Digital watermarking of physical objects

The present application is a continuation-in-part of application Ser. No. 09/127,502, filed Jul. 31, 1998 (now U.S. Pat. No. 6,345,104).

The present application is also related to applications Ser. No. 09/437,357, filed Nov. 10, 1999 now abandoned; Ser. No. 09/074,034, filed May 6, 1998 (now U.S. Pat. No. 6,449,377); Ser. No. 09/234,780, filed Jan. 20, 1999 (now abandoned); Ser. No. 09/433,104, filed Nov. 3, 1999 now U.S. Pat. No. 6,636, 615; Ser. No. 09/503,881, filed Feb. 14, 2000 (now U.S. Pat. No. 6,614,914 and application Ser. No. 09/553,112, filed Apr. 20, 2000 (now abandoned).

The present application is also related to applications Ser. No. 09/562,516 and 09/562,049 both filed herewith.

The present application is also related to the assignee's U.S. Pat. No. 5,862,260, 5,850,481 and 5,841,886.

The present invention relates to processing of physical media (e.g., blank printing stock, product packaging, catalogs, advertisements, etc.) to impart a machine-readable indicia (e.g., a plural-bit digital watermark) thereto.

Digital watermarking technology, a form of steganography, encompasses a great variety of techniques by which plural bits of digital data are hidden in some other object without leaving human-apparent evidence of alteration.

Most commonly, digital watermarking is applied to digital objects, such as digital image, video, and audio. In the case of images, slight changes can be made to local luminance or color values to effect the encoding. These changes can later be detected by a computer, and analyzed to discern the watermark information represented thereby.

Digital watermarking techniques can also be applied to traditional physical objects, including blank paper. Such blank media, however, presents certain challenges since there is no image that can serve as the carrier for the watermark signal.

The assignee's U.S. Pat. 5,850,481 notes that the surface of a paper or other physical object can be textured with a pattern of micro-indentations to steganographically encode plural-bit information. The texturing is optically discernible, e.g., by a scanner, permitting the  digital data to be decoded from scan data corresponding to the paper object.

In application Ser. No. 09/127,502, the present assignee taught various other arrangements by which blank media can be processed to encode a digital watermark. Some techniques employ very subtle printing, e.g., of fine lines or dots, which has the effect slightly tinting the media (e.g., a white media can be given a lightish-green cast). To the human observer the tinting appears uniform. Computer analysis of scan data from the media, however, reveals slight localized changes, permitting the multi-bit watermark payload to be discerned. Such printing can be by ink jet, dry offset, wet offset, xerography, etc.

Other techniques disclosed in the '502 application extend the texturing techniques first set forth in the '481 patent, e.g., by employing an intaglio press to texture the media as part of the printing process (either without ink, or with clear ink).

In one aspect, the present specification further develops and extends the techniques disclosed in the '502 application.

For example, printable media—especially for security documents (e.g., banknotes) and identity documents (e.g., passports)—is increasingly fashioned from synthetic materials. Polymeric films, such as are available from UCB Films, PLC of Belgium, are one example. Such films may be clear and require opacification prior to use as substrates for security documents. The opacification can be effected by applying plural layers of ink or other material, e.g., by gravure or offet printing processes. (Suitable inks are available, e.g., from Sicpa Securink Corp. of Springfield, Va.) In addition to obscuring the transparency of the film, the inks applied through the printing process form a layer that is well suited to fine-line printing by traditional intaglio methods. Such an arrangement is more particularly detailed in laid-open PCT publication WO98/33658. That application is a pending U.S. counterpart application, now published as U.S. 2003/0003275, A1 which claims priority to Australian application 4847, filed Jan. 29, 1997.

In accordance with certain embodiments of the present invention, a machine readable indicia encoding plural bits (e.g., a digital watermark) is embedded as part of such an opacification process.

Other security documents employ plural substrates bound together in a laminate structure. The laminates variously include paper, polypropylene film, polyethelyne film, polyurethane film, metal film, inks, adhesives, optically-variable devices (e.g., holograms), and other materials. (See, e.g., U.S. Pat. Nos. 5,935,696 and 5,618,630.) Again, in accordance with embodiments of the present invention, plural-bit machine readable indicia can be incorporated in such structures.

The foregoing and additional features and advantages of the invention will be more readily apparent from the following detailed description, which proceeds with reference to the accompanying drawings.

FIG. 1 shows a structure of a document according to one embodiment of the present invention.

FIG. 2 is a flowchart detailing a method according to one embodiment of the invention.

As noted, one technique for forming a blank medium suitable for use as a security document is to gravure-print, or otherwise apply, plural layers of material (e.g., ink) to a synthetic substrate. In addition to making the substrate opaque (if it was not already), the layers provide a surface well suited for fine line offset or intaglio printing.

In known techniques, the layers added to the substrate are substantially uniform. In the case of gravure-printing of ink, the gravure printing plates are generally formed with a pattern of grooves designed to apply a substantially uniform layer of ink across the substrate.

In accordance with one embodiment of the present invention, one or more of the layers is non-uniform, and instead exhibits localized changes in ink density or tone (e.g., color) to effect a pattern in the resulting medium that is optically detectable from the finished substrate. An exemplary method is shown in FIG. 2 .

For gravure opacification, the changes in ink density can be effected by maintaining the same groove patterns as are utilized to effect uniform inking, but changing the groove size on a localized basis to deposit more ink at some locations, less at others. Alternatively, the changes in ink density can be effected by changing the groove patterns, e.g., to move adjoining lines closer together or farther apart, again with the aim of depositing ink more densely in some regions than others. Related techniques, albeit in a somewhat different context, are detailed in the '034 application cited above, and can be employed here.

