fault pairs are successive meaning that their mutual distance is zero in this case the repeating interval p final of the fault the dent 22 resembling the letter z is h 5 when inspecting watermarked paper or printed adhesive paper for example a missing regularly repeated pattern is searched for when the distances are examined in this case the length of a distance line between similar areas is not allowed to be a multiple of the most common parallel distance present in the image signal in the machine and or cross direction because this means that one or more watermarks or imprints are missing between the areas at the ends of the distance line the fault is categorized at phase 35 the average appearance of the fault is determined with the help of a model image an outline of the fault is drawn on the basis of the model image and categorizing features such as area elongation average gray level variance roundness border line length per area etc can be calculated for the outline when searching for regularly repeated faults at phase 36 a fault has a cycle a location and a category this information can be used for reporting or providing an alarm of an anomaly or identifying.