1、前言
前一篇博客介绍了如何在OpenLayers中使用WebGLPoints加载海量数据点的方法，这篇博客就来介绍一下WebGLPoints图层的样式设置问题。
2、样式运算符
在VectorLayer图层中，我们只需要创建一个ol.style.Style对象即可，WebGLPoints则不同，它并不是基于Canvas进行绘制，因此其样式渲染不能直接使用ol.style.Style，取而代之的是使用样式表达式进行渲染。
2.1、读取运算符
1、['get', 'attributeName']2、['var', 'varName']3、['time']4、['zoom']5、['resolution']2.2、数学运算符
1、['*', value1, value2]2、['/', value1, value2]3、['+', value1, value2]4、['-', value1, value2]5、['clamp', value, low, high]6、['%', value1, value2]7、['^', value1, value2]2.3、变换运算符
1、['case', condition1, output1, ...conditionN, outputN, fallback]2、['match', input, match1, output1, ...matchN, outputN, fallback]3、['interpolate', interpolation, input, stop1, output1, ...stopN, outputN]2.4、逻辑运算符
1、['<', value1, value2]2、['<=', value1, value2]3、['>', value1, value2]4、['>=', value1, value2]5、['==', value1, value2]6、['!=', value1, value2]7、['!', value1]8、['between', value1, value2, value3]2.5、转换运算符
1、['array', value1, ...valueN]2、['color', red, green, blue, alpha]3、简单渲染
简单颜色渲染很简单，只需要使用['color', red, green, blue, alpha]即可，第一个参数为固定值‘color’，后面的参数依次为红、绿、蓝、透明度。下面的代码会将要素渲染为红色点：
<!DOCTYPE html><html lang="en"><head> <meta charset="UTF-8"> <meta name="viewport" content="width=device-width, initial-scale=1.0"> <title>WebGL</title> <style> html, body, #map { width: 100%; height: 100%; margin: 0; padding: 0; } </style> <link rel="stylesheet" href="ol/ol.css" /> <script src="ol/ol.js"></script></head><body> <div id="map"></div> <script> // 创建图层 var layer = new ol.layer.WebGLPoints({ source: new ol.source.Vector({ features: [ new ol.Feature({ geometry: new ol.geom.Point([120.0, 30.0]), "type": "学校", "dbm": 1 }), new ol.Feature({ geometry: new ol.geom.Point([120.0, 30.1]), "type": "学校", "dbm": 2 }), new ol.Feature({ geometry: new ol.geom.Point([120.1, 30.0]), "type": "超市", "dbm": 3 }), new ol.Feature({ geometry: new ol.geom.Point([120.1, 30.1]), "type": "超市", "dbm": 4 }), new ol.Feature({ geometry: new ol.geom.Point([120.2, 30.0]), "type": "医院", "dbm": 5 }), new ol.Feature({ geometry: new ol.geom.Point([120.2, 30.1]), "type": "医院", "dbm": 6 }), ] }), style: { symbol: { symbolType: 'circle', size: 40, color: ['color', 255, 0, 0, 1] } } }); // 创建地图 var map = new ol.Map({ target: 'map', layers: [ layer ], view: new ol.View({ projection: 'EPSG:4326', center: [120, 30], zoom: 10 }) }); </script></body></html>运行结果如下图所示：
4、分类渲染
测试数据中的type字段将要素分成了3类，即：学校、超市、医院。现在要求将学校渲染为红色、超市渲染为绿色、医院渲染为蓝色。此时需要使用match表达式，其形式如下所示：
['match', type的值, '学校', 红色, '超市', 绿色, '医院', 蓝色, 默认颜色]那么type的值又该如何获取？其实很简单，使用['get', 'attributeName']表达式即可，所以最后的样式表达式如下所示：
['match', ['get', 'type'], '学校', 红色, '超市', 绿色, '医院', 蓝色, 默认颜色]代码如下所示：
