Transparent forms

In the previous article we discussed about using of Windows 2000 function SetLayeredWindowAttributes for
creating transparent forms in Delphi.

const
WS_EX_LAYERED = $80000;
LWA_COLORKEY = 1;
LWA_ALPHA = 2;

type
TSetLayeredWindowAttributes = function (
hwnd : HWND; // handle to the layered window
crKey : TColor; // specifies the color key
bAlpha : byte; // value for the blend function
dwFlags : DWORD // action
): BOOL; stdcall;

Now I would like to talk about practical using of the layered windows.

Tip N1: Capturing screen

If you will try to capture the screen to bitmap using BitBlt function and have a layered (transparent) window visible, you will get only a picture of the screen without this window.
To fix it we have to use the new raster-operation code for BitBlt function CAPTUREBLT that introduced in Windows 2000 for including any windows that are layered on top of your window in the resulting image.

procedure CaptureScreen(AFileName : string);
const
CAPTUREBLT = $40000000;
var
hdcScreen : HDC;
hdcCompatible : HDC;
bmp : TBitmap;
hbmScreen : HBITMAP;
begin
// Create a normal DC and a memory DC for the entire screen. The
// normal DC provides a "snapshot" of the screen contents. The
// memory DC keeps a copy of this "snapshot" in the associated
// bitmap.

hdcScreen := CreateDC('DISPLAY', nil, nil, nil);
hdcCompatible := CreateCompatibleDC(hdcScreen);
// Create a compatible bitmap for hdcScreen.

hbmScreen := CreateCompatibleBitmap(hdcScreen,
GetDeviceCaps(hdcScreen, HORZRES),
GetDeviceCaps(hdcScreen, VERTRES));

// Select the bitmaps into the compatible DC.
SelectObject(hdcCompatible, hbmScreen);
bmp := TBitmap.Create;
bmp.Handle := hbmScreen;
BitBlt(hdcCompatible,
0,0,
bmp.Width, bmp.Height,
hdcScreen,
0,0,
SRCCOPY OR CAPTUREBLT);

bmp.SaveToFile(AFileName);
bmp.Free;
DeleteDC(hdcScreen);
DeleteDC(hdcCompatible);
end;

Tip N2: Transparent external program

OK, before we made our own forms transparent, but what about making external programs like Notepad transparent also? Yes, it's also possible.

First, we will start Notepad using ShellExecute function

ShellExecute(Application.Handle,'open','notepad.exe',nil,nil,SW_SHOW);

and after will make it transparent.

procedure ShowTransparentNotepad;
var
np:HWND;
begin
np:=FindWindow('Notepad',nil);
SetTransparentForm(np,100);
end;

The code of the SetTransparentForm procedure you can find in article Transparent Forms in Delphi 5 and 6

Tip N3: Transparent Hint

For customizing the standard Delphi hint window we have to override THintWindow class. After overriding THintWindow to create a new derived type TTransparentHint, we will assign the new type to the global HintWindowClass variable at application startup, so that the new transparent hint window type is used for Help Hints.

type
TTransparentHint = class(THintWindow)
private
FTransparency : byte;
protected
procedure CreateParams(var Params: TCreateParams); override;
procedure CreateWnd; override;
public
property Transparency : byte read FTransparency write FTransparency;
constructor Create(AOwner : TComponent); override;
end;

{ TTransparentHint }

constructor TTransparentHint.Create(AOwner: TComponent);
begin
inherited;
FTransparency := 100;
end;

procedure TTransparentHint.CreateParams(var Params: TCreateParams);
begin
inherited;
Params.ExStyle := Params.ExStyle or WS_EX_LAYERED;
end;

procedure TTransparentHint.CreateWnd;
var
SetLayeredWindowAttributes: TSetLayeredWindowAttributes;
begin
inherited;
SetLayeredWindowAttributes :=
GetProcAddress(GetModulehandle(user32), 'SetLayeredWindowAttributes');
SetLayeredWindowAttributes(Handle, 0, FTransparency, LWA_ALPHA);
end;

Now we have to assign TTransparentHint to the HintWindowClass variable at application startup.

procedure TForm1.FormCreate(Sender: TObject);
begin
HintWindowClass := TTransparentHint;
end;