A device comprises a first image phase with a first set of coded indicia positioned with respect to a background image, a second image phase with a second set of coded indicia positioned with respect to the background image, and means to alternate between the first and second phases. The second set of coded indicia are related to the first set of coded indicia by a mapping based on the background image, and means are provided to alternate between the image phases, the means configured to select the background image with the first set of coded indicia or the background image with the second set of coded indicia.
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