Author: Tomas Rutkauskas

When I override the CreateParams method of the TForm class and put an "or" clause 
to include a WS_EX_TRANSPARENT effect, all work fine. My Form appears with a 
transparent effect (or better, it does not appear. Only controls appear). But if I 
change the icons positions on the Desktop screen, my form does not get the changes 
and redraws itself. Well, I call redraw manually, but the form cannot get the 
desktop screen rectangle to correct the background of the form. How do I solve this 
problem? Is there a way to get a selected rectangle of the desktop screen to be 
copied to the canvas on the form?

Answer:

Here's a transparent form method that uses regions. It's so transparent, you can 
click right through to the underlying windows:

unit Unit1;

{The transparent form effect is done with regions. First create a region that 
encompasses the entire form. Then, find the client area of the form (Client vs. 
non-Client) and combine with the full region with RGN_DIFF to make the borders and 
title bar visible.  Then create a region for each of the controls and combine them 
with the original (FullRgn) region.}

1   interface
2   
3   uses
4     Windows, Messages, SysUtils, Classes, Graphics, Controls, Forms, Dialogs, 
5   ExtCtrls, StdCtrls;
6   
7   type
8     TForm1 = class(TForm)
9       Button1: TButton;
10      Panel1: TPanel;
11      Button2: TButton;
12      procedure FormDestroy(Sender: TObject);
13      procedure FormCreate(Sender: TObject);
14      procedure Button1Click(Sender: TObject);
15      procedure Button2Click(Sender: TObject);
16      procedure FormResize(Sender: TObject);
17    private
18      { Private declarations }
19      procedure DoVisible;
20      procedure DoInvisible;
21    public
22      { Public declarations }
23    end;
24  
25  var
26    Form1: TForm1;
27    FullRgn, ClientRgn, CtlRgn: THandle;
28  
29  implementation
30  
31  {$R *.DFM}
32  
33  procedure TForm1.DoInvisible;
34  var
35    AControl: TControl;
36    A, Margin, X, Y, CtlX, CtlY: Integer;
37  begin
38    Margin := (Width - ClientWidth) div 2;
39    {First, get form region}
40    FullRgn := CreateRectRgn(0, 0, Width, Height);
41    {Find client area region}
42    X := Margin;
43    Y := Height - ClientHeight - Margin;
44    ClientRgn := CreateRectRgn(X, Y, X + ClientWidth, Y + ClientHeight);
45    {'Mask' out all but non-client areas}
46    CombineRgn(FullRgn, FullRgn, ClientRgn, RGN_DIFF);
47    {Now, walk through all the controls on the form and 'OR' them into the existing 
48  Full region}
49    for A := 0 to ControlCount - 1 do
50    begin
51      AControl := Controls[A];
52      if (AControl is TWinControl) or (AControl is TGraphicControl) then
53        with AControl do
54        begin
55          if Visible then
56          begin
57            CtlX := X + Left;
58            CtlY := Y + Top;
59            CtlRgn := CreateRectRgn(CtlX, CtlY, CtlX + Width, CtlY + Height);
60            CombineRgn(FullRgn, FullRgn, CtlRgn, RGN_OR);
61          end;
62        end;
63    end;
64    {When the region is all ready, put it into effect:}
65    SetWindowRgn(Handle, FullRgn, TRUE);
66  end;
67  
68  procedure TForm1.FormDestroy(Sender: TObject);
69  begin
70    {Clean up the regions we created}
71    DeleteObject(ClientRgn);
72    DeleteObject(FullRgn);
73    DeleteObject(CtlRgn);
74  end;
75  
76  procedure TForm1.DoVisible;
77  begin
78    {To restore complete visibility:}
79    FullRgn := CreateRectRgn(0, 0, Width, Height);
80    CombineRgn(FullRgn, FullRgn, FullRgn, RGN_COPY);
81    SetWindowRgn(Handle, FullRgn, TRUE);
82  end;
83  
84  procedure TForm1.FormCreate(Sender: TObject);
85  begin
86    {We start out as a transparent form ...}
87    DoInvisible;
88  end;
89  
90  procedure TForm1.Button1Click(Sender: TObject);
91  begin
92    {This button just toggles between transparent and not transparent}
93    if Button1.Caption = 'Show Form' then
94    begin
95      DoVisible;
96      Button1.Caption := 'Hide Form';
97    end
98    else
99    begin
100     DoInvisible;
101     Button1.Caption := 'Show Form';
102   end;
103 end;
104 
105 procedure TForm1.Button2Click(Sender: TObject);
106 begin
107   Application.Terminate;
108 end;
109 
110 procedure TForm1.FormResize(Sender: TObject);
111 begin
112   {Need to address the transparency if the form gets resized. Also, note that Form1 
113 scroll bars are set to VISIBLE/FALSE. I did that to save a little coding here....}
114   if Button1.Caption = 'Show Form' then
115     DoInvisible
116   else
117     DoVisible;
118 end;
119 
120 end.