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Open Cascade Data Exchange STL

eryar@163.com

摘要Abstract：介绍了三维数据交换格式STL的组成，以及Open Cascade中对STL的读写。并将Open Cascade读进来的STL的三角面片在OpenSceneGraph中显示。 

关键字Key Words：STL, Open Cascade, OpenSceneGraph, Data Exchange 

STL（the Stereo Lithograpy）是快速原型系统所应用的标准文件类型。它的目的是将几何数据发送到可以读取和解释这些数据的机器，这种机器可将模型转换成塑料的物理模型。STL是用三角网格来表示三维模型的。STL文件格式简单，只能描述三维物体的几何信息，不支持颜色、材质等信息，是三维打印机支持的最常见的文件格式。由于STL文件的网格表示方法只能表示封闭的形状，所以要转换的形状必须是实体，或封闭的面和体。STL文件有两种：一种是明码（ASCII）格式，一种是二进制（Binary）格式。 

一、STL的明码（ASCII）格式

ASCII格式的STL文件逐行给出三角面片的几何信息，每行以1个或2个关键字开头。STL文件中的三角面片的信息单元facet是一个带法向方向的三角面片，STL三维模型就是由这一系列的三角面片构成。整个STL文件的首行给出了文件路径及文件名。在一个STL文件中，每个facet由7行数据组成：facet normal是三角面片指向实体外部的单位法矢量；outer loop说明随后的3行数据分别是三角面片的3个顶点坐标，3顶点沿指向实体外部的法矢量方向逆时针排列。 

ASCII格式的STL文件结构如下： 

说明如下： 

下面给出由Open Cascade中导出的一个长方体的STL文件： 

长方体的尺寸为长200，宽150，高100，原点在一个角点上。 

Figure 1.1 Box in Open Cascade 

solidfacet normal -1.000000e+000 -0.000000e+000 -0.000000e+000outer loopvertex  0.000000e+000  1.500000e+002  1.000000e+002vertex  0.000000e+000  1.500000e+002  0.000000e+000vertex  0.000000e+000  0.000000e+000  1.000000e+002endloopendfacetfacet normal -1.000000e+000  0.000000e+000  0.000000e+000outer loopvertex  0.000000e+000  1.500000e+002  0.000000e+000vertex  0.000000e+000  0.000000e+000  0.000000e+000vertex  0.000000e+000  0.000000e+000  1.000000e+002endloopendfacetfacet normal  1.000000e+000 -0.000000e+000  0.000000e+000outer loopvertex  2.000000e+002  0.000000e+000  1.000000e+002vertex  2.000000e+002  1.500000e+002  0.000000e+000vertex  2.000000e+002  1.500000e+002  1.000000e+002endloopendfacetfacet normal  1.000000e+000 -0.000000e+000  0.000000e+000outer loopvertex  2.000000e+002  0.000000e+000  1.000000e+002vertex  2.000000e+002  0.000000e+000  0.000000e+000vertex  2.000000e+002  1.500000e+002  0.000000e+000endloopendfacetfacet normal  0.000000e+000 -1.000000e+000  0.000000e+000outer loopvertex  0.000000e+000  0.000000e+000  0.000000e+000vertex  2.000000e+002  0.000000e+000  0.000000e+000vertex  2.000000e+002  0.000000e+000  1.000000e+002endloopendfacetfacet normal  0.000000e+000 -1.000000e+000  0.000000e+000outer loopvertex  0.000000e+000  0.000000e+000  1.000000e+002vertex  0.000000e+000  0.000000e+000  0.000000e+000vertex  2.000000e+002  0.000000e+000  1.000000e+002endloopendfacetfacet normal  0.000000e+000  1.000000e+000  0.000000e+000outer loopvertex  2.000000e+002  1.500000e+002  1.000000e+002vertex  2.000000e+002  1.500000e+002  0.000000e+000vertex  0.000000e+000  1.500000e+002  0.000000e+000endloopendfacetfacet normal  0.000000e+000  1.000000e+000 -0.000000e+000outer loopvertex  2.000000e+002  1.500000e+002  1.000000e+002vertex  0.000000e+000  1.500000e+002  0.000000e+000vertex  0.000000e+000  1.500000e+002  1.000000e+002endloopendfacetfacet normal  0.000000e+000  0.000000e+000 -1.000000e+000outer loopvertex  0.000000e+000  0.000000e+000  0.000000e+000vertex  0.000000e+000  1.500000e+002  0.000000e+000vertex  2.000000e+002  1.500000e+002  0.000000e+000endloopendfacetfacet normal  0.000000e+000  0.000000e+000 -1.000000e+000outer loopvertex  2.000000e+002  0.000000e+000  0.000000e+000vertex  0.000000e+000  0.000000e+000  0.000000e+000vertex  2.000000e+002  1.500000e+002  0.000000e+000endloopendfacetfacet normal  0.000000e+000  0.000000e+000  1.000000e+000outer loopvertex  2.000000e+002  1.500000e+002  1.000000e+002vertex  0.000000e+000  1.500000e+002  1.000000e+002vertex  0.000000e+000  0.000000e+000  1.000000e+002endloopendfacetfacet normal -0.000000e+000  0.000000e+000  1.000000e+000outer loopvertex  2.000000e+002  1.500000e+002  1.000000e+002vertex  0.000000e+000  0.000000e+000  1.000000e+002vertex  2.000000e+002  0.000000e+000  1.000000e+002endloopendfacet
endsolid

