Document authentication based on the detection of modified fonts

Abstract

A method for authenticating a document (D) is disclosed, the document comprising a plurality of text fields, an authentic document comprising, among the plurality of text fields, a plurality of characters printed in a reference font, and at least one character, whose position is determined, being printed in a font that is modified relative to the reference font. The method comprises: receiving an image of the document to be authenticated, extracting, from the image, a region of interest containing the character having the determined position, evaluating discrepancies between the character contained in the extracted region of interest and a character model, and determining the authenticity of the document based on the evaluated discrepancies. Figure for the abstract: Figure 2

Description

Title of the invention: Document authentication based on the detection of modified fonts technical field

[0001] This disclosure relates to the field of document authentication and fraud detection in documents, such as identity documents. Previous art

[0002] Identity documents are traditionally secured by incorporating a series of security features. These features aim to guarantee the authenticity, integrity, and protection against fraud or falsification of the documents, and thus to distinguish an authentic document from a fraudulent one. During an identity check, the presence and integrity of the security features are verified to authenticate the document. Examples of security features include holograms, watermarks, microprinting, UV ink, etc.

[0003] A document, particularly an identity document, generally comprises both static and variable text fields. Static text fields contain text that does not vary depending on the document owner, while variable text fields contain text that varies depending on the owner, also known as Personally Identifiable Information. Typically, a static text field does not contain any personal or document-related data, but it may indicate the type of personal data that populates a neighboring variable text field. In the case of an identity document, a static text field may contain words such as "Last Name," "First Name," "Date of Birth," "Date of Issue," "Signature," etc. Static text fields may also include text identifying the document type and the issuing authority.

[0004] Some documents may include dedicated fonts as a security feature, called modified fonts. The modified font may have the same font style as that used for the surrounding static text—also called the reference font—the modified font being different from but close to the reference font (with, for example, a slight modification to the reference font), or it may have a different font style.

[0005] The changes between the original font and the modified font can be subtle and require careful and time-consuming review when document authentication is performed by a human operator. Therefore, it is necessary to find a fast and reliable solution for the automatic review of such a security feature.

[0006] [Lu, 2020] knows of a method for detecting falsified text in a document that classifies a document as fraudulent or authentic based on a Discrete Cosine Transform (DCT) of the document followed by an inverse DCT applied to the positive and negative coefficients of the DCT. This method does not specifically detect an altered font, but rather aims to detect any change in general, and not specifically a change in font style, because it relies on a global change in the intensity distribution of the pixels of a document that has been altered. Summary of the invention

[0007] This disclosure is intended to improve the situation.

[0008] In particular, one of the objectives of this disclosure is to provide a fast and reliable solution for the automatic authentication of a document based on the verification of a modified font.

[0009] Another objective of this disclosure is to provide a method that can be adapted to various types of modified fonts, including various characters and various types of font modifications.

[0010] Accordingly, a computer-implemented method for authenticating a document is disclosed, the document comprising a plurality of text fields, in which an authentic document comprises, among the plurality of text fields, a plurality of characters printed in a reference font, and at least one character, having a determined position, which is printed in a font that is modified relative to the reference font, the method comprising: - receiving an image of the document to be authenticated, - the extraction, from the image, of a region of interest containing the character having the determined position, - the evaluation of discrepancies between the characteristic contained in the extracted region of interest and a characteristic model, and - determining the authenticity of the document based on the assessed discrepancies.

[0011] In some embodiments, the character model is a reference template of the character printed in the reference font or in the modified font, and the evaluation of deviations between the character contained in the extracted region of interest and the reference template includes calculating the difference in intensity between the two characters.

[0012] In some embodiments, the determination of the document's authenticity is based on the detection of intensity deviations or on the positions of intensity deviations. In one embodiment, the character model is a reference template of the The character printed in the reference font, and the evaluation of discrepancies between the character contained in the extracted region of interest and the reference template, further includes determining the extrema positions of the difference in intensity between the two characters, and comparing the determined extrema positions to reference positions of the differences. According to this embodiment, the document is deemed authentic when the distance between the determined extrema positions of the difference and the reference positions is less than a specified threshold.

[0013] In another embodiment, the character model is a reference template of the printed character in the modified font, and the evaluation of discrepancies between the character contained in the extracted region of interest and the reference template further includes determining whether at least one difference in intensity exceeds a predetermined threshold. According to this embodiment, the document is considered fraudulent when at least one difference in intensity exceeds a predetermined threshold.

