On the Chain Code of a Line

In 1970 Freeman suggested the following criteria which the chain code of a line must meet [1], [2]: 1) at most two basic directions are present and these can differ only by unity, modulo eight, 2) one of these values always occurs singly, 3) successive occurrences of the principal direction occurring singly are as uniformly spaced as possible. In this correspondence we give the following: 1) an algorithm presentation of Freeman´s three properties about the chain code of a line and the proof that it is also the algorithm recognizing whether a chain code is the chain code of a line, 2) the proof of the equivalence of the above presentation and Rosenfeld´s chord property [3].