The use of 2D barcodes is becoming increasingly popular and is driven by both the added payload (data carrying capacity) that is provided over 1D linear codes and the fact that the majority of current Smart- phones are capable of reading them. While it is desirable to associate a forensic print signature with the outline of the 2D barcode, the high resolution required to image the stochastic microscopic structure typical of the printing process makes it difficult, expensive and bulky to achieve the field of view required for reading a 2D barcode of sufficient payload that can independently be read by a hand-held smart-phonedevice. One solution to this problem is to use a technique of 'subcoding' (introduced by Omniplanar) whereby an inverted mark smaller than the size of an individual barcode module (or 'tile') is placed within each module and the position of the mark(s) within the module provides the ability to code data. This increase in coding density enables the complete contents of the barcode to be repeated in a sub-region. This results in the field of view of the high resolution reader being reduced significantly, whilst maintaining the ability for both the forensic authentication and extraction of the barcode identity to simplify the forensic referential lookup. However, there is a limit to the sub-coding density, as it ultimately affects the primary barcode reading ability through a reduction in contrast. This paper investigates the density sub-coding payload until it affects the primary payload of the 2D barcode. We also explore if there are sensitivities to the placement of the sub-coding mark within a module and what effect dot gain has on the design of the sub-coding scheme.
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