<!DOCTYPE html><html lang="en"><head> <meta charset="UTF-8"> <meta name="viewport" content="width=device-width, initial-scale=1.0"> <title>WebGL</title> <style> html, body, #map { width: 100%; height: 100%; margin: 0; padding: 0; } </style> <link rel="stylesheet" href="ol/ol.css" /> <script src="ol/ol.js"></script></head><body> <div id="map"></div> <script> // 创建图层 var layer = new ol.layer.WebGLPoints({ source: new ol.source.Vector({ features: [ new ol.Feature({ geometry: new ol.geom.Point([120.0, 30.0]), "type": "学校", "dbm": 1 }), new ol.Feature({ geometry: new ol.geom.Point([120.0, 30.1]), "type": "学校", "dbm": 2 }), new ol.Feature({ geometry: new ol.geom.Point([120.1, 30.0]), "type": "超市", "dbm": 3 }), new ol.Feature({ geometry: new ol.geom.Point([120.1, 30.1]), "type": "超市", "dbm": 4 }), new ol.Feature({ geometry: new ol.geom.Point([120.2, 30.0]), "type": "医院", "dbm": 5 }), new ol.Feature({ geometry: new ol.geom.Point([120.2, 30.1]), "type": "医院", "dbm": 6 }), ] }), style: { symbol: { symbolType: 'circle', size: 40, color: [ 'match', ['get', 'type'], '学校', ['color', 255, 0, 0, 1], '超市', ['color', 0, 255, 0, 1], '医院', ['color', 0, 0, 255, 1], ['color', 255, 0, 0, 1] ] } } }); // 创建地图 var map = new ol.Map({ target: 'map', layers: [ layer ], view: new ol.View({ projection: 'EPSG:4326', center: [120, 30], zoom: 10 }) }); </script></body></html>运行结果如下图所示：
5、分级渲染
测试数据中包含一个dbm字段，现在根据dbm的范围进行分级渲染，规定如下：
1、dbm∈[1, 2]，渲染为红色
2、dbm == 3，渲染为绿色
3、dbm == 4，渲染为蓝色
4、dbm∈[5, 6]，渲染为黄色
此时需要使用case表达式，其形式如下所示：
['case', 'dbm∈[1,2]', 红色, 'dbm==3', 绿色, 'dbm==4', 蓝色, 'dbm∈[5,6]', 红色， 默认颜色]在判断dbm的值的范围时，需要使用逻辑表达式：
['==', dbm, 3]['==', dbm, 4]['between', dbm, 1, 2]['between', dbm, 5, 6]最后，使用['get', 'attributeName']表达式获取字段值：
['get', 'dbm']代码如下所示：
<!DOCTYPE html><html lang="en"><head> <meta charset="UTF-8"> <meta name="viewport" content="width=device-width, initial-scale=1.0"> <title>WebGL</title> <style> html, body, #map { width: 100%; height: 100%; margin: 0; padding: 0; } </style> <link rel="stylesheet" href="ol/ol.css" /> <script src="ol/ol.js"></script></head><body> <div id="map"></div> <script> // 创建图层 var layer = new ol.layer.WebGLPoints({ source: new ol.source.Vector({ features: [ new ol.Feature({ geometry: new ol.geom.Point([120.0, 30.0]), "type": "学校", "dbm": 1 }), new ol.Feature({ geometry: new ol.geom.Point([120.0, 30.1]), "type": "学校", "dbm": 2 }), new ol.Feature({ geometry: new ol.geom.Point([120.1, 30.0]), "type": "超市", "dbm": 3 }), new ol.Feature({ geometry: new ol.geom.Point([120.1, 30.1]), "type": "超市", "dbm": 4 }), new ol.Feature({ geometry: new ol.geom.Point([120.2, 30.0]), "type": "医院", "dbm": 5 }), new ol.Feature({ geometry: new ol.geom.Point([120.2, 30.1]), "type": "医院", "dbm": 6 }), ] }), style: { symbol: { symbolType: 'circle', size: 40, color: [ 'case', ['between', ['get', 'dbm'], 1, 2], ['color', 255, 0, 0, 1], ['==', ['get', 'dbm'], 3], ['color', 0, 255, 0, 1], ['==', ['get', 'dbm'], 4], ['color', 0, 0, 255, 1], ['between', ['get', 'dbm'], 5, 6], ['color', 255, 255, 0, 1], ['color', 255, 0, 0, 1] ] } } }); // 创建地图 var map = new ol.Map({ target: 'map', layers: [ layer ], view: new ol.View({ projection: 'EPSG:4326', center: [120, 30], zoom: 10 }) }); </script></body></html>运行结果如下图所示：
