Hardcopy Watermarking for Document Authentication
Robinson Pizzio · InTech eBooks · 2012
Will-be-set-by-IN-TECHprovide instant recognition of the copyrights of the document.Although visible, the inserted logos are unobtrusive, which means that they are sufficiently transparent in order to allow a clear interpretation of the information conveyed by the text.We illustrate that the proposed technique is able to use a very transparent logo and yet provide a robust detection.In our method, in addition to the recognition of the visible logo, we also authenticate the document by coding information bits (a code generated from the sensitive parts of the document like names, dates, or values, for instance) associated with the position of the inserted logos.This coding approach, coined position based watermarking (PBW), was originally exploited by Borges and Mayer in Borges & Mayer (2003), but it was not designed specifically to be robust to the PS channel.Also, the PBW was originally designed and analyzed only to the insertion of invisible Gaussian marks in digital images.In order to be robust to the PS process, and able to be applied for document authentication, this paper proposes to improve the PBW technique.Moreover, our method can be used along with barcodes, TLM and other techniques.A combination of these methods with the proposed one can provide superior robustness and high payload when compared to an isolated technique.Moreover, these techniques can be designed to have a very small interference among them.Besides, we analyze the applicability of the developed method for the case where the authenticated document is printed on recycled paper.For this case, we characterize the noise imposed by the recycled paper, estimate the autocorrelation function of this noise, and a new detector is derived.Also, some new analyses are presented.To achieve the development of the new binary document authentication method proposed here, this paper brings the following contributions: (i) the definition of a simple but effective analytical model to the PS channel; (ii) the derivation of optimal detectors and optimal detection threshold for cases of white or recycled paper documents; (iii) identification and characterization of the segmentation noise; (iv) characterization of the pattern noise for a recycled paper; and (v) the determination of the position error identification during logo detection.The remain of this paper is organized as follows.In Section 2 some of the main distortions found in the PS channel are discussed and a simple but effective channel model is proposed.Details on the logos insertion and detection processes, the development of an optimal detection threshold for the proposed method, an error probability analysis, the lower bound capacity of the method, and the analysis of the position error are all presented in Section 3. The analysis of the applicability of the method for recycled paper documents is presented in Section 4. Also in this section we characterize the recycled paper noise and a new detector is derived.The new detector depends on the autocorrelation matrix of the recycled paper noise and this matrix is deduced in Section 5. Some experiments to illustrate the discussions and performance of the method are in Section 6. Section 7 concludes this paper and present some future works. Printing-scanning channel PS channel distortionsWhen a grayscale document is printed, digital halftoning (gray to binary conversion) takes place in order to print with bilevel devices.Moreover, the spreading of the toner or ink on the hardcopy introduces another distortion similar to a blurring effect Norris & Smith (2004).In the scanner, another blurring distortion is generated by the optics and motion of the inline CCD.Geometric distortions, including rotation, cropping and scaling take place in 134