EP3459755A1 - Method for inserting guilloche patterns, method for extracting guilloche patterns, authentication method for said guilloche patterns and implementation devices - Google Patents

Abstract

The invention relates to a method for inserting guilloche patterns on a document, each guilloche pattern being able to encode variable alphanumeric data ensuring a different appearance to each guilloche pattern so as to secure said document,
the method comprising an operation (300-600) for determining an insertion intensity of each previously generated guilloche pattern (100) according to a medium gray level of the document adjacent said guilloché pattern.

The invention also relates to a device implementing this insertion method.

The invention also relates to a method for extracting the guilloche patterns (601-608) from a secure document (600) that can be degraded, the method comprising the following operations:
-determining (710) a contrasts map (c) of at least a portion of the secure document including the guilloche patterns;
- identification (720), on this map of the contrasts, of an optimal percolation trajectory; and
extraction (730) of the optimal percolation trajectory, said trajectory corresponding to the guilloche pattern.

The invention further relates to a method for authenticating the guilloche patterns of a secure document that may be degraded, comprising:
- the previous extraction operations, and
an operation (740) for comparing each extracted guilloche pattern with a corresponding theoretical guilloche pattern.

Description

TECHNICAL AREA

The present invention relates to a method for inserting guilloché patterns containing coded data and a device for inserting said guilloche patterns. It further relates to a method for extracting and authenticating guilloche patterns containing encoded data. It also relates to an authentication device and a secure document that needs to be authenticated such as an identity card, a driver's license, a notarial act, etc. The invention finds applications in the field of securing documents and, in particular, authentication of secure documents even when they are damaged.

STATE OF THE ART

In a known manner, security documents, such as identity documents, generally include an identity photograph of its holder as well as alphanumeric data such as, for example, the name of the holder, his date of birth, his size, address, document number, date of issue of the document and / or the identity of the issuing authority.

To secure these identity documents, it is known to associate identity photography with guilloches. These guilloches form a network of wavy lines, superimposed on the photograph and visible to the naked eye. The guilloches are intended to make discernable, during an identity check, any fraudulent modification of the photograph. In particular, they make it possible to guard against changes in the hue of the photograph, for example when adding a beard or hair. Thus, the guilloches generally allow the implementation of level 1 (visual inspection with the naked eye) and / or level 2 (visual control with a simple optical equipment such as a magnifying glass or an ultraviolet lamp) .

Furthermore, it is also known to implement alphanumeric data protection means mentioned on identity documents. in order to guard against alterations of these data such as, for example, changes to these alphanumeric mentions.

The protection of alphanumeric data can be obtained by digital tattooing techniques (or watermarking, in English-speaking terms) intended to integrate elements invisible to the naked eye in an identity photograph, these elements coding predefined information. Digital tattoo provides level 3 security (automatic verification of encoded data) during a security check. Thus, by means of a digital processing and analysis apparatus, the identity photograph is analyzed and the data tattooed there are extracted. A consistency check of these data is then performed.

However, digital tattooing has a number of disadvantages. Indeed, since it is not visible to the naked eye, the digital tattoo does not allow to implement a level 1 or 2 security. The verification of secure tattooed documents therefore requires a trained staff, equipped with means analysis, which limits the security controls, especially the itinerant controls. In addition, digital tattoos may be sensitive to degradations of the identity document, degradations that can be induced, during the validity period of the document, by natural wear, exposure to UV rays, lack of care from the holder, etc. The analysis of degraded digital tattoos can then lead to an incorrect reading of tattoos, or to an impossibility of reading during the control.

Another known technique also makes it possible to secure the alphanumeric data mentioned on the identity documents. This technique proposes to insert, on the identity photograph, variable guilloches encoding all or part of the alphanumeric data. Examples of document security methods using such variable guilloches are described in particular in patent applications. EP 2 325 022 A1 and US 2010/0260372 filed in the name of the plaintiff.

These guilloches must allow both level 1 and level 3 security to be implemented. To implement level 1 security, the guilloches must not interfere with the visual perception of the underlying content. underlying of the document, in particular of the photograph, and not to prevent a good readability of the characteristic features of the holder of the secure document.

Moreover, and in an antithetical manner, the guilloches must be sufficiently marked to be able to be retrieved automatically by means of an authentication device and thus to meet a level of security 3. Indeed, the implementation of a security of Level 3 requires, prior to any extraction of the guilloches, a print phase of the photograph on the document and then a scanning phase of the scanned image of the photograph and the superimposed guilloches. However, these print and scan phases (called "print-scan" in English terms) generate a double degradation of the quality of the image, the degree of degradation depending in particular on the quality of the printing system and of digitization. An example of an original image and its degraded images is shown on the figure 1 . In particular, image (a) of the figure 1 represents an example of an original digital image, comprising a photograph of the holder of the secure document on which are superimposed guilloches. The images (b), (c) and (d) represent the image (a) degraded by a printing and scanning system, respectively, of high quality, medium quality and low quality. In addition to the double degradation of printing and scanning, secure documents tend to suffer additional degradation during their period of validity, such as natural wear, the effects of UV rays, physical aggression, etc. Also, so that the automatic extraction of the guilloche, security level 3, is not hampered by the damage suffered by the security document, it is important that these guilloches are inserted sufficiently marked in the document.

In addition, to implement a security level 3, the guilloches affixed to the photograph must be extracted automatically by means of an authentication device in order to be verified and / or authenticated. A level 3 verification / authentication of a document secured by guilloches requires, prior to any extraction of the guilloches, a print phase of the photograph on the document and then a scanning phase of the scanned image of the photograph and superimposed guilloches. However, these phases of printing and digitization (called "print-scan" in English-speaking terms) generates a double degradation of the quality of the image, the degree of degradation depending in particular on the quality of the printing and scanning system. An example of an original image and its degraded images is shown on the figure 6 . In particular, image (a) of the figure 6 represents an example of an original digital image, comprising a photograph of the holder on which are superimposed guilloches. The images (g), (h) and (j) represent the image (a) degraded by a printing and scanning system, respectively, of high quality, medium quality and low quality.

