This disclosure relates to image signal processing technology including signal encoding. One claim recites a method of detecting plural-bit code conflicts within an image, the image includes at least one color separation. The image includes a first plural-bit code carried by a first symbology, and a second plural-bit code carried by a second symbology, the first symbology and the second symbology comprising different symbology types. The method includes: accessing a subset of the image that comprises the first plural-bit code carried by the first symbology; analyzing the subset of the image to decode the first plural-bit code; analyzing the at least one color separation to spatially locate and decode the second plural-bit code carried by the second symbology; comparing the first plural-bit code and the second plural-bit code; and outputting information if a conflict is identified by said act of comparing, in which the information comprises a spatial location within the image of the conflict. Of course, other claims, features and combinations are described as well.