背景
地图可视化项目中往往有要求进行内阴影的需求，但是对于可视化能力比较弱的giser来说,这个没有现成的方案，一般都会说进行ui切图然后进行贴图就行，但是这样对于动态生成的场景就不太满足，本文基于canvas进行动态绘制内阴影。
原理
内阴影的原理主要是用的canvas的shadowBlur和globalCompositeOperation 属性，shadowBlur让它整体有模糊的效果,globalCompositeOperation为'source-out'在目标图像之外显示源图像。只会显示目标图像之外源图像部分，目标图像是透明的。另外高亮区域使用的是canvas的裁剪。
方案一
使用maptalks原生api,这里只要设置一下symbol和layer
xml
<!DOCTYPE html>
<html>
<meta charset="UTF-8">
<meta name="viewport" content="width=device-width, initial-scale=1">
<title>mtk内阴影-polong</title>
<style type="text/css">
html, body {
margin: 0px;
height: 100%;
width: 100%
}
.container {
width: 100%;
height: 100%
}
</style>
<link rel="stylesheet" href="https://unpkg.com/maptalks/dist/maptalks.css">
<script type="text/javascript" src="https://unpkg.com/maptalks/dist/maptalks.min.js"></script>
<body>
<div id="map" class="container"></div>
<script>
var map = new maptalks.Map('map', {
center: [109.876457, 19.386884],
zoom: 13,
baseLayer: new maptalks.TileLayer('base', {
urlTemplate: 'https://{s}.basemaps.cartocdn.com/dark_all/{z}/{x}/{y}.png',
subdomains: ['a', 'b', 'c', 'd']
})
})
var layer = new maptalks.VectorLayer('vector', {
globalCompositeOperation: 'source-out'
}).addTo(map)
var rect = new maptalks.Polygon([
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symbol: {
'polygonFill': 'rgb(52,149,233)',
shadowBlur: 200,
shadowColor: 'rgb(52,149,233)',
'polygonOpacity': 1,
'lineColor': '#1bbc9b',
'lineWidth': 0,
'lineJoin': 'round', //miter, round, bevel
'lineCap': 'round', //butt, round, square
'lineDasharray': null, //dasharray, e.g. [10, 5, 5]
'lineOpacity ': 1
}
}
).addTo(layer)
</script>
</body>
</html>展示效果
方案二
本方案比较复杂且只支持二维还不支持每个层级重绘，通过绘制图形到canvas上导出base64在贴到图层上，要确定能缩放到图形整体，这样才能使用地图的api转换(目前支持单面)，首先将图形的点转为屏幕坐标（小技巧是要减去最小经纬度的屏幕坐标，这样绘制的图片会小一点），绘制图形，设置shadowblur和globalCompositeOperation（'source-out'）， 绘制完图形之后通过imageLayer就行了
css
<!DOCTYPE html>
<html>
<meta charset="UTF-8">
<meta name="viewport" content="width=device-width, initial-scale=1">
<title>瓦片图层与地理投影 - 百度投影底图</title>
<style type="text/css">
html, body {
margin: 0px;
height: 100%;
width: 100%
}
.container {
width: 100%;
height: 100%
}
.pane {
background: #34495e;
line-height: 28px;
color: #fff;
z-index: 10;
position: absolute;
top: 20px;
right: 20px
}
.pane a {
display: block;
color: #fff;
text-align: left;
padding: 0 10px;
min-width: 28px;
min-height: 28px;
float: left
}
</style>
<link rel="stylesheet" href="https://unpkg.com/maptalks/dist/maptalks.css">
<script type="text/javascript" src="https://unpkg.com/maptalks/dist/maptalks.min.js"></script>
<body>
<div id="map" class="container"></div>
<div class="pane"><a href="javascript:save();">Export image</a></div>
<script>
