In this paper the problem of numerical encoding of pictures consisting of regions of differing contrast is discussed. For this purpose we define the containment code, a special case of the more general property code presented in [1]. The containment code leads in a natural way to procedures for following the boundaries of the regions of the picture. This permits a compression of data into a linear array that can be stored for later processing or for picture reconstruction. The boundary following algorithms presented lend themselves well to hardware implementation as can be seen from an explicit sequential machine description. Related codes such as the boundary index code and the directional code are obtained from the containment code. Procedures are described for extracting various topological features such as curve length, chord length, area, and moments.