Open Cascade Data Exchange --- STL
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
solid
facet normal -1.000000e+000 -0.000000e+000 -0.000000e+000
outer loop
vertex 0.000000e+000 1.500000e+002 1.000000e+002
vertex 0.000000e+000 1.500000e+002 0.000000e+000
vertex 0.000000e+000 0.000000e+000 1.000000e+002
endloop
endfacet
facet normal -1.000000e+000 0.000000e+000 0.000000e+000
outer loop
vertex 0.000000e+000 1.500000e+002 0.000000e+000
vertex 0.000000e+000 0.000000e+000 0.000000e+000
vertex 0.000000e+000 0.000000e+000 1.000000e+002
endloop
endfacet
facet normal 1.000000e+000 -0.000000e+000 0.000000e+000
outer loop
vertex 2.000000e+002 0.000000e+000 1.000000e+002
vertex 2.000000e+002 1.500000e+002 0.000000e+000
vertex 2.000000e+002 1.500000e+002 1.000000e+002
endloop
endfacet
facet normal 1.000000e+000 -0.000000e+000 0.000000e+000
outer loop
vertex 2.000000e+002 0.000000e+000 1.000000e+002
vertex 2.000000e+002 0.000000e+000 0.000000e+000
vertex 2.000000e+002 1.500000e+002 0.000000e+000
endloop
endfacet
facet normal 0.000000e+000 -1.000000e+000 0.000000e+000
outer loop
vertex 0.000000e+000 0.000000e+000 0.000000e+000
vertex 2.000000e+002 0.000000e+000 0.000000e+000
vertex 2.000000e+002 0.000000e+000 1.000000e+002
endloop
endfacet
facet normal 0.000000e+000 -1.000000e+000 0.000000e+000
outer loop
vertex 0.000000e+000 0.000000e+000 1.000000e+002
vertex 0.000000e+000 0.000000e+000 0.000000e+000
vertex 2.000000e+002 0.000000e+000 1.000000e+002
endloop
endfacet
facet normal 0.000000e+000 1.000000e+000 0.000000e+000
outer loop
vertex 2.000000e+002 1.500000e+002 1.000000e+002
vertex 2.000000e+002 1.500000e+002 0.000000e+000
vertex 0.000000e+000 1.500000e+002 0.000000e+000
endloop
endfacet
facet normal 0.000000e+000 1.000000e+000 -0.000000e+000
outer loop
vertex 2.000000e+002 1.500000e+002 1.000000e+002
vertex 0.000000e+000 1.500000e+002 0.000000e+000
vertex 0.000000e+000 1.500000e+002 1.000000e+002
endloop
endfacet
facet normal 0.000000e+000 0.000000e+000 -1.000000e+000
outer loop
vertex 0.000000e+000 0.000000e+000 0.000000e+000
vertex 0.000000e+000 1.500000e+002 0.000000e+000
vertex 2.000000e+002 1.500000e+002 0.000000e+000
endloop
endfacet
facet normal 0.000000e+000 0.000000e+000 -1.000000e+000
outer loop
vertex 2.000000e+002 0.000000e+000 0.000000e+000
vertex 0.000000e+000 0.000000e+000 0.000000e+000
vertex 2.000000e+002 1.500000e+002 0.000000e+000
endloop
endfacet
facet normal 0.000000e+000 0.000000e+000 1.000000e+000
outer loop
vertex 2.000000e+002 1.500000e+002 1.000000e+002
vertex 0.000000e+000 1.500000e+002 1.000000e+002
vertex 0.000000e+000 0.000000e+000 1.000000e+002
endloop
endfacet
facet normal -0.000000e+000 0.000000e+000 1.000000e+000
outer loop
vertex 2.000000e+002 1.500000e+002 1.000000e+002
vertex 0.000000e+000 0.000000e+000 1.000000e+002
vertex 2.000000e+002 0.000000e+000 1.000000e+002
endloop
endfacet
