High capacity data embedding for printed documents (2011)

Despite the increasing use of electronic communications, paper documents continue to play a very significant role in our day to day lives and methods that bridge the gap between the physical world of paper and the electronic world of digital data are therefore becoming increasingly important. Traditionally, machine readable barcodes with specialized readers have enabled communication of digital data via printed interfaces. Increasingly, it is becoming desirable to also enable communication of digital data using commonly available electronic imaging equipment such as desktop scanners, printers, and digital cameras built into mobile devices that have become ubiquitous in the world around us.
In this thesis, we investigate problems in high capacity data embedding for printed documents that operate using commonly available electronic imaging equipment and enable novel applications for security and connectivity. In this context, we first develop a new framework for data hiding in images printed with clustered-dot halftones. In particular, we introduce an analytical threshold function that generates elliptically shaped halftone dots in any desired orientation. Our embedding strategy then embeds a binary symbol as a particular choice of the orientation. At the receiver, we first develop a two stage synchronization algorithm to combat the distortions introduced by the print-scan process. Then we use suitable detection statistics and a statistical modeling of the communication channel, which is then incorporated with an error control methodology to recover the resulting errors caused by the print-scan process. We derive the capacity expressions for the effective communication channel in this embedding scenario and show that the experimentally observed error free data rates closely follow the variations in the capacity across different image graylevels. As an application of our data hiding framework, a method for content authentication of printed images is also developed.
Next, we develop a high capacity color barcode scheme that uses the cyan, magenta, and yellow (CMY) colorant separations available in color printers. In each colorant channel, payload data is conveyed by using a periodic array of elliptically shaped dots whose individual orientations are modulated to encode the data. The overall color barcode is obtained when these color separations are printed in overlay. A barcode reader recovers the barcode data from a color scan of the barcode, using red, green, and blue (RGB) channels complementary, respectively, to the print CMY channels. For each channel, first the periodic arrangement of dots is exploited at the reader to enable synchronization. To overcome the color interference resulting from colorant absorptions in non-complementary scanner channels, we use a novel interference minimizing data encoding approach and a statistical channel model that captures the characteristics of the interference. We employ an error correction methodology that effectively utilizes the channel model. Using per-channel data encoding, we also develop color image barcodes that achieve high capacity for encoding data while providing better aesthetics for the visual appearance. Finally, we develop a general framework to extend monochrome barcodes to color by using the per-channel method in conjunction with color interference cancellation.