Encoding Information in Clustered-Dot Halftones

Abstract

While barcodes are a popular means for encoding information for printed matter, they add unsightly overt content. If an image is already part of the composition of a printed label or page, hiding information in that image is an attractive alternative to barcodes. This paper offers a new method for encoding information in the halftone of an image. We focus on the class of techniques that perform clustered-dot halftoning, as commonly used in both dry toner and liquid toner electrophotographic processes. The method takes as input any grayscale image and a payload of data to be encoded and produces a bitonal clustereddot halftone of that image with selected halftone clusters shifted to carry varying numbers of bits from the payload. The resulting data-bearing steganographic halftone is referred to as a “Stegatone”. Because of the small size and large number of clustered-dot cells in printed halftones the bit density is quite high – over 2000 bytes/square-inch. Scans of test printed stegatones from a number of printers support the robustness of the method with high recovery rates.