BRep Builder

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1 Introduction

類BRep_Builder提供了創建、修改BRep模型的方法。使用這個類，你可以從底層自己構建BRep體，前提條件是你要對BRep模型的數據結構有一定理解。邊界表示法BRep的重點在邊界的定義，打開BRep_Builder的類圖：

 

可以看到其中重載了很多UpdateEdge函數，每個UpdateEdge函數都修改了Edge中的幾何數據，包括邊界的定義數據。若能理解每個UpdateEdge函數，則對OpenCASCADE的BREP數據結構就能理解了。本文主要介紹其中兩個函數的用法：Add和Remove。

 2 Add Shape

BRep_Builder的Add函數的字面意思是將一個Shape添加到另外一個Shape中。因這個函數的實現比較簡單，把源碼列出如下：

 

void TopoDS_Builder::Add (TopoDS_Shape& aShape, 
                          const TopoDS_Shape& aComponent) const
{
  // From now the Component cannot be edited
  aComponent.TShape()->Free(Standard_False);

  // Note that freezing aComponent before testing if aShape is free
  // prevents from self-insertion
  // but aShape will be frozen when the Exception is raised
  if (aShape.Free())
  {
    static const unsigned int aTb[9]=
    {
      //COMPOUND to:
      (1<<((unsigned int)TopAbs_COMPOUND)),
      //COMPSOLID to:
      (1<<((unsigned int)TopAbs_COMPOUND)),
      //SOLID to:
      (1<<((unsigned int)TopAbs_COMPOUND)) |
      (1<<((unsigned int)TopAbs_COMPSOLID)),
      //SHELL to:
      (1<<((unsigned int)TopAbs_COMPOUND)) |
      (1<<((unsigned int)TopAbs_SOLID)),
      //FACE to:
      (1<<((unsigned int)TopAbs_COMPOUND)) |
      (1<<((unsigned int)TopAbs_SHELL)),
      //WIRE to:
      (1<<((unsigned int)TopAbs_COMPOUND)) |
      (1<<((unsigned int)TopAbs_FACE)),
      //EDGE to:
      (1<<((unsigned int)TopAbs_COMPOUND)) |
      (1<<((unsigned int)TopAbs_SOLID)) |
      (1<<((unsigned int)TopAbs_WIRE)),
      //VERTEX to:
      (1<<((unsigned int)TopAbs_COMPOUND)) |
      (1<<((unsigned int)TopAbs_SOLID)) |
      (1<<((unsigned int)TopAbs_FACE)) |
      (1<<((unsigned int)TopAbs_EDGE)),
      //SHAPE to:
      0
    };
    //
    const unsigned int iC=(unsigned int)aComponent.ShapeType();
    const unsigned int iS=(unsigned int)aShape.ShapeType();
    //
    if ((aTb[iC] & (1<<iS)) != 0) {
      TopoDS_ListOfShape& L = aShape.TShape()->myShapes;
      L.Append(aComponent);
      TopoDS_Shape& S = L.Last();
      //
      // compute the relative Orientation
      if (aShape.Orientation() == TopAbs_REVERSED)
        S.Reverse();
      //
      // and the Relative Location
      const TopLoc_Location& aLoc=aShape.Location();
      if (!aLoc.IsIdentity())
        S.Move(aLoc.Inverted());
      //
      // Set the TShape as modified.
      aShape.TShape()->Modified(Standard_True);
    }
    else {
      throw TopoDS_UnCompatibleShapes("TopoDS_Builder::Add");
    }
  }
  else {
    throw TopoDS_FrozenShape("TopoDS_Buider::Add");
  }
}

 

Add函數通過一個靜態的檢查列表，來檢查添加的Shape是不是合法的，即FACE只能添加到SHELL和COMPOUND中，EDGE只能添加到WIRE，SOLID和COMPOUND中等。添加之后還檢查了Shape的ORIENTATION及位置信息並作相應調整。不滿足條件的情況都會拋出異常，所以對於Add函數需要增加異常處理邏輯。

使用這個函數需要注意的是這個Add只是簡單的將Shape添加到TShape的Shape表中，並沒有維護BREP的邊界信息。

3 Remove Shape

與Add對應的有Remove函數，可以從一個Shape中刪除一個子Shape。還是打開源碼，有源碼有真相：

//=======================================================================
//function : Remove
//purpose  : Remove a Shape from an other one
//=======================================================================

void TopoDS_Builder::Remove (TopoDS_Shape& aShape, 
                             const TopoDS_Shape& aComponent) const
{
  // check  if aShape  is  not Frozen
  TopoDS_FrozenShape_Raise_if (!aShape.Free(),"TopoDS_Builder::Remove");
  
  // compute the relative Orientation and Location of aComponent
  TopoDS_Shape S = aComponent;
  if (aShape.Orientation() == TopAbs_REVERSED)
    S.Reverse();
  S.Location(S.Location().Predivided(aShape.Location()));

  TopoDS_ListOfShape& L = aShape.TShape()->myShapes;
  TopoDS_ListIteratorOfListOfShape It(L);
  while (It.More()) {
    if (It.Value() == S) {
      L.Remove(It);
      aShape.TShape()->Modified(Standard_True);
      break;
    }
    It.Next();
  }
}

 

從源碼中可知，Remove實現的邏輯也是很簡單的：

l 檢查Shape是不是Free的，若不是則拋出異常；

l 計算要刪除Component的ORIENTATION和LOCATION；

l 從Shape列中查找Component，若找到將其從列表中刪除；

 

刪除操作比添加操作要簡單，一個是把已有的數據刪除，一個是從無到有的構建數據。從函數實現代碼來看，刪除操作也是簡單的從Shape列表中刪除指定的Shape。刪除之后多余的邊界信息還會存在原來的Shape中，要確保刪除的Shape之后沒有多余信息，還需要刪除沒有使用的PCurves。

 

上圖所示為刪除一個底面的圓柱體。

 4 Conclusion

類BRep_Builder的操作需要以充分理解OpenCASCADE的BREP數據結構為前提，因為其Add和Remove函數並沒有提供維護邊界的功能，只是將指定的Shape添加到列表中或從列表中刪除。