Classifications

• • G—PHYSICS
  • G03—PHOTOGRAPHY; CINEMATOGRAPHY; ANALOGOUS TECHNIQUES USING WAVES OTHER THAN OPTICAL WAVES; ELECTROGRAPHY; HOLOGRAPHY
  • G03G—ELECTROGRAPHY; ELECTROPHOTOGRAPHY; MAGNETOGRAPHY
  • G03G21/00—Arrangements not provided for by groups G03G13/00 - G03G19/00, e.g. cleaning, elimination of residual charge
  • G03G21/04—Preventing copies being made of an original
  • G03G21/043—Preventing copies being made of an original by using an original which is not reproducible or only reproducible with a different appearence, e.g. originals with a photochromic layer or a colour background
• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04N—PICTORIAL COMMUNICATION, e.g. TELEVISION
  • H04N1/00—Scanning, transmission or reproduction of documents or the like, e.g. facsimile transmission; Details thereof
  • H04N1/00838—Preventing unauthorised reproduction
  • H04N1/00883—Auto-copy-preventive originals, i.e. originals that are designed not to allow faithful reproduction
• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04N—PICTORIAL COMMUNICATION, e.g. TELEVISION
  • H04N2201/00—Indexing scheme relating to scanning, transmission or reproduction of documents or the like, and to details thereof
  • H04N2201/32—Circuits or arrangements for control or supervision between transmitter and receiver or between image input and image output device, e.g. between a still-image camera and its memory or between a still-image camera and a printer device
  • H04N2201/3201—Display, printing, storage or transmission of additional information, e.g. ID code, date and time or title
  • H04N2201/3269—Display, printing, storage or transmission of additional information, e.g. ID code, date and time or title of machine readable codes or marks, e.g. bar codes or glyphs
  • H04N2201/327—Display, printing, storage or transmission of additional information, e.g. ID code, date and time or title of machine readable codes or marks, e.g. bar codes or glyphs which are undetectable to the naked eye, e.g. embedded codes

