Public Sub InvertBitmap(ByVal bmp As Bitmap)
 
     Dim bd As New Imaging.BitmapData
     With bmp
        bd = .LockBits(New Rectangle(0, 0, .Width, .Height), _
            Imaging.ImageLockMode.ReadWrite, .PixelFormat)
 
        Dim byt(bd.Stride * bd.Height) As Byte
 
        Runtime.InteropServices.Marshal.Copy(bd.Scan0, byt, 0, byt.Length)
        For i As Integer = 0 To byt.Length - 1 
            byt(i) = CByte(255) - byt(i) 
        Next i
        Runtime.InteropServices.Marshal.Copy(byt, 0, bd.Scan0, byt.Length)
 
        .UnlockBits(bd)
     End With
 
    End Sub