opencascade获取面的法向

//遍历零件的面
for (TopExp_Explorer e(workpiece, TopAbs_FACE); e.More(); e.Next())
	{
		TopoDS_Face aFace = TopoDS::Face(e.Current());
		//拓扑面和几何曲面的方向相反时反转
		if (aFace.Orientation() == TopAbs_REVERSED)
		{
			aFace.Reversed();
		}
		BRepGProp_Face analysisFace(aFace);
		Standard_Real umin, umax, vmin, vmax;
		analysisFace.Bounds(umin, umax, vmin, vmax);//获取拓扑面的参数范围
		Standard_Real midU, midV;
		midU = (umin + umax) / 2;//拓扑面的参数中点
		midV = (vmin + vmax) / 2;
		gp_Vec norm;
		gp_Pnt midPoint;
		analysisFace.Normal(midU, midV, midPoint, norm);//返回面参数中点的坐标及其法向
		//绘制法线
		gp_Lin normLine(midPoint, gp_Dir(norm));
		TopoDS_Edge anEdge = BRepBuilderAPI_MakeEdge(normLine, 0, 20);
		Handle(AIS_Shape)ais_shape = new AIS_Shape(anEdge);
		m_context->Display(ais_shape, Standard_True);
		m_view->FitAll();
	}