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OpenCASCADE BRepTools

Posted on 2015-03-14 20:18 eryar 閱讀(5257) 評論(0) 編輯收藏引用所屬分類: 2.OpenCASCADE

OpenCASCADE BRepTools

Abstract. OpenCASCADE BRepTools provides utilities for BRep data structure. OuterWire method to find the outer wire of a face. Dump method to dump a BRep object. It also can be used as the data exchange for OpenCASCADE native shapes.

Key Words. OpenCASCADE, BRepTools, BRep, Topology

1. Introduction

OpenCASCADE提供了一個類BRepTools，其中有許多static函數(shù)，主要用來對BRep表示的拓樸形狀的數(shù)據(jù)進(jìn)行讀寫，也提供了查找一個面中外環(huán)（Outer Wire）的函數(shù)。因為OpenCASCADE中的邊界表示法BRep的數(shù)據(jù)結(jié)構(gòu)如下圖1.1所示：

Figure 1.1 BRep Data Structure of OpenCASCADE

因為OpenCASCADE中拓樸結(jié)構(gòu)采用了包含關(guān)系，當(dāng)需要將TopoDS_Shape數(shù)據(jù)保存到文件時，如何保持TopoDS_Shape中的關(guān)系，以便于從文件讀取這些數(shù)據(jù)時，可以重構(gòu)出TopoDS_Shape中的各種關(guān)系？

參考o(jì)pennurbs中的BRep表示時數(shù)據(jù)的存儲方式，可知直接在BRep中保存拓樸及幾何數(shù)據(jù)的索引，這樣對數(shù)據(jù)的存儲及讀取時重構(gòu)拓樸結(jié)構(gòu)還是很方便的。而在OpenCASCADE中拓樸數(shù)據(jù)是以Handle來保存的，且為組合關(guān)系，即一個父結(jié)構(gòu)中有一個列表（TopoDS_ListOfShape）給包含了子結(jié)構(gòu)數(shù)據(jù)。對于沒有索引的OpenCASCADE的拓樸結(jié)構(gòu)，如何進(jìn)行讀寫操作呢？

本文結(jié)合類BRepTools中的函數(shù)，對OpenCASCADE中TopoDS_Shape數(shù)據(jù)的保存和讀取功能的代碼進(jìn)行分析，從而對ModelingData中的BRep數(shù)據(jù)做進(jìn)一步的理解。

2.Topology Shape Serialization

OpenCASCADE的類BRepTools中提供了如下函數(shù)，可以TopoDS_Shape中的數(shù)據(jù)進(jìn)行導(dǎo)入導(dǎo)出：

v BRepTools::Dump()；

v BRepTools::Read()；

v BRepTools::Write()；

這幾個函數(shù)比較常用，因為可以方便地將TopoDS_Shape導(dǎo)出，或?qū)氲絆penCASCADE的Draw Test Harness中，來對程序一些算法進(jìn)行驗證。對于使用了組合關(guān)系的TopoDS_Shape如何確保數(shù)據(jù)的保存及讀取后，能夠維持這些關(guān)系？帶著這個問題去看BRep文件讀寫的功能，應(yīng)該更為清晰。

還是看看代碼，如下所示為輸出TopoDS_Shape的函數(shù)，在程序Debug時比較常用：

//=======================================================================
//function : Dump
//purpose  :
//=======================================================================
void  BRepTools::Dump(const TopoDS_Shape& Sh, Standard_OStream& S)
{
  BRepTools_ShapeSet SS;
  SS.Add(Sh);
  SS.Dump(Sh,S);
  SS.Dump(S);
}

其中使用了類BRepTools_ShapeSet，這里的Set的意思我理解為集合的意思，其Add函數(shù)如下：

//=======================================================================
//function : Add
//purpose  :
//=======================================================================
Standard_Integer  TopTools_ShapeSet::Add(const TopoDS_Shape& S)
{
  if (S.IsNull()) return 0;
  myLocations.Add(S.Location());
  TopoDS_Shape S2 = S;
  S2.Location(TopLoc_Location());
  Standard_Integer index = myShapes.FindIndex(S2);
  if (index == 0) {
    AddGeometry(S2);

    for (TopoDS_Iterator its(S2,Standard_False,Standard_False);
         its.More(); its.Next())
      Add(its.Value());
    index = myShapes.Add(S2);
  }
  return index;
}

這是一個遞歸函數(shù)，通過AddGeometry函數(shù)，將TopoDS_Shape中的幾何信息都保存到相應(yīng)的集合Set中，Set中使用了Map，即給每個幾何信息一個唯一的編號與之對應(yīng)。

