature; for (var i = originalFeatures.length - 1; i >= 0; --i) { originalFeature = originalFeatures[i]; styles.push(createEarthquakeStyle(originalFeature)); } return styles; }

4.添加聚合图层，并绑定点击事件

 var map = new ol.Map({ layers: [raster, vector], interactions: ol.interaction.defaults().extend([new ol.interaction.Select({  condition: function(evt) { return evt.originalEvent.type == 'mousemove' || evt.type == 'singleclick'; }, style: selectStyleFunction })]), target: 'map', view: new ol.View({ center: [0, 0], zoom: 2 }) });