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function GetScreenShot(form:TForm): TBitmap;
var
  Desktop: HDC;
begin
  Result  := TBitmap.Create;
  form.Visible:=false;
  Desktop := GetDC(0);
  try
    try
      Result.PixelFormat := pf32bit;
      Result.Width := form.Width; //Screen.Width
      Result.Height := form.Width; //Screen.Height
      BitBlt(Result.Canvas.Handle, 0, 0, Result.Width, Result.Height,Desktop,0,0,SRCCOPY);
      Result.Modified := True;
      form.Visible:=true;
    finally
      ReleaseDC(0, Desktop);
    end;
  except
    Result.Free;
    form.Visible:=true;
    Result := nil;
  end;
end;

procedure makeAlpha(form:TForm;alpha:integer);
var Overlay: TBitmap;
    BlendFunction: TBlendFunction;
    t1,t2:ttime;
begin
     t1:=time;

     Overlay := TBitmap.Create;
     Overlay := GetScreenShot(form);
     {Overlay.Height := Form1.Height; //Change these 500s to 1 or 2500 or 499
     Overlay.Width := Form1.Width; //and you can see the time difference!
    overlay.canvas.brush.color:=$33CC66;
    overlay.canvas.fillrect(rect(0,0,overlay.width,overlay.height));}

     BlendFunction.AlphaFormat := 0;
     BlendFunction.BlendFlags := 0;
     BlendFunction.BlendOp := AC_SRC_OVER;
     BlendFunction.SourceConstantAlpha := alpha; //(0..255)  128

     Windows.AlphaBlend(Form.Canvas.Handle, 0, 0, Form.Height, Form.Width,
                         Overlay.Canvas.Handle, 0, 0, Overlay.Width, Overlay.Height, BlendFunction);
     t2:=time;
     showmessage(floatTostr(t2-t1));

     Overlay.Free;
end;					

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const
LWA_ALPHA = $2;
WS_EX_LAYERED = $80000;

function SetLayeredWindowAttributes (const hWnd : HWND;
crKey : Integer; bAlpha : Byte;
dwFlags : Integer) : Integer; stdcall; external 'user32.dll';

procedure TForm1.FormCreate(Sender: TObject);
var temp : Integer;
begin
temp := GetWindowLong(Handle, GWL_EXSTYLE);
temp := temp or WS_EX_LAYERED;
SetWindowLong(Handle, GWL_EXSTYLE, temp);
SetLayeredWindowAttributes(Handle, 0, 200, LWA_ALPHA);
end;

//erst ab Windows 2000