//=======================================================================
//function : AddGeometry
//purpose  :
//=======================================================================

void BRepTools_ShapeSet::AddGeometry(const TopoDS_Shape& S)
{
  // Add the geometry

  if (S.ShapeType() == TopAbs_VERTEX) {

    Handle(BRep_TVertex) TV = Handle(BRep_TVertex)::DownCast(S.TShape());
    BRep_ListIteratorOfListOfPointRepresentation itrp(TV->Points());

    while (itrp.More()) {
      const Handle(BRep_PointRepresentation)& PR = itrp.Value();

      if (PR->IsPointOnCurve()) {
        myCurves.Add(PR->Curve());
      }

      else if (PR->IsPointOnCurveOnSurface()) {
        myCurves2d.Add(PR->PCurve());
        mySurfaces.Add(PR->Surface());
      }

      else if (PR->IsPointOnSurface()) {
        mySurfaces.Add(PR->Surface());
      }

      ChangeLocations().Add(PR->Location());
      itrp.Next();
    }

  }
  else if (S.ShapeType() == TopAbs_EDGE) {

    // Add the curve geometry
    Handle(BRep_TEdge) TE = Handle(BRep_TEdge)::DownCast(S.TShape());
    BRep_ListIteratorOfListOfCurveRepresentation itrc(TE->Curves());

    while (itrc.More()) {
      const Handle(BRep_CurveRepresentation)& CR = itrc.Value();
      if (CR->IsCurve3D()) {
        if (!CR->Curve3D().IsNull()) {
          myCurves.Add(CR->Curve3D());
          ChangeLocations().Add(CR->Location());
        }
      }
      else if (CR->IsCurveOnSurface()) {
        mySurfaces.Add(CR->Surface());
        myCurves2d.Add(CR->PCurve());
        ChangeLocations().Add(CR->Location());
        if (CR->IsCurveOnClosedSurface())
          myCurves2d.Add(CR->PCurve2());
      }
      else if (CR->IsRegularity()) {
        mySurfaces.Add(CR->Surface());
        ChangeLocations().Add(CR->Location());
        mySurfaces.Add(CR->Surface2());
        ChangeLocations().Add(CR->Location2());
      }
      else if (myWithTriangles) { // for XML Persistence
        if (CR->IsPolygon3D()) {
          if (!CR->Polygon3D().IsNull()) {
            myPolygons3D.Add(CR->Polygon3D());
            ChangeLocations().Add(CR->Location());
          }
        }
        else if (CR->IsPolygonOnTriangulation()) {
          myTriangulations.Add(CR->Triangulation());
          myNodes.Add(CR->PolygonOnTriangulation());
          ChangeLocations().Add(CR->Location());
          if (CR->IsPolygonOnClosedTriangulation())
            myNodes.Add(CR->PolygonOnTriangulation2());
        }
        else if (CR->IsPolygonOnSurface()) {
          mySurfaces.Add(CR->Surface());
          myPolygons2D.Add(CR->Polygon());
          ChangeLocations().Add(CR->Location());
          if (CR->IsPolygonOnClosedSurface())
          myPolygons2D.Add(CR->Polygon2());
        }
      }
      itrc.Next();
    }
  }

  else if (S.ShapeType() == TopAbs_FACE) {

    // Add the surface geometry
    Handle(BRep_TFace) TF = Handle(BRep_TFace)::DownCast(S.TShape());
    if (!TF->Surface().IsNull())  mySurfaces.Add(TF->Surface());

    if (myWithTriangles) { // for XML Persistence
      Handle(Poly_Triangulation) Tr = TF->Triangulation();
      if (!Tr.IsNull()) myTriangulations.Add(Tr);
    }

    ChangeLocations().Add(TF->Location());
  }
}

由上述代碼可知，Edge中的幾何信息較多，F(xiàn)ace中的幾何信息最少，只是幾何曲面或其用于顯示的網(wǎng)格數(shù)據(jù)。在將拓樸數(shù)據(jù)輸出時，拓樸面、邊及頂點中包含的幾何信息都是前面幾何數(shù)據(jù)的編號，即相當(dāng)于索引號的形式輸出，代碼如下所示：

//=======================================================================
//function : WriteGeometry
//purpose  :
//=======================================================================

void  BRepTools_ShapeSet::WriteGeometry(const TopoDS_Shape& S,
                                        Standard_OStream&   OS)const
{
  // Write the geometry

  if (S.ShapeType() == TopAbs_VERTEX) {

    // Write the point geometry
    TopoDS_Vertex V = TopoDS::Vertex(S);
    OS << BRep_Tool::Tolerance(V) << "\n";
    gp_Pnt p = BRep_Tool::Pnt(V);
    OS<<p.X()<<" "<<p.Y()<<" "<<p.Z()<<"\n";

