procedure TForm1.Button1Click(Sender: TObject);
 var
 b1, b2: TBitmap;
 c1, c2: PByte;
 x, y, i,
 different: Integer; // Counter for different pixels 
begin
 b1 := Image1.Picture.Bitmap;
 b2 := Image2.Picture.Bitmap;
 Assert(b1.PixelFormat = b2.PixelFormat); // they have to be equal 
 different := 0;
 for y := 0 to b1.Height - 1 do
 begin
 c1 := b1.Scanline[y];
 c2 := b2.Scanline[y];
 for x := 0 to b1.Width - 1 do
 for i := 0 to BytesPerPixel - 1 do // 1, to 4, dep. on pixelformat 
 begin
 Inc(different, Integer(c1^ <> c2^));
 Inc(c1);
 Inc(c2);
 end;
 end;
 end;