In addition, like any secure document, the documents secured by guilloches undergo additional degradations during their period of validity, such as natural wear, the effects of UV rays, physical aggression, etc.

The double degradation of the printing and scanning phase, to which can be added additional degradations, has the effect of hindering the extraction of the guilloches, which leads to an impossibility to check / authenticate the alphanumeric data encoded in guilloches and compromises the implementation of level 3 security.

SUMMARY OF THE INVENTION

To respond to the double problem mentioned above of readability with the naked eye of the document with guilloches and extraction of the guilloches when the secure document is degraded, the applicant proposes to insert the guilloches, or guilloche patterns, with an intensity of Insertion determined according to the average gray level of the document in the vicinity of the guilloche patterns.

To address the problem mentioned above of the impossibility of controlling a secure document by guilloches when the quality of the guilloches is degraded, the applicant proposes to extract the guilloches, or guilloche patterns, from trajectories generated according to the contrasts of the secure document.

According to a first aspect, the invention relates to a method of inserting guilloche patterns on a document, each guilloche pattern being able to encode variable alphanumeric data ensuring a different appearance to each guilloche pattern so as to secure said document. This method is characterized in that it comprises an operation for determining an insertion intensity of each guilloche pattern previously generated according to an average gray level of the document in the vicinity of said guilloche pattern.

This process is used to insert the guilloche patterns with an intensity that depends on the gray level of the document where the guilloche pattern is affixed. The marking of the guilloche pattern on the document is thus more or less marked is a function of the photographic environment.

• aux documents d'identité tels que les cartes d'identité, les passeports, les permis de conduire, etc., où les motifs guillochés sont généralement appliqués sur la photographie du détenteur,
• à des documents sécurisés avec ou sans photographie, tels que les actes d'état civil, les actes notariés (« brider documents » en termes anglo-saxons) ou encore les documents d'identité mobiles (« mobile ID » en termes anglo-saxons), où la totalité du document contient des motifs guillochés, ou
• à des documents de protection des marques (« brand protection » en termes anglo-saxons) ou autre marquage physique (par exemple emballages et objets de luxe) dont on cherche à authentifier l'origine.

This process can be applied to all kinds of secure documents. It can apply in particular:

• identity documents such as identity cards, passports, driving licenses, etc., where the guilloché motifs are generally applied to the holder's photograph,
• secure documents with or without photographs, such as civil status documents, notarised documents ("brider documents" in English-speaking terms) or even mobile identity documents ("mobile ID" in Anglo-Saxon terms) ), where the entire document contains guilloché motifs, or
• trademark protection documents ("brand protection" in Anglo-Saxon terms) or other physical markings (eg packaging and luxury goods) whose authenticity is sought to be authenticated.

Advantageously, the insertion intensity of a guilloche pattern comprises a contrast value and a thickness of said guilloche pattern. The variation of the thickness and the contrast of a guilloche pattern make it possible to more or less mark said pattern on the document.

According to some embodiments, the determination of the insertion intensity of the guilloche pattern comprises a calculation operation, at each pixel of said guilloche pattern, of the contrast value between an internal zone with the guilloche pattern and an external zone, close to said internal area.

According to one or more embodiments, the contrast value is obtained by comparing the average gray level of the inner zone and the average gray level of the outer zone.

According to some embodiments, the comparison between the average gray level of the inner zone and the average gray level of the outer zone is performed by a logarithmic image processing (called "Logarithmic Image Processing"). or LIP).

According to some embodiments, the determination of the insertion intensity of the guilloche pattern includes a step of selecting a thickness of the guilloche pattern.

In some embodiments, the outer area is an area of pixels contiguous to the inner area.

According to some embodiments, the guilloche pattern is a sinusoidal pattern, affixed substantially horizontally to the document, the outer zone being positioned vertically to the pixel (s) of the guilloché pattern.

According to some embodiments, the guilloche pattern is a set of streaks corresponding to a fingerprint, for example that of the holder of the document, the inner and outer zones being positioned along a normal to the curve at the pixel in question. According to a second aspect, the invention relates to a device for inserting guilloche patterns on a document, comprising a computer comprising a set of instructions that cause said computer to implement the method for inserting guilloche patterns as defined herein. -above. This device allows to insert guilloche patterns with more or less marked marking depending on the photographic environment.

According to a third aspect, the invention relates to a secure document on which are affixed guilloche patterns encoding alphanumeric data, characterized in that the guilloche patterns are inserted on the document by the insertion method defined above.

According to a fourth aspect, the invention relates to an identity document comprising an identity photograph and alphanumeric data relating to the holder, characterized in that the identity photograph comprises visibly guilloche patterns encoding the alphanumeric data, inserted by the insertion method defined above.

• détermination d'une carte des contrastes d'une portion au moins du document sécurisé incluant les motifs guillochés ;
• identification, sur cette carte des contrastes, d'une trajectoire de percolation optimale ; et
• extraction de la trajectoire de percolation optimale, ladite trajectoire correspondant au motif guilloché.

According to a fifth aspect, the invention relates to a method for extracting guilloche patterns from a secure document that may be degraded, the method comprising the following operations:

• determining a contrasts map of at least a portion of the secure document including the guilloche patterns;
• identification, on this map of contrasts, of an optimal percolation trajectory; and
• extraction of the optimal percolation trajectory, said trajectory corresponding to the guilloche pattern.

This process makes it possible to extract guilloche patterns even when they are difficult to perceive with the naked eye. It thus makes it possible to extract the guilloche motifs from secured documents having undergone damage.

