vc++ mfc 里保存缩放的bmp图片 不失真

void CSaveView::OnFileSave()
{
 BITMAP info;//原始图片
 m_bitmap.GetBitmap(&info);
 CDC DC1;
 DC1.CreateCompatibleDC(NULL);
 DC1.SelectObject(&m_bitmap);

 DC1.MoveTo(,);
 DC1.LineTo(info.bmWidth,info.bmHeight);

 CDC DC2;
 CBitmap bmp;//缩放后的图片
 bmp.CreateCompatibleBitmap(&DC1,,);
 DC2.CreateCompatibleDC(NULL);
 DC2.SelectObject(&bmp);
 int mode = SetStretchBltMode(DC2, COLORONCOLOR); //设置不失真缩放
 DC2.StretchBlt(,,,,&DC1,,,info.bmWidth,info.bmHeight,SRCCOPY);
 SetStretchBltMode(DC2, mode); 

 CFileDialog dlg(false,"bmp",NULL,OFN_HIDEREADONLY | OFN_OVERWRITEPROMPT,NULL,this);
 if(IDOK == dlg.DoModal())
 {
  　　　 CString bmpfile = dlg.GetPathName();
        CFile file(bmpfile,CFile::modeCreate|CFile::modeWrite);
        BITMAP bInfo;
　　　　 bmp.GetBitmap(&bInfo);
        //计算调色板大小
        int panelsize = ;
        if (bInfo.bmBitsPixel<) //非真彩色
        {
            panelsize = int(pow((double),bInfo.bmBitsPixel)*sizeof(RGBQUAD));
        }     

        //定义位图信息
        BITMAPINFO*  bMapInfo = (BITMAPINFO*)LocalAlloc(LPTR,sizeof(BITMAPINFOHEADER)+panelsize);
        bMapInfo->bmiHeader.biBitCount = bInfo.bmBitsPixel;
        bMapInfo->bmiHeader.biClrImportant = ;
        bMapInfo->bmiHeader.biCompression = ;
        bMapInfo->bmiHeader.biHeight = bInfo.bmHeight;
        bMapInfo->bmiHeader.biPlanes = bInfo.bmPlanes;
        bMapInfo->bmiHeader.biSize = sizeof(BITMAPINFO);
        bMapInfo->bmiHeader.biSizeImage = bInfo.bmHeight*bInfo.bmWidthBytes;
        bMapInfo->bmiHeader.biWidth = bInfo.bmWidth;
        bMapInfo->bmiHeader.biXPelsPerMeter = ;
        bMapInfo->bmiHeader.biYPelsPerMeter = ;     

        //获取位图的实际数据
        char* pData = new char[bMapInfo->bmiHeader.biSizeImage];
 　　　　int len = GetDIBits(DC2.m_hDC,bmp,,bInfo.bmHeight,pData,bMapInfo,DIB_RGB_COLORS); 

        BITMAPFILEHEADER bFileHeader;
        bFileHeader.bfType = 0x4D42;
        bFileHeader.bfReserved1 = ;
        bFileHeader.bfReserved2 = ;
        bFileHeader.bfSize = sizeof(BITMAPFILEHEADER);
        bFileHeader.bfOffBits = sizeof(BITMAPFILEHEADER)+sizeof(BITMAPINFOHEADER)+panelsize;   

        //向文件中写入位图数据
        file.Write(&bFileHeader,sizeof(BITMAPFILEHEADER));
        file.Write(&bMapInfo->bmiHeader,sizeof(BITMAPINFOHEADER));
        file.Write(pData,bMapInfo->bmiHeader.biSizeImage+panelsize);
        file.Close();
        delete pData;
        LocalFree(bMapInfo);
　　}
}