[0014] In one embodiment, the character model is a reference template of the character printed in the reference font or the modified font, and the method includes, before evaluating discrepancies between the character contained in the region of interest and the reference template, preprocessing the region of interest to align the character contained in the region of interest with the reference template. The preprocessing may include iteratively modifying the region of interest by implementing a rotation, translation, and scaling of the character until the modified region of interest best fits the reference template. This increases the reliability of the comparison with the reference template.

[0015] In one embodiment, the extraction of a region of interest containing a character includes the extraction of an area of ​​the image containing the character, the normalization of the intensity of the area, the extraction and optionally, the resizing of a bounding box of the character, the extracted bounding box forming the region of interest.

[0016] In one embodiment, the method further comprises: - the processing of the received image to determine a document type, - access to a database comprising, for each of a plurality of types of documents, character data descriptors based on the modified font, including at least the determined position of said character, and character model data descriptors, and - the retrieval, from the database, of the data descriptors corresponding to the type of document determined.

[0017] In some embodiments, the character model is a reference template of the character printed in the reference font.

[0018] In some embodiments, the document includes several static text fields, and the character printed in the modified font is located in one of the static text fields.

[0019] In some embodiments, an authentic document comprises at least two occurrences of the same character, at least one first occurrence of the character being printed in the modified font, and at least one second occurrence of the character being printed in a reference font. The method further comprises acquiring the reference template from the document to be authenticated, at a position corresponding to the second occurrence of the character. Acquiring the reference template from the image includes extracting an area containing the character in the reference font from the image, normalizing the intensity of the area, and extracting a bounding box of the character, the extracted bounding box forming the reference template. In some embodiments, the bounding box may be compressed around the contours of the character contained within the region of interest.

[0020] The disclosed method allows for the automatic verification of a modified font as a security feature of a document by comparing a region of interest containing a specific character that is supposed to be printed in the modified font (if the document is authentic) to a model of the character. In one embodiment, the determination of the document's authenticity is based on the position of the gaps between the compared characters. Consequently, it is sufficient to rely on the reference positions of the gaps and not, for example, on the exact shape of the compared characters. The method is therefore not designed for a specific character or type of modification and can thus be adapted to a wide variety of font modification types, for example, depending on the type of document (identity card, passport, visa, driver's license, etc.) and the issuing authority.

[0021] Consequently, during document authentication, the region of interest extracted from the document can be subjected to preprocessing consisting of font alignment (to find the best-fitting position) and normalization of font intensity and size. During input, a reference template can also be created by performing the same extraction of a region of interest, but without the alignment step (since the exact font position for the reference template is known and the model acquired during input must be used as a reference). Consequently, size normalization allows for the comparison of fonts with identical width and aspect ratio.

[0022] The method may be based on the use of a database of printed character descriptors in the modified font and, optionally, in the reference font for each of a plurality of document types. At each document authentication, the supervisory authority may access said database to Extract the relevant descriptors for the document type in question and verify the modified font based on those descriptors. The database can be updated regularly (including, for example, by changing the character affected by the modified font, its position, or the type of modification) without altering the implementation of the process.

[0023] According to another object, a computer-implemented method for generating a database for document authentication is disclosed, comprising adding to the database, for each of a plurality of document types, data descriptors of at least one character model and at least one character printed in a font modified from a reference font, comprising at least one determined position, in an authentic document, of the character printed in the modified font.

[0024] In some embodiments, the database generation process includes preliminary steps, for each type of document, comprising: - receiving an image of an authentic document template, - extracting, from the received image, a region of interest containing a character, - the processing of the extracted region of interest to obtain a model of the character, and - the recording, in the database, of template data descriptors.

[0025] In some embodiments, the template data descriptors that are recorded in the database include the template itself.

[0026] According to another object, a computer program product is disclosed, comprising instructions for implementing the process as described above, when executed by a computer.

[0027] According to another object, a non-transient computer-readable recording medium is disclosed, on which are stored code instructions which, when executed by a computer, cause said computer to implement a process according to the above description.

[0028] According to another object, a document authentication system is disclosed, comprising at least one image sensor, designed to acquire an image of a document to be authenticated, a database storing, for each of a plurality of document types, data descriptors of the character model and of the character printed in the modified font, comprising at least one determined position, in an authentic document, of the character in the modified font, and a computer, configured to receive images acquired by the image sensor and to implement the method according to the above description.

