B4J Question 2's complement

Sub Prepara
Dim larg, lung, fill, padding, length, i, y, centro As Number
Dim mom, mom2
    centro = 0
    If FileExist("/Int/Img/Img1.bmp") Then
            FileOpen(l1, "/Int/Img/Img1.bmp", cRandom)
    Else
        Return
    End If
    binprint.New1(l1, True)
    length = binprint.ReadBytes(serbuff(), 8192)
    FileClose(l1)
    ''''''''preparazione buffer grafico
    larg = ((serbuff(19) * 256 ) + serbuff(18)) /8
    fill = larg Mod 4
    For i = 62 To length - 1
        For y = 0 To larg - 1
            mom2 = Chr(main.btw.Complement(serbuff(y+i)) + 256)
            mom = mom & mom2
        Next y
        '''''''''''''segue correzzione per divisibilità per 4
        If fill > 0 Then i = i + fill
        '''''''''''''''''''''''''''''''''''''''''''''''''''''
        i = i + (larg - 1)
    Next i
    larg = larg + centro
    '''''''''''''''''''''''''''''''''''''''''''''''''''''
    lung = serbuff(22)
    buffer_st = buffer_st & Chr(29) & Chr(118) & Chr(48) & Chr(51) & Chr(larg) & Chr(0) & Chr(lung) & Chr(0) & mom & Chr(10)
End Sub

Sub Prepara
Dim larg, lung, fill, padding, length, i, y, centro As Int
Dim mom, mom2 As String
    centro = 0
    If File.Exists(Main.Workingfolder, "Img1.bmp") Then
        Dim instream As InputStream = File.OpenInput(Main.Workingfolder, "Img1.bmp")
    Else
        Return
    End If
    length = instream.BytesAvailable
    Dim BufferImg(length) As Byte
    instream.ReadBytes(BufferImg, 0, instream.BytesAvailable)
    ''''''''preparazione buffer grafico
    larg = ((Bit.And(255, BufferImg(19)) * 256 ) + Bit.And(255,BufferImg(18))) /8
    fill = larg Mod 4
    For i = 62 To length - 1
        For y = 0 To larg - 1
            mom2 = Chr(Bit.Not(BufferImg(y+i)) + 256)
            mom = mom & mom2
        Next
        '''''''''''''segue correzzione per divisibilità per 4
        If fill > 0 Then i = i + fill
        '''''''''''''''''''''''''''''''''''''''''''''''''''''
        i = i + (larg - 1)
    Next
    larg = larg + centro
    '''''''''''''''''''''''''''''''''''''''''''''''''''''
     lung = Bit.And(255, BufferImg(22))
     buffer_st.Append(Chr(29) & Chr(118) & Chr(48) & Chr(51) & Chr(larg) & Chr(0) & Chr(lung) & Chr(0) & mom & Chr(10)
End Sub