由上面的STL明码文件可知，上述数据将一个长方体的6个面用12个三角形来表示。在OpenSceneGraph中显示效果如下图所示，分别为此长方体的实体渲染模式和线框渲染模式： 

Figure 1.2 Shaded and Wireframe box in OpenSceneGraph 

二、STL的二进制（Binary）格式

二进制的STL文件用固定的字节数来给出三角面片的几何信息。文件起始80个字节是文件头，用于存贮零件名；紧接着4个字节的整数来描述模型的三角面片个数；后面逐个给出每个三角面片的几何信息。每个三角面片用固定的50个字节，依次是表示三角面片的法矢量的3个4字节浮点数；表示三角面片三个顶点的3x3个4字节浮点数；最后2个字节用来描述三角面片的属性信息。 

三、OCC中STL文件的读写Read/Write STL in Open Cascade

在Open Cascade中STL文件的读写分别使用类：StlAPI_Reader/StlAPI_Writer来实现。查看源程序可知，写STL文件的步骤如下： 

l 遍历一个TopoDS_Shape所有的面Face； 

l 使用工具BRep_Tool::Triangulation将每个面Face三角面片化； 

l 计算每个三角面片的法矢量； 

l 将结果写入文件。 

类RWStl对STL的读定也是有两种格式，即ASCII格式和Binary格式： 

n RWStl::WriteBinary 

n RWStl::WriteAscii 

n RWStl::ReadBinary 

n RWStl::ReadAscii 

程序的具体实现可以查看Open Cascade源代码，将读写部分主要代码RWStl.cxx列出如下： 

// Created on: 1994-10-13
// Created by: Marc LEGAY
// Copyright (c) 1994-1999 Matra Datavision
// Copyright (c) 1999-2012 OPEN CASCADE SAS
//
// The content of this file is subject to the Open CASCADE Technology Public
// License Version 6.5 (the "License"). You may not use the content of this file
// except in compliance with the License. Please obtain a copy of the License
// at http://www.opencascade.org and read it completely before using this file.
//
// The Initial Developer of the Original Code is Open CASCADE S.A.S., having its
// main offices at: 1, place des Freres Montgolfier, 78280 Guyancourt, France.
//
// The Original Code and all software distributed under the License is
// distributed on an "AS IS" basis, without warranty of any kind, and the
// Initial Developer hereby disclaims all such warranties, including without
// limitation, any warranties of merchantability, fitness for a particular
// purpose or non-infringement. Please see the License for the specific terms
// and conditions governing the rights and limitations under the License.
#include <RWStl.ixx>
#include <OSD_Protection.hxx>
#include <OSD_File.hxx>
#include <Message_ProgressSentry.hxx>
#include <TCollection_AsciiString.hxx>
#include <Standard_NoMoreObject.hxx>
#include <Standard_TypeMismatch.hxx>
#include <Precision.hxx>
#include <StlMesh_MeshExplorer.hxx>
#include <OSD.hxx>
#include <OSD_Host.hxx>
#include <gp_XYZ.hxx>
#include <gp.hxx>
#include <stdio.h>
#include <gp_Vec.hxx>// constants
static const int HEADER_SIZE           =  84;
static const int SIZEOF_STL_FACET      =  50;
static const int STL_MIN_FILE_SIZE     = 284;
static const int ASCII_LINES_PER_FACET =   7;
static const int IND_THRESHOLD         = 1000; // increment the indicator every 1k triangles//=======================================================================
//function : WriteInteger
//purpose  : writing a Little Endian 32 bits integer
//=======================================================================

