OpenCascade源码分析之BRepMesh_IncrementalMesh(网格离散化操作)

一、引言

在使用opencascade读取连续曲面模型的时候，一般来说我们都会调用BRepMesh_IncrementalMesh对其进行离散化，然而可能对于离散过程的执行还是没有什么概念，接下来从源码出发来看看它进行了什么操作。

二、源码分析

在调用离散的时候，我们一般会用到下面的代码

TopoDS_Shape cur;    //需要离散的拓扑
IMeshTools_Parameters aMeshParams;  //离散化的参数
BRepMesh_IncrementalMesh::BRepMesh_IncrementalMesh(cur, this->aMeshParams);

BRepMesh_IncrementalMesh.cpp 的代码为

//=======================================================================
//function : Constructor
//purpose  :
//=======================================================================
BRepMesh_IncrementalMesh::BRepMesh_IncrementalMesh(const TopoDS_Shape&          theShape,const IMeshTools_Parameters& theParameters,const Message_ProgressRange& theRange): myParameters(theParameters)
{myShape = theShape;Perform(theRange);
}
//=======================================================================
//function : Perform
//purpose  :
//=======================================================================
void BRepMesh_IncrementalMesh::Perform(const Message_ProgressRange& theRange)
{//新建一个上下文//myParameters.MeshAlgo返回了参数中的IMeshTools_MeshAlgoType//如果没有设置IMeshTools_MeshAlgoType默认为Delaunay三角化Watson方法//也可以设置为Delabella方法Handle(BRepMesh_Context) aContext = new BRepMesh_Context (myParameters.MeshAlgo);//theRange消息进度条，没什么用 不用管Perform (aContext, theRange);
}//=======================================================================
//function : Perform
//purpose  :
//=======================================================================
void BRepMesh_IncrementalMesh::Perform(const Handle(IMeshTools_Context)& theContext, const Message_ProgressRange& theRange)
{//对于我们自己设置的离散参数进行判断，修改一些不合理的值//例如当Deflection<Precision::Confusion()时，提醒用户值无效 精度太低initParameters();//IMeshTools_Context继承至IMeshData_Shape//IMeshData_Shape中的SetShape()设置了成员变量TopoDS_Shape myShapetheContext->SetShape(Shape());//设置离散参数theContext->ChangeParameters()            = myParameters;// 算法结束时不清理临时的数据类型theContext->ChangeParameters().CleanModel = Standard_False;//进度条的设置Message_ProgressScope aPS(theRange, "Perform incmesh", 10);//创建一个IMeshTools_MeshBuilder对象，将上下文传入IMeshTools_MeshBuilder aIncMesh(theContext);//执行操作//这边是核心部分！！将会放到后面单独讲解//这边是核心部分！！将会放到后面单独讲解//这边是核心部分！！将会放到后面单独讲解aIncMesh.Perform(aPS.Next(9));if (!aPS.More()){myStatus = IMeshData_UserBreak;return;}myStatus = IMeshData_NoError;//返回了上下文中的成员变量Handle (IMeshData_Model)  myModel;const Handle(IMeshData_Model)& aModel = theContext->GetModel();if (!aModel.IsNull()){//遍历所有的面for (Standard_Integer aFaceIt = 0; aFaceIt < aModel->FacesNb(); ++aFaceIt){//IMeshData是一个namespace具体里面包含的东西在下面的链接中//https://dev.opencascade.org/doc/refman/html/namespace_i_mesh_data.htmlconst IMeshData::IFaceHandle& aDFace = aModel->GetFace(aFaceIt);myStatus |= aDFace->GetStatusMask();for (Standard_Integer aWireIt = 0; aWireIt < aDFace->WiresNb(); ++aWireIt){const IMeshData::IWireHandle& aDWire = aDFace->GetWire(aWireIt);myStatus |= aDWire->GetStatusMask();}}}aPS.Next(1);//设置已经完成标志setDone();
}

总体流程就是如上啦，当然其中的核心代码还没有讲，接下来讲核心代码

aIncMesh.Perform(aPS.Next(9));   //核心代码！！

Perform中的内容如下

void IMeshTools_MeshBuilder::Perform (const Message_ProgressRange& theRange)
{//清除状态位ClearStatus ();//获得之前设置的上下文const Handle (IMeshTools_Context)& aContext = GetContext ();if (aContext.IsNull ()){SetStatus (Message_Fail1);return;}Message_ProgressScope aPS(theRange, "Mesh Perform", 10);//开始离散化的六个步骤 这边的if中每个都执行了相应的步骤，然后返回是否成功if (aContext->BuildModel ()){if (aContext->DiscretizeEdges ()){if (aContext->HealModel ()){if (aContext->PreProcessModel()){if (aContext->DiscretizeFaces(aPS.Next(9))){if (aContext->PostProcessModel()){SetStatus(Message_Done1);}else{SetStatus(Message_Fail7);}}else{if (!aPS.More()){SetStatus(Message_Fail8);aContext->Clean();return;}SetStatus(Message_Fail6);}}else{SetStatus(Message_Fail5);}}else{SetStatus(Message_Fail4);}}else{SetStatus (Message_Fail3);}}else{const Handle (IMeshTools_ModelBuilder)& aModelBuilder =aContext->GetModelBuilder ();if (aModelBuilder.IsNull ()){SetStatus (Message_Fail1);}else{// Is null shape or another problem?SetStatus (aModelBuilder->GetStatus ().IsSet (Message_Fail1) ?Message_Warn1 : Message_Fail2);}}aPS.Next(1);aContext->Clean ();
}