• aux documents d'identité tels que les cartes d'identité, les passeports, les permis de conduire, etc., où les motifs guillochés sont généralement appliqués sur la photographie du détenteur,
• à des documents sécurisés avec ou sans photographie, tels que les actes d'état civil, les actes notariés (brider documents, en termes anglo-saxons) ou encore les documents d'identité mobiles (mobile ID, en termes anglo-saxons), où la totalité du document contient des motifs guillochés, ou
• à des documents de protection des marques (brand protection, en termes anglo-saxons) ou autre marquage physique (par exemple emballages et objets de luxe) dont on cherche à authentifier l'origine.

This process can be applied to all kinds of secure documents. It can apply in particular:

• identity documents such as identity cards, passports, driving licenses, etc., where the guilloché motifs are generally applied to the holder's photograph,
• secure documents with or without photographs, such as civil status documents, notarised documents (in English terms), or mobile ID documents (mobile ID, in English terms), where the entire document contains guilloché motifs, or
• trademark protection documents (brand protection, in Anglo-Saxon terms) or other physical markings (for example packaging and luxury goods) whose authenticity is sought to be authenticated.

Advantageously, the contrast map is constructed using a logarithmic image processing (LIP) model. This LIP model makes it possible to determine contrasts that are compatible with human vision.

• une identification, sur la carte des contrastes, de toutes les trajectoires susceptibles de traverser le document sécurisé pour relier un point de départ d'un motif guilloché à un point d'arrivée dudit motif ;
• une détermination d'une fonction de coût de chacune des trajectoires, ladite fonction de coût évaluant une variabilité du contraste entre le contraste déterminé pour chaque trajectoire et un contraste attendu ;
• une comparaison des fonctions coûts de toutes les trajectoires ; et
• une détermination de la fonction coût minimale, la trajectoire comportant la fonction coût minimale correspondant au motif guilloché.

According to some embodiments, the operation of identifying the optimal percolation trajectory comprises:

• an identification, on the contrasts map, of all the trajectories likely to cross the secure document to connect a starting point of a guilloche pattern to a point of arrival of said pattern;
• a determination of a cost function of each of the trajectories, said cost function evaluating a variability of the contrast between the contrast determined for each trajectory and an expected contrast;
• a comparison of the cost functions of all the trajectories; and
• a determination of the minimum cost function, the trajectory comprising the Minimum cost function corresponding to the guilloche pattern.

According to one or more embodiments, when the document and in particular the photograph is in color, the contrast map is calculated from the luminance image associated with the color image.

• les opérations d'extraction des motifs guillochés définies précédemment, et
• une opération de comparaison de chaque motif guilloché extrait avec un motif guilloché théorique correspondant.

According to a sixth aspect, the invention relates to a method for authenticating the guilloche patterns of a secure document that may be degraded, this method comprising:

• the extraction operations of the guilloche motifs defined previously, and
• a comparison operation of each extracted guilloche pattern with a corresponding theoretical guilloche pattern.

This authentication method makes it possible to control a secure document with a security level 3, even when the secure document is damaged and / or the quality of the guilloche patterns is degraded.

According to one or more embodiments, when each guilloche pattern encodes variable alphanumeric data mentioned on said secure document, the theoretical guilloche pattern is obtained by optical character recognition (OCR) alphanumeric data mentioned on the secure document and generation guilloché patterns corresponding theoretical.

According to some embodiments, when the secure document comprises an electronic chip, the theoretical guilloche pattern is an original guilloche pattern, previously stored in the chip during the generation of the guilloche pattern affixed to the secure document.

According to one or more embodiments, the comparison operation of the extracted and theoretical guilloche patterns comprises an estimate of a score based on a distance between each point of the extracted guilloché pattern and a corresponding point of the theoretical guilloche pattern.

According to one variant, the score is calculated by accumulating distances for all the points of the guilloche pattern.

According to some embodiments, the distance is calculated in number of pixels.

According to some embodiments, a weight is assigned to each distance, the distant distances having a greater weight than short distances. This weight can be flexible.

According to a seventh aspect, the invention relates to a guilloche pattern authentication device comprising a computer comprising a set of instructions that cause said computer to implement the guilloche pattern authentication method defined above.

According to an eighth aspect, the invention relates to an identity document comprising alphanumeric data relating to the holder and an identity photograph on which are affixed guilloche patterns encoding the alphanumeric data, characterized in that the guilloche patterns are capable of being authenticated by the method defined above.

BRIEF DESCRIPTION OF THE FIGURES

• La figure 1, déjà décrite, représente un exemple d'un document sécurisé d'origine et de documents sécurisés détériorés ;
• La figure 2 représente un exemple d'une photographie d'origine, de cette même photographie avec guilloches et de cette photographie sécurisée après impression et numérisation ;
• La figure 3 représente un exemple d'une tranche d'un motif guilloché à partir duquel est calculée la valeur de contraste ;
• La figure 4 représente des exemples de photographies avec des motifs guillochés insérés selon le procédé de l'invention et, pour chacune de ces photographies, un exemple des motifs guillochés détectés lors d'une vérification de sécurité de niveau 3 ;
• La figure 5 représente un diagramme fonctionnel d'un exemple de procédé d'insertion selon l'invention.
• La figure 6, déjà décrite, représente un exemple d'un document sécurisé d'origine et de documents sécurisés détériorés ;
• La figure 7 représente un exemple des différents états d'un document sécurisé pendant le procédé d'extraction selon l'invention ;
• La figure 8 représente un exemple d'un motif guilloché extrait et d'un motif guilloché théorique ;
• La figure 9 représente des exemples de documents sécurisés authentifiables et de documents sécurisés non-authentifiables selon le procédé de l'invention ;
• La figure 10 représente un diagramme fonctionnel d'un exemple de procédé d'authentification selon l'invention.

