Show-through watermarking of duplex printed documents

A technique for watermarking duplex printed pages is presented. The technique produces visible watermark patterns 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. Illumination 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.