SECURITY SOLUTION WITH A DIGITAL IMAGE WITH INTEGRATED SECURITY FEATURE AS WELL AS AN IMAGE CONVERSION METHOD AND AN IMAGE CONVERSION DEVICE FOR ITS MANUFACTURING

Description

[0001] The present invention relates to a digital image with an integrated security feature, an image conversion method and an image acquisition device for integrating a security feature into a source image, as well as a computer program or computer program product for carrying out the image conversion method.

[0002] One way to equip objects with security features to protect against counterfeiting is to affix an image that is difficult to forge. Personalized documents are among the objects where this method is regularly used.

[0003] A wide variety of personalized documents, particularly in card or book form, are known from the state of the art. For example, book-like passport documents or individual pages thereof (e.g., the so-called "passport holder page" or paper pages), identity cards, and many types of personalized chip cards, such as bank cards, credit cards, ID cards, membership cards, access cards, etc., or personal (mostly card-shaped) labels, all belong to the group of personalized documents.

[0004] Personalization means that the document in question contains or bears document-specific information that is typically associated with a document holder. In some cases, this information can identify the holder, for example, by means of their name, photograph, identification number, or other characteristics that are printed on, affixed to, or stored within the document, particularly in the form of data. Personalization can refer specifically to an individual person or to a particular, limited group of people, such as employees of a company.

[0005] Laser marking is a well-known method for applying personalization information, particularly when the surface of an object to be personalized is made of a polymer material. Using a laser beam, the object surface can be selectively treated so that a chemical transformation of the polymer material occurs at the points where the laser beam strikes the surface, resulting in a color change. This allows for the creation of various shades of gray. This technique can be used, in particular, to create a grayscale image, such as a passport photo, on the object surface.

[0006] Because laser marking systems are commercially available, criminal organizations regularly manage to acquire them and use them to produce and circulate counterfeit objects, especially personalized documents or other security documents. One way to create such forgeries is to alter an existing laser marking on the object's surface by applying additional laser radiation or by covering it with a foreign substance.

[0007] US 2007 / 211913 A1 discloses an image processing device with a raster processing unit that shifts the position of a raster cell applied to an input image in at least one direction of a main scanning direction and a subordinate scanning direction of the input image by a first displacement amount that changes according to the applied position of the raster cell when raster processing is performed on the input image using the raster cell.

[0008] US 2017 / 124795 A1 discloses a system, method and software that assist in identifying the source of an unauthorized publication of a document, such as a publication of a check, receipt or other type of document.

[0009] US 2010 / 260985 A1 discloses a method for producing a polymer layer composite, wherein the polymer layer composite has a plurality of polymer layers and wherein at least one polymer layer contains a laser-sensitive component.

[0010] One objective of the invention is to further improve the protection against counterfeiting of images, in particular images for the personalization of objects.

[0011] The solution to this problem is achieved according to the teaching of the independent claims. Various embodiments and further developments of the invention are the subject of the dependent claims.

[0012] In the following, a comprehensive security solution for the protection against forgery of digital images is presented, which, in addition to various aspects of a solution for integrating a security feature into a digital image to be protected and such a protected digital image itself, also includes a method, a device and / or a computer program, each of which can be used to verify such a protected digital image.

[0013] A first aspect of the security solution concerns an image conversion method, particularly a computer-implemented one, for integrating a security feature into a digital source image to generate a target image secured by the integrated security feature. The image conversion method includes: (i) Capture of source image data representing a digital source image to be protected by the security feature, which has image points (pixels) arranged in a grid, in particular a rectangular pixel matrix, from straight rows of pixels parallel to each other, each pixel having at least one image point value, e.g. gray or color value;(ii) Generating intermediate image data representing an intermediate image resulting from the source image by applying a distortion rule, according to which, for each pixel row of the source image, the respective pixel values ​​of, in particular all, pixels of the pixel row within the grid are transferred along a direction angled to the pixel row, in particular orthogonal, to a respective other pixel of the grid determined or determinable by the distortion rule, such that the arrangement of these other pixels in the grid has a transverse waveform;(iii) Generating target image data representing the target image, wherein: (iii-1) the intermediate image is sampled pixel by pixel to define, for each pixel row of the intermediate image, a sequence of consecutive pixels according to the sampling, the pixel values ​​of which result from the transfer of corresponding pixel values ​​from the source image according to the distortion rule; (iii-2) each pixel of the sequence is transformed into a corresponding pixel of the target image by determining its position in the target image from its position in the intermediate image by compensating for the distortion suffered by applying the distortion rule during the generation of the intermediate image, such that the arrangement of the respective pixels of each sequence in the target image represents a transverse wave train;and (iii-3) the integrated security feature is defined by the wave trains, in particular by their shape, size and / or arrangement within the target image.

[0014] The term "capture" of the source image data, as used herein, can in particular refer to the sensor-based generation of the source image, for example using at least one image sensor (camera), or to receiving or reading from a memory of already existing source image data.

[0015] The term "pixel array", as used herein, can in particular refer to a row or column of a rectangular two-dimensional pixel matrix forming the grid.

[0016] The term "transverse wave," as used herein, refers to a waveform in which the wave oscillates perpendicular to its direction of propagation. Similarly, the term "transverse wave-like" refers to a waveform of such a transverse wave. This term must be distinguished from a longitudinal wave or longitudinal waveform, in which the oscillation occurs along the direction of propagation.

