Digital topology: Regular sets and root images of the cross-median filter

In the study of topological properties of digital images, which is the central topic of digital topology, one is often interested in special operations on object boundaries and their properties. Examples are contour filling or border following. In classical topology there exists the strong concept of regularity: regular sets in R-2 show no "exotic behaviour'' and are extensively used in the theory of boundary value problems. In this paper we transfer the concept of regularity to digital topology within the framework of semi-topology. It is shown that regular open sets in (a special) semi-topology can be characterized graphically. A relationship between digital topology and image processing is established by showing that regular open digital sets, interpreted as digital pictures, are left unchanged when the cross-median filter is applied.