Other advantages and characteristics of the invention will appear on reading the description, illustrated by the figures in which:

• The figure 1 , already described, represents an example of a secure original document and damaged security documents;
• The figure 2 represents an example of an original photograph, this same photograph with guilloches and this secure photograph after printing and digitization;
• The figure 3 represents an example of a slice of a guilloche pattern from which the contrast value is calculated;
• The figure 4 represents examples of photographs with guilloché motifs inserted according to the method of the invention and, for each of these photographs, an example of the guilloche patterns detected during a level 3 security check;
• The figure 5 represents a functional diagram of an exemplary insertion method according to the invention.
• The figure 6 , already described, represents an example of a secure original document and damaged security documents;
• The figure 7 represents an example of the different states of a secure document during the extraction process according to the invention;
• The figure 8 represents an example of an extracted guilloche pattern and a theoretical guilloche pattern;
• The figure 9 represents examples of authenticatable secure documents and non-authenticatable secure documents according to the method of the invention;
• The figure 10 represents a functional diagram of an exemplary authentication method according to the invention.

DETAILED DESCRIPTION OF AT LEAST ONE EMBODIMENT

An exemplary embodiment of a method for inserting guilloche patterns in a secure document is described in detail below with reference to the accompanying drawings. An exemplary embodiment of a method for authenticating guilloche patterns of a secure document, even when said secure document is degraded, is also described in detail below, with reference to the accompanying drawings. These examples illustrate the features and advantages of the invention. However, it is recalled that the invention is not limited to these examples.

In the figures, the identical elements are identified by identical references. For questions of readability of the figures, the size scales between elements represented are not respected.

The method of the invention proposes to insert guilloche patterns on a portion or on the entirety of a secure document such as those mentioned above. In the following description, the method will be described when it is applied to a portion of a secure document such as a photograph on which are superimposed the guilloche patterns, also called guilloches. In the description, the term "photograph" will be understood indifferently as a portion of secure document or an underlying content. Those skilled in the art will also understand that the method as it will be described may also be implemented for a document whose entire document comprises guilloche patterns.

These guilloche patterns are, for example, one-dimensional patterns, or 1D guilloche patterns, adapted to encode alphanumeric data such as the holder's personal data mentioned on the identity documents. Each 1D guilloche pattern is in the form of a variable wavy line, visible with the naked eye, on an underlying content, and in particular on a photograph.

According to the invention, the visual saliency is sufficiently low that it does not disturb the visual perception of the underlying content, that is to say the portion of the secure document positioned under the guilloche patterns - for example the photograph - all by being sufficiently marked to allow automatic extraction even when the secure document is degraded for reasons such as those mentioned above.

Examples of a photograph of a secure document are shown on the figure 2 . The image (a) of this figure 2 represents a digital photograph of the holder. Image (b) of this figure 2 represents the photograph (a) on which guilloché motifs according to the invention have been affixed. As shown in this image (b), the guilloche patterns do not disturb the readability of the photograph; in other words, the holder is recognizable to the naked eye, despite the presence of guilloche patterns. Level 1 security is assured. The image (c) of this figure 2 represents an example of the image (b) when it has been deteriorated by a printing and scanning process. This image (c) shows that, even in the presence of deterioration, the guilloche patterns inserted according to the method of the invention remain sufficiently marked to be detected automatically by an authentication machine, so as to ensure the level 3 security.

In order for the guilloche patterns to be inserted into the photograph sufficiently marked to ensure the level 3 of security while allowing the readability of the photograph, the method of the invention proposes to adapt the insertion intensity of each guilloche pattern. depending on the gray level of the photograph. The insertion intensity is determined, for each pixel of the guilloche pattern, as a function of the average gray level of the pixels of the photograph around said guilloche pattern. According to the invention, the insertion intensity is defined by the thickness of the guilloche pattern and the level of contrast between the photograph and said guilloché pattern.

In the method of the invention, the thickness is the number of pixels aligned in the same direction (for example horizontal or vertical) to form the guilloché pattern inserted in the photograph.

In general, contrast is a property of an image that quantifies the difference in brightness between the light and dark parts of the image. In the method of the invention, the contrast is the difference in brightness between the guilloche pattern and the area of the photograph in the vicinity of said guilloche pattern. Contrast is a unitless value, calculated for each pixel of each guilloche pattern.

In order to calculate the contrast, the method of the invention proposes to calculate the contrast between the average gray level of an area internal to the guilloche pattern and the average gray level of at least one zone external to said guilloché pattern. The figure 3 is an example of a photograph portion having an internal zone zi with a guilloche pattern and two outer zones ze with said guilloche pattern. This portion of photography comprises several pixels p1 - pn aligned in the same direction. The inner zone zi comprises the pixels of a slice of the guilloche pattern. Each outer zone ze comprises pixels of the photograph, aligned with the pixels of the inner zone zi. In the example of the figure 3 , the pixels p1 - pn are aligned vertically because the guilloche pattern is a pattern having a sinusoidal shape extending in a generally horizontal direction, as will be described in more detail later.

The photograph portion of the figure 3 comprises those pixels p1 - pn which form the edge of the guilloche pattern. In particular, it comprises the pixel px, called the current pixel, whose contrast is sought to be determined. These pixels of the inner zone zi constitute the thickness of the guilloche pattern. This thickness is variable. In the example of the figure 3 the thickness is 5. It can also be, for example, 7 or 9, as in the examples of the figure 4 described later. The thickness of the guilloche as well as the contrast - the insertion force - are chosen according to the psycho-visual impact (the guilloches must not disturb the observation of the photograph) and the robustness (the extraction guilloché patterns should work properly, even after aging). The thickness chosen is, furthermore, highly dependent on the quality of the printing and scanning system: the printing step and the scanning step each have their own resolution (pixel dimension), in general worse than that of the initial image. This has the effect of disturbing the colors of the pixels located at the border of the guilloche pattern: the thickness of the guilloche pattern must be sufficient for the colors of the central pixels (near the axis of said guilloche pattern) to be correct, so that to prepare a future extraction.