[0029] In certain embodiments, the data descriptors further include at least one of the following elements: - a reference template of the character printed in the modified font or printed in the reference font, - an expected position for each deviation between the character model and the printed character in the modified font, - a height, width, or aspect ratio of the character model, - expected extreme positions of intensity differences between the character in the modified font and the character in the reference font, - threshold values ​​concerning the differences in intensity, or their positions, between a character and a reference template. Brief description of the drawings

[0030] Other features, details and advantages will appear in the detailed description that follows and in the figures, in which: Figure 1

[0031] Fig. 1 represents examples of the same character (the letter "T") printed with different fonts. Figure 2

[0032] Figure [Fig.2] schematically represents a document authentication system according to certain embodiments. Figure 3

[0033] Fig. 3 schematically represents the main steps of a document authentication process, according to certain embodiments. Figure 4

[0034] Fig. 4 schematically represents the main steps of a process for generating a database for document authentication according to certain embodiments. Figure 5

[0035] Figure 5 schematically represents an example of discrepancies between two characters corresponding to different fonts. Description of the implementation methods

[0036] With reference to the figures, a method and a system for authenticating documents will now be described.

[0037] The document may be a document issued by an authority that attests to specific information about the owner, such as their identity and / or rights. For example, the document may be an identity document such as an identity card or passport. Other types of documents are also covered by this Disclosure, such as a visa, a permit (e.g., a driver's license), a health insurance card, a membership card, etc. The document includes several static text fields and several variable text fields, the latter of which may contain personally identifiable information.

[0038] The composition of a document, in particular the number and position of text fields, the content of static text fields, the choice of font and its size, depends on the type of document. "Type" refers both to the nature of the document, i.e., the nature of the information or right attested by the document (e.g., passport, visa, driver's license), and to the issuing authority (e.g., the State). For example, an identity card issued by one country may not have the same layout, text fields, fonts, etc., as an identity card issued by another country.

[0039] According to this disclosure, an authentic document comprises, among a plurality of text fields, a plurality of characters printed in a reference font, and at least one character printed in a font modified from the reference font. In this disclosure, the term "character" means a letter, number, or symbol printed on a document. The word "font" refers to a consistent design and style with which the characters are printed, whereas different fonts may vary with respect to certain attributes such as spacing, size, line spacing, and weight (the thickness of the character strokes).

[0040] The modified font may differ from the reference font. With reference to [Fig. 1], several instances of the same letter "T" printed in four different typefaces are shown. The modified font may also be a font comprising slight modifications to the reference font, such as a change in font weight or height, or a line through the font, etc.

[0041] The character printed in the modified font is a security element of the document. Consequently, this character is located in a specific position. The specific position may be a predetermined and constant position within the document. For example, the character printed in the modified font may be a specific character from one of the static text fonts (for example, the letter "E" in the static text font "NAME"). In this case, both the type of character (letter, number, or symbol) and its position in the document are known and constant. The position of the character may also be determined according to a predefined rule. In this case, the character printed in the modified font may also be a character from one of the variable fields, and consequently, the specific type of character may not be fully determined or may be selected from a list.According to non-exhaustive examples, the predefined rule may be that the . character in the modified font corresponds to the second digit of the month of the date of birth, or to the first vowel found in a given variable field, etc.

[0042] With reference to [Fig. 2], the method can be implemented by means of a document authentication system 1. The document authentication system can be installed in any location where the identity or rights of the user need to be verified. For example, the document authentication system can be located at a boarding gate or terminal (in airports, train stations, ports, etc.), at customs, in embassies, in police stations, etc. The document authentication system 1 includes an image sensor 10, for example a camera, configured to acquire an image of the document D. In some embodiments, the camera is configured to acquire images of the documents at a resolution greater than 150 DPI (dots per pixel).The document authentication system 1 further includes a computer 20, which includes at least one processor 21 configured to implement the method disclosed below, and memory storing code instructions executed by the processor for the execution of the method. The computer 20 may be installed in the same location as the image sensor 10, i.e., on the premises where document authentication is implemented, or it may be remote and accessible remotely via a telecommunications network.

