Sub invert()
        Dim b As Bitmap = pb.Image
        Dim x, y As Integer
        Dim bmpData As BitmapData
        Dim p As Byte
        Dim ptr As IntPtr
        Dim offset As Integer = 0
        bmpData = b.LockBits(New Rectangle(0, 0, b.Width, b.Height), _
          ImageLockMode.ReadWrite, PixelFormat.Format24bppRgb)
        ptr = bmpData.Scan0
        For y = 0 To b.Height - 1
            For x = 0 To b.Width - 1
                p = 255 - Marshal.ReadByte(ptr, offset)
                Marshal.WriteByte(ptr, offset, p)
                offset += 1
                p = 255 - Marshal.ReadByte(ptr, offset)
                Marshal.WriteByte(ptr, offset, p)
                offset += 1
                p = 255 - Marshal.ReadByte(ptr, offset)
                Marshal.WriteByte(ptr, offset, p)
                offset += 1
            Next
        Next
        b.UnlockBits(bmpData)
        pb.Refresh()
    End Sub