Make a Form semi-/ transparent

Make your forms 25%, 50% Transparent…

TMyForm = class (TForm);

TrackBar1: TTrackBar;
//..
private
FColorKey: TCOLOR;

end;

const
  // Use crKey as the transparency color.
  LWA_COLORKEY = 1;
  // Use bAlpha to determine the opacity of the layered window..
  LWA_ALPHA     = 2;
  WS_EX_LAYERED = $80000;

implementation

{$R *.DFM}

// The SetLayeredWindowAttributes function sets the opacity and transparency
// color key of a layered window.
// Note : This function is only available with Windows 2000 and XP!
function SetLayeredWindowAttributes(
  // Handle to the layered window.
  Wnd: hwnd;
  // Pointer to a COLORREF value that specifies the transparency
  // color key to be used when composing the layered window
  crKey: ColorRef;
  // Alpha value used to describe the opacity of the layered window
  Alpha: Byte;
  // Specifies an action to take
  // LWA_COLORKEY or LWA_ALPHA
  // This parameter can be one or more of the following values:
  dwFlags: DWORD): Boolean;
  stdcall; external 'user32.dll';


procedure TMyForm.TrackBar1Change(Sender: TObject);
  // Trackbar.Position must run at range 1-255...
begin
  SetWindowLong(Handle, GWL_EXSTYLE, GetWindowLong(Handle, GWL_EXSTYLE) or WS_EX_LAYERED);
  SetLayeredWindowAttributes(Handle, ColorToRGB(FColorKey), TrackBar1.Position, LWA_ALPHA);
end;

{******************************************************************}

{ To load the SetLayeredWindowAttributes() function dynamically, use this function:}

function MakeWindowTransparent(Wnd: HWND; nAlpha: Integer = 10): Boolean;
type
  TSetLayeredWindowAttributes = function(hwnd: HWND; crKey: COLORREF; bAlpha: Byte;
    dwFlags: Longint): Longint;
  stdcall;
const
  // Use crKey as the transparency color.
  LWA_COLORKEY = 1;
  // Use bAlpha to determine the opacity of the layered window..
  LWA_ALPHA     = 2;
  WS_EX_LAYERED = $80000;
var
  hUser32: HMODULE;
  SetLayeredWindowAttributes: TSetLayeredWindowAttributes;
  i: Integer;
begin
  Result := False;
  // Here we import the function from USER32.DLL
  hUser32 := GetModuleHandle('USER32.DLL');
  if hUser32 <> 0 then
  begin @SetLayeredWindowAttributes := GetProcAddress(hUser32, 'SetLayeredWindowAttributes');
    // If the import did not succeed, make sure your app can handle it!
    if @SetLayeredWindowAttributes <> nil then
    begin
      // Check the current state of the dialog, and then add the WS_EX_LAYERED attribute
      SetWindowLong(Wnd, GWL_EXSTYLE, GetWindowLong(Wnd, GWL_EXSTYLE) or WS_EX_LAYERED);
      // The SetLayeredWindowAttributes function sets the opacity and
      // transparency color key of a layered window
      SetLayeredWindowAttributes(Wnd, 0, Trunc((255 / 100) * (100 - nAlpha)), LWA_ALPHA);
      Result := True;
    end;
  end;
end;