    Handle(BRep_TVertex) TV = Handle(BRep_TVertex)::DownCast(S.TShape());
    BRep_ListIteratorOfListOfPointRepresentation itrp(TV->Points());

    while (itrp.More()) {
      const Handle(BRep_PointRepresentation)& PR = itrp.Value();

      OS << PR->Parameter();
      if (PR->IsPointOnCurve()) {
        OS << " 1 " << myCurves.Index(PR->Curve());
      }

      else if (PR->IsPointOnCurveOnSurface()) {
        OS << " 2 " <<  myCurves2d.Index(PR->PCurve());
        OS << " " << mySurfaces.Index(PR->Surface());
      }

      else if (PR->IsPointOnSurface()) {
        OS << " 3 " << PR->Parameter2() << " ";
        OS << mySurfaces.Index(PR->Surface());
      }

      OS << " " << Locations().Index(PR->Location());
      OS << "\n";

      itrp.Next();
    }

    OS << "0 0\n"; // end representations

  }

  else if (S.ShapeType() == TopAbs_EDGE) {

    // Write the curve geometry

    Handle(BRep_TEdge) TE = Handle(BRep_TEdge)::DownCast(S.TShape());

    OS << " " << TE->Tolerance() << " ";
    OS << ((TE->SameParameter()) ? 1 : 0) << " ";
    OS << ((TE->SameRange())     ? 1 : 0) << " ";
    OS << ((TE->Degenerated())   ? 1 : 0) << "\n";

    Standard_Real first, last;
    BRep_ListIteratorOfListOfCurveRepresentation itrc = TE->Curves();
    while (itrc.More()) {
      const Handle(BRep_CurveRepresentation)& CR = itrc.Value();
      if (CR->IsCurve3D()) {
        if (!CR->Curve3D().IsNull()) {
          Handle(BRep_GCurve) GC = Handle(BRep_GCurve)::DownCast(itrc.Value());
          GC->Range(first, last);
          OS << "1 ";                               // -1- Curve 3D
          OS << " "<<myCurves.Index(CR->Curve3D());
          OS << " "<<Locations().Index(CR->Location());
          OS << " "<<first<<" "<<last;
          OS << "\n";
        }
      }
      else if (CR->IsCurveOnSurface()) {
        Handle(BRep_GCurve) GC = Handle(BRep_GCurve)::DownCast(itrc.Value());
        GC->Range(first, last);
        if (!CR->IsCurveOnClosedSurface())
          OS << "2 ";                             // -2- Curve on surf
        else
          OS << "3 ";                             // -3- Curve on closed surf
        OS <<" "<<myCurves2d.Index(CR->PCurve());
        if (CR->IsCurveOnClosedSurface()) {
          OS <<" " << myCurves2d.Index(CR->PCurve2());
          PrintRegularity(CR->Continuity(),OS);
        }
        OS << " " << mySurfaces.Index(CR->Surface());
        OS << " " << Locations().Index(CR->Location());
        OS << " "<<first<<" "<<last;
        OS << "\n";

        // Write UV Points // for XML Persistence higher performance
        if (FormatNb() == 2)
        {
          gp_Pnt2d Pf,Pl;
          if (CR->IsCurveOnClosedSurface()) {
            Handle(BRep_CurveOnClosedSurface) COCS =
              Handle(BRep_CurveOnClosedSurface)::DownCast(CR);
            COCS->UVPoints2(Pf,Pl);
          }
          else {
            Handle(BRep_CurveOnSurface) COS =
              Handle(BRep_CurveOnSurface)::DownCast(CR);
            COS->UVPoints(Pf,Pl);
          }
          OS << Pf.X() << " " << Pf.Y() << " " << Pl.X() << " " << Pl.Y() << "\n";
        }
      }
      else if (CR->IsRegularity()) {
        OS << "4 ";                              // -4- Regularity
        PrintRegularity(CR->Continuity(),OS);
        OS << " "<<mySurfaces.Index(CR->Surface());
        OS << " "<<Locations().Index(CR->Location());
        OS << " "<<mySurfaces.Index(CR->Surface2());
        OS << " "<<Locations().Index(CR->Location2());
        OS << "\n";
      }

