The present invention relates generally to image or video display devices. For example, there are many devices (e.g., cell phones, video games, calculators, positioning systems and personal computing devices) that include such display devices. One claim recites a device including: an image or video display including a first set of elements and a second set of elements, the first set of elements being varied relative to at least the second set of elements, the first set of elements being arranged to provide a plural-bit device identifier obtainable from optical scan data representing at least a portion of the image or video display, the first set of elements and the second set of elements being arranged so as to conceal the presence of the plural-bit device identifier from a human observer of the display. Of course, other claims and combinations are provided as well.