<title>Hidden and scrambled images: a review</title>

Abstract

This paper reviews screen-decoded images, images that are invisible or illegible to the naked eye but that are visualized or decoded by means of periodic phenomena, such as an absorptive grating, a lenticular screen or the sampling frequency of a copying system. Two basic types are distinguished: carrier screen images and scrambled images. Carrier screen images consist of periodical arrays of screen elements, such as dots and lines, which serve as a carrier on which the encoded information is modulated. The counterpart of the carrier screen images is the scrambled image, which consists of numerous separate dissections of the original image. A classification of screen-decoded images by the type of carrier screen is presented in an attempt to clear up the existing confusion in nomenclature.