The photograph portion of the figure 3 also has external zones ze contiguous to the inner zone zi. Each of these external zones ze includes the pixels of the photograph that are in the vicinity of the inner zone zi. The number of pixels of each external zone ze is predefined. In the example of the figure 3 , the external zone ze1, like the external zone ze2, comprises two pixels selected in the continuity and the alignment of the pixels of the inner zone zi. Thus, the pixels of the outer zones ze1, ze2 and of the inner zone zi are aligned in the same direction. In the example of the figure 3 , the pixels of the external ze1 and ze2 are aligned vertically with the pixels of the inner zone zi, respectively above and below the inner zone zi, insofar as the guilloche pattern is globally horizontal.

Indeed, the skilled person will understand that if the guilloche pattern is affixed in a horizontal direction in the photograph, a slice of said guilloche pattern comprises vertically aligned pixels; the inner and outer zones are then aligned vertically, as in the example of the figure 3 . On the contrary, if the guilloche pattern is affixed in a vertical direction on the photograph, then the edge of said guilloche pattern includes horizontally aligned pixels; the inner and outer zones are then aligned horizontally.

In some embodiments, the guilloche pattern is a sinusoidal pattern, passing right through the photograph horizontally or vertically, or even at an angle. When the guilloche pattern is sinusoidal, the direction of said pattern is the overall direction of travel of the photograph.

In some embodiments, the guilloche pattern may consist of the streaks of an imprint, for example a fingerprint of the holder. In this case, the inner and outer zones are positioned along a normal to the curve at the point considered, that is to say orthogonally to a direction of tangent estimated near said point.

According to the invention, the contrast is calculated for each pixel px of the inner zone zi by comparing the average gray level of the inner zone zi and the average gray level of the outer zones ze. In the case of a color image, the average gray level is determined from the luminance image associated with the color image.

The comparison between the average gray level of the inner zone and the average gray level of the outer zone is achieved by a logarithmic image processing known as LIP (or "Logarithmic Image Processing", in English terms). ) and describes, for example, in the document Mr. Jourlin and JC. Pinoli, "A Model for Logarithmic Image Processing," Journal of Microscopy, 149 (1), pp 21-35, 1988 or in the document M. Jourlin, "Logarithmic Image Processing: Theory and Applications," Advances in Imaging and Electron Physics, Vol. 195, 253 p, 2016 .

Thus, at each pixel of the guilloche pattern, a contrast value is calculated which is added to or subtracted from the LIP image so as to obtain an additive contrast LIP selected from the average gray levels of the inner zone zi and the zones external ze. The same contrast value is then applied to all points of the inner zone zi. Thus, the pixels of the same guilloche pattern slice all have the same contrast value. On the other hand, the guilloche pattern may have a variable contrast value from one slice to another. In other words, the same guilloche pattern may have a contrast that varies over its length. In the example of the figure 2 , guilloche patterns may be more pronounced in the character's hair areas than in the background areas

The figure 4 represents several examples of the same photograph on which are affixed guilloche motifs of different insertion intensity. Although it may be different from one guilloche pattern to another, the intensity of insertion of the guilloche motifs is chosen to be identical for all the motifs of the same image, in the examples of the figure 4 . In these examples of figure 4 , the insertion intensity differs from one image to another. The images (a1) (b1) and (c1) are images, after printing and scanning, of the same holder. The guilloche patterns of the image (a1) have a thickness of 5, which corresponds to a guilloché pattern slice of 5 pixels aligned vertically. The guilloche patterns of the image (b1) have a thickness of 7 and the guilloche patterns of the image (c1) have a thickness of 9. The contrast value of each of these examples was determined as the thicknesses, depending the quality of the printing and scanning system: a stronger contrast makes extraction easier. We can see that, although the guilloche motifs are inserted more intensely on the image (c1) than on the image (a1), the character of the image (c1) is clearly recognizable to the naked eye.

The images (a2) (b2) and (c2) of the figure 4 are images after automatic detection of guilloche patterns during a level 3 check. The guilloches visible in these figures (a2) (b2) and (c2) are the guilloches detected (reconstructed) by the authentication machine when level 3 security. We can see that even if the guilloche patterns are inserted less intensely in the image (a1) than in the image (c1), the respective images (a2) and (c2) show that the detection of the guilloche patterns gives equally good results with the patterns guilloche of the image (a1) with those of the image (c1).

On the figure 5 is shown an example of a functional diagram showing the various steps of the insertion method according to the invention. This method comprises a first step 100 of generating a guilloche pattern. The guilloche pattern is a curve resulting from a function, for example sinusoidal, which encodes alphanumeric data. Many documentations describe methods of generating guilloches. The generation of guilloche motifs will therefore not be described in this application.

The guilloche patterns generated are then positioned - step 200 - on the photograph (or other document) with a predefined distance which depends, for example, on the number of guilloche patterns to be affixed and / or the importance of the modulation used during the printing. generation of guilloche motifs.

The method then comprises a step 300 for selecting and applying a thickness of guilloche. When the guilloche is generated and its thickness has been defined, a step 400 for calculating the contrast value is performed for each pixel of the guilloche. As long as all the N pixels of the guilloche pattern have not been processed, the operation 400 for calculating the contrast value is reiterated for the next n pixel (steps 410 and 420).

When the contrast value is determined, the guilloche is inserted on the document in step 500. As long as all the K guilloches have not been inserted on the document (steps 510 and 520), the process is reiterated from step 300 of applying the thickness for the following guilloche k. When all the guilloches have been inserted (step 510), the document is secured (step 600).

