个人写的双缓冲 CDC绘图函数

void CDRImageStatic::ShowImg(CDC *dc, CRect rect)
{
 CBitmap bmp;
    bmp.CreateCompatibleBitmap(dc, m_nWidth, m_nHeight);
    CBitmap* pOld=NULL;
 CDC memDC;
 memDC.CreateCompatibleDC( dc );
 if (m_nColor!=NULL)
 {  
  EnterCriticalSection(&csImgCriticalSection);
  bmp.SetBitmapBits(sizeof(*m_nColor)*m_nHeight*m_nWidth, m_nColor);
  LeaveCriticalSection(&csImgCriticalSection); 
  pOld = memDC.SelectObject( &bmp );  
 }
 if((m_nDXA==0))
 {
  EnterCriticalSection(&csMyCriticalSection);
  int oldmode=memDC.SetBkMode(TRANSPARENT);
  CPen * pOldPen=memDC.SelectObject(&m_Pen);  
  memDC.MoveTo(rect.right-m_nCurPosX,0);
  memDC.LineTo(rect.right-m_nCurPosX,rect.bottom); 
  memDC.SelectObject(pOldPen);
  memDC.SetBkMode(oldmode);
  BOOL bbR=m_nCurPosX!=m_nOldPosX;
  if (!this->m_blCreateLine || bbR)
  {
   this->m_blCreateLine=TRUE;
   m_nOldPosX=m_nCurPosX;
   InvalidateRect(rect,FALSE);
  }
  memDC.MoveTo(CPoint(0,0));
  LeaveCriticalSection(&csMyCriticalSection);
 }
 dc->BitBlt( rect.left, rect.top, rect.Width(), rect.Height(), &memDC, 0, 0, SRCCOPY );
 if (pOld!=NULL)
 {
  memDC.SelectObject(pOld);
 }
 memDC.DeleteDC();
 bmp.DeleteObject();
}