The simple register mark of the last section is ambiguous because of the periodic nature of the screened patterns. We introduce frequency warping with the aim of destroying the periodicity and thus the ambiguity of the resulting mark.Specifically, we want a higher frequency at the center of the mark and a lower frequency at the edges. We obtain the desired result by interposing a hyperbolic tangent. Thus, the bottom pattern is defined by the reflectance function: and the top pattern by