/// <summary>
/// 三点法创建圆弧，逆向。（起始点、圆心、终止点）
/// </summary>
/// <param name="arc">所要创建的圆弧</param>
/// <param name="startPoint">起始点</param>
/// <param name="centerPoint">圆心</param>
/// <param name="endPoint">终止点</param>
public static void CreateArcOfCenter(this Arc arc, Point3d startPoint, Point3d centerPoint, Point3d endPoint)
{
//确定圆弧的圆心与半径
arc.Center = centerPoint;
Vector2d vec = new Vector2d(startPoint.X - centerPoint.X, startPoint.Y - centerPoint.Y);
arc.Radius = vec.Length;
//计算起始和终点角度
arc.StartAngle = startPoint.AngleFromXAxis(centerPoint);
arc.EndAngle = endPoint.AngleFromXAxis(centerPoint);
}
//根据三点创建圆弧
public static void CreateArc(this Arc arc, Point3d startPoint, Point3d pointOnArc, Point3d endPoint)
{
//创建一个几何类的圆弧对象
CircularArc3d geArc = new CircularArc3d(startPoint, pointOnArc, endPoint);
//将几何类圆弧对象的圆心和半径赋值给圆弧
Point3d centerPoint = geArc.Center;
arc.Center = centerPoint;
arc.Radius = geArc.Radius;
//计算起始和终止角度
arc.StartAngle = startPoint.AngleFromXAxis(centerPoint);
arc.EndAngle = endPoint.AngleFromXAxis(centerPoint);
}