inline static void WriteInteger(OSD_File& ofile,const Standard_Integer value)
{union {Standard_Integer i;// don't be afraid, this is just an unsigned intchar c[4];} bidargum;bidargum.i = value;Standard_Integer entier;entier  =  bidargum.c[0] & 0xFF;entier |= (bidargum.c[1] & 0xFF) << 0x08;entier |= (bidargum.c[2] & 0xFF) << 0x10;entier |= (bidargum.c[3] & 0xFF) << 0x18;ofile.Write((char *)&entier,sizeof(bidargum.c));
}//=======================================================================
//function : WriteDouble2Float
//purpose  : writing a Little Endian 32 bits float
//=======================================================================

inline static void WriteDouble2Float(OSD_File& ofile,Standard_Real value)
{union {Standard_ShortReal f;char c[4];} bidargum;bidargum.f = (Standard_ShortReal)value;Standard_Integer entier;entier  =  bidargum.c[0] & 0xFF;entier |= (bidargum.c[1] & 0xFF) << 0x08;entier |= (bidargum.c[2] & 0xFF) << 0x10;entier |= (bidargum.c[3] & 0xFF) << 0x18;ofile.Write((char *)&entier,sizeof(bidargum.c));
}//=======================================================================
//function : readFloat2Double
//purpose  : reading a Little Endian 32 bits float
//=======================================================================

inline static Standard_Real ReadFloat2Double(OSD_File &aFile)
{union {Standard_Boolean i; // don't be afraid, this is just an unsigned int
    Standard_ShortReal f;}bidargum;char c[4];Standard_Address adr;adr = (Standard_Address)c;Standard_Integer lread;aFile.Read(adr,4,lread);bidargum.i  =  c[0] & 0xFF;bidargum.i |=  (c[1] & 0xFF) << 0x08;bidargum.i |=  (c[2] & 0xFF) << 0x10;bidargum.i |=  (c[3] & 0xFF) << 0x18;return (Standard_Real)(bidargum.f);
}//=======================================================================
//function : WriteBinary
//purpose  : write a binary STL file in Little Endian format
//=======================================================================

Standard_Boolean RWStl::WriteBinary (const Handle(StlMesh_Mesh)& theMesh,const OSD_Path& thePath,const Handle(Message_ProgressIndicator)& theProgInd)
{OSD_File aFile (thePath);aFile.Build (OSD_WriteOnly, OSD_Protection());Standard_Real x1, y1, z1;Standard_Real x2, y2, z2;Standard_Real x3, y3, z3;// writing 80 bytes of the trash?char sval[80];aFile.Write ((Standard_Address)sval,80);WriteInteger (aFile, theMesh->NbTriangles());int dum=0;StlMesh_MeshExplorer aMexp (theMesh);// create progress sentry for domainsStandard_Integer aNbDomains = theMesh->NbDomains();Message_ProgressSentry aDPS (theProgInd, "Mesh domains", 0, aNbDomains, 1);for (Standard_Integer nbd = 1; nbd <= aNbDomains && aDPS.More(); nbd++, aDPS.Next()){// create progress sentry for triangles in domainMessage_ProgressSentry aTPS (theProgInd, "Triangles", 0,theMesh->NbTriangles (nbd), IND_THRESHOLD);Standard_Integer aTriangleInd = 0;for (aMexp.InitTriangle (nbd); aMexp.MoreTriangle(); aMexp.NextTriangle()){aMexp.TriangleVertices (x1,y1,z1,x2,y2,z2,x3,y3,z3);//pgo      aMexp.TriangleOrientation (x,y,z);gp_XYZ Vect12 ((x2-x1), (y2-y1), (z2-z1));gp_XYZ Vect13 ((x3-x1), (y3-y1), (z3-z1));gp_XYZ Vnorm = Vect12 ^ Vect13;Standard_Real Vmodul = Vnorm.Modulus ();if (Vmodul > gp::Resolution()){Vnorm.Divide(Vmodul);}else{// si Vnorm est quasi-nul, on le charge a 0 explicitementVnorm.SetCoord (0., 0., 0.);}WriteDouble2Float (aFile, Vnorm.X());WriteDouble2Float (aFile, Vnorm.Y());WriteDouble2Float (aFile, Vnorm.Z());WriteDouble2Float (aFile, x1);WriteDouble2Float (aFile, y1);WriteDouble2Float (aFile, z1);WriteDouble2Float (aFile, x2);WriteDouble2Float (aFile, y2);WriteDouble2Float (aFile, z2);WriteDouble2Float (aFile, x3);WriteDouble2Float (aFile, y3);WriteDouble2Float (aFile, z3);aFile.Write (&dum, 2);// update progress only per 1k trianglesif (++aTriangleInd % IND_THRESHOLD == 0){if (!aTPS.More())break;aTPS.Next();}}}aFile.Close();Standard_Boolean isInterrupted = !aDPS.More();return !isInterrupted;
}
//=======================================================================
//function : WriteAscii
//purpose  : write an ASCII STL file
//=======================================================================

