i cannot call pDC->SetMapMode(MM_LOMETRIC)?

OpenGL OpenGL: Advanced Coding
1

void CMyGLView::OnPrepareDC(CDC* pDC, CPrintInfo* pInfo)
{
pDC->SetMapMode(MM_LOMETRIC);
CView::OnPrepareDC(pDC, pInfo);
}
when i call "pDC->SetMapMode(MM_LOMETRIC);
"in my OpenGL/MFC,no line drawed out.
why?