Fade in an alpha-blended PNG form in Delphi - delphi

Question

I asked a question about this some years back when Vista was first released, but never resolved the problem and shelved it as something to consider later.
I have a splash screen that I went to great effort to make look great. It's a 32bpp alpha-blended PNG. I have some code (which I can dig up if required!) that works great under Windows XP or under Vista+ when desktop composition is turned off. However, under Vista+ all the transparent parts are black, destroying everything that looks great about it!
So, my question is this: as anyone been able to display a 32bpp alpha-blended PNG as a splash screen in a way that works both with and without desktop composition activated? I'm not adverse to using third-party components if required, free or otherwise.
Ideally, this would work in Delphi 7.
Update: Besides the answers below, which work very well, I found that the TMS TAdvSmoothSplashScreen component also handles this task very well, if somewhat more complex.

Answer 1

Tim, I just tried this on Vista/D2007 with 'Windows Classic' theme selected:
Alpha Blended Splash Screen in Delphi - Part 2
http://melander.dk/articles/alphasplash2/2/
no black background that I could see... it still looks great.

Answer 2

The article Bob S links to gives the correct answer. Since that article contains quite a bit extra information that you actually need, here is the form/unit I create through it (Note that you'll need the GraphicEx library from here:
unit Splash2Form;
interface
uses
Windows, Messages, SysUtils, Variants, Classes, Graphics, Controls, Forms,
Dialogs, ExtCtrls, GraphicEx;
type
TSplash2 = class(TForm)
private
{ Private declarations }
procedure PreMultiplyBitmap(Bitmap: TBitmap);
public
constructor Create(Owner: TComponent);override;
{ Public declarations }
procedure CreateParams(var Params: TCreateParams);override;
procedure Execute;
end;
var
Splash2: TSplash2;
implementation
{$R *.dfm}
{ TSplash2 }
constructor TSplash2.Create(Owner: TComponent);
begin
inherited;
Brush.Style := bsClear;
end;
procedure TSplash2.CreateParams(var Params: TCreateParams);
begin
inherited;
end;
procedure TSplash2.Execute;
var exStyle: DWORD;
BitmapPos: TPoint;
BitmapSize: TSize;
BlendFunction: TBlendFunction;
PNG: TPNGGraphic;
Stream: TResourceStream;
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);
PNG := TPNGGraphic.Create;
try
Stream := TResourceStream.Create(HInstance, 'SPLASH', RT_RCDATA);
try
PNG.LoadFromStream(Stream);
finally
Stream.Free;
end;
PreMultiplyBitmap(PNG);
ClientWidth := PNG.Width;
ClientHeight := PNG.Height;
BitmapPos := Point(0, 0);
BitmapSize.cx := ClientWidth;
BitmapSize.cy := ClientHeight;
// Setup alpha blending parameters
BlendFunction.BlendOp := AC_SRC_OVER;
BlendFunction.BlendFlags := 0;
BlendFunction.SourceConstantAlpha := 255;
BlendFunction.AlphaFormat := AC_SRC_ALPHA;
// ... and action!
UpdateLayeredWindow(Handle, 0, nil, #BitmapSize, PNG.Canvas.Handle,
#BitmapPos, 0, #BlendFunction, ULW_ALPHA);
Show;
finally
PNG.Free;
end;
end;
procedure TSplash2.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;
end.