Patent ID: 8345314

Claim:
A method of operating a printing system, the printing system including one or more controllers configured to perform a method of embedding a watermark into a contone image comprising: receiving a contone data representation of an image; defining a watermark area within the contone data representation of the image; spatially modulating the contone data representation of the image substantially adjacent the watermark area to produce a contone data representation of the image substantially adjacent the watermark area according to a first polarization; and spatially modulating the contone data representation of the image substantially within the watermark area to produce a contone data representation of the image substantially within the watermark area according to a second polarization, thereby producing a contone data representation of the image with an embedded watermark, wherein the steps of spatially modulating the contone data representation of the image substantially adjacent the watermark area of the image and substantially within the watermark area comprises: spatially modulating the contone data representation of the image according to G ( x,y,g o )= p o ( g o )+ p A ( g o )cos [2π( f Ax x+f Ay y )]+ p B ( g o )cos [2π( f Bx x+f By y )] where g ( x,y,g o )= G ( x,y,g o ),if G≧ 0 and G≦ 255, g ( x,y,g o )=0,if G< 0, g ( x,y,g o )=255,if G> 255, and p o , p A and p B are selected to satisfy ∫ x ⁢ ∫ y ⁢ g ⁡ ( x , y , g o ) = ∫ x ⁢ ∫ y ⁢ g o .