Question

I have a “caution” image on a dialog that is shown if there are questionable parameter values. Users do not always notice it, so I want to fade it in and out cyclically over a second or so (yes, I could just toggle the Visible property, but that would look a bit like I was just toggling the Visible property). Is there a simpler way than putting it on it’s own form and floating it over the dialog (and changing the AlphaBlendValue property of the form)?

Answer 1

You can do this using the Opacity parameter of TCanvas.Draw. Behind the scenes this calls TGraphic.DrawTransparent which in turn calls the Windows AlphaBlend API function. An easy way to implement this is with a TPaintBox:

procedure TAlphaBlendForm.FormCreate(Sender: TObject);
begin
  FBitmap := TBitmap.Create;
  FBitmap.Assign(Image1.Picture.Graphic);//Image1 contains a transparent PNG
  PaintBox1.Width := FBitmap.Width;
  PaintBox1.Height := FBitmap.Height;
  Timer1.Interval := 20;
end;

procedure TAlphaBlendForm.PaintBox1Paint(Sender: TObject);
begin
  PaintBox1.Canvas.Draw(0, 0, FBitmap, FOpacity);
end;

procedure TAlphaBlendForm.Timer1Timer(Sender: TObject);
begin
  FOpacity:= (FOpacity+1) mod 256;
  PaintBox1.Invalidate;
end;

If you are using an older version of Delphi without the Opacity parameter of TCanvas.Draw you can use AlphaBlend directly.

procedure TAlphaBlendForm.PaintBox1Paint(Sender: TObject);
var
  fn: TBlendFunction;
begin
  fn.BlendOp := AC_SRC_OVER;
  fn.BlendFlags := 0;
  fn.SourceConstantAlpha := FOpacity;
  fn.AlphaFormat := AC_SRC_ALPHA;
  Windows.AlphaBlend(
    PaintBox1.Canvas.Handle,
    0,
    0,
    PaintBox1.Width,
    PaintBox1.Height,
    FBitmap.Canvas.Handle,
    0,
    0,
    FBitmap.Width,
    FBitmap.Height,
    fn
  );
end;

Thanks to Giel for suggesting the Opacity parameter of TCanvas.Draw, and for Sertac for pointing out that it is quite a recent addition to TCanvas.Draw.