OpenCASCADE Make Face With Holes

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OpenCASCADE Make Face With Holes

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OpenCASCADE提供了構造Face的類BRepBuilderAPI_MakeFace，使用這個類可以構造出帶孔的面。如所示：

當然，要得到所示的結果，還可以使用Boolean操作，用一個面去Cut幾個圓柱。當使用布爾操作就會涉及到一些複雜演算法，如求交，重構Topo體等，比較耗時。既然可以直接在產生面的時候挖孔，這個不涉及複雜演算法，速度、穩定性都比使用布爾操作要好。本文主要來介紹如何使用BRepBuilderAPI_MakeFace來產生帶孔的面，及其注意事項。直接上代碼：

#include <gp_Circ.hxx>#include <gp_Pln.hxx>#include <BRepBuilderAPI_MakeEdge.hxx>#include <BRepBuilderAPI_MakeWire.hxx>#include <BRepBuilderAPI_MakeFace.hxx>#include <BRepTools.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")void makeFaceTest(){    gp_Pln aPlane;    gp_Circ aCircle1(gp::XOY(), 1.0);    gp_Circ aCircle2(gp::XOY(), 1.0);    gp_Circ aCircle3(gp::XOY(), 1.0);    aCircle1.SetLocation(gp_Pnt(3.0, 3.0, 0.0));    aCircle2.SetLocation(gp_Pnt(7.0, 3.0, 0.0));    aCircle3.SetLocation(gp_Pnt(3.0, 7.0, 0.0));    BRepBuilderAPI_MakeEdge anEdgeMaker1(aCircle1);    BRepBuilderAPI_MakeEdge anEdgeMaker2(aCircle2);    BRepBuilderAPI_MakeEdge anEdgeMaker3(aCircle3);    BRepBuilderAPI_MakeWire aWireMaker1(anEdgeMaker1.Edge());    BRepBuilderAPI_MakeWire aWireMaker2(anEdgeMaker2.Edge());    BRepBuilderAPI_MakeWire aWireMaker3(anEdgeMaker3.Edge());    BRepBuilderAPI_MakeFace aFaceMaker(aPlane, 0.0, 10.0, 0.0, 10.0);    if (aWireMaker1.IsDone())    {        aFaceMaker.Add(aWireMaker1.Wire());    }    if (aWireMaker2.IsDone())    {        aFaceMaker.Add(aWireMaker2.Wire());    }    if (aWireMaker3.IsDone())    {        aFaceMaker.Add(aWireMaker3.Wire());    }    if (aFaceMaker.IsDone())    {        BRepTools::Write(aFaceMaker.Shape(), "d:/face.brep");    }}int main(int argc, char* argv[]){    makeFaceTest();    return 0;}

上面代碼就是在一個平面上開三個孔，最後在D盤產生face.brep檔案。在Draw Test Harness中載入並顯示這個檔案得到：

當切換到線框顯示模式時，和預期效果一致。但是當切換到著色顯示模式時，發現產生的面和預期的效果剛好相反。這是什麼原因呢？

在Draw Test Harness中輸入命令pcruve來檢查，如所示：

根據pcurve命令的提示得知，藍色的方嚮應該反向，紅色的為外環。所以得知，內環孔的方向反了。修改代碼，直接將Wire的方向Reverse。修改後的代碼如下：

#include <gp_Circ.hxx>#include <gp_Pln.hxx>#include <TopoDS_Wire.hxx>#include <BRepBuilderAPI_MakeEdge.hxx>#include <BRepBuilderAPI_MakeWire.hxx>#include <BRepBuilderAPI_MakeFace.hxx>#include <BRepTools.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")void makeFaceTest(){    gp_Pln aPlane;    gp_Circ aCircle1(gp::XOY(), 1.0);    gp_Circ aCircle2(gp::XOY(), 1.0);    gp_Circ aCircle3(gp::XOY(), 1.0);    aCircle1.SetLocation(gp_Pnt(3.0, 3.0, 0.0));    aCircle2.SetLocation(gp_Pnt(7.0, 3.0, 0.0));    aCircle3.SetLocation(gp_Pnt(3.0, 7.0, 0.0));    BRepBuilderAPI_MakeEdge anEdgeMaker1(aCircle1);    BRepBuilderAPI_MakeEdge anEdgeMaker2(aCircle2);    BRepBuilderAPI_MakeEdge anEdgeMaker3(aCircle3);    BRepBuilderAPI_MakeWire aWireMaker1(anEdgeMaker1.Edge());    BRepBuilderAPI_MakeWire aWireMaker2(anEdgeMaker2.Edge());    BRepBuilderAPI_MakeWire aWireMaker3(anEdgeMaker3.Edge());    BRepBuilderAPI_MakeFace aFaceMaker(aPlane, 0.0, 10.0, 0.0, 10.0);    if (aWireMaker1.IsDone())    {        TopoDS_Wire aWire1 = aWireMaker1.Wire();        aWire1.Reverse();        aFaceMaker.Add(aWire1);    }    if (aWireMaker2.IsDone())    {        TopoDS_Wire aWire2 = aWireMaker2.Wire();        aWire2.Reverse();        aFaceMaker.Add(aWire2);    }    if (aWireMaker3.IsDone())    {        TopoDS_Wire aWire3 = aWireMaker3.Wire();        aWire3.Reverse();        aFaceMaker.Add(aWire3);    }    if (aFaceMaker.IsDone())    {        BRepTools::Write(aFaceMaker.Shape(), "d:/face.brep");    }}int main(int argc, char* argv[]){    makeFaceTest();    return 0;}

重新載入新產生的face.brep並使用pcurve查看，得到如所示的效果：

從可知，這時得到的為預期的效果。綜上所述，如果要直接使用BRepBuilderAPI_MakeFace來產生帶有孔的面，需要自己為外環和內環的方向負責，opencascade對此不作檢查。當面顯示不正確時，可以使用Draw Test Harness的pcurve命令來檢查。一個帶孔的面的pcurve的規則為：外環為逆時針方向；內環孔的方向為順時針方向。

基於直接產生的帶孔的面，還可以進一步使用放樣演算法來造型，如展開，旋轉等，如所示。這樣就可以避免使用布爾操作，提高造型演算法的效能和穩定性。

OpenCASCADE Make Face With Holes