The determined contrast value, associated with the chosen thickness of the guilloche pattern, constitutes the intensity of insertion of a guilloche. The insertion intensity allows more or less intense marking of each guilloche pattern, which depends directly on the gray level of the document on which it is affixed. The intensity of the marking is therefore directly dependent on the gray level of the document in the vicinity of the guilloche. This allows that, whatever the state of the document, the underlying content is visible to the naked eye and the guilloche patterns can be detected and the encoded data authenticated.

The insertion method as just described can be implemented in a guilloché pattern insertion device on a document. This device comprises at least one computer executing a set of instructions that cause said computer to implement this method of inserting the guilloches.

The method of the invention proposes to authenticate guilloché patterns inserted in a portion or in the entirety of a secure document such as those mentioned above. In the following description, the method will be described when it is applied to a portion of a secure document such as a photograph on which are superimposed guilloche patterns, also called guilloches. Those skilled in the art will understand that the method as it will be described can also be implemented for a document whose entire document comprises guilloche patterns. These guilloche patterns are, for example, one-dimensional patterns, or 1D guilloche patterns, adapted to encode alphanumeric data such as personal data of the holder, mentioned on the identity documents. Each 1D guilloche pattern is in the form of a variable wavy line, visible to the naked eye, on an underlying content, and in particular on a photograph, but whose visual saliency is sufficiently low enough not to disturb the visual perception of the underlying content, that is, the portion of the secure document positioned under the guilloche patterns.

An example of a document secured by guilloche motifs 601-608 is shown in the image (a) of the figure 7 . In this example, the portion 600 of the secure document is a photograph on which are superimposed guilloche patterns 1D, referenced 601 - 608. As shown in the secure document 600, the guilloche patterns 601-608 are sets of wavy lines and irregular , whose irregularities correspond to coded data. The guilloche patterns 601-608 are different from each other. They are all weakly prominent, which allows not to visually disturb the main features of the character of the photograph 600. In other words, the salience of guilloche patterns on the underlying content is limited so that the attention of the controller is not focused on the guilloches but on said underlying content.

In the remainder of the description, the underlying content on which the guilloche patterns are affixed will be indifferently called photograph, underlying content or image (a).

An example of the authentication method according to the invention is represented on the figure 10 . This method comprises several operations 710 - 730 making it possible to extract the guilloche patterns from the digitized secure document 600. Indeed, as previously explained, the security level 3 requires, before the implementation of the authentication method, a phase of authentication. scanner printing and scanning of the secure document. However, this phase of printing and digitization degrades the quality of information contained in the secure document, and in particular that of photography and guilloche patterns. An example of a photograph with original guilloches, named image (a), and the same photograph with guilloches after degradation by the printing and scanning phase, named image (b), are represented on the figure 7 . As shown by the comparison between the images (a) and (b), the guilloche patterns of the image (b) are substantially degraded compared to those of the image (a), which makes the automatic extraction of these guilloché patterns more complex. To extract the guilloche patterns from the image (b), it is therefore necessary to perform the following operations described below.

The extraction method according to the invention firstly comprises an operation 710 for determining a contrast map of the portion 600 of the secure document. This operation, which aims to reveal the contrasts along the guilloche patterns of the image (b), comprises a calculation, at each point of each guilloche pattern, of the contrast between the average gray level of an internal zone and the average gray level of an external zone of said point. The internal zone of the guilloché pattern is defined as the vertical neighborhood of the pixel in question, with a predetermined thickness, for example of two pixels, on either side of said pixel. The outer area of the guilloché pattern is defined as the vertical neighborhood around the inner area, with a predetermined thickness, for example four pixels, around said inner area.

The contrast calculation, according to some embodiments, is based on the Logarithmic Image Processing (LIP) model described, for example, in Mr. Jourlin and JC. Pinoli, "A Model for Logarithmic Image Processing," Journal of Microscopy, 149 (1), pp 21-35, 1988 or in the document M. Jourlin, "Logarithmic Image Processing: Theory and Applications," Advances in Imaging and Electron Physics, Vol. 195, 253 p, 2016 . The general principle of this contrast calculation is that, in each pixel of the guilloche pattern, the additive contrast LIP chosen between the average gray values of the internal zone to the guilloche pattern and the external zone is calculated.

The computation of the contrast at each point of the guilloche motifs makes it possible to generate a map of the contrasts, as represented by the image (c) of the figure 7 .

When the document, and in particular the photograph, is in color, the contrast map is calculated from the luminance image associated with the color image. For this, a luminance image (grayscale) calculated from the three color planes is previously extracted.

The extraction method then comprises an operation 720 of identification, on this contrast map, of the optimal percolation trajectory. According to some embodiments, this operation 720 includes an identification of all the percolation trajectories that may pass through the image (c) to connect a starting point of a guilloche pattern to an end point of said guilloché pattern. Indeed, the guilloche patterns 1D are positioned in the secure document in a horizontal overall direction. Each guilloche pattern thus crosses the image from side to side, from left to right. The method then proposes to search, in the image (c) of contrasts, all the trajectories that cross the image from left to right. Those skilled in the art will understand that in the case where the guilloche patterns have a non-horizontal global direction, for example vertical or bias, the starting points and the arrival points of the desired percolation paths are not necessarily on the left. and to the right of the image but, for example, at the top and bottom of the image.

