/// <summary>
/// 将CurveLoop压平在xoy平面
/// </summary>
/// <param name="loop"></param>
/// <returns>如果loop不能确定一个平面，则返回空</returns>
public static CurveLoop ProjectXoy(this CurveLoop loop)
{
Plane plane = null;
try
{
plane = loop.GetPlane();
}
catch { /*loop没有在平面内*/ }
if (plane != null && !plane.Normal.IsVertical())
{
return(null);
}
XYZ vector = XYZ.Zero - new XYZ(0, 0, loop.FirstOrDefault().GetEndPoint(0).Z);
Transform transform = Transform.CreateTranslation(vector);
return(CurveLoop.CreateViaTransform(loop, transform));
}