[0043] The document authentication system 30 further includes a database storing, for each of a plurality of document types, data characterizing the document type and data enabling the implementation of the authentication process. In particular, the database includes, for each document type, at least data descriptors of the character printed in the modified font, which may include at least the position of said character. The position may be expressed directly, for example as coordinates in the document, when the modified font is located in a static text field, or it may be expressed by a rule allowing the character to be retrieved based on the modified font.As explained in more detail below, the database can also store other data descriptors relating to the printed character in the modified font, as well as data descriptors relating to a character model, for example a reference template of the printed character in the reference font.

[0044] Examples of a document authentication method will now be described with reference to [Fig. 3]. The method includes a first step 100 of acquiring and processing an image of the document D to be authenticated. When a user accesses the document authentication area, they may be prompted by a system or a person to present the document, and the image sensor 10 can capture an image of the document. Computer 20 therefore receives, during a step 110, an image of the document D to be authenticated which has been acquired by the image sensor.

[0045] The process then includes a step 200 of extracting, from the received image, a region of interest containing a character corresponding to the determined position of the printed character in the modified font, in an authentic document.

[0046] In some embodiments, the document authentication system 1 can be configured to authenticate several types of documents. In this case, the document authentication system may have to determine the position of the region of interest to be extracted based on the document type. Consequently, before implementing step 200, the image can be processed 120 to determine the type of document to which it relates. This processing may consist of extracting information from the document that allows its type to be determined. This information may include an identification of the nature of the document appearing on the document and the identification of the issuing authority.Before determining the document type, the processing may also include image normalization processing, which may include cropping or segmenting the document within the image, or other operations such as rotating, resizing, and flattening the document.

[0047] Once the document type is determined, computer 20 can then access database 30 to retrieve, based on the document type, the position of the character printed in the modified font in the authentic documents. As mentioned above, the position may be explicit or correspond to a rule for determining the character in the document. The extraction of the region of interest is then performed based on this determined position.

[0048] In some embodiments, the extraction 200 of the region of interest comprises the extraction 210 of an area of ​​the image containing the character, the normalization 220 of the intensity of this area (for example, to bring the darkest pixels of the area to a predefined maximum value, and the lightest pixels of the area to a predefined minimum value), and the extraction 230 of a bounding box around the contours of the character contained in the area. The bounding box may be tightly bound to the contours of the character, i.e., contain nothing other than the character itself. The size of the bounding box may also be normalized. The extracted bounding box then forms the region of interest.

[0049] The process then includes evaluating 300 discrepancies between the character contained in the extracted region of interest and a character model, and determining 400 the authenticity of document D, at least on the basis of the evaluated discrepancies. According to variations of this disclosure, the document may include several Security features, and the determination of the document's authenticity, are based not only on verifying the character in the modified font, but also on verifying other security features. Nevertheless, evaluating discrepancies between the character and the character model can, on its own, determine that the document is fraudulent. In this case, the process may also include a step of rejecting authentication, issuing an alert, inviting the user to retry document authentication, etc.

[0050] The character model is data representing the basic truth for the printed character in either the reference font or the modified font. In some embodiments, the character model may be a reference template of the printed character in the reference font or the modified font. A reference template of a character is an explicit representation of the character that serves as the basic truth for the deviation evaluation step. The deviation evaluation step may also be performed based on a deep learning approach, in which the character model is not a single model representing the character, but is learned during the training of the deep learning model.

[0051] In certain embodiments, particularly when the reference template of the character is printed in the reference font, the reference template can also be extracted from the document image. Indeed, an authentic document may include, in its plurality of text fields, and particularly in its plurality of static text fields, a plurality of occurrences of the same character, at least one of which is printed in the reference font and at least one other occurrence is printed in the modified font. In this case, the character extracted in step 200 can be compared to a reference template obtained from the occurrence of the same character printed in the reference font.

[0052] In this case, the process includes an additional step 200' of extracting, from the image, the reference template used as a character model. The same steps as those of step 200 can be carried out, that is to say, an area containing the character in the reference font can be extracted 210' from the image, then the intensity of the area can be normalized 220' and a bounding box surrounding the contours of the character can be extracted 230', thus forming the reference template.

[0053] In some embodiments, the database 30 further includes, for each type of document, a position in the document where the reference template can be extracted, and the computer retrieves said position during step 140 before proceeding to the extraction 200' of the reference template.

[0054] Furthermore, the reference template is not extracted from the document, but can be stored in the database 30. In this case, the reference template is retrieved by the computer during step 140, from the determined document type.

