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procedure Blend(FromBit, ToBit, NewBit: TBitmap);
var
  pt, pf, pn: PByteArray;
  h, w: Integer;
begin
  for h:=0 to ToBit.Height-1 do
   begin
    pt:= ToBit.ScanLine[h];
    pf:= FromBit.ScanLine[h];
    pn:= NewBit.ScanLine[h];
    for w:=0 to (ToBit.Width *3)-1 do
     pn^[w]:= round(pf^[w] - ((pf^[w]-pt^[w]) / 100 * ws));
  end;
end;

procedure TForm1.Button1Click(Sender: TObject);
var
 bg,window,kl : TBitmap;
begin
 bg := TBitmap.Create;
 window := TBitmap.Create;
 kl := TBitmap.Create;
 bg.LoadFromfile('F:\My Delphi\bali\pics\hintergrund_strand.bmp');
 window.LoadFromfile('F:\My Delphi\bali\pics\windows\taskbar_top.bmp');
 window.transparentcolor := clwhite;
 window.transparent := true;
 Image1.picture.LoadFromfile('F:\My Delphi\bali\pics\hintergrund_strand.bmp');
 Image2.canvas.CopyRect(Rect(0,0,400,400),Image1.Canvas,Rect(0,0,400,400));
 Image2.picture.SaveToFile('c:\bali.bmp');
 kl.loadfromfile('C:\bali.bmp');
 image2.picture.Assign(kl);
// Image2.picture.bitmap := kl;
 Blend(Image2.Picture.Bitmap,window,Image1.picture.Bitmap);
end;					

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    for w:=0 to (ToBit.Width *3)-1 do
     pn^[w]:= round(pf^[w] - ((pf^[w]-pt^[w]) / 100 * ws));