最近项目有个需要求是通过真实的地理坐标截取不规则多边形的形状，首先想到的就是Canvas上使用画笔勾画，先看效果：
这是在地图软件上勾画的，下面是截取的：
本质是将坐标系转换后在canvas上描绘
其中coordinateListToCanvas就是canvas的坐标点
转换坐标系的代码如下 中间使用了一个过渡的坐标系优化计算
其中坐标系转换的时候用到了拉伸系数 而拉伸系数需要确定拉伸的方向再通过地理坐标的最大最小经纬度值和容器的长宽比确定。
完整代码如下
<template>
<div>
<canvas :width="width" :height="height" id="map-canvas"></canvas>
<img src="../assets/timg.jpg" alt="" id="img-canvas">
</div>
</template>
<script>
export default {
props: {
width: {
type: Number,
default: 825
},
height: {
type: Number,
default: 220
},
// 经纬度坐标
coordinateList: {
type: Array,
default () {
return [];
}
}
},
computed: {
// 获取缩放拉伸的方向（为了计算拉伸系数）
direction () {
let DeltaX = this.getMaxLongitude(this.coordinateList) - this.getMinLongitude(this.coordinateList);
let DeltaY = this.getMaxLatitude(this.coordinateList) - this.getMinLatitude(this.coordinateList);
return DeltaX / DeltaY > this.width / this.height ? 'horizontal' : 'vertical';
},
coordinateListToCanvas () {
return this.coordinateList.map(item => this.computedGeographicCoordinatesToCanvasCoordinates(item));
}
},
watch: {
coordinateListToCanvas (val) {
if (val.length) {
this.draw();
}
}
},
methods: {
// 获得最大维度
getMaxLatitude (coordinateList) {
return Math.max(...coordinateList.map(item => item.latitude));
},
// 获得最小维度
getMinLatitude (coordinateList) {
return Math.min(...coordinateList.map(item => item.latitude));
},
// 获得最大经度
getMaxLongitude (coordinateList) {
return Math.max(...coordinateList.map(item => item.longitude));
},
// 获得最小经度
getMinLongitude (coordinateList) {
return Math.min(...coordinateList.map(item => item.longitude));
},
draw () {
let cvs = document.querySelector('#map-canvas');
let ctx = cvs.getContext('2d');
ctx.beginPath();
ctx.clearRect(0, 0, cvs.width, cvs.height);
ctx.moveTo(...this.coordinateListToCanvas[0]);
for (let i = 1; i < this.coordinateListToCanvas.length; i++) {
ctx.lineTo(...this.coordinateListToCanvas[i]);
}
let img = document.querySelector('#img-canvas');
let pat = ctx.createPattern(img, 'repeat');
ctx.fillStyle = pat;
ctx.fill();
},
// 坐标系转换中间A->B->C 其中含有过渡坐标系B 为canvas正中心为（0,0）原点 方向与浏览器相同 拉伸系数为地理坐标在canvas容器里的最大拉伸系数
// 将地理坐标系转换成canvas坐标系
computedGeographicCoordinatesToCanvasCoordinates (coordinate) {
let coefficient = 0; // 缩放系数
// 获得地图canvas中心点对应的经纬度坐标
let center = {
longitude: (this.getMaxLongitude(this.coordinateList) + this.getMinLongitude(this.coordinateList)) / 2,
latitude: (this.getMaxLatitude(this.coordinateList) + this.getMinLatitude(this.coordinateList)) / 2
};
// 计算拉伸系数
if (this.direction === 'horizontal') {
coefficient = (this.getMaxLongitude(this.coordinateList) - this.getMinLongitude(this.coordinateList)) / this.width;
} else {
coefficient = (this.getMaxLatitude(this.coordinateList) - this.getMinLatitude(this.coordinateList)) / this.height;
}
coordinate = [
(coordinate.longitude - center.longitude) / coefficient,
-(coordinate.latitude - center.latitude) / coefficient
];
return [coordinate[0] + this.width / 2, coordinate[1] + this.height / 2];
},
// 将canvas坐标系转换成地理坐标系
computedCanvasCoordinatesToGeographicCoordinates (coordinate) {
let coordinateToCenter = [coordinate[0] - this.width / 2, coordinate[1] - this.height / 2];
let coefficient = 0;
// 获得地图canvas中心点对应的经纬度坐标
let center = {
longitude: (this.getMaxLongitude(this.coordinateList) + this.getMinLongitude(this.coordinateList)) / 2,
latitude: (this.getMaxLatitude(this.coordinateList) + this.getMinLatitude(this.coordinateList)) / 2
};
// 计算拉伸系数
if (this.direction === 'horizontal') {
coefficient = (this.getMaxLongitude(this.coordinateList) - this.getMinLongitude(this.coordinateList)) / this.width;
} else {
coefficient = (this.getMaxLatitude(this.coordinateList) - this.getMinLatitude(this.coordinateList)) / this.height;
}
return [coordinateToCenter[0] * coefficient + center.longitude, -coordinateToCenter[1] * coefficient + center.latitude];
}
}
};
</script>
<style lang="less" scoped>
canvas{
vertical-align: top;
}
img{
display: none;
}
</style>
