Data encoding and workpiece authentication using halftone information

#The invention relates generally to data encoding and decoding and, more particularly, to encoding data into and decoding data from an image. The invention further relates to authenticating an image using as authentication information a description of associated halftone characteristics.

#There are numerous applications in which machine readable data must be encoded on a paper document or other substrate. Typically such data is encoded using a printable symbology, either as a form of barcode, or as characters optimized for Optical Character Recognition, i.e., in an “OCR type font.” There are numerous applications of machine readable printed symbologies, for example: product identification—notably the ubiquitous Universal Product Code symbol, document identification, postal mailpiece identification and sorting, and so on. Even postage itself may be represented by a machine readable printed symbology, as for example in the United States Postal Service's Information Based Indicia Program.

#A barcode specifies data symbols as an arrangement of lines that vary in relative size and spacing. In one dimensional (1-D) barcodes the lines vary by width and relative horizontal spacing. In two dimensional (2-D) barcodes, the lines vary also by relative vertical length or relative position. As is common, the barcodes may be overlaid on an otherwise blank portion of, for example, a price tag.

#The invention description below refers to the accompanying drawings, of which:

#FIGS. 1A and 1B depict a K channel and a corresponding Fourier plane.

#FIG. 2 is a graph that depicts halftone screen angle and spatial frequency ranges;

#Most printing systems have very limited dynamic range at the lowest level. At each point on the paper, a mark (e.g., ink) is either present or absent, and this corresponds to a single bit of dynamic range. In order to print a continuous tone image with wide dynamic range in each color channel, a halftoning process is used. This process creates the illusion of dynamic range at one spatial resolution by forming appropriate ink patterns at a finer spatial resolution. Ideally the structure at the finer spatial resolution is difficult or impossible for humans to perceive, and thus, the printing press creates in a printed image the illusion of dynamic range at perceptually relevant resolutions.

#There are two basic classes of halftoning processes, so-called conventional and stochastic processes. We describe the invention in terms of the conventional halftoning process, which places inks of various colors on a document under the control of corresponding halftone screens. Each halftone screen typically has a distinct orientation on the document, or what is commonly referred to as a halftone “screen angle.” The screen angles are selected to avoid unpleasant visual artifacts in the image, such as Moiré patterns. Choosing the halftone properties appropriately is an important part of the art practiced by those skilled in the trade of printing, with printing establishments often achieving a characteristic “look” by judiciously choosing the properties of their halftone process.

#The software and/or hardware for converting a digital continuous tone image to a halftone image is called a Raster Image Processor, or RIP. Commercial printing presses use dedicated RIPing software and/or hardware that may be included in an associated printer driver or may be integrated into the printer.