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Proceedings Paper

Sub-pixel analysis to support graphic security after scanning at low resolution
Author(s): Bertrand Haas; Robert Cordery; Hongmei Gou; Steve Decker
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Paper Abstract

Whether in the domain of audio, video or finance, our world tends to become increasingly digital. However, for diverse reasons, the transition from analog to digital is often much extended in time, and proceeds by long steps (and sometimes never completes). One such step is the conversion of information on analog media to digital information. We focus in this paper on the conversion (scanning) of printed documents to digital images. Analog media have the advantage over digital channels that they can harbor much imperceptible information that can be used for fraud detection and forensic purposes. But this secondary information usually fails to be retrieved during the conversion step. This is particularly relevant since the Check-21 act (Check Clearing for the 21st Century act) became effective in 2004 and allows images of checks to be handled by banks as usual paper checks. We use here this situation of check scanning as our primary benchmark for graphic security features after scanning. We will first present a quick review of the most common graphic security features currently found on checks, with their specific purpose, qualities and disadvantages, and we demonstrate their poor survivability after scanning in the average scanning conditions expected from the Check-21 Act. We will then present a novel method of measurement of distances between and rotations of line elements in a scanned image: Based on an appropriate print model, we refine direct measurements to an accuracy beyond the size of a scanning pixel, so we can then determine expected distances, periodicity, sharpness and print quality of known characters, symbols and other graphic elements in a document image. Finally we will apply our method to fraud detection of documents after gray-scale scanning at 300dpi resolution. We show in particular that alterations on legitimate checks or copies of checks can be successfully detected by measuring with sub-pixel accuracy the irregularities inherently introduced by the illegitimate process.

Paper Details

Date Published: 9 February 2006
PDF: 13 pages
Proc. SPIE 6075, Optical Security and Counterfeit Deterrence Techniques VI, 60750B (9 February 2006); doi: 10.1117/12.641516
Show Author Affiliations
Bertrand Haas, Pitney Bowes (United States)
Robert Cordery, Pitney Bowes (United States)
Hongmei Gou, Univ. of Maryland, College Park (United States)
Steve Decker, Heron Innovation (United States)

Published in SPIE Proceedings Vol. 6075:
Optical Security and Counterfeit Deterrence Techniques VI
Rudolf L. van Renesse, Editor(s)

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