[0017] The terms "scan," "scanned" (and variations thereof) used herein in relation to an image or a series of pixels thereof refer to any method of acquiring image values, particularly those of an intermediate frame. This includes, in particular, measuring, reading, or receiving data representing the image values. Specifically, scanning can be performed serially along a scanning direction, so that the respective pixel values ​​of the pixels to be scanned successively along the scanning direction are acquired. In the case of scanning a series of pixels, this means the pixel values ​​of the successively reached pixels of the series are acquired.

[0018] Any terms used herein, such as "comprises," "includes," "features," "has," "with," or any other variant thereof, are intended to cover non-exclusive inclusion. For example, a method or apparatus comprising or featuring a list of elements is not necessarily limited to those elements but may include other elements not expressly listed or inherent in such method or apparatus.

[0019] Furthermore, unless explicitly stated otherwise, "or" refers to an inclusive or and not an exclusive "or". For example, a condition A or B is satisfied by any of the following: A is true (or present) and B is false (or not present), A is false (or not present) and B is true (or present), and both A and B are true (or present).

[0020] The terms "ein" or "eine," as used here, are defined as "one or more." The terms "ein anderer" and "ein Weitere," as well as any other variant thereof, are to be understood as "at least one more."

[0021] The term "plural", as used here, is to be understood in the sense of "two or more".

[0022] The terms "configured" or "set up" (and any variations thereof) used herein to fulfill a specific function, as defined in the invention, mean that the device in question is already in a configuration or setting in which it can perform the function, or at least is adjustable—i.e., configurable—so that it can perform the function after appropriate adjustment. Configuration can be achieved, for example, by adjusting parameters of a process sequence or by using switches or similar devices to activate or deactivate functionalities or settings. In particular, the device can have several predetermined configurations or operating modes, allowing configuration to be performed by selecting one of these configurations or operating modes.

[0023] The image conversion method described in the first aspect is thus capable of converting a raster graphic representing the source image into a graphic represented by the target image or target image data, in which the pixels are arranged in a plurality of parallel transverse wave trains. Such a graphic is significantly more difficult to create than a conventional raster graphic. In particular, personalization systems, especially laser marking systems, which could produce such a non-raster target image with sufficiently high image quality, are not readily available to counterfeiters. Therefore, the image conversion method can increase the counterfeit protection of a source image by integrating the security feature defined by the transverse wave trains.

[0024] The following section describes various exemplary embodiments of the image conversion method, which, unless expressly excluded or technically impossible, can be combined with each other and with the other described aspects of the security solution as desired.

[0025] The source image represented by the source image data can, in particular, represent a photograph of a person or one or more of their body regions, especially the face or a part thereof. The source image can, in particular, represent a section of a larger image.

[0026] The distortion rule can be the same for all image series to which it is applied.

[0027] In some embodiments, the image conversion method further comprises generating a physical image of the target image on the surface of a substrate. The image of the target image is generated by serially generating pixels on the substrate. To generate a series of pixels on the substrate that corresponds to a specific sequence of pixels in the target image, these pixels are generated on the substrate according to the pixel sequence defined by the sequence of corresponding pixels in the target image. The substrate can, in particular, be a page of a document, such as a personalized or customizable document, like an identity document.Thus, in addition to image conversion, the method can also be used to generate a physical image of the target image and, in particular, to personalize an object with a feature that contains the target image in whole or in part.

[0028] In some of these embodiments, the physical image of the target image is created on the substrate by means of laser marking, in which the image points are sequentially generated on the substrate using a laser beam. In this way, it is possible to carry out the novel process of personalization using proven marking technology, particularly with high image resolution.

[0029] In some such embodiments, the various positions in the series where the laser beam strikes the substrate to generate the image pixels are controlled by variable deflection of the laser beam in a mirror-based laser galvanometer. The use of such laser galvanometers can be advantageous in several respects. In particular, high precision and resolution, as well as high processing speeds despite serial pixel generation, can be achieved. Furthermore, the required installation space can be kept small.

[0030] Furthermore, such laser galvanometers regularly offer the possibility of specifically compensating for or correcting any distortions in the marking to be produced, which can be caused in particular by uneven object surfaces or imaging errors in the laser galvanometer's optics, using correction information to at least partially compensate for such distortions. The mirrors of the laser galvanometer are controlled with a specific offset profile defined in the correction information, so that, in principle, any possible substrate shape can be marked with the same inscription without distortion. This capability of many laser galvanometers can now be used – especially in the sense of "dual-use" – within the image conversion process to compensate for the distortions caused by applying the distortion formula during the generation of the intermediate image in order to generate the target image.The correction information can then be considered the target image data itself, since it defines and thus represents the target image, which is to be generated as an image on the substrate by the laser marking, by means of the corrections to be used as compensation (bias) in laser marking.

[0031] In some embodiments, at least one, and in particular all, mirrors of the laser galvanometer are controlled to position a specific pixel of the image to be generated on the substrate by means of a control signal that is defined as a function of the compensation determined for the pixel of the intermediate image that corresponds to the pixel of the image to be generated. In this way, the compensation according to the method can be achieved for each pixel.

