Author: Erwin Molendijk

How do I give my MDI window a background image or tile?

Answer:

This is a handy trick I found somewhere: 

Put an image called Image1 on your main form. 
Add the following routine to your main form: 

Make sure you have the following variables in your main form object: 
1   
2   FClientInstance: TFarProc;
3   FPrevClientProc: TFarProc;
4   
5   { MDI Background code }
6   
7   procedure TMainForm.ClientWndProc(var message: TMessage);
8   var
9     Dc: hDC;
10    Row: Integer;
11    Col: Integer;
12  begin
13    with message do
14      case Msg of
15        WM_ERASEBKGND:
16          begin
17            Dc := TWMEraseBkGnd(message).Dc;
18            // Tile Image on DC
19            for Row := 0 to ClientHeight div Image1.Picture.Height do
20              for Col := 0 to ClientWidth div Image1.Picture.Width do
21                BitBlt(Dc,
22                  Col * Image1.Picture.Width,
23                  Row * Image1.Picture.Height,
24                  Image1.Picture.Width,
25                  Image1.Picture.Height,
26                  Image1.Picture.Bitmap.Canvas.Handle,
27                  0,
28                  0,
29                  SRCCOPY);
30            Result := 1;
31          end;
32      else // Pass on other msg's
33        Result := CallWindowProc(FPrevClientProc,
34          ClientHandle,
35          Msg,
36          wParam,
37          lParam);
38      end;
39  end;


And put this in your mainform OnShow event: 

// MDI background tiles stuff, chain in de WndProc.
FClientInstance := MakeObjectInstance(ClientWndProc);
FPrevClientProc := Pointer(GetWindowLong(ClientHandle, GWL_WNDPROC));
SetWindowLong(ClientHandle, GWL_WNDPROC, LongInt(FClientInstance));

You now have a background!