The method then proposes to determine, among all the percolation trajectories detected, the optimal percolation trajectory, that is to say the trajectory that has the most preferred direction. This detection of the optimal percolation trajectory is obtained by calculating a cost function for each of the trajectories and by determining the minimum cost function. The cost function of a trajectory corresponds to the variability of the contrast between the contrast determined for each trajectory and an expected contrast, previously determined. The closer the contrast is to the expected contrast, the lower the cost function. The cost function therefore favors points with contrast levels that are close to the contrast value chosen for inserting the guilloche pattern into the secure document. The trajectory of Optimal percolation is therefore, among all the percolation trajectories detected, that which has the smallest cost function.

According to the method, it is considered that this optimal percolation trajectory is the guilloche pattern. The guilloche pattern is therefore extracted (step 730 of the figure 10 ) when the optimum percolation trajectory has been determined. In the example of the figure 7 , image (d) shows the optimal percolation trajectories, that is to say the guilloche patterns 611-618 from the contrast image (c). A comparison of the image (d) with the image (a) of the figure 7 allows to understand that the guilloche patterns extracted 611-618 with the method described above - image (d) - are close to the original guilloche patterns 601-608 of the image (a). These guilloche patterns extracted 611-618, although having an image (b) where they are not very visible, can be used at security level 3. Authentication of these guilloche patterns can then be implemented.

The authentication method according to the invention comprises an operation 740 for comparing each extracted guilloche pattern with the corresponding theoretical guilloche pattern. According to some embodiments, the theoretical guilloche pattern is the original guilloche pattern, that is to say the one that was initially generated and affixed to the content of the document to be secured. In these embodiments, each guilloche pattern is stored in an electronic chip housed in the secure document. At the time of the authentication of the guilloche pattern, the guilloche pattern extracted (for example the pattern 618) is compared with the corresponding guilloche pattern which is stored in the chip (for example the pattern 608). Comparing each of the extracted guilloché motifs with the original guilloche patterns can be used to determine whether the secure document has been tampered with or not.

In other embodiments, the theoretical guilloche pattern is obtained by means of OCR type optical character recognition, carried out for example during the scan (scanning by scanner) of the secure document. In these embodiments, the alphanumeric data mentioned on the secure document next to the image (a) are determined by optical recognition and theoretical guilloche patterns are generated from these alphanumeric data recognized. The guilloche patterns extracted are then compared to these theoretical guilloche motifs and their comparison makes it possible to determine whether the secure document has been falsified or not.

Whatever the technique for obtaining the theoretical guilloche motifs, the extracted guilloche patterns are compared with the theoretical guilloche patterns, for example by superimposing said extracted patterns and said theoretical patterns. This comparison can be based on an estimate of a score calculated for each pair of guilloche patterns, a pair consisting of an extracted guilloche pattern and a theoretical guilloche pattern. This score estimation operation, referenced 741 on the figure 10 , can be obtained by calculating, at each point of the x-axis (when the guilloche patterns extend horizontally), the distance between the extracted guilloche pattern and the theoretical guilloche pattern. Thus, in an orthonormal frame XY, the score at a point x of the X axis corresponds to the distance along the Y axis between the extracted guilloche pattern and the theoretical guilloche pattern. This distance is preferably calculated in number of pixels. An example of calculating the score at a point x is represented on the figure 8 . In this example, at the point x of the X axis, the distance - referenced d - separating the extracted guilloche pattern 618 from the theoretical guilloche pattern 608 is the length along the Y axis between the two patterns 608 and 618. The scores calculated in each point of the X axis, for a pair of guilloche motifs, are cumulated to form the score of the extracted guilloche pattern. The smaller the score, the closer the guilloche pattern is to the theoretical guilloche pattern. On the figure 10 the comparison of the score with a predetermined threshold is represented by step 742. Below a predefined score threshold, the guilloche pattern is considered to be authenticated (block 780 of the figure 10 ). Above the predefined score threshold, the guilloche pattern is considered unauthenticated (block 790 of the figure 10 ).

The scores of all pairs of guilloche patterns may also be cumulated to authenticate or not the secure document as a whole. Two examples of images (b) after printing and scanning are represented on the figure 9 , each of these images (b) being associated with an example of an authenticated image (e) and an example of an unauthenticated image (f). Each of the images (e) and (f) has, superimposed on the photograph, the theoretical guilloche motifs completed by the distances between extracted and theoretical guilloche motifs. This distance is symbolized by the thickness of the guilloche line. In the case of the images (e), the lines representing the guilloche patterns are thin. The cumulative score is less than or equal to the predetermined threshold. The image (e) is authenticated. On the contrary, on the images (f), the lines representing the guilloche motifs are thick over a great length of each motif. The cumulative score is greater than the predetermined threshold. The image (f) is not authenticated.

In some embodiments, a weight is assigned to each score before being accumulated. Indeed, the extraction is generally not perfect, it is normal that small distances exist between two patterns of a same pair of guilloche patterns, as is the case in the example of the images (e) of the figure 9 . On the other hand, when the distances are large, as in the example of the images (f) of the figure 9 there is suspicion of falsification. The method therefore proposes to assign a greater weight to large distances than to small distances so as to optimize the cumulative score. A "long distance" is a distance between a point of an extracted guilloche pattern and the corresponding point (for example of the same abscissa) of the theoretical guilloche pattern, which generates an extra thickness of the guilloche. On the other hand, a "small distance" does not generate extra thickness in the guilloche.

Thus, in some variants, the weights assigned to the scores may be adjustable. For example, they can be proportional to the square of the distance, to its exponential, and so on. in order to maximize the cumulative score and make the falsified guilloche patterns more easily detectable.

Although described through a number of examples, variants and embodiments, the method of inserting guilloche patterns according to the invention comprises various variants, modifications and improvements which will become apparent to those skilled in the art, being understood that these variants, modifications and improvements are within the scope of the invention.

Although described through a number of examples, variations and embodiments, the methods for extracting and authenticating guilloche patterns according to the invention include various variations, modifications, and improvements that will become apparent to humans. of the trade, it being understood that these variants, modifications and improvements fall within the scope of the invention.

