We present a method for embedding and detection of visual watermark patterns in printed images that use clustered-dot halftones in the printing process. The method allows two independent watermark patterns to be multiplexed, i.e. embedded in the same region of the printed image, thereby offering an improvement over prior techniques. The watermark patterns are embedded via phase modulation along the two periodicity directions of the halftone. Each embedded pattern can be visually detected, without interference from the other watermark, when the printed image, or a scan thereof, is overlaid with a decoder mask consisting of periodic lines oriented along the corresponding halftone periodicity direction. We experimentally demonstrate that our proposed multiplexing technique is effective. We also present analysis that demonstrates that the embedding has minimal visual impact and explains the pattern observed in the watermark detection process.Index Terms-Halftone watermarks, clustered-dot halftones, halftone phase modulation, print watermarks
INTRODUCTIONHardcopy watermarks have been extensively used for content authentication and counterfeit protection for over seven centuries [1,2]. Paper watermarks, which represent the most prevalent form of hardcopy watermarks, are embedded within the printing substrate at the time of manufacture [2, Chap. 3]. As digital printing methods become commonplace, the flexibility that these methods offer over conventional analog printing has also been exploited in order to develop a number of alternative digital watermarking methods for hardcopy prints.Halftoning is typically utilized for the reproduction of images in most digital printing systems [3] and many of the digital hardcopy watermarking techniques exploit the characteristics of the halftoning process for embedding watermarks in the printed images [4][5][6][7][8][9][10]. A majority of these methodologies [4][5][6][7][8] mimic the conventional paper watermarks and utilize visual watermark patterns such as bi-level text images or logos. The watermarks are embedded in the form of changes in the halftone structures that are normally imperceptible but can be revealed by appropriate detection methods.In this paper, we present a technique that enables embedding and detection of two independent visual watermark patterns in the same region of a halftone image. Our technique is specifically designed for periodic clustered-dot halftones that are widely utilized in electrophotographic (laser) and lithographic (offset) printing. Based on the analogy with multiplexing in communications, we refer to
