/// <summary>
/// 找到线圈南侧的点
/// </summary>
/// <returns></returns>
public static List <Vector2d> SourthVertices(this Polygon2d polygon2d)
{
if (polygon2d.IsClockwise) // 为true则是顺时针,
{
polygon2d.Reverse(); // 强制所有线圈为逆时针方向
}
// 1 找到线圈点集的最左、最右，同时偏下的角点
Vector2d leftDownPoint = polygon2d.MaxLeftMaxDownPoint();
Vector2d rightDownPoint = polygon2d.MaxRightMaxDownPoint();
// 2 基于点进行重新排序
List <Vector2d> path_Vector2d = polygon2d.ReSortPolygon2dByPoint(leftDownPoint).Vertices.ToList();
int leftDownIndex = path_Vector2d.IndexOf(leftDownPoint);
int rightDownIndex = path_Vector2d.IndexOf(rightDownPoint);
int length = rightDownIndex - leftDownIndex + 1;
return(path_Vector2d.GetRange(leftDownIndex, length));
}