Standard_Boolean RWStl::WriteAscii (const Handle(StlMesh_Mesh)& theMesh,const OSD_Path& thePath,const Handle(Message_ProgressIndicator)& theProgInd)
{OSD_File theFile (thePath);theFile.Build(OSD_WriteOnly,OSD_Protection());TCollection_AsciiString buf ("solid\n");theFile.Write (buf,buf.Length());buf.Clear();Standard_Real x1, y1, z1;Standard_Real x2, y2, z2;Standard_Real x3, y3, z3;char sval[512];// create progress sentry for domainsStandard_Integer aNbDomains = theMesh->NbDomains();Message_ProgressSentry aDPS (theProgInd, "Mesh domains", 0, aNbDomains, 1);StlMesh_MeshExplorer aMexp (theMesh);for (Standard_Integer nbd = 1; nbd <= aNbDomains && aDPS.More(); nbd++, aDPS.Next()){// create progress sentry for triangles in domainMessage_ProgressSentry aTPS (theProgInd, "Triangles", 0,theMesh->NbTriangles (nbd), IND_THRESHOLD);Standard_Integer aTriangleInd = 0;for (aMexp.InitTriangle (nbd); aMexp.MoreTriangle(); aMexp.NextTriangle()){aMexp.TriangleVertices (x1,y1,z1,x2,y2,z2,x3,y3,z3);//      Standard_Real x, y, z;
//      aMexp.TriangleOrientation (x,y,z);
gp_XYZ Vect12 ((x2-x1), (y2-y1), (z2-z1));gp_XYZ Vect23 ((x3-x2), (y3-y2), (z3-z2));gp_XYZ Vnorm = Vect12 ^ Vect23;Standard_Real Vmodul = Vnorm.Modulus ();if (Vmodul > gp::Resolution()){Vnorm.Divide (Vmodul);}else{// si Vnorm est quasi-nul, on le charge a 0 explicitementVnorm.SetCoord (0., 0., 0.);}Sprintf (sval," facet normal % 12e % 12e % 12e\n""   outer loop\n""     vertex % 12e % 12e % 12e\n""     vertex % 12e % 12e % 12e\n""     vertex % 12e % 12e % 12e\n""   endloop\n"" endfacet\n",Vnorm.X(), Vnorm.Y(), Vnorm.Z(),x1, y1, z1,x2, y2, z2,x3, y3, z3);buf += sval;theFile.Write (buf, buf.Length()); buf.Clear();// update progress only per 1k trianglesif (++aTriangleInd % IND_THRESHOLD == 0){if (!aTPS.More())break;aTPS.Next();}}}buf += "endsolid\n";theFile.Write (buf, buf.Length()); buf.Clear();theFile.Close();Standard_Boolean isInterrupted = !aDPS.More();return !isInterrupted;
}
//=======================================================================
//function : ReadFile
//Design   :
//Warning  :
//=======================================================================