[0032] In some embodiments, a laser galvanometer is used for variable deflection of the laser beam. The inertia of at least one of the mirrors used for deflection is so high that, when the wave trains of the target image are generated on the substrate by creating the image points, abrupt changes in direction along the wave train path result in deviations of the image from the target image. In this way, the image on the substrate only approximates the target image. The deviations between the two images occur primarily where the wave train path in the target image exhibits abrupt changes in direction that, due to the inertia of the mirrors, are not transferred 1:1 to the image to be generated on the substrate. In particular, small roundings typically occur at such points in the wave train path in the target image compared to the corresponding wave train path in the target image.This effect, which is very difficult for counterfeiters to reproduce, can be used to further increase the counterfeit protection achievable with the image conversion process and provided by the security feature.

[0033] In some embodiments, the arrangement of the respective pixels of each sequence in the target image is defined such that it represents a transverse wave train that is at least sectionally periodic. In particular, it can be at least sectionally sinusoidal. Providing such periodicity is particularly advantageous with regard to subsequent verification of the image on the substrate, especially when a verification method, such as the one described below, is to be used, which is based on detecting periodic structures in the image. Sinusoidal waveforms are particularly advantageous when a (discrete or continuous) Fourier transform is to be used, or can be used, for the detection of such periodic structures in the image during verification.

[0034] According to the invention, the arrangement of the respective pixels of each sequence in the target image is determined such that any two adjacent wave trains are separated from each other by a gap. The substrate is selected or processed to have a color that differs from the mean color of the pixels forming the wave trains of the target image, as determined by the pixel values, such that adjacent wave trains are optically distinguishable. Separating the wave trains serves, firstly, to make them easily distinguishable from one another, particularly optically, for example with the naked eye or using image magnification optics such as a magnifying glass or a microscope.On the other hand, this also facilitates the previously mentioned recognition of periodic structures in the image on the substrate, whereby the provision of the spaces themselves can be used to provide further periodic structures in the image, which can (also) be used as a basis for later verification.

[0035] In some embodiments, each of the wave trains resulting from a given sequence has at least two inflection points. This improves the recognizability of the wave pattern represented by the wave trains and can also increase the reliability and / or accuracy achievable during verification, particularly in the detection of periodic structures.

[0036] In some embodiments, the grid of pixels in the source image has rows and columns, with the resolution of the rows differing from the resolution of the columns. This is particularly advantageous when—as described above—spaces are provided between adjacent wave trains. By using a grid with different resolutions, the space occupied by the spaces can be compensated, at least partially, by a correspondingly lower resolution or pixel density, especially along the transverse direction of the wave trains, so that no or only a slight, tolerable distortion of the target image or representation on the substrate occurs.

[0037] In some of these embodiments, the resolution of the source image in the direction orthogonal to the pixel arrays is at most 70% of the resolution in the direction along the pixel arrays. Experiments have shown that this range is particularly suitable, as it regularly allows for largely distortion-free target images or representations on the substrate, and also provides sufficiently large intervals between adjacent wave trains for reliable and accurate verification.

[0038] In particular, in some embodiments, the resolution of the source image in the direction orthogonal to the pixel arrays is at least 200 pixels per inch or per 2.54 cm (PPI). This range is especially advantageous with regard to the aforementioned goals of achieving a largely distortion-free target image and reliable and accurate verification.

[0039] In some embodiments, the pixels of the target image are defined such that their respective dimensions are the same along and orthogonal to the transverse direction of the wave trains. This can particularly include cases where the pixels of the source image have different dimensions depending on their direction, for example, if they have a non-square, rectangular shape.In order to achieve the same extent of the pixels of the target image in the transverse direction and orthogonally to it according to these embodiments, the respective corresponding pixels of the source image can be "cropped", in particular so that the proportion of pixels thereby eliminated corresponds at least approximately to the space required by the associated space in the target image and thus a resulting image distortion (not to be confused with the distortion according to the distortion rule) during the transition from source image to target image can at least be largely avoided.

[0040] In some embodiments, the distortion rule is defined such that, when applied to pixel sequences of the source image, the resulting waveform exhibits a first waveform in one or more sections of its path and a second waveform different from the first in at least one other section. In particular, this can be achieved such that the first waveform or the second waveform is rectilinear or contains at least a rectilinear section. This further increases the complexity of the wave pattern that defines the security feature and thus also further enhances the achievable counterfeit protection.

[0041] A second aspect of the security solution concerns an image conversion device, in particular a data processing device, which is configured to perform the process steps of the image conversion method according to the first aspect, in particular according to one or more of its embodiments described herein.

[0042] A third aspect of the security solution concerns a computer program or computer program product comprising instructions that cause the image conversion device according to the second aspect to perform the process steps of the image conversion method according to the first aspect, in particular according to one or more of its embodiments described herein.

[0043] The computer program can be stored, in particular, on a non-volatile data carrier. Preferably, this is a data carrier in the form of an optical data carrier or a flash memory module. This can be advantageous if the computer program itself is to be handled independently of a processor platform on which the one or more programs are to be executed. In another implementation, the computer program can exist as a file on a data processing unit, in particular on a server, and be downloadable via a data connection, for example, the Internet or a dedicated data connection, such as a proprietary or local network. Furthermore, the computer program can comprise a plurality of interacting individual program modules. The modules can, in particular, be configured, or at least be usable, in such a way that they function in the sense of distributed computing (i.e., distributed computing)."Distributed computing" is performed on different devices (computers or processor units) that are geographically separated and connected via a data network.