[0055] In certain embodiments, for example when the character in the modified font is a character from a variable text field, and can therefore be chosen from a plurality of different characters, the database can store a reference template of each character, and the computer can extract the reference template corresponding to the character extracted from document D on the basis of a similarity comparison between the extracted region of interest containing the character and the reference templates stored in the database.

[0056] Optionally, before evaluating 300 of the discrepancies between the extracted region of interest containing the character and the reference template, the method may include a preprocessing 250 of the region of interest to align the character contained in the region of interest with the reference template. This alignment may include an iterative modification of the region of interest by implementing at least one rotation, one translation, and one scaling, until the modified region of interest best fits the reference template, in order to improve the accuracy of the subsequent evaluation step 300. This preprocessing may take into account data descriptors associated with the reference template, such as the width, aspect ratio, and font height, which may either be stored in the database and retrieved in step 140 or derived from the reference template.

[0057] The evaluation of the 300 discrepancies between the extracted region of interest and the reference template is carried out to determine whether the character is actually printed in the modified font and whether the modification of the font relative to the reference font is authentic. This evaluation is performed on the basis of the intensities of the two characters, without taking into account the background of each of them.

[0058] In some embodiments, the gap evaluation 300 includes calculating 310 the difference in intensity between the two characters and determining the extrema positions 320 of the intensity difference between the two characters. Determining the extrema positions allows only font changes to be considered, and not potential intensity variations that may exist between the extracted region of interest and the template. With reference to [Fig. 5], examples of the gap positions between the intensities of the two compared characters are shown schematically (the positions are indicated by the squares on the right-hand character).

[0059] When the reference template is a model of the character in the reference font, the extreme positions of the intensity differences between the two compared characters can be compared to reference positions of the deviations between the modified model and the reference template, and a distance between these positions can be calculated and compared to a predetermined threshold. Indeed, in this case, deviations are expected between the modified font and the reference template, and the positions of these deviations are known. The reference positions, as well as the comparison threshold(s), are... They can also be part of the descriptors stored in the database for each type of document and retrieved by the computer in step 140. The document is then considered authentic when the calculated distance between the extreme positions of intensity difference and the reference positions is less than a determined threshold.

[0060] Furthermore, when the character in the region of interest is compared to a reference template of the character in the modified font, no discrepancy is expected. Consequently, a comparison is performed in step 330 between the intensity differences between the two compared characters, or the maximum intensity difference, and a predetermined threshold, which can also be stored in the database for each document type and retrieved by the computer in step 140. When an intensity difference exceeding a predetermined threshold is detected, the document is considered fraudulent.

[0061] According to another object, a method 900 for generating a database 30 for document authentication is disclosed, with reference to [Fig. 4]. The database can be initialized, for each document type, with the nature of the document and an indication of the issuing authority. Furthermore, the method includes adding 940 to the database, for each document type, data descriptors enabling the subsequent implementation of the authentication method, comprising data descriptors of at least one character pattern, and at least one character printed in a modified font, the latter comprising at least one determined position, in an authentic document, of the character printed in the modified font.

[0062] In some embodiments, the character model data descriptors may include a position in an authentic document where a reference template of the character printed in the reference font can be extracted. In other embodiments, the character model data descriptors may include a reference template of the character printed in the reference font or a reference template of the character in a modified font. In this case, the database generation includes the input of at least one reference template for each document type.

[0063] This introduction may include receiving at least one image of an authentic document template. By "authentic document template" is meant a standardized template or layout that describes the structure and design of the document. It includes predefined elements, in particular the static text fields of the document. The received image may be processed to normalize the image, which may include cropping or segmenting the document within the image, or other operations such as rotating, resizing, and flattening the document.

[0064] The introduction may also include the extraction 920, from the received image, of a region of interest comprising a character, and the processing 930 of said region to obtain a model of the character, which will be used as a reference template during the implementation of the authentication process. The processing 930 may be carried out according to the steps 200, 200' mentioned above, that is to say, it may include the extraction of an area containing the character, the normalization of the intensity of the area, and the extraction of a bounding box tightly around the contours of the character, the size of which may also be normalized.

[0065] The resulting reference template can be saved in the database. In some embodiments, the introduction may also include saving data descriptors concerning the reference template in the database, such as the height, width, aspect ratio, etc. of the font.

