Show controls on PNG Alpha Blended Form

Hi there,

Please see my code attached below - this allows me to show a PNG graphic as a alphablended splash screen. This only shows my bitmap, I need to see other controls on my form and be able to click on them, but still retain the alphablended effect. I think the problem lies in that the form style is set to WS_EX_LAYERED.

uses
  GraphicEx;
...
procedure PremultiplyBitmap(Bitmap: TBitmap);
var
  Row, Col: integer;
  p: PRGBQuad;
  PreMult: array[byte, byte] of byte;
begin
  // Precalculate all possible values of a*b
  for Row := 0 to 255 do
    for Col := Row to 255 do
    begin
      PreMult[Row, Col] := Row * Col div 255;
      if (Row <> Col) then
        PreMult[Col, Row] := PreMult[Row, Col]; // a*b = b*a
    end;

  for Row := 0 to Bitmap.Height-1 do
  begin
    Col := Bitmap.Width;
    p := Bitmap.ScanLine[Row];
    while (Col > 0) do
    begin
      p.rgbBlue := PreMult[p.rgbReserved, p.rgbBlue];
      p.rgbGreen := PreMult[p.rgbReserved, p.rgbGreen];
      p.rgbRed := PreMult[p.rgbReserved, p.rgbRed];
      inc(p);
      dec(Col);
    end;
  end;
end;

procedure TFormSplash.Execute;
var
  Ticks: DWORD;
  BlendFunction: TBlendFunction;
  BitmapPos: TPoint;
  BitmapSize: TSize;
  exStyle: DWORD;
  Bitmap: TBitmap;
  Stream: TStream;

begin
  // Enable window layering
  exStyle := GetWindowLongA(Handle, GWL_EXSTYLE);
  if (exStyle and WS_EX_LAYERED = 0) then
    SetWindowLong(Handle, GWL_EXSTYLE, exStyle or WS_EX_LAYERED);

  // Bitmap := TBitmap.Create;
  Bitmap := TPNGGraphic.Create;
  try
    //Bitmap.LoadFromFile('DLLSplashForm\splash.png');
    Stream := TResourceStream.Create(HInstance, 'SPLASH', RT_RCDATA);
    try
      Bitmap.LoadFromStream(Stream);
    finally
      Stream.Free;
    end;

    ASSERT(Bitmap.PixelFormat = pf32bit, 'Wrong bitmap format - must be 32 bits/pixel');

    // Perform run-time premultiplication
    PremultiplyBitmap(Bitmap);

    // Resize form to fit bitmap
    ClientWidth := Bitmap.Width;
    ClientHeight := Bitmap.Height;

    // Position bitmap on form
    BitmapPos := Point(0, 0);
    BitmapSize.cx := Bitmap.Width;
    BitmapSize.cy := Bitmap.Height;

    // Setup alpha blending parameters
    BlendFunction.BlendOp := AC_SRC_OVER;
    BlendFunction.BlendFlags := 0;
    BlendFunction.SourceConstantAlpha := 0; // Start completely transparent
    BlendFunction.AlphaFormat := AC_SRC_ALPHA;

    Show;
    Width := 0;
    Height := 0;
    // ... and action!
    Ticks := 0;
    while (BlendFunction.SourceConstantAlpha < 255) do
    begin
      while (Ticks = GetTickCount) do
        Sleep(25); // Don't fade too fast
      Ticks := GetTickCount;
      inc(BlendFunction.SourceConstantAlpha,
        (255-BlendFunction.SourceConstantAlpha) div 32+1); // Fade in
      UpdateLayeredWindow(Handle, 0, nil, @BitmapSize, Bitmap.Canvas.Handle,
        @BitmapPos, 0, @BlendFunction, ULW_ALPHA);
    end;
  finally
    Bitmap.Free;
  end;
end;