[0044] The image conversion device according to the second aspect can accordingly have a program memory in which the computer program is stored. Alternatively, the image conversion device can also be configured to access an external computer program, available, for example, on one or more servers or other data processing units, via a communication link, in particular to exchange data with it that is used during the execution of the procedure or computer program or represents outputs of the computer program.

[0045] A fourth aspect of the security solution concerns a digital image with an integrated security feature as the target image or image thereof on a substrate, obtainable by the image conversion method according to the first aspect, in particular according to one or more of its embodiments described herein.

[0046] A fifth aspect of the security solution concerns a digital image with an integrated security feature, particularly according to the fourth aspect, comprising a plurality of parallel rows of pixels, each exhibiting a transverse wave-like progression, wherein adjacent rows of pixels are separated from each other by a space that is at least partially distinguished by color from the rows of pixels separated by it, and wherein the security feature is defined by the wave-like progression of the rows of pixels and the spaces between them.

[0047] In some embodiments, the digital image, according to the fourth or fifth aspect, is formed on a document page serving as a substrate, in particular a data page, for a valuable or security document. In particular, the digital image can be a picture, such as a passport photo, of the holder of the document, especially if it is an identity document.

[0048] The features and benefits explained in relation to the first aspect of the security solution also apply accordingly to the other aforementioned aspects of the security solution.

[0049] Furthermore, a verification methodology is described below in various other aspects of the security solution, each of which can be used to verify a digital image, in particular a digital image according to the fourth or fifth aspect of the security solution. The verification methodology, or its individual aspects, thus represent one or more further possible elements of the aforementioned security solution for the protection against forgery of digital images.

[0050] A sixth aspect of the security solution concerns a verification procedure for verifying a digital image with an integrated security feature, in particular a digital image according to the fourth or fifth aspect of the security solution. It indicates: (i) Transforming the digital image from a spatial space defined by the spatial arrangement of its pixels into a frequency space by applying a mathematical transformation which has the property of mapping distances between adjacent lines in spatial space into the frequency space such that different distances in spatial space correspond to different frequencies in the frequency space; (ii) matching the spectrum resulting from the transformation with at least one reference spectrum representing a spectrum generated or generateable by means of the transformation of a genuine digital original image; and (iii) classifying the digital image to be verified as genuine or fake depending on the result of the matching.

[0051] The term "transformation" and variations thereof, as used herein, can in particular refer to a discrete or continuous transformation, for example a discrete or continuous Fourier transformation.

[0052] The verification procedure thus opens up the possibility of verifying the authenticity of a digital image in the frequency domain, instead of solely or additionally in the spatial domain. Since the transformation to the frequency domain is based on periodic functions, particularly sinusoidal functions, frequency domain testing allows for the identification of image structures exhibiting periodicity. This applies especially to digital images, such as those described in the fourth or fifth aspect of the security solution, which contain multiple image sequences composed of pixels that are periodically spaced and / or each exhibits a periodic transverse wave pattern.Even if such digital images essentially represent the same image as a purely conventional raster graphic image, making it difficult or only possible with special analytical tools to reliably distinguish between the two images in spatial space, such a desired distinction can be successfully achieved with the help of the verification procedure based on testing in the frequency domain and thus a different testing concept.

[0053] Due to the wave-personalization shape, the frequency domain representations of different images produced using the same personalization method are readily comparable. In particular, no database is required; it is sufficient, for example, to store only the spectrum or frequency image corresponding to the original image (e.g., Fourier transform) "offline" on a verification device intended for carrying out the verification procedure.

[0054] The following section describes various exemplary embodiments of the verification procedure, which, unless expressly excluded or technically impossible, can be combined with each other and with the other aspects of the verification methodology described below.

[0055] In some embodiments, the transformation is a two-dimensional transformation that transforms a two-dimensional spatial space defined by the planar extent of the digital image to be verified into an associated two-dimensional frequency space. This allows, in particular, the reliable detection and use of periodicities occurring within the image, even those appearing along different directions, for verification.

[0056] In some embodiments, a plurality of distinct image sectors are defined in the image to be verified before the transformation is performed, particularly by subdividing (such as tessellation) the image into sectors that cumulatively cover the entire image. The transformation is performed individually for several, and in particular all, of the image sectors in order to obtain a spectrum in the frequency domain assigned to the respective image sector. The alignment process involves an image sector-based alignment, in which, for each of the image sectors that have undergone the transformation, the spectrum resulting from the transformation is aligned with at least one reference spectrum (of the original image) assigned to the respective image sector. This reference spectrum represents a spectrum of the corresponding image sector of a genuine digital original image, generated or generable by the transformation.The classification of the digital image to be verified as genuine or fake then depends on the results of the comparisons carried out on an image sector basis.

