无需cad，从dwg文件中读取缩略图

using namespace SNXH;
using namespace SNMFC;
//无需cad，从dwg中读取缩略图，有的dwg没缩略图,CAD2014打开界面，可以看到缩略图
void ReadThumbnailBmp(const CString strBmpFileName,const CString& strDWGFileName)
{
 CFile file;
 if( !file.Open(strDWGFileName,CFile::modeRead))
 {
  return ;
 }
 int iPosSentinel = 0 ;
 file.Seek(13,CFile::begin);
 NFile::Read(file,iPosSentinel);
 unsigned char iNum = 0 ;
 file.Seek(iPosSentinel+20,CFile::begin);
 NFile::Read(file,iNum);
 for( int i = 0 ; i < iNum ; i++)
 {
  unsigned char iType = 0;
  int iPos = 0 ,iLen = 0 ;
  NFile::Read(file,iType);
  NFile::Read(file,iPos);
  NFile::Read(file,iLen);
  if( 2 == iType )
  {
   file.Seek(iPos,CFile::begin);
   CArray<unsigned char,unsigned char> v;
   v.SetSize(iLen);
   file.Read(v.GetData(),iLen);
   {
    BITMAPFILEHEADER fileHead;
    CBmp::InitFileHead(fileHead,iLen);
    CFile f2;
    f2.Open(strBmpFileName,CFile::modeCreate|CFile::modeWrite);
    f2.Write(&fileHead,sizeof(fileHead));
    f2.Write(v.GetData(),iLen);
    f2.Close();
   }
   break;
  }
 }
 file.Close();
}
void CSNPictureDlg::OnButton1()
{
 ReadThumbnailBmp(_T("d:\\3.dwg"),_T("d:\\1.bmp") ); 
}