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18.743734], [109.905859, 18.744968], [109.908962, 18.746459], [109.913418, 18.750503], [109.917011, 18.754442], [109.920972, 18.760579], [109.922318, 18.761175], [109.924457, 18.76096], [109.925955, 18.761307], [109.927434, 18.76225], [109.928887, 18.763942], [109.930792, 18.764254], [109.932924, 18.763949], [109.935393, 18.764358], [109.936828, 18.765752], [109.937653, 18.767624], [109.938866, 18.769066], [109.941976, 18.770557], [109.944109, 18.771868], [109.947092, 18.772436], [109.952944, 18.772526], [109.958575, 18.773615], [109.96628, 18.773594], [109.969702, 18.771126], [109.974996, 18.766355], [109.975865, 18.766279], [109.979217, 18.767367], [109.980454, 18.767139], [109.982048, 18.765731], [109.983279, 18.763914], [109.98519, 18.758193], [109.987913, 18.756994], [109.991004, 18.754386], [109.992553, 18.751876], [109.993143, 18.746772], [109.993791, 18.745641], [109.996076, 18.743595], [109.998291, 18.742215], [110.001071, 18.739732], [110.00321, 18.737118], [110.005737, 18.734739], [110.009342, 18.733109], [110.014921, 18.732818], [110.023764, 18.735363], [110.029578, 18.737249], [110.030866, 18.737894], [110.031019, 18.741924], [110.031863, 18.744906], [110.030917, 18.747167], [110.031089, 18.749525], [110.031615, 18.750142], [110.036782, 18.753263], [110.037531, 18.754317], [110.040007, 18.760405], [110.041905, 18.761487], [110.044482, 18.761626], [110.047211, 18.760502], [110.051655, 18.756356], [110.053863, 18.755974], [110.057266, 18.758491], [110.058345, 18.760066], [110.05971, 18.763685], [110.060947, 18.76605], [110.06341, 18.769469], [110.0656, 18.771882], [110.066939, 18.772859], [110.070659, 18.773095], [110.071789, 18.774586], [110.068767, 18.778254], [110.068767, 18.780077], [110.069605, 18.78231], [110.070564, 18.78724], [110.072366, 18.788474], [110.079482, 18.790103], [110.081951, 18.791441], [110.084973, 18.794728], [110.090736, 18.800427], [110.094824, 18.805786], [110.095782, 18.811491], [110.093142, 18.817779], [110.090133, 18.821099], [110.08762, 18.824635], [110.083855, 18.833258], [110.08218, 18.839517], [110.077159, 18.846801], [110.073465, 18.851292], [110.06951, 18.855333], [110.062718, 18.86085], [110.057005, 18.871612], [110.049896, 18.880212], [110.045313, 18.884772], [110.047859, 18.889851], [110.054968, 18.892907], [110.06666, 18.897487], [110.076302, 18.901561], [110.080948, 18.90427], [110.086877, 18.907041], [110.094005, 18.909771], [110.097712, 18.911704], [110.104478, 18.91742], [110.106421, 18.923551], [110.106351, 18.92768], [110.106021, 18.930409], [110.105437, 18.931663], [110.105748, 18.932702], [110.105266, 18.936672], [110.108249, 18.944589], [110.113022, 18.948773], [110.13157, 18.962661], [110.138146, 18.967724], [110.141694, 18.970252], [110.14966, 18.978279], [110.14966, 18.982247], [110.149121, 18.985959], [110.149774, 18.989027], [110.150511, 18.990072], [110.15258, 18.991284], [110.154065, 18.993002], [110.157874, 18.996249], [110.158566, 18.999372], [110.157779, 19.00057], [110.15795, 19.002246], [110.156903, 19.004808], [110.155754, 19.005992], [110.153989, 19.006872], [110.15284, 19.009039], [110.152834, 19.010327], [110.154465, 19.015264], [110.155182, 19.016745], [110.158071, 19.019321], [110.15484, 19.023157], [110.151628, 19.02414], [110.148829, 19.02423], [110.144988, 19.0254], [110.140494, 19.03269], [110.140278, 19.034664], [110.139441, 19.036145], [110.13767, 19.03762], [110.134039, 19.03879], [110.131538, 19.040652], [110.129253, 19.042722], [110.128288, 19.045096], [110.128402, 