Handle_StlMesh_Mesh RWStl::ReadFile (const OSD_Path& thePath,const Handle(Message_ProgressIndicator)& theProgInd)
{OSD_File file (thePath);file.Open(OSD_ReadOnly,OSD_Protection(OSD_RWD,OSD_RWD,OSD_RWD,OSD_RWD));Standard_Boolean IsAscii;unsigned char str[128];Standard_Integer lread,i;Standard_Address ach;ach = (Standard_Address)str;// we skip the header which is in Ascii for both modes
  file.Read(ach,HEADER_SIZE,lread);// we read 128 characters to detect if we have a non-ascii charfile.Read(ach,sizeof(str),lread);IsAscii = Standard_True;for (i = 0; i< lread && IsAscii; ++i) {if (str[i] > '~') {IsAscii = Standard_False;}}
#ifdef DEBcout << (IsAscii ? "ascii\n" : "binary\n");
#endiffile.Close();return IsAscii ? RWStl::ReadAscii  (thePath, theProgInd): RWStl::ReadBinary (thePath, theProgInd);
}//=======================================================================
//function : ReadBinary
//Design   :
//Warning  :
//=======================================================================

Handle_StlMesh_Mesh RWStl::ReadBinary (const OSD_Path& thePath,const Handle(Message_ProgressIndicator)& /*theProgInd*/)
{Standard_Integer NBFACET;Standard_Integer ifacet;Standard_Real fx,fy,fz,fx1,fy1,fz1,fx2,fy2,fz2,fx3,fy3,fz3;Standard_Integer i1,i2,i3,lread;char buftest[5];Standard_Address adr;adr = (Standard_Address)buftest;// Open the file
  OSD_File theFile (thePath);theFile.Open(OSD_ReadOnly,OSD_Protection(OSD_RWD,OSD_RWD,OSD_RWD,OSD_RWD));// the size of the file (minus the header size)// must be a multiple of SIZEOF_STL_FACET// compute file sizeStandard_Integer filesize = theFile.Size();if ( (filesize - HEADER_SIZE) % SIZEOF_STL_FACET !=0|| (filesize < STL_MIN_FILE_SIZE)) {Standard_NoMoreObject::Raise("RWStl::ReadBinary (wrong file size)");}// don't trust the number of triangles which is coded in the file// sometimes it is wrong, and with this technique we don't need to swap endians for integerNBFACET = ((filesize - HEADER_SIZE) / SIZEOF_STL_FACET);// skip the header
  theFile.Seek(HEADER_SIZE,OSD_FromBeginning);// create the StlMesh_Mesh objectHandle(StlMesh_Mesh) ReadMesh = new StlMesh_Mesh ();ReadMesh->AddDomain ();for (ifacet=1; ifacet<=NBFACET; ++ifacet) {// read normal coordinatesfx = ReadFloat2Double(theFile);fy = ReadFloat2Double(theFile);fz = ReadFloat2Double(theFile);// read vertex 1fx1 = ReadFloat2Double(theFile);fy1 = ReadFloat2Double(theFile);fz1 = ReadFloat2Double(theFile);// read vertex 2fx2 = ReadFloat2Double(theFile);fy2 = ReadFloat2Double(theFile);fz2 = ReadFloat2Double(theFile);// read vertex 3fx3 = ReadFloat2Double(theFile);fy3 = ReadFloat2Double(theFile);fz3 = ReadFloat2Double(theFile);i1 = ReadMesh->AddOnlyNewVertex (fx1,fy1,fz1);i2 = ReadMesh->AddOnlyNewVertex (fx2,fy2,fz2);i3 = ReadMesh->AddOnlyNewVertex (fx3,fy3,fz3);ReadMesh->AddTriangle (i1,i2,i3,fx,fy,fz);// skip extra bytestheFile.Read(adr,2,lread);}theFile.Close ();return ReadMesh;}
//=======================================================================
//function : ReadAscii
//Design   :
//Warning  :
//=======================================================================

