Method and apparatus for encoding paper with information

Description

RELATED APPLICATION DATA

This application is a continuation of Ser. No. 09/515,545 (now U.S. Pat. No. 6,608,919), filed Feb. 29, 2000, which is a continuation-in-part of application Ser. No. 09/437,357 (now abandoned), filed Nov. 10, 1999.

FIELD OF THE INVENTION

The present invention relates to paper-making, and more particularly relates to techniques useful in encoding information in paper during its formation.

BACKGROUND AND SUMMARY OF THE INVENTION

Digital watermarking is a burgeoning science with an extensive literature. Some of it, by the present assignee, relates to texturing the surface of a paper to convey optically-detectable digital information.

The assignee's U.S. Pat. No. 5,850,481, for example, details how the microtopology of a paper can be shaped, e.g., by a Braille-like machine, to encode digital data. The assignee's application Ser. No. 09/127,502 (now U.S. Pat. No. 6,345,104) details how the high-pressures used in intaglio printing can be used to similar effect.

In accordance with a preferred embodiment of the invention, paper is textured during the paper-making process, rather than in a subsequent process, thereby affording various advantages. The texture yields a known signature when transformed to the frequency domain. This signature can be used for various purposes, including determining the angular orientation of the paper when scanned and analyzed for the presence of watermark data, and for simple document recognition purposes (e.g., photocopiers that refuse to reproduce banknotes). One way of effecting the desired texturing is by shaping the surface of a de-watering element accordingly.

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

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows the frequency spectrum attributed to surface texture of a banknote paper.

DETAILED DESCRIPTION

On optical scanning of an existing banknote, it was found that that the image data had components that were not attributable to the artwork printed on the note. Rather, further examination found that texturing left during the paper-making process had a pattern that appeared —when transformed into the frequency domain by a FFT process—as a well defined array of spectral impulses (FIG. 1). This spectrum of this particular note had the appearance of a 2-dimensional sampled sync function. (The size of the points in FIG. 1 signifies amplitude of the corresponding frequency component; each is an impulse.)

In the digital watermarking field, subliminal frequency domain patterns are sometimes deliberately inserted into images to serve as calibration signals by which scale, rotation, and offset can be computer-determined. (See, e.g., the assignee's U.S. Pat. No. 5,862,260.) Instead of adding such a pattern during printing, the patterns formed during paper-making might alternatively be used.

In accordance with one embodiment of the present invention, the printing on a paper is oriented so as to align in a predetermined manner with the texture markings on the paper surface. For example, the printing process can be tightly integrated with the paper-making process so that the orientation of the printing applied to the paper coincides in the desired manner with the paper texture. In a related embodiment, the paper-making and printing processes are not tightly integrated, but the paper is formed or cut so that its edges correspond in a known relationship to the texture pattern on the paper surface. By such arrangement, careful alignment of the paper's edge with a reference during later printing can assure that the image is oriented in the desired manner with the substrate texture. In still another embodiment, the paper is scanned just before printing, and the resulting data is processed so as to determine orientation of the texture signal. Printing is then oriented relative to the paper in the desired manner.

When a document printed by such a technique is thereafter placed on the platen of a scanner and scanned, the resulting image data is of uncertain rotation. The scan data can be converted to a frequency domain (e.g., by the FFT), and the known frequency pattern due to the paper texture can then be used as a calibration signal permitting the rotational state of the image data to be determined. Decoding of the watermark can then proceed once the rotation is known. (In some watermarking systems, scale and/or offset may also need to be determined. However, their determination is facilitated by knowledge of rotation. Moreover, the frequency domain signal may result in some ambiguity in the document orientation, e.g., rotations beyond 180 degrees may appear as rotations in the range 0–180 degrees. Such cases can be resolved through other means.)

As is familiar to those skilled in the art, paper is typically formed by depositing a layer of pulp slurry on a planar or cylindrical surface, extracting the water from the pulp, and drying. The middle step, sometimes known as de-watering, can be performed using various structures. Porous ceramic rollers are used in some processes, with a vacuum applied to the inside of the roller that serves to draw water away from the wet layer, through the pores, and to the inside of the roller. Other processes involve meshes or screens through which water is drawn from the pulp. The surfaces of such de-watering structures can be textured to impart a complementary texture (and a corresponding frequency-domain signal) to the paper.

In particular, a desired frequency domain signal can first be defined. The signal can be comprised of one or more impulses in the frequency domain, or can be spread across the spectrum. (The signal shown in FIG. 29A of U.S. Pat. No. 5,862,260 is exemplary of a suitable signal.) This signal is then inverse—transformed into the spatial domain, and the de-watering element then shaped in accordance with the resulting pattern.

In many embodiments, the frequency domain signal is manifested as a fine continuous “weave”-like pattern in the spatial domain. In other embodiments, pseudo-random patterns can be used. If desired, certain discrete markings in the spatial domain can also be provided on the de-watering element surface to provide additional functionality in the texture signal (e.g., permitting image translation to be determined in addition to image rotation).

In other embodiments, the frequency domain signal effected by the texturing does not serve as a calibration adjunct to another signal (e.g., a watermark signal). Rather, the frequency domain signal is employed alone. One application is in simple document recognition. For example, a photocopier may be arranged to look for the frequency domain signature characteristic of the texturing associated with banknote paper and, if detected on a document-to-be-copied, refuse to copy. (An example of another banknote sensing photocopier, albeit one that looks for a pattern in the printing on the document rather than in the paper texture, is shown in Canon's U.S. Pat. No. 5,583,614.) The texturing may be arbitrarily complex so as to reduce false triggering of such a detector.

In still other embodiments, the texturing can convey a plural-bit watermark payload. For example, the presence or absence of different frequency-domain components can signal “1” or “0” bits. Many other such arrangements will be apparent.

To provide a comprehensive disclosure without unduly lengthening this specification, applicant incorporates by reference the patents and applications cited above.

Having described an illustrated the principles of my invention with reference to specific embodiments, it will be recognized that the principles thereof can be implemented in many other, different, forms. For example, while the detailed embodiment discussed use of the FFT transform, the patterns formed during paper-making can likewise be discerned in other transform domains. Similarly, while a de-watering element was particularly noted as suitable for texturing, other elements in a paper-making apparatus can be alternatively be textured with similar effect. Still further, while the invention was illustrated with reference to conventional “paper,” the principles thereof are equally applicable to other printable media, including plastic and the material marketed by DuPont under the brand name Tyvek. Yet further, while the illustrated embodiment textured the paper during its formation, post-formation texturing—as detailed in the assignee's other patents/applications—can be used in the particular applications above-detailed (e.g., to determine image rotation, for simple document recognition, etc.).

Moreover, 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.