6、根据地图缩放等级渲染
现做如下规定：
1、地图缩放等级zoom∈（0, 10]，渲染为红色
2、地图缩放等级zoom∈（10, 12]，渲染为绿色
3、地图缩放等级zoom∈（12, 14]，渲染为蓝色
4、其余缩放等级，渲染为黄色
看了上面的例子之后，相信同志们应该想到了：利用['zoom']获取地图缩放等级，然后利用case表达式进行情况分类，最后每种情况利用逻辑表达式判断即可。代码如下：
<!DOCTYPE html><html lang="en"><head> <meta charset="UTF-8"> <meta name="viewport" content="width=device-width, initial-scale=1.0"> <title>WebGL</title> <style> html, body, #map { width: 100%; height: 100%; margin: 0; padding: 0; } </style> <link rel="stylesheet" href="ol/ol.css" /> <script src="ol/ol.js"></script></head><body> <div id="map"></div> <script> // 创建图层 var layer = new ol.layer.WebGLPoints({ source: new ol.source.Vector({ features: [ new ol.Feature({ geometry: new ol.geom.Point([120.0, 30.0]), "type": "学校", "dbm": 1 }), new ol.Feature({ geometry: new ol.geom.Point([120.0, 30.1]), "type": "学校", "dbm": 2 }), new ol.Feature({ geometry: new ol.geom.Point([120.1, 30.0]), "type": "超市", "dbm": 3 }), new ol.Feature({ geometry: new ol.geom.Point([120.1, 30.1]), "type": "超市", "dbm": 4 }), new ol.Feature({ geometry: new ol.geom.Point([120.2, 30.0]), "type": "医院", "dbm": 5 }), new ol.Feature({ geometry: new ol.geom.Point([120.2, 30.1]), "type": "医院", "dbm": 6 }), ] }), style: { symbol: { symbolType: 'circle', size: 40, color: [ 'case', ['<=', ['zoom'], 10], ['color', 255, 0, 0, 1], ['<=', ['zoom'], 12], ['color', 0, 255, 0, 1], ['<=', ['zoom'], 14], ['color', 0, 0, 255, 1], ['color', 255, 255, 0, 1] ] } } }); // 创建地图 var map = new ol.Map({ target: 'map', layers: [ layer ], view: new ol.View({ projection: 'EPSG:4326', center: [120, 30], zoom: 10 }) }); </script></body></html>运行结果如下图所示：
7、根据地图分辨率渲染
根据地图分辨率渲染也很简单，只需要把上面的['zoom']替换成['resolution']即可。不过考虑到resolution值是一个小数，因此这里将resolution乘以10000之后再进行判断，代码如下：
<!DOCTYPE html><html lang="en"><head> <meta charset="UTF-8"> <meta name="viewport" content="width=device-width, initial-scale=1.0"> <title>WebGL</title> <style> html, body, #map { width: 100%; height: 100%; margin: 0; padding: 0; } </style> <link rel="stylesheet" href="ol/ol.css" /> <script src="ol/ol.js"></script></head><body> <div id="map"></div> <script> // 创建图层 var layer = new ol.layer.WebGLPoints({ source: new ol.source.Vector({ features: [ new ol.Feature({ geometry: new ol.geom.Point([120.0, 30.0]), "type": "学校", "dbm": 1 }), new ol.Feature({ geometry: new ol.geom.Point([120.0, 30.1]), "type": "学校", "dbm": 2 }), new ol.Feature({ geometry: new ol.geom.Point([120.1, 30.0]), "type": "超市", "dbm": 3 }), new ol.Feature({ geometry: new ol.geom.Point([120.1, 30.1]), "type": "超市", "dbm": 4 }), new ol.Feature({ geometry: new ol.geom.Point([120.2, 30.0]), "type": "医院", "dbm": 5 }), new ol.Feature({ geometry: new ol.geom.Point([120.2, 