19.046723], [110.124073, 19.047485], [110.123876, 19.050088], [110.121763, 19.055314], [110.120988, 19.060298], [110.123902, 19.062859], [110.123051, 19.067511], [110.124568, 19.070882], [110.125628, 19.078869], [110.123121, 19.088607], [110.12128, 19.096933], [110.120887, 19.104753], [110.121414, 19.111839], [110.119097, 19.120191], [110.116799, 19.128993], [110.115276, 19.133179], [110.109766, 19.138001], [110.10663, 19.138575], [110.101851, 19.139904], [110.094595, 19.140388], [110.084408, 19.14007], [110.078054, 19.138686], [110.068983, 19.136479], [110.06579, 19.135441], [110.057272, 19.13459], [110.051109, 19.138472], [110.048601, 19.142222], [110.047732, 19.144782], [110.046976, 19.150081], [110.043663, 19.157995], [110.039201, 19.161586], [110.033278, 19.165225], [110.031457, 19.167646], [110.030943, 19.169465], [110.029305, 19.172419], [110.030517, 19.175366], [110.031488, 19.176715], [110.028467, 19.177005], [110.0282, 19.176113], [110.026512, 19.175691], [110.020279, 19.176549], [110.018368, 19.177143], [110.015874, 19.176673], [110.013144, 19.177226], [110.011716, 19.178665], [110.009355, 19.177863], [110.007869, 19.178243], [110.0046, 19.177904], [110.002677, 19.180837], [110.002226, 19.18256], [110.000132, 19.182663], [109.99944, 19.183154], [109.99963, 19.184752], [109.999053, 19.185395], [110.000506, 19.186945], [110.0001, 19.187477], [109.997828, 19.188107], [109.997542, 19.189933], [109.995879, 19.190535], [109.99447, 19.189801], [109.993873, 19.191745], [109.977319, 19.192125], [109.973396, 19.190742], [109.957642, 19.18866], [109.95363, 19.195902], [109.953262, 19.198668], [109.953947, 19.20369], [109.954861, 19.208247], [109.954322, 19.212853], [109.950704, 19.219402], [109.945162, 19.226476], [109.937869, 19.23223], [109.930455, 19.234774], [109.923606, 19.239601], [109.91736, 19.240963], [109.911235, 19.239566], [109.900311, 19.238813], [109.896445, 19.239179], [109.891761, 19.243017], [109.887349, 19.24776], [109.885521, 19.250353], [109.879256, 19.25746], [109.875625, 19.26333], [109.87009, 19.270181], [109.866802, 19.272829], [109.862537, 19.275048], [109.856037, 19.275974], [109.849499, 19.27576], [109.847652, 19.273887], [109.846008, 19.273845], [109.844192, 19.274751], [109.840194, 19.278747], [109.837178, 19.279991], [109.834532, 19.279521], [109.831878, 19.279963], [109.830431, 19.280648], [109.822706, 19.289683], [109.82197, 19.29121], [109.821868, 19.29271], [109.822706, 19.295634], [109.823442, 19.309278], [109.825258, 19.312292], [109.823747, 19.316736], [109.82435, 19.318899], [109.825556, 19.321041], [109.826451, 19.328229], [109.826908, 19.330378], [109.827054, 19.333253], [109.827841, 19.335651], [109.827175, 19.337047], [109.824065, 19.338954], [109.824337, 19.340143], [109.826445, 19.343156], [109.830215, 19.344876], [109.832164, 19.346742], [109.834436, 19.348117], [109.837052, 19.348608], [109.843291, 19.353417], [109.843793, 19.36005], [109.843462, 19.363961], [109.845951, 19.371672], [109.849277, 19.376135], [109.852393, 19.381751], [109.855859, 19.383693], [109.859839, 19.386297], [109.864549, 19.386726], [109.867754, 19.385454], [109.869563, 19.385171], [109.869874, 19.387658], [109.871518, 19.388052], [109.873562, 19.386401], [109.876457, 19.386884]]]