注：下面的详细步骤都在BRep_XXXX中实现，IMeshTools_ModelAlgo只是作为基类存在，其中的函数都是virtual的

1、aContext->BuildModel () 创建一个需要离散化的模型

此阶段建立了一个模型

virtual Standard_Boolean BuildModel (){if (myModelBuilder.IsNull()){return Standard_False;}//创建一个需要离散化的模型的HandlemyModel = myModelBuilder->Perform(GetShape(), myParameters);return !myModel.IsNull();}

Peform执行了下面的代码

//! Exceptions protected method to create discrete model for the given shape.
//! Returns nullptr in case of failure.
Handle (IMeshData_Model) Perform (const TopoDS_Shape&          theShape,const IMeshTools_Parameters& theParameters){ClearStatus ();try{OCC_CATCH_SIGNALS//返回模型Handle (IMeshData_Model)return performInternal (theShape, theParameters);}catch (Standard_Failure const&){SetStatus (Message_Fail2);return NULL;}}

performInternal

Handle (IMeshData_Model) BRepMesh_ModelBuilder::performInternal (const TopoDS_Shape&          theShape,const IMeshTools_Parameters& theParameters)
{Handle (BRepMeshData_Model) aModel;//包围盒Bnd_Box aBox;//给模型添加包围盒BRepBndLib::Add (theShape, aBox, Standard_False);if (!aBox.IsVoid ()){// Build data model for further processing.aModel = new BRepMeshData_Model (theShape);//Relative为是否启用相对计算的边缘公差if (theParameters.Relative){Standard_Real aMaxSize;//获取给定包围盒的最大维度BRepMesh_ShapeTool::BoxMaxDimension (aBox, aMaxSize);aModel->SetMaxSize(aMaxSize);}else{//用于面的挠度将是其边缘和面中的最大挠度aModel->SetMaxSize(Max(theParameters.Deflection,theParameters.DeflectionInterior));}//通过添加面孔和自由边缘来构建形状的离散模型。//计算相应形状的挠度，并检查它是否适合现有的多边形表示形式。Handle (IMeshTools_ShapeVisitor) aVisitor =new BRepMesh_ShapeVisitor (aModel);IMeshTools_ShapeExplorer aExplorer (theShape);aExplorer.Accept (aVisitor);SetStatus (Message_Done1);}else{SetStatus (Message_Fail1);}return aModel;
}

2、aContext->DiscretizeEdges ()对边进行离散

  virtual Standard_Boolean DiscretizeEdges(){if (myModel.IsNull() || myEdgeDiscret.IsNull()){return Standard_False;}// Discretize edges of a model.return myEdgeDiscret->Perform(myModel, myParameters, Message_ProgressRange());}

同样的Perform调用了BRepMesh_EdgeDiscret.cxx中的performInternal

Standard_Boolean BRepMesh_EdgeDiscret::performInternal (const Handle (IMeshData_Model)& theModel,const IMeshTools_Parameters&    theParameters,const Message_ProgressRange&    theRange)
{(void )theRange;myModel      = theModel;myParameters = theParameters;if (myModel.IsNull()){return Standard_False;}//For中的格式为: begin,end,func,isParallel//多线程或者单线程跑OSD_Parallel::For (0, myModel->EdgesNb (), *this, !myParameters.InParallel);myModel.Nullify(); // Do not hold link to model.return Standard_True;
}

具体的代码太长了感兴趣的话阅读BRepMesh_EdgeDiscret.cxx吧

3、aContext->HealModel () 修复模型

virtual Standard_Boolean HealModel(){if (myModel.IsNull()){return Standard_False;}return myModelHealer.IsNull() ?Standard_True :myModelHealer->Perform (myModel, myParameters, Message_ProgressRange());}