[0057] This image sector-based approach can be particularly advantageous and improve the verification process's capabilities for detecting forgeries when the forged image exhibits only or primarily minor alterations (falsifications) compared to the original image. When converting the entire image (e.g., a photograph of a document holder) from spatial space to the frequency domain, these alterations result in a spectrum for the entire image that differs only slightly from that of the original image due to frequency overlap. In contrast, generating spectra sector by sector in the frequency domain makes it easier to identify deviations in the corresponding spectra of the image being verified and the original image (reference spectrum) within the image sectors affected by the forgeries, as these deviations are typically larger than those for the entire image.

[0058] In some of these embodiments, the digital image to be verified is classified as fake if a number N of image sector-based comparisons, in which a deviation beyond a predetermined threshold is detected between the spectrum of the respective image sector and its associated reference spectrum, exceeds a predetermined verification threshold M. In particular, N=1 can be chosen. This threshold approach allows for a particularly simple and efficient implementation of the aforementioned image sector-based verification approach.

[0059] In some embodiments, the digital image to be verified represents at least a portion of a person's body region, in particular the face, and the verification method further comprises: (i) performing an image analysis, in particular facial feature recognition, of the digital image to detect at least one predetermined biometric feature of the person and to locate this at least one detected biometric feature within the digital image; and (ii) selecting an image area of ​​the digital image representing the respective biometric feature as one image sector from a plurality of distinct image sectors. In this way, the definition of the image sectors can be optimized to specifically define image sectors where forgery is particularly likely, namely in areas with high density of detail, especially those representing biometrically relevant body regions.This will further increase the efficiency of the verification process.

[0060] In some embodiments, the comparison of a spectrum obtained from the transformation with an assigned reference spectrum is based on (a) a point-by-point comparison of a plurality of spectral values ​​corresponding to each other in the frequency domain of both spectra, in particular of all corresponding spectral values ​​of both spectra, and a comparison of the deviations that may result, individually or cumulatively, with a correspondingly defined deviation threshold, or (b) a comparison function that calculates an evaluation from the measured spectrum and the reference spectrum, and a comparison of the evaluation with a correspondingly defined deviation threshold, to determine a result of the comparison that indicates whether a relevant deviation was detected during the comparison. In particular, the desired balance between the efficiency and effectiveness of the verification procedure can be set by selecting the number of spectral values ​​used.

[0061] In some embodiments, the transformation corresponds to at least one of the following transformation types or is based on at least one of them: (i) Fourier transform; (ii) Cosine transform; (iii) Laplace transform; (iv) Wavelet transform; (v) Gabor transform.

[0062] A seventh aspect of the security solution concerns an image verification device, in particular a data processing system, which is configured to execute the procedural steps of the aforementioned verification procedure.

[0063] An eighth aspect of the security solution concerns a computer program or computer program product, comprising commands that cause the aforementioned image verification device to perform the procedural steps of the aforementioned verification procedure.

[0064] The features and advantages explained in relation to the verification procedure according to the sixth aspect of the security solution also apply accordingly to the aforementioned further aspects of the security solution with regard to the verification methodology.

[0065] Further advantages, features and application possibilities of the security solution presented here will become apparent from the following detailed description in conjunction with the figures.

[0066] This shows: Fig. 1 schematically a system for image conversion according to an exemplary embodiment of the security solution with an image conversion device including a laser galvanometer for laser marking of substrates; Fig. 2 a flowchart to illustrate an exemplary embodiment of the safety-solution-compliant image conversion method; Fig. 3 a schematic representation to illustrate an exemplary generation of a raster graphic intermediate image from a raster graphic source image within the framework of the procedure from Fig. 2 ; Fig. 4 a schematic representation to illustrate an exemplary generation of a target image from an intermediate image within the framework of the procedure from Fig. 2 ; Fig. 5 a schematic representation to illustrate an exemplary generation of an image of the target image on a substrate by means of laser marking within the framework of the process from Fig. 2 ; Fig. 6 a schematic representation to illustrate a process that takes place within the framework of the procedure Fig. 2 generable wave structure in the target image and, if applicable, the resulting image of the target image on a substrate; Fig. 7 a schematic representation to illustrate a resolution adjustment or pixel shape change within the framework of the procedure from Fig. 2 ; Fig. 8 a flowchart illustrating an exemplary embodiment of a verification procedure for verifying digital images, in particular those using the security-solution-compliant image conversion method (e.g. according to Fig. 2 ), digital images that can be generated; Fig. 9 a schematic representation to illustrate a verification of a digital image based on its two-dimensional (2D) Fourier spectrum within the framework of the procedure from Fig. 8 .

[0067] The same reference symbols are regularly used in the figures for the same or corresponding elements of the security solution.

[0068] The in Fig. 1 The illustrated system 100 for image conversion according to an exemplary embodiment of the security solution comprises an image conversion device 105, which may in particular include a data processing device with a processor 105a and a memory 105b. The memory 105b may in particular serve as program memory for a computer program containing instructions which, when executed on the processor 105a, cause the image acquisition device 105 to perform an image conversion method in accordance with the security solution (for example, according to Fig. 2 ). Accordingly, the image conversion method can be designed, in particular, as a computer-implemented method.