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 int char 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 domains Standard_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 domain Message_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 explicitement Vnorm.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 triangles if (++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 domains Standard_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 domain Message_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 explicitement Vnorm.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 triangles if (++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 char file.Read(ach,sizeof(str),lread); IsAscii = Standard_True; for (i = 0; i< lread && IsAscii; ++i) { if (str[i] > '~') { IsAscii = Standard_False; } }#ifdef DEB cout << (IsAscii ? "ascii\n" : "binary\n");#endif file.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 size Standard_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 integer NBFACET = ((filesize - HEADER_SIZE) / SIZEOF_STL_FACET); // skip the header theFile.Seek(HEADER_SIZE,OSD_FromBeginning); // create the StlMesh_Mesh object Handle(StlMesh_Mesh) ReadMesh = new StlMesh_Mesh (); ReadMesh->AddDomain (); for (ifacet=1; ifacet<=NBFACET; ++ifacet) { // read normal coordinates fx = ReadFloat2Double(theFile); fy = ReadFloat2Double(theFile); fz = ReadFloat2Double(theFile); // read vertex 1 fx1 = ReadFloat2Double(theFile); fy1 = ReadFloat2Double(theFile); fz1 = ReadFloat2Double(theFile); // read vertex 2 fx2 = ReadFloat2Double(theFile); fy2 = ReadFloat2Double(theFile); fz2 = ReadFloat2Double(theFile); // read vertex 3 fx3 = 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 bytes theFile.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_ShortReal x[4],y[4],z[4]; Standard_Integer i1,i2,i3; Handle(StlMesh_Mesh) ReadMesh; thePath.SystemName (filename); // Open the file FILE* 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 lines for (ipos = 0; ipos < filesize; ++ipos) { if (getc(file) == '\n') nbLines++; } // compute number of triangles nbTris = (nbLines / ASCII_LINES_PER_FACET); // go back to the beginning of the file// fclose(file);// file = fopen(filename.ToCString(),"r"); rewind(file); // skip header while (getc(file) != '\n');#ifdef DEB cout << "start mesh\n";#endif ReadMesh = new StlMesh_Mesh(); ReadMesh->AddDomain(); // main reading Message_ProgressSentry aPS (theProgInd, "Triangles", 0, (nbTris - 1) * 1.0 / IND_THRESHOLD, 1); for (iTri = 0; iTri < nbTris && aPS.More();) { // reading the facet normal fscanf(file,"%*s %*s %f %f %f\n",&x[0],&y[0],&z[0]); // skip the keywords "outer loop" fscanf(file,"%*s %*s"); // reading vertex fscanf(file,"%*s %f %f %f\n",&x[1],&y[1],&z[1]); fscanf(file,"%*s %f %f %f\n",&x[2],&y[2],&z[2]); fscanf(file,"%*s %f %f %f\n",&x[3],&y[3],&z[3]); // here the facet must be built and put in the mesh datastructure i1 = ReadMesh->AddOnlyNewVertex ((Standard_Real)x[1],(Standard_Real)y[1],(Standard_Real)z[1]); i2 = ReadMesh->AddOnlyNewVertex ((Standard_Real)x[2],(Standard_Real)y[2],(Standard_Real)z[2]); i3 = ReadMesh->AddOnlyNewVertex ((Standard_Real)x[3],(Standard_Real)y[3],(Standard_Real)z[3]); ReadMesh->AddTriangle (i1,i2,i3,(Standard_Real)x[0],(Standard_Real)y[0],(Standard_Real)z[0]); // skip the keywords "endloop" fscanf(file,"%*s"); // skip the keywords "endfacet" fscanf(file,"%*s"); // update progress only per 1k triangles if (++iTri % IND_THRESHOLD == 0) aPS.Next(); }#ifdef DEB cout << "end mesh\n";#endif fclose(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;
static
const
int IND_THRESHOLD = 1000;
//
increment the indicator every 1k triangles
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