概述：
本文讲述如何在Openlayers3中结合canvas实现对地图的切割。
效果：
全图
切割北京区域
切割河北区域
实现：
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
<title>Ol3 wms</title>
<link rel="stylesheet" type="text/css" href="../../../plugin/ol3/css/ol.css"/>
<style type="text/css">
body, #map {
border: 0px;
margin: 0px;
padding: 0px;
width: 100%;
height: 100%;
font-size: 13px;
}
</style>
<script type="text/javascript" src="../../../plugin/ol3/build/ol-debug.js"></script>
<script type="text/javascript" src="../../../plugin/jquery/jquery-1.8.3.js"></script>
<script type="text/javascript">
function init(){
var bounds = [73.4510046356223, 18.1632471876417,
134.976797646506, 53.5319431522236];
var vec_c = getTdtLayer("vec_c");
var cva_c = getTdtLayer("cva_c");
var map = new ol.Map({
controls: ol.control.defaults({
attribution: false
}),
target: 'map',
layers: [vec_c, cva_c],
view: new ol.View({
projection: 'EPSG:4326'
})
});
map.getView().fit(bounds, map.getSize());
function getTdtLayer(lyr){
var url = "http://t0.tianditu.com/DataServer?T="+lyr+"&X={x}&Y={y}&L={z}";
var projection = ol.proj.get("EPSG:4326");
var projectionExtent = [ -180, -90, 180, 90 ];
var maxResolution = (ol.extent.getWidth(projectionExtent) / (256 * 2));
var resolutions = new Array(16);
var z;
for (z = 0; z < 16; ++z) {
resolutions[z] = maxResolution / Math.pow(2, z);
}
var tileOrigin = ol.extent.getTopLeft(projectionExtent);
var layer = new ol.layer.Tile({
extent: [ -180, -90, 180, 90 ],
source: new ol.source.TileImage({
tileUrlFunction: function(tileCoord) {
var z = tileCoord[0]+1;
var x = tileCoord[1];
var y = -tileCoord[2]-1;
var n = Math.pow(2, z + 1);
x = x % n;
if (x * n < 0) {
x = x + n;
}
return url.replace('{z}', z.toString())
.replace('{y}', y.toString())
.replace('{x}', x.toString());
},
projection: projection,
tileGrid: new ol.tilegrid.TileGrid({
origin: tileOrigin,
resolutions: resolutions,
tileSize: 256
})
})
});
return layer;
}
$.get("data/province.geojson",null,function(result){
var image = new ol.style.Circle({
radius: 5,
fill: null,
stroke: new ol.style.Stroke({color: 'red', width: 1})
});
var styles = new ol.style.Style({
stroke: new ol.style.Stroke({
color: 'blue',
width: 2
}),
fill: new ol.style.Fill({
color: 'rgba(0, 0, 255, 0)'
})
});
var vectorSource = new ol.source.Vector({
features: (new ol.format.GeoJSON()).readFeatures(result)
});
var vector = new ol.layer.Vector({
source: vectorSource,
style: styles
});
map.addLayer(vector);
var _styles = new ol.style.Style({
stroke: new ol.style.Stroke({
color: '#00ffff',
lineDash: [4],
width: 3
}),
fill: new ol.style.Fill({
color: 'rgba(0, 0, 255, 0.1)'
})
});
var select = new ol.interaction.Select({
condition: ol.events.condition.click,
style: _styles
});
map.addInteraction(select);
var highlight;
select.on('select', function (e) {
var feature = e.target.getFeatures().item(0);
if (feature) {
highlight = feature;
var _coord = feature.getGeometry().getCoordinates();
console.log(_coord);
map.render();
map.on('precompose',clip)
}
});
var center,pixelScale,offsetX,offsetY,rotation;
function clip(evt) {
var canvas=evt.context;
canvas.save();
var coords=highlight.getGeometry().getCoordinates();
var frameState = evt.frameState;
var pixelRatio = frameState.pixelRatio;
var viewState = frameState.viewState;
center = viewState.center;
var resolution = viewState.resolution;
rotation = viewState.rotation;
var size = frameState.size;
var size1=map.getSize();
offsetX = Math.round(pixelRatio * size[0] / 2);
offsetY = Math.round(pixelRatio * size[1] / 2);
pixelScale = pixelRatio / resolution;
canvas.beginPath();
if(highlight.getGeometry().getType() == 'MultiPolygon'){
for(var i=0;i<coords.length;i++){
createClip(coords[i][0], canvas);
}
}
else if(highlight.getGeometry().getType() == 'Polygon'){
createClip(coords[0], canvas);
}
canvas.clip();
}
function createClip(coords, canvas) {
for (var i = 0, cout = coords.length; i < cout; i++) {
var xLen = Math.round((coords[i][0] - center[0]) * pixelScale);
var yLen = Math.round((center[1] - coords[i][1]) * pixelScale);
var x = offsetX;
var y = offsetY;
if (rotation) {
x = xLen * Math.cos(rotation) - yLen * Math.sin(rotation) + offsetX;
y = xLen * Math.sin(rotation) + yLen * Math.cos(rotation) + offsetY;
} else {
x = xLen + offsetX;
y = yLen + offsetY;
}
if (i == 0) {
canvas.moveTo(x, y);
} else {
canvas.lineTo(x, y);
}
}
canvas.closePath();
}
map.on('postcompose', function(event) {
var ctx = event.context;
ctx.restore();
});
});
}
</script>
</head>
<body onLoad="init()">
<div id="map">
</div>
</body>
</html>
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