OpenLayers基础教程——WebGLPoints图层样式的设置方法

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：

在这里插入图片描述

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：

在这里插入图片描述

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：

在这里插入图片描述

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：

在这里插入图片描述

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：

在这里插入图片描述

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：

在这里插入图片描述
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：

在这里插入图片描述
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：

在这里插入图片描述

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>