Two-dimensional comma-free and cylindric codes

A two-dimensional code of pictures is defined as a set X⊆Σ⁎⁎ such that any picture over Σ is tilable in at most one way with pictures in X. It has been proved that it is undecidable whether a finite set of pictures is a code. Here we introduce two classes of picture codes: the comma-free codes and the cylindric codes, with the aim of generalizing the definitions of comma-free (or self-synchronizing) code and circular code of strings. The properties of these classes are studied and compared, in particular in relation to maximality and completeness. As a byproduct, we introduce self-covering pictures and study their periodicity issues.