30.1]), "type": "医院", "dbm": 6 }), ] }), style: { symbol: { symbolType: 'circle', size: 40, color: [ 'case', ['<=', ['*', ['resolution'], 10000], 2], ['color', 255, 0, 0, 1], ['<=', ['*', ['resolution'], 10000], 3], ['color', 0, 255, 0, 1], ['<=', ['*', ['resolution'], 10000], 4], ['color', 0, 0, 255, 1], ['<=', ['*', ['resolution'], 10000], 5], ['color', 255, 255, 0, 1], ['<=', ['*', ['resolution'], 10000], 6], ['color', 255, 0, 255, 1], ['<=', ['*', ['resolution'], 10000], 7], ['color', 0, 255, 255, 1], ['color', 300, 200, 100, 1] ] } } }); // 创建地图 var map = new ol.Map({ target: 'map', layers: [ layer ], view: new ol.View({ projection: 'EPSG:4326', center: [120, 30], zoom: 10 }) }); </script></body></html>运行结果如下图所示：
8、设置要素的形状和透明度
在symbol对象中，symbolType参数用于定义要素的形状，它可以设置为circle、triangle、square、image。例如将symbolType设置为triangle：
symbol: { symbolType: 'circle', size: 40, color: ['color', 255, 0, 0, 1]}此时要素会被渲染为三角形，如下图所示：
将symbolType设置为square：
symbol: { symbolType: 'square', size: 40, color: ['color', 255, 0, 0, 1]}此时要素会被渲染为正方形，如下图所示：
如果希望设置透明度，只需要添加opacity属性即可，例如将透明度设置为0.3：
symbol: { symbolType: 'circle', size: 40, color: ['color', 255, 0, 0, 1], opacity: 0.3}如下图所示：
9、设置要素的尺寸
上面的代码主要针对color属性进行设置，其实不仅仅是color，size属性同样可以使用样式表达式。现在根据dbm值生成不同大小的要素，代码如下：
<!DOCTYPE html><html lang="en"><head> <meta charset="UTF-8"> <meta name="viewport" content="width=device-width, initial-scale=1.0"> <title>WebGL</title> <style> html, body, #map { width: 100%; height: 100%; margin: 0; padding: 0; } </style> <link rel="stylesheet" href="ol/ol.css" /> <script src="ol/ol.js"></script></head><body> <div id="map"></div> <script> // 创建图层 var layer = new ol.layer.WebGLPoints({ source: new ol.source.Vector({ features: [ new ol.Feature({ geometry: new ol.geom.Point([120.0, 30.0]), "type": "学校", "dbm": 1 }), new ol.Feature({ geometry: new ol.geom.Point([120.0, 30.1]), "type": "学校", "dbm": 2 }), new ol.Feature({ geometry: new ol.geom.Point([120.1, 30.0]), "type": "超市", "dbm": 3 }), new ol.Feature({ geometry: new ol.geom.Point([120.1, 30.1]), "type": "超市", "dbm": 4 }), new ol.Feature({ geometry: new ol.geom.Point([120.2, 30.0]), "type": "医院", "dbm": 5 }), new ol.Feature({ geometry: new ol.geom.Point([120.2, 30.1]), "type": "医院", "dbm": 6 }), ] }), style: { symbol: { symbolType: 'circle', size: [ 'case', ['==', ['get', 'dbm'], 1], 10, ['==', ['get', 'dbm'], 2], 20, ['==', ['get', 'dbm'], 3], 30, ['==', ['get', 'dbm'], 4], 40, ['==', ['get', 'dbm'], 5], 50, ['==', ['get', 'dbm'], 6], 60, 20 ], color: ['color', 255, 0, 0, 1], } } }); // 创建地图 var map = new ol.Map({ target: 'map', layers: [ layer ], view: new ol.View({ projection: 'EPSG:4326', center: [120, 30], zoom: 10 }) }); </script></body></html>运行结果如下图所示：
10、结语
本文主要介绍了OpenLayers中WebGLPoints图层的样式设置方法。其实刚接触样式表达式的时候觉得这种方法很反人类，但是习惯之后发现它的灵活度很高，本文也只列举了一些常见的用法，有兴趣的同志可以去官网查看更详细的文档。