Handle_StlMesh_Mesh RWStl::ReadAscii (const OSD_Path& thePath,const Handle(Message_ProgressIndicator)& theProgInd)
{TCollection_AsciiString filename;long ipos;Standard_Integer nbLines = 0;Standard_Integer nbTris = 0;Standard_Integer iTri;Standard_Integer i1,i2,i3;Handle(StlMesh_Mesh) ReadMesh;thePath.SystemName (filename);// Open the fileFILE* file = fopen(filename.ToCString(),"r");fseek(file,0L,SEEK_END);long filesize = ftell(file);fclose(file);file = fopen(filename.ToCString(),"r");// count the number of linesfor (ipos = 0; ipos < filesize; ++ipos) {if (getc(file) == '\n')nbLines++;}// compute number of trianglesnbTris = (nbLines / ASCII_LINES_PER_FACET);// go back to the beginning of the file
//  fclose(file);
//  file = fopen(filename.ToCString(),"r");
  rewind(file);// skip headerwhile (getc(file) != '\n');
#ifdef DEBcout << "start mesh\n";
#endifReadMesh = new StlMesh_Mesh();ReadMesh->AddDomain();// main readingMessage_ProgressSentry aPS (theProgInd, "Triangles", 0, (nbTris - 1) * 1.0 / IND_THRESHOLD, 1);for (iTri = 0; iTri < nbTris && aPS.More();){char x[256]="", y[256]="", z[256]="";// reading the facet normalif (3 != fscanf(file,"%*s %*s %80s %80s %80s\n", x, y, z))break; // error should be properly reported
    gp_XYZ aN (Atof(x), Atof(y), Atof(z));// skip the keywords "outer loop"fscanf(file,"%*s %*s");// reading vertexif (3 != fscanf(file,"%*s %80s %80s %80s\n", x, y, z))break; // error should be properly reported
    gp_XYZ aV1 (Atof(x), Atof(y), Atof(z));if (3 != fscanf(file,"%*s %80s %80s %80s\n", x, y, z))break; // error should be properly reported
    gp_XYZ aV2 (Atof(x), Atof(y), Atof(z));if (3 != fscanf(file,"%*s %80s %80s %80s\n", x, y, z))break; // error should be properly reported
    gp_XYZ aV3 (Atof(x), Atof(y), Atof(z));// here the facet must be built and put in the mesh datastructure
i1 = ReadMesh->AddOnlyNewVertex (aV1.X(), aV1.Y(), aV1.Z());i2 = ReadMesh->AddOnlyNewVertex (aV2.X(), aV2.Y(), aV2.Z());i3 = ReadMesh->AddOnlyNewVertex (aV3.X(), aV3.Y(), aV3.Z());ReadMesh->AddTriangle (i1, i2, i3, aN.X(), aN.Y(), aN.Z());// skip the keywords "endloop"fscanf(file,"%*s");// skip the keywords "endfacet"fscanf(file,"%*s");// update progress only per 1k trianglesif (++iTri % IND_THRESHOLD == 0)aPS.Next();}
#ifdef DEBcout << "end mesh\n";
#endiffclose(file);return ReadMesh;
}

程序开始定义了一些常量： 

// constants
static const int HEADER_SIZE           =  84; 
static const int SIZEOF_STL_FACET      =  50; 
static const int STL_MIN_FILE_SIZE     = 284; 
static const int ASCII_LINES_PER_FACET =   7; 

分别对应二进制文件中相关信息，即文件头84个字节，每个三角面片50个字节，STL文件最小为284字节。ASCII的STL中每个三角面有7行。 

在数据的读写过程中，对数据进行了小端转换。将double数据转换成小端表示的代码如下所示： 

//=====================================================================
//function : WriteDouble2Float
//purpose  : writing a Little Endian 32 bits float
//=====================================================================

inline static void WriteDouble2Float(OSD_File& ofile,Standard_Real value)
{union {Standard_ShortReal f;char c[4];} bidargum;bidargum.f = (Standard_ShortReal)value;Standard_Integer entier;entier  =  bidargum.c[0] & 0xFF;entier |= (bidargum.c[1] & 0xFF) << 0x08;entier |= (bidargum.c[2] & 0xFF) << 0x10;entier |= (bidargum.c[3] & 0xFF) << 0x18;ofile.Write((char *)&entier,sizeof(bidargum.c));
}

