procedure TForm1.Button1Click(Sender: TObject);
const
  count = 100;
var
  i: integer;
  x, y: integer;
  bm, bm1, bm2: TBitMap;
  p1, p2, p: PByteArray;
  c: integer;
  k: integer;
begin
  bm := TBitMap.Create;
  bm1 := TBitMap.Create;
  bm2 := TBitMap.Create;
  bm1.LoadFromFile('Bitmap1.bmp');
  bm2.LoadFromFile('Bitmap2.bmp');
  if bm1.Height < bm2.Height then
  begin
    bm.Height := bm1.Height;
    bm2.Height := bm1.Height;
  end
  else
  begin
    bm.Height := bm2.Height;
    bm1.Height := bm2.Height;
  end;
  if bm1.Width < bm2.Width then
  begin
    bm.Width := bm1.Width;
    bm2.Width := bm1.Width;
  end
  else
  begin
    bm.Width := bm2.Width;
    bm1.Width := bm2.Width;
  end;
  bm.PixelFormat := pf24bit;
  bm1.PixelFormat := pf24bit;
  bm2.PixelFormat := pf24bit;

  Form1.Canvas.Draw(0, 0, bm1);
  for i := 1 to count - 1 do
  begin
    for y := 0 to bm.Height - 1 do
    begin
      p := bm.ScanLine[y];
      p1 := bm1.ScanLine[y];
      p2 := bm2.ScanLine[y];
      for x := 0 to bm.Width * 3 - 1 do
        p^[x] := round((p1^[x] * (count - i) + p2^[x] * i) / count);
    end;
    Form1.Canvas.Draw(0, 0, bm);
    Form1.Caption := IntToStr(round(i / count * 100)) + '%';
    Application.ProcessMessages;
    if Application.Terminated then
      break;
  end;
  Form1.Canvas.Draw(0, 0, bm2);
  Form1.Caption := 'done';
  bm1.Destroy; bm2.Destroy; bm.Destroy;
end;