cesium系列10：根据中心点绘制圆和返回离某点最近的一个点的方法

1.背景

有需求写一个方法cesium 提供一个代码方法接收参数为function aaa(point1,point2,radius,num)其中点1和点2是经度纬度高度对象， 以point2为圆心，radius为半径生成一个圆弧，并创建一个数组在圆弧上均匀生成 num个点,点需要包含经纬高, 返回圆弧上的点数组 和点数组内距离point1最近的一个点。

2.结果

point1:蓝色点；

point 2:圆心 绿色点 ；

radius:半径 米；

num:返回的圆上的点的个数；

closestpoint:离点1 最近的数组里的点 黄色；

 3.方法如下：

function aaa(point1, point2, radius, num) {
    // 计算点1到点2的距离和方位角
    var dist = Cesium.Cartesian3.distance(Cesium.Cartesian3.fromDegrees(point1.longitude, point1.latitude, point1.height), 
                                           Cesium.Cartesian3.fromDegrees(point2.longitude, point2.latitude, point2.height));
    var heading = Cesium.Math.heading(
        Cesium.Cartesian3.fromDegrees(point1.longitude, point1.latitude),
        Cesium.Cartesian3.fromDegrees(point2.longitude, point2.latitude));

    // 以点2为圆心创建圆弧并生成点数组
    var positions = [];
    for (var i = 0; i < num; i++) {
        var angle = heading + i * Cesium.Math.toRadians(360.0 / num);
        var latlon = Cesium.Ellipsoid.WGS84.cartographicToCartesian(
            new Cesium.Cartographic(
                Cesium.Math.toRadians(point2.longitude) + Math.cos(angle) * radius / dist,
                Cesium.Math.toRadians(point2.latitude) + Math.sin(angle) * radius / dist,
                point2.height
            )
        );
        positions.push(latlon.x, latlon.y, latlon.z);
    }

    // 将数组转换为经纬高度对象
    var result = [];
    for (var j = 0; j < positions.length; j += 3) {
        var carto = Cesium.Cartographic.fromCartesian(new Cesium.Cartesian3(positions[j], positions[j + 1], positions[j + 2]));
        result.push({
            longitude: Cesium.Math.toDegrees(carto.longitude),
            latitude: Cesium.Math.toDegrees(carto.latitude),
            height: carto.height
        });
    }

    // 计算距离point1最近的点
    var closestDistance = Number.MAX_VALUE;
    var closestPoint = null;
    for (var k = 0; k < result.length; k++) {
        var distance = Cesium.Cartesian3.distance(Cesium.Cartesian3.fromDegrees(point1.longitude, point1.latitude, point1.height), 
                                                   Cesium.Cartesian3.fromDegrees(result[k].longitude, result[k].latitude, result[k].height));
        if (distance < closestDistance) {
            closestDistance = distance;
            closestPoint = result[k];
        }
    }


    return {point1:point1,closestPoint: closestPoint,points: result };
}

=================================================
// 调用 aaa 函数生成圆弧点数组
var result = aaa(point1, point2, radius, num);
var point1=result.point1,closestPoint=point2;
// 将点1和最近的点转换为 Cesium.Entity 实体
var entity1 = viewer.entities.add({
    position: Cesium.Cartesian3.fromDegrees(point1.longitude, point1.latitude, point1.height),
    point: {
        pixelSize: 10,
        color: Cesium.Color.BLUE
    }
});
var entity2 = viewer.entities.add({
    position: Cesium.Cartesian3.fromDegrees(closestPoint.longitude, closestPoint.latitude, closestPoint.height),
    point: {
        pixelSize: 10,
        color: Cesium.Color.BLUE
    }
});

// 将圆弧点数组转换为 Cesium.Cartesian3 数组
var positions = [];
for (var i = 0; i < result.points.length; i++) {
    var position = Cesium.Cartesian3.fromDegrees(result.points[i].longitude, result.points[i].latitude, result.points[i].height);
    positions.push(position);
}

// 创建圆弧实体
var entity3 = viewer.entities.add({
    polyline: {
        positions: positions,
        width: 3,
        material: Cesium.Color.RED
    }
});