Definitions

• the present invention relates to a method of marking documents, intended to avoid confusion of the copies with the originals.
• the object of the present invention is precisely to propose a method the application of which makes it possible to distinguish surely a photocopy of an original, even when the photocopy is performed by a photocopier of the most sophisticated type.
• the invention relates to a document marking method according to which a mark is incorporated on the original document during the preparation of said original document by digitally combining the information relating to this mark with the information relating to the image to be protected, so that this mark is invisible on the original document but appears on the copy of this original document made using a photocopier, the scanning of the original document being carried out with a sample step swims at least twice as thin as that of the photocopier.
• the method according to the present invention therefore intervenes at the level of the information which must go on the document, by digitally combining the camouflage information and the image information to be protected before the document is printed, without changing the procedure.
• print Journal ⁇ ment to processes that have been proposed to date, which generally use different ways to print the original infor ⁇ tion, or by a mixture of different colors, to mark the document. either by marking by photographic printing.
• This process makes it possible to incorporate marking information on the originals, this information being invisible on the originals and. appearing only after photocopy. It is based on the fact that the photocopies obtained with photocopiers, even the most modern, have a lower definition of the document (in general by a factor of 2) than the originals which are printed. It is of course likely that the photocopier technique will continue to progress in order to further refine the definition of the document. It will then suffice to print the originals which one wishes to protect with an even greater definition.
• the luminance of a document can be represented by a two-dimensional analog function x "(u, v).
• the information is represented by three functions of the same type for the primary colors red, green and blue.
• this or these functions are sampled in space with a very fine step which should normally be chosen in accordance with the two-dimensional sampling theorem. (see M. Kunt, "Digital signal processing", Electricity treaty, EPFL, Vol. XX, 402p.).
• the sampling step is generally chosen small enough so that the digitally reconstructed document does not have a pointil ⁇ list effect in the eyes of the observer.
• Common photocopiers use sampling ranging from 300 to 600 dots per inch, that is, dots on the order of 0.042 mm to 0.085 mm in width. This is what allows them to have a good quality of reproduction.
• the sampling theorem says that any pattern in the original, the fineness of which is less than double these values, cannot be correctly represented.
• X - . (F, g) be the Fourier transform of the function x * (u, v).
• the sampling of the function x "- (u, v) causes the periodic repetition of the Fourier transform Xa (f, g) in both directions, with periods equal to the reverse of double the periods of spatial sampling, which will be called ⁇ u and ⁇ v.
• the original document contains components at frequencies higher than l / 2 ⁇ u and / or l / 2 ⁇ v
• the repeated versions of the function X * (f, g) overlap and a covering effect takes place. This covering is at the origin of the figures of Moiré which one observes on certain copied documents.
• the present invention is based on this overlap and implies that the original must have a definition at least twice better than that of photocopiers.
• FIG. 2 represents the four quadrants of the discrete Fourier transform of the marking image
• FIG. 3 illustrates the arrangement of the Fourier transforms, so as to place the marking image in the high frequencies
• FIG. 4 is a block diagram of a first variant of the method for marking the original image in the space domain
• FIG. 5 is a block diagram of a second variant of the method for marking the original image in the space domain
• FIG. 6 is a block diagram of a third variant of the method for marking the original image in the space domain
• FIG. 7 is a block diagram of a first variant of the method of marking the original image in the frequency domain
• FIG. 8 illustrates the distribution of the quadrants of the Fourier transform for the execution of the method according to a second variant in the frequency domain
• FIG. 9 is a block diagram of the second variant of the method of marking the original image in the frequency domain.
• FIG. 10 is a block diagram of a third variant of the method of marking the original image in the frequency domain.
• Figure 1 shows the different stages of sampling.
• Figure la is a representation of the function ⁇ representing the luminance of an image.
• Figure 1b is a representation of the sampling function. The figure illustrates the multiplication of the functions shown in the first two figures.
• Figure 1d represents the Fourier transform of the original image and Figure 1d represents the Fourier transform after sampling.
• the method of the invention consists precisely in introducing into the original document, during its preparation, one or more marking images whose components will be placed at frequencies higher than those of the photoco ⁇ pious, so that this or these marking images are invisible on the original, but they appear on photocopies.
• the user is free to choose the marking image he wishes to appear on the photocopy.
• Such an image can for example be that of a text which says "This is a copy” or "This is a false note”.
• the marking image can also be any grayscale or color image. This image is digitized during the preparation of the original document with a sampling step comparable to that of photocopiers. Its discrete two-dimensional Fourier transform is calculated and also represented in digital form.
• FIG. 2 illustrates the four quadrants 1 to 4 of the discrete Fourier transform of the marking image.
• the original document in image form must be scanned with a step at least twice as fine as that of the best photocopier.
• Factor 2 is not critical. Any higher value is fine.
• the protection of the document by the introduction of the invisible marking image on the original can be carried out according to two equivalent methods, the spatial method and the frequency method. We will describe below, by way of example, three variants for each of them.
• this involves calculating a new function y (k, l) where k and 1 represent the indices of the discrete points after digitization, by adding to the digitized image of the original document. initial unprotected nal x (k, l) the marking image z (k, l) multiplied by cosine and sine functions judiciously chosen to place the marking image in the high frequencies, as illustrated in the domain of Fourier in Figure 3.
• this operation is known as amplitude modulation. The exact form of this operation is:
• FIG. 4 A block diagram of this variant of the process is shown in FIG. 4.
• the marking image does not replace the original image, but just superimposed on it.
• FIG. 5 A block diagram of this variant of the process is shown in FIG. 5.
• a third variant of the spatial method lying between the two preceding variants, it is possible to weight the original image with respect to the marking image.
• this third variant it is also a question of hiding a weighted version of the original image in the high frequencies, so that, after photocopying, the appearance of the marking image can be measured. For this, we subtract the modulated and weighted version of the original. The corresponding relation for the calculation is therefore:
• y (k, l) x (k, I) + [z (k, T) - ax (k, I)] lcos [ ⁇ (- + -L)] + cos [ ⁇ ⁇ - - * - )]]] m
• a first variant of the frequency method it suffices to distribute the quadrants of the Fourier transform of the marking image of FIG. 2, in accordance with the locations indicated in FIG. 3 (halftone parts) and to add the corresponding values to the Fourier transform of the original unprotected image. It then suffices to take the inverse Fourier transform of this resulting image to obtain the protected image.
• a block diagram of the process according to this variant is shown in FIG. 7.
• the quadrants 1 to 4 of the Fourier transform of the marking image of FIG. 2 are distributed in accordance with the locations indicated in FIG. 8, by subtracting from them the corresponding quadrants lo, 2o, 3o and 4o of the Fourier transform of the original image (halftone parts). It is then necessary to add the values corresponding to the Fourier transform of the original unprotected image. Finally, it suffices to take the inverse Fourier transform of this resulting image to obtain the protected image.
• FIG. 9 A block diagram of the method according to this second variant of the frequency method is shown in FIG. 9.
• the quadrants of the Fourier transform of the marking image of FIG. 2 are distributed, in accordance with the locations indicated in FIG. 8 by subtracting from them the corresponding quadrants, weighted by ⁇ of the Fourier transform of the original image (halftone parts).
• the factor ⁇ is the same as that of equation (3). It is then necessary to add the values corresponding to the Fourier transform of the original unprotected image. Finally, it suffices to take the inverse Fourier transform of this resulting image to obtain the protected image.
• FIG. 10 A block diagram of the method according to this third variant of the frequency method is shown in FIG. 10.

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• Engineering & Computer Science (AREA)
• Computer Security & Cryptography (AREA)
• Multimedia (AREA)
• Signal Processing (AREA)
• Physics & Mathematics (AREA)
• General Physics & Mathematics (AREA)
• Editing Of Facsimile Originals (AREA)
• Printing Methods (AREA)
• Cleaning In Electrography (AREA)

Applications Claiming Priority (3)

Publications (1)

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• 1993
  • 1993-05-26 EP EP93909744A patent/EP0597056A1/de not_active Withdrawn
  • 1993-05-26 US US08/190,021 patent/US5524933A/en not_active Expired - Lifetime
  • 1993-05-26 JP JP6500060A patent/JPH06511611A/ja active Pending
  • 1993-05-26 WO PCT/CH1993/000135 patent/WO1993025038A1/fr not_active Application Discontinuation
  • 1993-05-26 CA CA002114548A patent/CA2114548A1/en not_active Abandoned

Non-Patent Citations (1)

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