背景
最近看到ArcGIS Maps SDK for JavaScript官网有个聚焦效果的例子，于是想在MapTalks上也试试。
原理
滤镜的原理主要是用的canvas的cssfilter属性，让它整体有模糊或者高亮的功能。另外高亮区域使用的是canvas的裁剪。
方案一
使用maptalks原生api，对底图进行模糊和蒙版操作，由于这里要两种滤镜，所以需要有两个底图方便处理，一个进行模糊处理，一个设置蒙版。
设置模糊滤镜效果
var map = new maptalks.Map('map', {
center: [109.191369116306319, 19.736612649767874],
zoom: 16,
pitch : 0,
dragPitch:false,
baseLayer : new maptalks.TileLayer('tile1', {
urlTemplate: 'https://services.arcgisonline.com/ArcGIS/rest/services/World_Imagery/MapServer/tile/{z}/{y}/{x}',
cssFilter : 'blur(10px) grayscale(1.2)',
})
});
设置模蒙版和高亮滤镜效果
var boundary = [ [ [ 109.191369116306319, 19.736612649767874 ], [ 109.191347658634214, 19.736794423924326 ], [ 109.19259220361711, 19.736531861187487 ], [ 109.195274412632031, 19.735562395035053 ], [ 109.209500849247036, 19.730068642352247 ], [ 109.209565222263379, 19.72653395894347 ], [ 109.209264814853711, 19.726453165265738 ], [ 109.209028780460415, 19.726069394738637 ], [ 109.203707277774839, 19.726089593210407 ], [ 109.203664362430601, 19.726594554174969 ], [ 109.202827513217969, 19.727099513543749 ], [ 109.201840460300488, 19.728028634612741 ], [ 109.2000594735146, 19.728715372799819 ], [ 109.195961058139844, 19.730270621899336 ], [ 109.192656576633482, 19.731401702643819 ], [ 109.192999899387388, 19.731745064856661 ], [ 109.192849695682554, 19.732048030902128 ], [ 109.192163050174742, 19.73196724001286 ], [ 109.191154539585142, 19.732674158908559 ], [ 109.19169098138812, 19.735461408639065 ], [ 109.191369116306319, 19.736612649767874 ] ] ]
var mask = new maptalks.Polygon(boundary, {
'symbol' : [
{
'polygonOpacity': 1,
'polygonFill': 'rgb(0,0,0)',
'lineColor': 'rgb(0,0,0)',
'lineWidth': 1,
}
]
});
var maskedLayer = new maptalks.TileLayer('tile2', {
zIndex:100,
urlTemplate: 'https://services.arcgisonline.com/ArcGIS/rest/services/World_Imagery/MapServer/tile/{z}/{y}/{x}',
cssFilter : 'drop-shadow(0px 0px 20px black) brightness(1.0)',
})
.setMask(mask) // set boundary as the mask to the tilelayer
.addTo(map);
由于底图的drop-shadow滤镜效果不太明显，需要绘制一个vectorlayer
var outline = new maptalks.Polygon(boundary, {
'symbol' : [
{
'polygonOpacity': 1,
'polygonFill': 'rgb(0,0,0)',
'lineColor': 'rgb(0,0,0)',
'lineWidth': 1,
}
]
});
new maptalks.VectorLayer('v', [outline],{
enableSimplify:false,
cssFilter : 'drop-shadow(-6px -6px 16px black)',
}
).addTo(map);
优点：二三维均可使用，图形绘制简单。缺点：一个图层使用cssfilter影响过大
方案二
使用底图监听绘制事件，绘制完成以后，通过canvas上下文获取到图片，对图片进行上述模糊蒙版高亮等处理。
function onRenderEnd(e) {
console.time('draw')
// map's canvas context
var ctx = e.context;
ctx.filter= 'drop-shadow(-6px -6px 16px black)';
var c=ctx.canvas;
var imgData=ctx.getImageData(0,0,c.width,c.height);
ctx.drawImage(createBg(imgData, c.width,c.height,'blur(10px) '),0,0);
ctx.drawImage(createMagCircle(imgData,c.width,c.height,boundary), 0, 0);
console.timeEnd('draw')
}
map.on('renderend', onRenderEnd);
新建canvas，转换坐标后绘制图形，接着绘制当前底图图片，返回canvas元素
function createMagCircle(imageData,width,height,coords) {
var magImg = document.createElement('canvas');
var magCircle = document.createElement('canvas');
//console.log(sw,sh)
magImg.width = magCircle.width =width
magImg.height = magCircle.height = height
magImg.getContext('2d').putImageData(imageData, 0, 0);
var ctx = magCircle.getContext('2d');
ctx.beginPath();
for(let i=0;i<coords[0].length;i++){
var center = new maptalks.Coordinate(coords[0][i]);
let containerPoint = map.coordinateToContainerPoint(center).round();
let cx = containerPoint.x
let cy = containerPoint.y
if(i==0){
ctx.moveTo(cx,cy);
}else{
ctx.lineTo(cx,cy);
}
}
ctx.stroke();
ctx.clip();
ctx.drawImage(magImg, 0, 0);
return magCircle;
}
新建canvas元素，绘制绘制当前底图图片，进行模糊滤镜处理
function createBg(imageData, width,height,cssfilter) {
var magImg = document.createElement('canvas');
var magCircle = document.createElement('canvas');
magImg.width =width
magImg.height = height
magCircle.width = width
magCircle.height = height;
magImg.getContext('2d').putImageData(imageData, 0, 0);
var ctx = magCircle.getContext('2d');
ctx.filter= cssfilter
ctx.drawImage(magImg, 0, 0);
return magCircle;
}
优点：只要一个地图并且不用再绘制vectorlayer。缺点：只能二维使用，三维没有处理所以不可用，图形绘制处理比较复杂需要知道一些canvas知识