Claims (26)

A method of inserting guilloche patterns on a document, each guilloche pattern being able to encode variable alphanumeric data ensuring a different appearance to each guilloche pattern so as to secure said document,
the method comprising an operation (300-600) for determining an insertion intensity of each previously generated guilloche pattern (100) according to a medium gray level of the document adjacent said guilloché pattern. A guilloché pattern insertion method according to claim 1, characterized in that the insertion intensity of a guilloche pattern comprises a contrast value and a thickness of said guilloché pattern. Method for inserting guilloche patterns according to claim 1 or 2, characterized in that the determination of the insertion intensity of the guilloche pattern comprises a calculation operation (400), at each pixel of said guilloche pattern, of the value of contrast between an internal zone (zi) with the guilloche pattern and an external zone (ze), close to said internal zone. A guilloché pattern insertion method according to claim 3, characterized in that the contrast value is obtained by comparing the average gray level of the inner zone (zi) with the average gray level of the outer zone (ze). . Method for inserting guilloche patterns according to claim 4, characterized in that the comparison between the average gray level of the inner zone and the average gray level of the outer zone is performed by a logarithmic image processing (LIP) . A method of inserting guilloche patterns according to any one of claims 1 to 5, characterized in that the determination of the insertion intensity of the guilloche pattern comprises an operation (300) for selecting a thickness of the guilloche pattern. A method of inserting guilloché patterns according to any one of claims 3 to 6, characterized in that the outer zone (ze) is an area of pixels (p1-pn) contiguous to the inner area (zi). A method of inserting guilloché patterns according to any one of claims 3 to 7, characterized in that the guilloche pattern is a sinusoidal pattern, affixed substantially horizontally on the document, the outer zone (ze) being positioned vertically to the pixel (px) of the guilloche pattern. Method for inserting guilloche patterns according to any one of claims 3 to 7, characterized in that the guilloche pattern is a set of striations corresponding to a fingerprint, the inner (zi) and outer (ze) zones being positioned on the along a normal to the curve at the pixel considered. A guilloché pattern insertion device on a document, comprising a computer having a set of instructions that cause said computer to implement the guilloché pattern insertion method according to any one of claims 1 to 9. Secure document on which are affixed guilloche patterns encoding alphanumeric data, characterized in that the guilloche patterns are inserted on the document by the method according to any one of claims 1 to 9. Identity document comprising an identity photograph and alphanumeric data relating to the holder, characterized in that the identity photograph visibly includes guilloche patterns encoding the alphanumeric data, inserted by the method according to any one of the claims. 1 to 9. A method for extracting the guilloche patterns (601-608) from a degradable secure document (600), the method comprising the following operations: - determining (710) a contrasts map (c) of at least a portion of the secure document including the guilloche patterns; - identification (720), on this map of contrasts, of a trajectory of optimal percolation; and extraction (730) of the optimal percolation trajectory, said trajectory corresponding to the guilloche pattern. A guilloché pattern extraction method according to claim 1, characterized in that the contrast map (c) is constructed using a logarithmic image processing model (LIP). Method for extracting guilloche patterns according to claim 1 or 2, characterized in that the identification operation (720) of the optimal percolation trajectory comprises: an identification, on the contrast map, of all the trajectories likely to cross the secure document to connect a starting point of a guilloche pattern to a point of arrival of said pattern; a determination of a cost function of each of the trajectories, said cost function evaluating a variability between the contrast determined at each point of a trajectory and an expected contrast; - a comparison of the cost functions of all the trajectories; and a determination of the minimum cost function, the trajectory comprising the minimum cost function corresponding to the guilloche pattern. Method for extracting guilloche patterns according to any one of claims 1 to 3, characterized in that , when the secure document is in color, the contrast map is calculated from a luminance image associated with the color document . A method for authenticating guilloche patterns of a secure document that may be degraded, characterized in that it comprises: the operations for extracting the guilloche motifs according to any one of claims 1 to 4, and an operation (740) for comparing each extracted guilloche pattern with a corresponding theoretical guilloche pattern. A method of authenticating the guilloche patterns according to claim 5, in which each guilloche pattern encodes variable alphanumeric data mentioned on said secure document,
characterized in that the theoretical guilloche pattern is obtained by optical character recognition (OCR) of the alphanumeric data mentioned on the secure document and generation of the corresponding theoretical guilloche patterns. A method for authenticating guilloche patterns according to claim 5 or 6, wherein the secure document comprises an electronic chip,
characterized in that the theoretical guilloche pattern is an original guilloche pattern, previously stored in the chip during the generation of the guilloche pattern affixed to the secure document. A method for authenticating guilloche patterns according to any of claims 5 to 7, characterized in that the comparison operation of the extracted and theoretical guilloche patterns comprises an estimate (741) of a score based on a distance between each point of the guilloché motif extracted and a corresponding point of the theoretical guilloche motive. A method of authenticating guilloche patterns according to claim 8, characterized in that the score is calculated by accumulating the distances for all the points of the guilloche pattern. A method of authenticating guilloche patterns according to claim 8 or 9, characterized in that the distance is calculated in number of pixels. A method of authenticating guilloche patterns according to any one of claims 8 to 10, characterized in that a weight is assigned to each distance, the distant distances having a greater weight than the short distances. A method for authenticating guilloche patterns according to claim 11, characterized in that the weight assigned to each distance is adjustable. Guilloche motif authentication device comprising a computer comprising a set of instructions that cause said computer to implement the guilloche pattern authentication method according to any one of claims 5 to 12. Identity document comprising alphanumeric data relating to the holder and an identity photograph on which guilloché patterns encoding the alphanumeric data are affixed, characterized in that the guilloche patterns are able to be authenticated by the method according to any one of the Claims 5 to 12.

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