Another technique of forming a pattern in one or more opacification layers is to use to laser to ablate portions of the layer, selectively removing material to form the desired pattern. Again, such a process results in a change visibly detectable from the exterior of the medium. (Laser ablation of an opacification layer is known in other contexts; see, e.g., laid-open PCT application WO9836913.)

In one embodiment, the watermark pattern is effected in only a single of the layers formed on the film substrate. In another embodiment, the pattern is effected in two or more of the layers. (The former approach avoids problem of plate-registration that arise in the latter. The latter may provide a marking that is more durable in the presence of wear than the former.) In still other embodiments, one layer may represent a first set of watermark data, and another may represent a second set of watermark data. One such arrangement is depicted in FIG. 1 .

In another form of the invention, a security document medium is prepared in laminate fashion, with one or more of the interior layers formed so as to impart an optically-discernible pattern of contrast discernible from the exterior of the medium. The layer can be printed to form this pattern, or can be shaped to give such a result. The latter approach can employ a swiss cheese-like layer (formed, e.g., by laser cutting), where the holes in the layer give rise to an optical contrast effect from the exterior of the substrate that encodes the plural-bit data payload. The degree of contrast depends on the color of the layer, and the translucency of the exterior layer(s).

In some such embodiments an internal layer is paper, and exterior layers are synthetic. In others, an opposite arrangement is employed.

In the foregoing embodiments, or in embodiments in which a watermark signal is applied by printing to a finished substrate, the watermark can be deposited by ink jet printing or other dithered-technique, where the dither pattern is tailored to effect a luminance profile across a region that encodes a watermark.

The watermarking arrangement detailed in the '112 application can be employed both to the outside of a substrate as well as to a non-exterior component layer (whether laminate or opacification). The algorithm employed in the '112 application can be altered, if desired, to provide a control by which a user can establish the degree of randomness to be included in the traversed path as the line(s) extend from one point to another. Likewise, a control can be provided to change a modulation effect (e.g., sinusoidal) applied to the direction or width of the line as it traverses a region.

It can be advantageous to apply various digital watermarking techniques to security badges and identity cards—of the sort commonly worn by employees at industrial facilities. Such cards are commonly printed by dedicated print stations that include both a personal computer and a specialized card printer. Datacard is one vendor of such systems. Information can be embedded on such cards (or in a non-surface layer and yet still sensible from the card exterior) in watermark form, embodying various data, e.g., the date of issuance, the owner name, an identification number, etc. The information can have the appearance of a tint or wash, as noted above, yet permit ready recovery of the embedded information.

In the foregoing embodiments, the watermark signal can be represented as a checkerboard pattern comprising, e.g., a 96×96 array of elements, where each element is 0.012 inch on a side. Each component element can be light or dark, or intermediate grey-scale values may be used to further reduce visibility. Such checkerboards may be tiled together to span the full width and length of the media.

Alternatively, patterns other than checkerboards can be used. Such patterns (e.g., weave-like patterns), and methods for their generation, are detailed in the '112 and '502 applications cited above.

In most embodiments, the watermark payload is uniform across the medium. In some applications, however, it may be desirable to encode different payloads in different regions of a medium. Such may be the case, for example, in pre-encoding blank pages for magazine stock. Each sheet (ultimately defining two magazine pages on its front and two on its back) may be arranged in columnar form (e.g., 3 columns per page), with each column bearing a different watermark. Still more complex arrangements, e.g., segregating each column into top, middle, and bottom thirds, can of course be used.

In other arrangements, the same watermark may be encoded in different places (e.g., on different sheets of media), but not by using the same pattern. Instead, different patterns can be used in different places to encode the same watermark payload.

The watermark can convey a payload of arbitrary length, commonly in the 2-256 bit range, and perhaps most commonly between 24 and 72 bits. Error correcting coding, such as convolutional coding or BCH coding, can be employed to transform the base payload (e.g., 50 bits) to a longer data string (e.g., 96-1024 bits), assuring robustness in detection notwithstanding some data corruption (e.g., due to wear and tear of the medium, artifacts from scanning, etc.). The bits of this longer string are mapped, e.g., pseudo-randomly, to define the pattern (e.g., checkerboard).

An illustrative watermark-encoding technique is more particularly detailed in application Ser. No. 09/503,881 (but is used without gain control and perceptual analysis since no image is present on the blank medium). The calibration signal there-detailed can be employed to permit decoding notwithstanding rotation or certain other corruptions of the detected watermark data.

While the watermarking technique detailed in the just-cited application is preferred by the present assignee, it should be understood that the principles of the present invention can be employed with essentially any other watermarking technology. A great variety of such techniques are known.

Moreover, the invention finds application beyond “watermarking.” Any form of machine-readable indicia, including 1-and 2-D barcodes and data glyphs, may be formed as noted above, and serve to facilitate machine-recognition of the media.

To provide a comprehensive disclosure without unduly lengthening this specification, the above-detailed patents and applications are incorporated herein by reference.

The particular combinations of elements and features in the above-detailed embodiments are exemplary only; the interchanging and substitution of these teachings with other teachings in this and the incorporated-by-reference patents/applications are also contemplated.

In view of the wide variety of embodiments to which the principles and features discussed above can be applied, it should be apparent that the detailed embodiments are illustrative only and should not be taken as limiting the scope of the invention. Rather, we claim as our invention all such modifications as may come within the scope and spirit of the following claims and equivalents thereof.