使用联合体（union）来处理显得很优雅。关于大端、小端的相关信息请参考： 

http://www.cppblog.com/tx7do/archive/2009/01/06/71276.html 

四、在OpenSceneGraph中显示STL

结合OpenCascade中对STL文件读写的功能和OpenSceneGraph的显示功能，将STL读取所得数据进行显示。源程序如下所示： 

// Open Cascade
#include <gp_Vec.hxx>
#include <OSD_Path.hxx>
#include <RWStl.hxx>
#include <StlMesh_Mesh.hxx>
#include <StlMesh_MeshExplorer.hxx>#pragma comment(lib, "TKernel.lib")
#pragma comment(lib, "TKMath.lib")
#pragma comment(lib, "TKSTL.lib")// OpenSceneGraph
#include <osgDB/ReadFile>
#include <osgViewer/Viewer>
#include <osgViewer/ViewerEventHandlers>
#include <osgGA/StateSetManipulator>#pragma comment(lib, "osgd.lib")
#pragma comment(lib, "osgDbd.lib")
#pragma comment(lib, "osgGAd.lib")
#pragma comment(lib, "osgViewerd.lib")osg::Node* readSTLFile(const std::string& fileName)
{osg::Group* root = new osg::Group();OSD_Path stlFile(fileName.c_str());Handle_StlMesh_Mesh stlMesh = RWStl::ReadFile(stlFile);Standard_Integer nDomains = stlMesh->NbDomains();StlMesh_MeshExplorer meshExplorer(stlMesh);Standard_Real x[3] = {0};Standard_Real y[3] = {0};Standard_Real z[3] = {0};Standard_Real n[3] = {0};gp_XYZ p1;gp_XYZ p2;gp_XYZ p3;gp_XYZ normal;gp_Vec vecNormal;for (int i = 1; i <= nDomains; i++){for (meshExplorer.InitTriangle(i); meshExplorer.MoreTriangle(); meshExplorer.NextTriangle()){meshExplorer.TriangleVertices(x[0], y[0], z[0], x[1], y[1], z[1], x[2], y[2], z[2]);meshExplorer.TriangleOrientation(n[0], n[1], n[2]);p1.SetCoord(x[0], y[0], z[0]);p2.SetCoord(x[1], y[1], z[1]);p3.SetCoord(x[2], y[2], z[2]);normal.SetCoord(n[0], n[1], n[2]);//gp_Vec vec12((x[1] - x[0]), (y[1] - y[0]), (z[1] - z[0]));//gp_Vec vec23((x[2] - x[1]), (y[2] - y[1]), (z[2] - z[1]));//vecNormal = vec12.Crossed(vec23).Normalized();
osg::ref_ptr<osg::Geode> geode = new osg::Geode();osg::ref_ptr<osg::Geometry> triGeom = new osg::Geometry();osg::ref_ptr<osg::Vec3Array> vertices = new osg::Vec3Array();osg::ref_ptr<osg::Vec3Array> normals = new osg::Vec3Array();vertices->push_back(osg::Vec3(x[0], y[0], z[0]));vertices->push_back(osg::Vec3(x[1], y[1], z[1]));vertices->push_back(osg::Vec3(x[2], y[2], z[2]));normals->push_back(osg::Vec3(n[0], n[1], n[2]));triGeom->setVertexArray(vertices.get());triGeom->addPrimitiveSet(new osg::DrawArrays(osg::PrimitiveSet::TRIANGLES, 0, vertices->size()));triGeom->setNormalArray(normals);triGeom->setNormalBinding(osg::Geometry::BIND_PER_PRIMITIVE);geode->addDrawable(triGeom);root->addChild(geode);}}return root;
}int main(int argc, char* argv[])
{osgViewer::Viewer myViewer;osg::ref_ptr<osg::Group> root = new osg::Group();//root->addChild(readSTLFile("D:\\OpenCASCADE6.5.0\\data\\stl\\propeller.stl"));root->addChild(readSTLFile("D:\\OpenCASCADE6.5.0\\data\\stl\\sh1.stl"));//root->addChild(readSTLFile("D:\\OpenCASCADE6.5.0\\data\\stl\\motor.stl"));//root->addChild(readSTLFile("D:\\box.stl"));
    myViewer.setSceneData(root);myViewer.addEventHandler(new osgGA::StateSetManipulator(myViewer.getCamera()->getOrCreateStateSet()));myViewer.addEventHandler(new osgViewer::StatsHandler);myViewer.addEventHandler(new osgViewer::WindowSizeHandler);return myViewer.run();
}

以下所示为OpenCascade提供的几个STL文件在OpenSceneGraph中显示的效果： 

Figure 4.1 Shaded Piston 

Figure 4.2 Wireframe Piston 

Figure 4.3 Shaded Propeller 

Figure 4.4 Wireframe Propeller 

五、结论 

通过使用OpenCascade的类RWStl来读取STL格式的文件，理解了STL文件格式；通过将读取的三角面面片数据在OpenSceneGraph中显示，对三维物体在计算机中的表示有了感性的认识。 

六、参考资料 

1. OpenCascade中类RWStl.cxx 

2. OpenCascade中STL模型数据 

3. 字节序、大端、小端：http://www.cppblog.com/tx7do/archive/2009/01/06/71276.html

 

PDF Version: Open Cascade DataExchange STL