Split Shape by Plane in OpenCASCADE

Split Shape by Plane in OpenCASCADE

eryar@163.com

Abstract. Sometimes you want to split a shape by plane or even split a shape by a B Spline surface, OpenCASCADE provide a feature class BRepFeat_SplitShape to implement the function. The paper give a sample code to split a cylinder by plane.

Key Words. Split Shape, BRep Feature Algorithms.

1. Introduction

OpenCASCADE提供了Boolean Operation实现了任意两个形状的交、并、差的布尔操作。但是如何实现用一个面将一个形状切割成两半呢？其实Boolean Operation中已经有求交分割的算法，但是没有直接提供一个分割的功能类，而是在BRepFeat_SplitShape提供了分割功能。

Figure 1. Split Cylinder by Plane

2. Code Demo

使用类BRepAlgoAPI_Section和类BRepFeat_SplitShape相结合来实现分割Split形状的功能。完整代码示例如下：

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// Visual Studio 2013 & OpenCASCADE7.1.0

#include <gp_Pln.hxx>

#include <TopoDS.hxx>

#include <TopExp.hxx>
#include <TopExp_Explorer.hxx>

#include <BRepTools.hxx>

#include <BRepPrimAPI_MakeCylinder.hxx>

#include <BRepAlgoAPI_Section.hxx>

#include <BRepFeat_SplitShape.hxx>

#pragma comment(lib, "TKernel.lib")
#pragma comment(lib, "TKMath.lib")

#pragma comment(lib, "TKG2d.lib")
#pragma comment(lib, "TKG3d.lib")
#pragma comment(lib, "TKGeomBase.lib")
#pragma comment(lib, "TKGeomAlgo.lib")

#pragma comment(lib, "TKBRep.lib")
#pragma comment(lib, "TKTopAlgo.lib")

#pragma comment(lib, "TKBO.lib")
#pragma comment(lib, "TKPrim.lib")
#pragma comment(lib, "TKFeat.lib")

//! Test split a cylinder by plane.
//! You can use the algorithm to split other shapes.
void testSplit()
{
    BRepPrimAPI_MakeCylinder aCylinderMaker(10.0, 20.0);
    TopoDS_Shape aCylinder = aCylinderMaker.Shape();

    // Build section by the split plane for the cylinder.
    BRepAlgoAPI_Section aSection(aCylinder, gp_Pln(gp_Pnt(0.0, 0.0, 15.0), gp::DZ()), Standard_False);
    aSection.ComputePCurveOn1(Standard_True);
    aSection.Approximation(Standard_True);
    aSection.Build();

    // Split the cylinder shape.
    BRepFeat_SplitShape aShapeSpliter(aCylinder);

    for (TopExp_Explorer i(aSection.Shape(), TopAbs_EDGE); i.More(); i.Next())
    {
        TopoDS_Shape anEdge = i.Current();
        TopoDS_Shape aFace;

        if (aSection.HasAncestorFaceOn1(anEdge, aFace))
        {
            TopoDS_Edge E = TopoDS::Edge(anEdge);
            TopoDS_Face F = TopoDS::Face(aFace);

            aShapeSpliter.Add(E, F);
        }
    }

    aShapeSpliter.Build();

    // Rebuild left and right shape.
    BRep_Builder aBuilder;
    TopoDS_Compound aLeftCompound;
    TopoDS_Compound aRightCompound;

    aBuilder.MakeCompound(aLeftCompound);
    aBuilder.MakeCompound(aRightCompound);

    // Left shape.
    TopTools_MapOfShape aLeftShapeMap;
    const TopTools_ListOfShape& aLeftShapes = aShapeSpliter.Left();
    for (auto i = aLeftShapes.cbegin(); i != aLeftShapes.cend(); i++)
    {
        aLeftShapeMap.Add(*i);

        aBuilder.Add(aLeftCompound, *i);
    }

    // Right shape.
    TopTools_IndexedMapOfShape aShapeMap;
    TopExp::MapShapes(aShapeSpliter.Shape(), TopAbs_FACE, aShapeMap);

    for (auto i = aShapeMap.cbegin(); i != aShapeMap.cend(); i++)
    {
        if (!aLeftShapeMap.Contains(*i))
        {
            aBuilder.Add(aRightCompound, *i);
        }
    }

    // Output left and right shape.
    BRepTools::Write(aLeftCompound, "d:/left.brep");
    BRepTools::Write(aRightCompound, "d:/right.brep");
}

int main(int argc, char* argv[])
{
    testSplit();

    return 0;
}

先创建一个圆柱体，再使用类BRepAlgoAPI_Section将圆柱体用平面进行分割，最后使用类BRepFeat_SplitShape进行分类，得到切割后的形状及Left()形状，把切割后形状中的Left过滤后剩下就是切割另一半的形状；最后导出被平面切割后得到的两半形状。结果用动画演示如下：

Figure 2. Split Shape animation demo

3. Conclusion

OpenCASCADE提供类BRepFeat_SplitShape来实现对一个形状进行切割的功能，但是要配合BRepAlgoAPI_Section使用。因为Boolean Operation中已经实现了求交、分类的功能，所以在最新版本的源码7.2.0中已经将分割功能集成到了Boolean Operation中。分割后如果没有被改变的面还是原来的面。