[0069] The system 100 can further comprise an image sensor (camera) 110 for capturing a digital image, in particular a two-dimensional digital image, of an object, such as a person P. Additionally or alternatively, a data storage device 115 can be provided, containing image data representing an image of such an object. In particular, it is possible for the image data to be generated by the image sensor 110 and temporarily stored in the data storage device 115 in order to be made available from there to the image acquisition device 105. The system 100 can optionally also be designed such that it already contains the image sensor 110 and / or the data storage device 115 as components themselves, in particular as a single unit. An image provided accordingly to the image conversion device 105 as input is hereinafter referred to as the "source image," and the data representing it as "source image data."

[0070] If the image acquisition device 105 uses a safety-compliant image conversion method, in particular the image conversion method 200 according to Fig. 2 , and thereby produces as a result a transformed image, which is hereinafter referred to as the "target image" and the data representing it as "target image data", it is also possible to transfer this target image as a physical image 165 of the same onto a substrate 160. This can be done in particular, as in Fig. 1 The substrate 160 can, in particular, be a page of a document suitable for marking by means of the marking technology used to generate the image 165, in the case of laser marking, in particular a page which has at least one polymer material which can be altered by means of laser light in order to achieve a change in properties, in particular a change in color.

[0071] System 100 specifically includes a laser galvanometer 120 as a device for laser marking of substrates 160. It comprises a laser 125 configured to emit a laser beam 130 that strikes a first mirror 135. There, depending on the position of the first mirror 135, which is adjustable by means of a first mirror drive 140, the laser beam is deflected onto a second mirror 145. The second mirror 145, in turn, has a mirror drive 150 to allow its position to be variably adjusted. The laser light 130 deflected by the second mirror 145 then passes through a focusing optic 155, which may, in particular, be or comprise an F-theta lens. From the focusing optics 155, the focused laser beam 130 then hits the substrate 160 to be marked, in order to create a pixel 170 of the image 165 of the target image at the point of its impact on the substrate 160.By appropriately controlling the two mirror drives 140 and 150, the laser beam 130 can be variably deflected by the combination of mirrors 135 and 145, so that over time its point of impact on the substrate exhibits a line-like profile. Since the target image, as will be explained in detail below, has wave-like image components, these wave trains are transferred to the substrate 160 during laser marking as wave-like marking lines in the form of wave trains 175 formed from pixels of the image 165.

[0072] Fig. 2 Figure 200 illustrates an exemplary embodiment of an image conversion method, which can be carried out in particular using the system 100. Further details are provided below. Figuren 3 bis 7 Individual steps of the image conversion process 200 are illustrated in greater detail.

[0073] As part of the image conversion process 200, in a first step 205, as already described above with reference to Fig. 1 In more detail, a source image 305 is captured and, based on the source image data representing it, is made available as input for the further steps of the image conversion process 200. The following describes the process for the Figuren 3 bis 5 For the sake of simplicity, it is assumed that source image 305 is a digital raster graphic consisting of matrix-arranged two-dimensional pixels, representing a set of parallel straight lines running along its line direction in source image 305. Fig. 3 Different image values ​​are indicated by different hatching patterns or black or white coloring. For example, it is assumed here that the source image 305 and the intermediate image 400 are both grayscale images, and that each pixel is assigned exactly one grayscale value as its image value (pixel value). The resolution in the grid can differ, as assumed here, for rows and columns of the grid, so that the pixels have a rectangular, non-square shape, where, for example, the height of pixels 315 and 325 is greater than their width. This will be explained further with reference to Fig. 7 This will be explained in more detail.

[0074] In a further step 210, which is described in more detail in Fig. 3 As illustrated, an intermediate image 400 is generated from the source image 305, which is represented by corresponding intermediate image data. The intermediate image 400 results from the source image 305 by applying a distortion rule that defines a transverse wave-like distortion, such that the respective image values ​​of pixels 315 of the same pixel row 310 (in particular, a row or column of the raster graphic) are shifted within the raster such that the pixels 325, to which the image values ​​of the pixel row 310 are shifted, have an arrangement within the raster that resembles a transverse wave running along a "propagation direction" 330 parallel to the pixel rows 310. The shape of the transverse wave can, in particular, be sinusoidal, at least section by section, which can be advantageous especially with regard to the verification procedure described below, which is based on a Fourier analysis.

[0075] In the image conversion process 200, a further step 215 follows, which starts from the intermediate image 400 generated in step 210. Depending on the distortion formula used in step 210 to distort the source image 305, correction information is determined. This information, based on the grid of the intermediate image 400, defines a shift in the pixel positions of the grid, resulting in the target image 455 appearing, or would appear, at least largely undistorted to a viewer. The matrix-like grid is thus modified in such a way that straight rows of pixels 310 of the grid are each converted into transverse wave-like wave trains consisting of pixels arranged in a corresponding transverse wave pattern, thereby at least largely compensating for the distortion generated in step 210.

[0076] Step 215 is an example and simplified in Fig. 4 Illustrated. This is shown in the upper section of Fig. 4 The intermediate image 400 is shown again in a simplified representation with some exemplary transverse wave-shaped arrangements of pixels resulting from the distortion in step 210, in particular arrangements 405, 410, and 415. Each of these transverse wave-shaped arrangements originated in step 210 from a straight row of pixels, in particular a line, 310 of the source image 305. To get from the intermediate image 400 to the target image 455, the intermediate image 400 is scanned along straight scan paths 420, of which only one is shown as an example for the sake of simplicity. Fig. 4 This is illustrated. Along a scan direction of scan path 420, the positions of pixels 425 to 450 are recorded, each belonging to one of the transverse wave-shaped arrangements of pixels, such as the arrangements 405, 410, and 415 shown here. A displacement vector v is shown for pixel 430 as an example, which corresponds to the displacement of the pixel value of an (output) pixel in the source image 305 to the corresponding "other" pixel 430 in the intermediate image 400, which occurred during the distortion in step 210. This applies accordingly to the other pixels 425 and 435 to 450 identified here.