这块的具体实现在BRepMesh_ModelHealer.hxx和BRepMesh_ModelHealer.cxx中，感兴趣的可以自己进行阅读
其中主要的功能有下：

connectClosestPoints 将非常近的点进行合并
fixFaceBoundaries 修复面的边界（边界可能会出现断连的情况）

4、aContext->PreProcessModel() 前处理

和上面一样的Perform函数然后调用了performInternal

Standard_Boolean BRepMesh_ModelPreProcessor::performInternal(const Handle(IMeshData_Model)& theModel,const IMeshTools_Parameters&   theParameters,const Message_ProgressRange&   theRange)
{(void )theRange;if (theModel.IsNull()){return Standard_False;}const Standard_Integer aFacesNb    = theModel->FacesNb();const Standard_Boolean isOneThread = !theParameters.InParallel;//并行运行或者单线程运行//对锥面进行分割 对所有3d的曲线以及参数曲线进行分割和缝合OSD_Parallel::For(0, aFacesNb, SeamEdgeAmplifier        (theModel, theParameters),                      isOneThread);//判断三角化的一致性 判断是否有过期的三角数据OSD_Parallel::For(0, aFacesNb, TriangulationConsistency (theModel, theParameters.AllowQualityDecrease), isOneThread);// 从过时的多边形中清理边缘和面.Handle(NCollection_IncAllocator) aTmpAlloc(new NCollection_IncAllocator(IMeshData::MEMORY_BLOCK_SIZE_HUGE));NCollection_Map<IMeshData_Face*> aUsedFaces(1, aTmpAlloc);for (Standard_Integer aEdgeIt = 0; aEdgeIt < theModel->EdgesNb(); ++aEdgeIt){const IMeshData::IEdgeHandle& aDEdge = theModel->GetEdge(aEdgeIt);if (aDEdge->IsFree()){if (aDEdge->IsSet(IMeshData_Outdated)){TopLoc_Location aLoc;BRep_Tool::Polygon3D(aDEdge->GetEdge(), aLoc);BRepMesh_ShapeTool::NullifyEdge(aDEdge->GetEdge(), aLoc);}continue;}for (Standard_Integer aPCurveIt = 0; aPCurveIt < aDEdge->PCurvesNb(); ++aPCurveIt){// Find adjacent outdated face.const IMeshData::IFaceHandle aDFace = aDEdge->GetPCurve(aPCurveIt)->GetFace();if (!aUsedFaces.Contains(aDFace.get())){aUsedFaces.Add(aDFace.get());if (aDFace->IsSet(IMeshData_Outdated)){TopLoc_Location aLoc;const Handle(Poly_Triangulation)& aTriangulation =BRep_Tool::Triangulation(aDFace->GetFace(), aLoc);// Clean all edges of oudated face.for (Standard_Integer aWireIt = 0; aWireIt < aDFace->WiresNb(); ++aWireIt){const IMeshData::IWireHandle& aDWire = aDFace->GetWire(aWireIt);for (Standard_Integer aWireEdgeIt = 0; aWireEdgeIt < aDWire->EdgesNb(); ++aWireEdgeIt){const IMeshData::IEdgeHandle aTmpDEdge = aDWire->GetEdge(aWireEdgeIt);BRepMesh_ShapeTool::NullifyEdge(aTmpDEdge->GetEdge(), aTriangulation, aLoc);}}BRepMesh_ShapeTool::NullifyFace(aDFace->GetFace());}}}}return Standard_True;
}

总结一下前处理基本上是对一些过期的三角数据和边数据进行清理

5、aContext->DiscretizeFaces(aPS.Next(9)) 面的离散

这里的算法比较庞大，基本上是根据平面不同的类型来执行不同的算法

switch (theSurfaceType){case GeomAbs_Plane:return theParameters.InternalVerticesMode ?new NodeInsertionMeshAlgo<BRepMesh_DefaultRangeSplitter>::Type :new BaseMeshAlgo::Type;break;case GeomAbs_Sphere:return new NodeInsertionMeshAlgo<BRepMesh_SphereRangeSplitter>::Type;break;case GeomAbs_Cylinder:return theParameters.InternalVerticesMode ?new NodeInsertionMeshAlgo<BRepMesh_CylinderRangeSplitter>::Type :new BaseMeshAlgo::Type;break;case GeomAbs_Cone:return new NodeInsertionMeshAlgo<BRepMesh_ConeRangeSplitter>::Type;break;case GeomAbs_Torus:return new NodeInsertionMeshAlgo<BRepMesh_TorusRangeSplitter>::Type;break;case GeomAbs_SurfaceOfRevolution:return new DeflectionControlMeshAlgo<BRepMesh_BoundaryParamsRangeSplitter>::Type;break;default:return new DeflectionControlMeshAlgo<BRepMesh_NURBSRangeSplitter>::Type;}

这里可以读一下这个大佬的文章，总结的还不错
https://blog.csdn.net/YongsenDY/article/details/115304488

6、aContext->PostProcessModel() 后处理

后处理将三角剖分中的多边形存储到TopoDS_Edge
具体的源码在BRepMesh_ModelHealer.cxx中可以自行查看

三、总结

离散的步骤主要如下：

设置离散的参数
执行IncrementalMesh算法开始离散
设置上下文的参数（包括用什么算法离散、以及初始化六个步骤用到的类的实例）
开始离散
4.1. 创建离散化模型
4.2. 对边进行离散
4.3. 修复模型
4.4. 前处理
4.5. 对不同类型的面用不同的算法离散
4.6. 后处理，将数据存储到TopoDS上