var mask = new maptalks.Polygon(boundary, {
'symbol': [
{
'polygonOpacity': 1,
'polygonFill': 'rgb(0,0,0)',
'lineColor': 'rgb(31,89,158)',
'lineWidth': 0
}
]
})
let lock = true
var map = new maptalks.Map('map', {
center: mask.getCenter(),
zoom: 10,
minZoom: 1,
maxZoom: 19,
baseLayer: new maptalks.TileLayer('base', {
'urlTemplate': 'https://{s}.basemaps.cartocdn.com/dark_all/{z}/{x}/{y}.png',
'subdomains': ['a', 'b', 'c', 'd']
}),
attribution: false
})
map.fitExtent(mask.getExtent())
let layer2 = new maptalks.VectorLayer('vector2').addTo(map)
var imageLayer = new maptalks.ImageLayer('images')
map.addLayer(imageLayer)
function createMagCircle(width, height, coords, mask) {
let {xmin, ymin, xmax, ymax} = mask.getExtent()
var center1 = new maptalks.Coordinate(xmin, ymin)
var center2 = new maptalks.Coordinate(xmax, ymax)
let containerPoint1 = map.coordinateToContainerPoint(center1).round()
let containerPoint2 = map.coordinateToContainerPoint(center2).round()
let sw = containerPoint2.x - containerPoint1.x
let sh = containerPoint1.y - containerPoint2.y
var magImg = document.createElement('canvas')
magImg.width = sw
magImg.height = sh
var ctx = magImg.getContext('2d')
//ctx.beginPath();
const path2D = new Path2D()
for (let i = 0; i < coords[0].length; i++) {
var center = new maptalks.Coordinate(coords[0][i])
let containerPoint = map.coordinateToContainerPoint(center).round()
console.log(containerPoint)
let cx = containerPoint.x - containerPoint1.x
let cy = containerPoint.y - containerPoint2.y
if (i === 0) {
path2D.moveTo(cx, cy)
} else {
path2D.lineTo(cx, cy)
}
}
path2D.closePath()
ctx.save()
ctx.globalCompositeOperation = 'source-out'
ctx.lineWidth = 50
ctx.shadowBlur = 2000
ctx.shadowColor = 'rgba(37,188,47,1)'
ctx.fillStyle = 'rgba(37,188,47,1)'
ctx.strokeStyle = 'rgba(37,188,47,1)'
ctx.stroke()
ctx.fill(path2D)
ctx.imageSmoothingEnabled = true
return magImg
}
function onRenderEnd(e) {
if (lock === false) {
return
}
lock = false
console.time('draw')
var ctx = e.context
var c = ctx.canvas
let can = createMagCircle(c.width, c.height, boundary, mask)
//ctx.imageSmoothingQuality = "low";
let {xmin, ymin, xmax, ymax} = mask.getExtent()
// var image = can.toDataURL("image/png").replace("image/png", "image/octet-stream"); // here is the most important part because if you dont replace you will get a DOM 18 exception.
// window.location.href=image;
let base64 = can.toDataURL('image/png', 1)
let a = {}
a[base64] = 1
imageLayer.setImages([ { url: base64, extent: [xmin, ymin, xmax, ymax],
opacity: 1
}
])
console.timeEnd('draw')
}
map.on('renderend', onRenderEnd)
function save() {
console.log(map.getExtent())
var data = map.toDataURL({
'mimeType': 'image/jpeg', // or 'image/png'
'save': true, // to pop a save dialog
'fileName': 'map' // file name
})
}
</script>
</body>
</html>效果
结论
看结果都知道方案一简单很多，不过我是通过做方案二时候才想出来方案一，两个方案各有优略，方案二可以自定义生成贴图用于不同的底图api,方案一非常简单适合新手。
参考资料：
w3chool canvas globalcompositeoperation参考
# canvas可视化效果之内阴影效果
# canvas内阴影?