      else if (myWithTriangles) { // for XML Persistence
        if (CR->IsPolygon3D()) {
          Handle(BRep_Polygon3D) GC = Handle(BRep_Polygon3D)::DownCast(itrc.Value());
          if (!GC->Polygon3D().IsNull()) {
            OS << "5 ";                            // -5- Polygon3D
            OS << " "<<myPolygons3D.FindIndex(CR->Polygon3D());
            OS << " "<<Locations().Index(CR->Location());
            OS << "\n";
          }
        }
        else if (CR->IsPolygonOnTriangulation()) {
          Handle(BRep_PolygonOnTriangulation) PT =
            Handle(BRep_PolygonOnTriangulation)::DownCast(itrc.Value());
          if (!CR->IsPolygonOnClosedTriangulation())
            OS << "6 ";                            // -6- Polygon on triangulation
          else
            OS << "7 ";                            // -7- Polygon on closed triangulation
          OS << " " <<  myNodes.FindIndex(PT->PolygonOnTriangulation());
          if (CR->IsPolygonOnClosedTriangulation()) {
            OS << " " << myNodes.FindIndex(PT->PolygonOnTriangulation2());
          }
          OS << " " << myTriangulations.FindIndex(PT->Triangulation());
          OS << " "<<Locations().Index(CR->Location());
          OS << "\n";
        }
      }

      itrc.Next();
    }
    OS << "0\n"; // end of the list of representations
  }

  else if (S.ShapeType() == TopAbs_FACE) {

    Handle(BRep_TFace) TF = Handle(BRep_TFace)::DownCast(S.TShape());
    const TopoDS_Face& F = TopoDS::Face(S);

    if (!(TF->Surface()).IsNull()) {
      OS << ((BRep_Tool::NaturalRestriction(F)) ? 1 : 0);
      OS << " ";
      // Write the surface geometry
      OS << " " <<TF->Tolerance();
      OS << " " <<mySurfaces.Index(TF->Surface());
      OS << " " <<Locations().Index(TF->Location());
      OS << "\n";
    }
    else //For correct reading of null face
      {
    OS << 0;
    OS << " ";
    OS << " " <<TF->Tolerance();
    OS << " " << 0;
    OS << " " << 0;
    OS << "\n";
      }
    if (myWithTriangles) { // for XML Persistence
      if (!(TF->Triangulation()).IsNull()) {
        OS << 2;
        OS << " ";
        // Write the triangulation
        OS << " " <<myTriangulations.FindIndex(TF->Triangulation());
      }
    }
  }

}

通過先將幾何數(shù)據(jù)收集到相應(yīng)的集合（映射）中，再在拓樸結(jié)構(gòu)對應(yīng)的地方以索引號的方式輸出，這樣就便于從文件讀取數(shù)據(jù)時，以類似的方式來重構(gòu)BRep邊界表示的拓樸Shape的結(jié)構(gòu)。即讀取文件重構(gòu)拓樸結(jié)構(gòu)數(shù)據(jù)是輸出的逆過程。

在實現(xiàn)從文件讀取BRep表示的數(shù)據(jù)時，先將幾何信息讀取到對應(yīng)的集合中，再讀取拓樸結(jié)構(gòu)數(shù)據(jù)時，若拓樸結(jié)構(gòu)中包含幾何信息，則以索引的方式，找到對應(yīng)的幾何數(shù)據(jù)即可。詳細(xì)實現(xiàn)可參考源程序。

3. For Debugging

由于BRepTools為Toolkit TKBRep中的類，所以依賴的動態(tài)庫較少，所以在編程時，若要驗證一些算法的正確性時，經(jīng)常需要將TopoDS_Shape的數(shù)據(jù)導(dǎo)出，甚至可以直接先在Draw Test Harness中使用相關(guān)命令來將導(dǎo)出的數(shù)據(jù)導(dǎo)入來查看結(jié)果。

4. Conclusion

通過BRepTools中對TopoDS_Shape數(shù)據(jù)的輸出及導(dǎo)入的代碼分析可知，對于只有組合關(guān)系的數(shù)據(jù)，若想維持這種關(guān)系，就需要引入集合映射的類來產(chǎn)生索引，進(jìn)而在讀取數(shù)據(jù)時，可以根據(jù)索引來重構(gòu)拓樸關(guān)系。由于opennurbs中的BRep在內(nèi)存中本來就是索引的方式，所以在數(shù)據(jù)存取時，實現(xiàn)要簡單很多。

5. References

1. OpenCASCADE Team. BRep Format. 2014.12

2. Shing Liu. Topology and Geometry in OpenCascade-Topology.

http://www.shnenglu.com/eryar/archive/2013/09/21/203338.html

3. Shing Liu. Topology and Geometry in OpenCascade-Vertex

http://www.shnenglu.com/eryar/archive/2013/08/20/202678.html

4. Shing Liu. Topology and Geometry in OpenCascade-Edge

http://www.shnenglu.com/eryar/archive/2013/08/24/202739.html

5. Shing Liu. Topology and Geometry in OpenCascade-Face

http://www.shnenglu.com/eryar/archive/2013/09/12/203199.html

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