MFC MDI: How to change the background of the MDI main frame?

[ovidiucucu]

Q: I have changed the MDI client background like in example shown above. Next, I have tried to draw a centered bitmap image, but encountered a strange behavior when resizing the frame window: the image is not entirely drawn or appears multiplied. I have tried both in 'WM_ERASEBKGND' and 'WM_PAINT' message handlers but without success. What I'm missing?

A: That happens because 'MDIClient' window class has not 'CS_VREDRAW' and 'CS_HREDRAW' styles (if 'CS_VREDRAW' and 'CS_HREDRAW' are set, the entire window is redrawn if a movement or size adjustment changes the width of the client area).

You can fix the problem by invalidate the client area in the 'WM_SIZE' message handler:

void CMDIClientWnd::OnSize(UINT nType, int cx, int cy) 
{
   CWnd::OnSize(nType, cx, cy);
   Invalidate(FALSE);
}

void CMDIClientWnd::OnPaint() 
{
   CPaintDC dc(this); // device context for painting

   CDC dcMem;
   dcMem.CreateCompatibleDC(&dc);
   CBitmap bitmap;
   bitmap.LoadBitmap(IDB_LOGO);
   CBitmap* pOldBitmap = dcMem.SelectObject(&bitmap);

   BITMAP bmp;
   bitmap.GetBitmap(&bmp);
   CRect rc;
   GetClientRect(rc);
   const int nWidth  = bmp.bmWidth;
   const int nHeight = bmp.bmHeight;
   const int x       = (rc.Width() / 2) - (nWidth / 2);
   const int y       = (rc.Height() / 2) - (nHeight / 2);

   dc.BitBlt(x, y, nWidth, nHeight, &dcMem, 0, 0, SRCCOPY);  

   dcMem.SelectObject(pOldBitmap);
}

Q: The centered bitmap drawn as in previous example flickers when resizing the frame window. Can be this avoided?

A: Yes.

• In 'WM_ERASEBKGND' do nothing, just return 'FALSE'
  
  

  BOOL CMDIClientWnd::OnEraseBkgnd(CDC* pDC) 
  {
     // All drawing moved in OnPaint
     return FALSE;
  }

• Perform all drawing (including filling with the brush) in WM_PAINT mesage handler using 'double buffering', i.e. draw in an additional memory device context, then copy the memory device context contents in the client device context.
  
  

  void CMDIClientWnd::OnPaint() 
  {
     CPaintDC dc(this);
     CRect rcClient;
     GetClientRect(rcClient);
  
     CDC dcMem1; // memory DC for double buffering
     dcMem1.CreateCompatibleDC(&dc);
     CBitmap bitmap1;
     bitmap1.CreateCompatibleBitmap(&dc, rcClient.Width(), rcClient.Height());
     CBitmap* pOldBitmap1 = dcMem1.SelectObject(&bitmap1);
     // fill memory device context with the desired brush
     dcMem1.FillRect(rcClient, &m_brush);
  
     CDC dcMem2; // memory DC for the bitmap to draw
     dcMem2.CreateCompatibleDC(&dc);
     CBitmap bitmap2;
     bitmap2.LoadBitmap(IDB_LOGO);
     CBitmap* pOldBitmap2 = dcMem2.SelectObject(&bitmap2);
  
     BITMAP bmp;
     bitmap2.GetBitmap(&bmp);
     int nWidth  = bmp.bmWidth;
     int nHeight = bmp.bmHeight;
     int x       = (rcClient.Width() / 2) - (nWidth / 2);
     int y       = (rcClient.Height() / 2) - (nHeight / 2);
     // copy the bitmap in memory device context 
     dcMem1.BitBlt(x, y, nWidth, nHeight, &dcMem2, 0, 0, SRCCOPY);
  
     CRect rcClip;
     dc.GetClipBox(rcClip);
  
     nWidth  = rcClip.Width();
     nHeight = rcClip.Height();
     x       = rcClip.left;
     y       = rcClip.top;
     // copy memory device context in client device context
     dc.BitBlt(x, y, nWidth, nHeight, &dcMem1, x, y, SRCCOPY); 
  
     dcMem1.SelectObject(pOldBitmap1);
     dcMem2.SelectObject(pOldBitmap2);
  }

See also

• A Custom MDI Client Class.