[0066] In certain embodiments, when the reference template conforms to the reference font, the database generation may also include recording, in the database, additional data descriptors related to the modified font, which may include height, width, aspect ratio, and / or data descriptors relating to the deviations between the character in the reference font and the same character in the modified font, such as the expected positions of extrema of intensity deviations between the characters, and one or more threshold(s) concerning said positions.

[0067] When the reference template is in the modified font, the 900 generation of the database may also include recording, in the database, a threshold value concerning the maximum acceptable intensity difference between the characters compared. Bibliography

[0068] [Lu, 2020] : Y. Lu et al., « A New Method for Detecting Altered Text in Document Images », International Conférence on Pattern Récognition and Artificial Intelligence (ICPRAI), LNCS 12068, pp. 93-108, 2020.

Claims

Demands

1. A computer-implemented method for authenticating a document (D), the document comprising a plurality of text fields, in which an authentic document comprises, among the plurality of text fields, a plurality of characters printed in a reference font, and at least one character, having a determined position, which is printed in a font modified relative to the reference font, the method comprising: - receiving (110) an image of the document to be authenticated, - extracting (200), from the image, a region of interest containing the character having the determined position, - evaluating (300) discrepancies between the character contained in the extracted region of interest and a character model, and - determining (400) the authenticity of the document on the basis of the evaluated discrepancies.

2. A method according to claim 1, wherein the character model is a reference template of the character printed in the reference font or in the modified font, and the evaluation of deviations (300) between the character contained in the extracted region of interest and the reference template includes the calculation (310) of the difference in intensities between the two characters.

3. A method according to claim 2, wherein the determination (400) of the authenticity of the document is based on the detection of intensity deviations or on the positions of intensity deviations.

4. A method according to claim 2 or 3, wherein the character model is a reference template of the character printed in the reference font, and the evaluation of deviations between the character contained in the extracted region of interest and the reference template further comprises determining positions (320) of extrema of the difference in intensity between the two characters, and comparing (330) the determined positions of the extrema to reference positions of the differences.

5. A method according to claim 4, wherein it is determined (400) that the document is authentic when a distance between the determined positions of the difference extrema and the reference positions is less than a determined threshold.

6. A method according to any one of the preceding claims, wherein the extraction of a region of interest (200, 200') containing a character comprises the extraction (210, 210') of an area of ​​the image containing the character, the normalization (220, 220') of the intensity of the area, the extraction and optionally, the resizing (230, 230') of a bounding box of the character, the extracted bounding box forming the region of interest.

7. A method according to any one of the preceding claims, wherein the character model is a reference template of the character printed in the reference font.

8. A method according to any one of the preceding claims, wherein the document comprises several static text fields, and the character printed in the modified font is located in one of the static text fields.

9. A method according to claims 7 and 8 in combination, wherein an authentic document comprises at least two occurrences of the same character, at least a first occurrence of the character being printed in the modified font, and at least a second occurrence of the character being printed in a reference font, and wherein the method further comprises acquiring the reference template from the document to be authenticated, at a position corresponding to the second occurrence of the character, and acquiring the reference template from the image comprises extracting (210') an area comprising the character in the reference font from the image, normalizing (220') the intensity of the area and extracting (230') a bounding box (tightened around the contours) of the character, the extracted bounding box forming the reference template.

10. A computer-implemented method for generating (900) a database for document authentication, comprising adding (940) to the database, for each of a plurality of document types, data descriptors of at least one character pattern and at least one character printed in a font modified from a reference font, comprising at least one determined position, in an authentic document, of the character printed in the modified font.

11. Document authentication system (1), comprising at least one image sensor (10), designed to acquire an image of a

12. document to be authenticated, a database (30) storing, for each of a plurality of document types, data descriptors of the character model and of the printed character in the modified font, including at least one determined position, in an authentic document, of the character in the modified font, and a computer (20), configured to receive images acquired by the image sensor and to implement the method according to any one of claims 1 to 9. Document authentication system (1) according to the preceding claim, wherein the data descriptors further comprise at least one of the following: - a reference template of the character printed in the modified font or printed in the reference font, - an expected position for each deviation between the character model and the printed character in the modified font, - a height, width, or aspect ratio of the character model, - expected extreme positions of intensity differences between the character in the modified font and the character in the reference font, - threshold values ​​concerning differences in intensity, or their positions, between a characteristic and a corresponding reference template.

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