[0077] In step 220, this distortion is compensated using a displacement vector v' that is the inverse of v. Unlike distortion, however, compensation does not transfer image values ​​from one pixel to another. Instead, it shifts the positions of the pixels themselves, which retain their respective pixel values. For example, pixel 430 of intermediate image 400 is shifted by the displacement vector v', as shown in the lower part of the Fig. 4 As shown. If this is also done for the other image points 425 and 435 to 450 lying on the scan path 420, whereby their individual displacement vector v' is determined and used for compensation, then a transverse wave-shaped wave train 460 is formed in the target image 455 from the correspondingly shifted image points 425a and 435a to 450a.

[0078] Overall, this displacement process is carried out in step 220 for all defined (parallel) scan paths 420, resulting in a family of transverse wave-shaped wave trains 460 in the target image 455.

[0079] In a further step 225, which is in Fig. 5 As illustrated in more detail, a physical image 465 of the target image 455 can now be generated on the substrate 160 by means of laser marking, for which the point of impact 170 of the laser beam is guided along a respective laser path 455a by appropriate control of the laser marking device, in particular the laser galvanometer 120 of the system 100, such that the wave trains 460 of the target image 455 are mapped onto the substrate 160 by forming corresponding image points of the image along the laser path 455a.

[0080] As in Fig. 5 As illustrated, in practice the conversion of the target image 455 into the image 465 is usually not perfect. This can be due, in particular, to the fact that the mirrors 135 and 145 have a non-zero moment of inertia and, at high laser marking speeds, are no longer able to precisely map abrupt changes in direction along the wave trains 460 onto the substrate 160. Therefore, deviations between the exact shape of the respective wave train 460 and the corresponding laser path 455a derived from it can regularly occur at points of sharp changes in direction. In particular, "angular" sections of a wave train 460 will generally have a rounded shape in the image 465, especially in the sense of "overshooting". Fig. 5 This is shown as an example, with pixels 425a to 450a of the target image 455 displayed as a reference to clearly show the deviations. This imperfect mapping from the target image 455 to the image 465 is actually advantageous, as it represents a further aspect of the security feature defined by the wave patterns of image 465, making counterfeiting even more difficult.

[0081] Fig. 6 Figure 470a, an enlarged representation of an exemplary image section 470, illustrates a wave structure that can be generated within the framework of the process 200 in the target image 455 or the image 465 generated therefrom on a substrate 160 (without showing the aforementioned deviations between target image 455 and image 465). The distinctive mouth area contained here in image section 470 is represented in the target image 455 and equally in the image 465 by corresponding gray values ​​of the pixels (here forming the thick line areas of the wave train segments) on the wave trains, which stand out against adjacent image areas. The adjacent wave trains are separated from each other by a space of a different color (e.g., white or in the background color of the substrate 160), so that they are individually optically recognizable and detectable, at least at a corresponding magnification.

[0082] Fig. 7 illustrates a resolution adjustment or pixel shape change within the framework of procedure 200, as described above with reference to Fig. 3 As described in detail, an image grid of the source image 305 and the intermediate image 400 was used as an example, in which the rows and columns have different resolutions and thus different edge lengths of the pixels in the two orthogonal directions (x and y). In order to realize the aforementioned spaces between adjacent wave trains in the target image 455 and, if applicable, the subsequently generated image 465, without introducing an undesirable distortion (not to be confused with the desired temporary distortion in step 210 according to the distortion rule) of the respective image, the pixels are cropped during the transition to the target image 455 so that they have a symmetrical shape with respect to width and height, in particular a square shape, so that the cropped portion(s) are available to form the spaces.

[0083] This is Fig. 7 It is shown where a pixel 500 of the intermediate image 400 is decomposed into a square portion 505 to be transferred to the target image, and two remaining portions 510 not to be transferred to the target image. Instead of the latter, a (half) space is formed in the target image 455 between each adjacent pixel.

[0084] Additionally, in Fig. 7 Exemplary dimensions and resolutions (in dpi) are given, the latter referring specifically to the source image 305 and equally to the intermediate image 400.

[0085] Fig. 8 illustrates, by means of a flowchart, an exemplary embodiment 600 of a verification method, in particular a computer-implemented method, for verifying digital images, in particular by means of the security solution-compliant image conversion method (e.g. according to Fig. 2 ), generable digital images. Fig. 9 shows a schematic representation to illustrate a verification of a digital image based on its two-dimensional (2D) Fourier spectrum within the framework of the 600 method. Fig. 8 .

[0086] In the verification procedure 600, image data 605 are captured in one step, representing a digital image 700 to be verified. For the following example, it is assumed that this digital image 700 represents a person P (see below). Fig. 9 ). Similar to the System 100 from Fig. 1 The image 700 can be captured, in particular, by means of a camera, by receiving image data via a communication link, or by reading it from a memory. The image 700 can, in particular, be a physical image projected onto an object surface, especially onto a sheet-shaped substrate 160, and especially onto a page of a document (such as a passport document).

