Show-through watermarking of duplex printed documents.pdfVIP

Show-through Watermarking of Duplex Printed Documents Gaurav Sharmaa and Shen-ge Wangb aECE Dept, Univ. of Rochester, Rochester, NY 14627-0126, USA; bXerox Corporation, 800 Phillips Road, Webster, NY 14580 ABSTRACT A technique for watermarking duplex printed pages is presented. The technique produces visible watermark pat- terns like conventional watermarks embedded in paper fabric. Watermark information is embedded in halftones used to print images on either side. The watermark pattern is imperceptible when images printed on either side are viewed independently but becomes visible when the sheet of paper is held up against a light. The technique employs clustered dot halftones and embeds the watermark pattern as controlled local phase variations. Illu- mination with a back-light superimposes the halftone patterns on the two sides. Regions where the front and back-side halftones are in phase agreement appear lighter in show-through viewing, whereas regions over which the front and back side halftones are in phase disagreement appear darker. The image printed on one side has a controlled variation of the halftone phase and the one printed on the other side provides a constant phase reference. The watermark pattern is revealed when the sheet is viewed in “show-through mode” superimposing the halftones on the two sides. Threshold arrays for the halftone screens are designed to allow incorporation of a variety of halftone patterns while minimizing artifacts in images printed using these halftones. Keywords: watermarking, hardcopy watermarking, halftoning, data embedding 1. INTRODUCTION Traditional watermarks are visual patterns embedded in a sheet of paper. These watermarks are produced by creating a variation in the thickness of paper ?ber during the wet-phase of