Using StretchBlt to reduce a bitmap size by 50%

I want to view a loaded 8bit bitmap on the screen.  

I want to shrink the bitmap by 50%

Using the StretchBlt (in code below) I specify the bitmap height and width of the destination to be 1/2 that of the source.

The bitmap is drawn however the quality is reduced why? How can I retain the quality to the naked eye?



void CImageDialog::ShowBitmap(CPaintDC* pdc)
{
      CDemoDlg * pWnd = (CDemoDlg*)GetParent();
      BITMAP bm;
      CDC dcMem;
      CRect rRect;


      pWnd->m_bmpBitmap.GetBitmap(&bm);
      dcMem.CreateCompatibleDC(pdc);
      dcMem.SelectObject(pWnd->m_bmpBitmap);
      
      GetClientRect(rRect);
      rRect.NormalizeRect();
      
      
      pdc->StretchBlt(0,0,bm.bmWidth/2,bm.bmHeight/2,&dcMem,0,0,bm.bmWidth,bm.bmHeight,SRCCOPY);
      }Start Free Trial