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(0,0);
DC1.LineTo(info.bmWidth,info.bmHeight);

CDC DC2;
CBitmap bmp;//缩放后的图片
bmp.CreateCompatibleBitmap(&DC1,200,200);
DC2.CreateCompatibleDC(NULL);
DC2.SelectObject(&bmp);
int mode = SetStretchBltMode(DC2, COLORONCOLOR); //设置不失真缩放
DC2.StretchBlt(0,0,200,200,&DC1,0,0,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 = 0;
if (bInfo.bmBitsPixel<24) //非真彩色
{
panelsize = int(pow((double)2,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 = 0;
bMapInfo->bmiHeader.biCompression = 0;
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 = 0;
bMapInfo->bmiHeader.biYPelsPerMeter = 0;

//获取位图的实际数据
char* pData = new char[bMapInfo->bmiHeader.biSizeImage];
　　　　int len = GetDIBits(DC2.m_hDC,bmp,0,bInfo.bmHeight,pData,bMapInfo,DIB_RGB_COLORS);

BITMAPFILEHEADER bFileHeader;
bFileHeader.bfType = 0x4D42;
bFileHeader.bfReserved1 = 0;
bFileHeader.bfReserved2 = 0;
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);
　　}
}