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
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_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();)
{
char x[256]="", y[256]="", z[256]="";
// reading the facet normal
if (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 vertex
if (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 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;
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