[0087] In a further (optional) step 610 of the verification procedure 600, an image analysis is performed in which the digital image 700 is analyzed, particularly with regard to especially relevant image components. This may include, in particular, image segmentation. In the case of a person P depicted at least partially by the image 700, the image analysis may include, in particular, a facial analysis in which biometric features of the face, such as the position of the eyes, especially the pupils, the nose, ears, corners of the mouth, etc., are located. This may, in particular, serve the purpose of defining the image sectors 705 in a further step 615 of the procedure 600, in which the digital image is divided into different image sectors 705, depending on the localized biometric features, for example, such that at least one image sector 705 is defined for each biometric feature. In Fig. 9 For example, image sector 710 represents such a selected image sector from the set of image sectors 705.

[0088] The in Fig. 9 The illustrated subdivision of the image into several image sectors 705 serves in particular to enable, in a further step 620, a transformation from spatial space to frequency space based on individual image sectors (e.g., image sector 710) by means of a two-dimensional transformation, specifically a two-dimensional Fourier transform as an example. The image sectors considered in this process can be, in particular, all defined image sectors 705 or only a specific selection thereof. This selection can be determined, in particular, by including only image sectors 710 defined for the biometric features, and especially all of them.

[0089] For example, in Fig. 9 A matrix-shaped (discrete) 2D Fourier spectrum 715 of the selected image sector 710 resulting from step 620 is shown, in which each point of the matrix is ​​described by its point value (in Fig. 9 The matrix (indicated by different colors or hatching) represents the value of a specific Fourier coefficient or spectral value of the spectrum 715. The center of the spectrum corresponds to zero frequency, and the position of each point in the matrix identifies its corresponding wave vector or Fourier coefficient, such that the distance of each point from the center corresponds to the frequency (or wavelength) represented by the point. The direction vector represented by the center and the point indicates the direction of propagation in two-dimensional space.

[0090] In the present example, it is assumed that image sector 710 – similar to image section 470a from Figur 6 - exhibits a plurality of wave trains. The bright line in the spectrum, which indicates that the Fourier coefficients corresponding to the points of the line are heavily populated in spectrum 715, is essentially caused by the very frequent, approximately horizontal regions of the minima and maxima of the wave trains (in spatial space) due to the numerous wave trains present in image sector 710.

[0091] In a further step 625 of the procedure 600, several comparisons are now performed, in each of which a spectrum 715 obtained from a selected image sector in step 620 is compared with a reference spectrum 720, which results from the actual original image for the respective image sector when the same transformation is applied. The reference spectra 720 can, in particular, be stored in advance in a memory in a manner secured against unauthorized access, so that they can be read from it and made available for the purpose of comparison. For example, it is possible to design this so that the reference spectra 720 can be retrieved from a remote server via a secure communication connection.

[0092] In the example of the Fig. 9A comparison of spectrum 715, derived from image 700 to be verified, with the corresponding reference spectrum 720, derived from the genuine original image, reveals a significant deviation. A corresponding comparison is then performed individually for all selected image sectors. To definitively determine whether image 700 is the original image or a forgery, a test criterion can be used that is based on a number N of selected image sectors of image 700 for which a deviation exceeding a defined threshold was detected in their respective assigned spectrum comparisons. This number N can then be compared with a predetermined threshold M, which allows the sensitivity of the test procedure to be adjusted.If it turns out that N is less than M (630 - yes), then the digital image 700 is classified as genuine and this result is output in step 635. Otherwise, (630 - no), the image 700 is classified as fake and this result is output in step 640. In particular, M = 1 can be chosen, so that then even a deviation in the spectrum 715 of a single image sector is sufficient to classify the image 700 as fake.

[0093] The comparison of the two spectra 715 and 720 for the respective image sector can be carried out, in particular, on a Fourier coefficient basis. This involves comparing the pairwise corresponding Fourier coefficients of the two spectra 715 and 720, checking whether their values ​​differ by more than an acceptable threshold. The total number of values ​​exceeding this threshold can then be compared with an acceptance threshold to determine whether there is a (significant) deviation between the two spectra in the respective image sector.

[0094] While at least one exemplary embodiment has been described above, it should be noted that a large number of variations exist. It should also be noted that the described exemplary embodiments are merely non-limiting examples, and it is not intended to restrict the scope, applicability, or configuration of the devices and methods described herein. Rather, the preceding description will provide the person skilled in the art with guidance for implementing at least one exemplary embodiment. It is understood that various modifications to the function and arrangement of the elements described in an exemplary embodiment can be made without deviating from the subject matter defined in the appended claims. REFERENCE MARK LIST

[0095] PObject, in particular person vDisplacement vector for distortion v'to vInverse displacement vector for compensation 100Image conversion system, exemplary embodiment 105Image conversion device, in particular data processing device 105aProcessor 105b(Program) memory 110Image sensor, camera 115Image memory 120Laser galvanometer 125Laser 130Laser beam 135First mirror 140Mirror drive for first mirror 145Second mirror 150Mirror drive for second mirror 155Optics, in particular F-theta lens 160Substrate 165Image of the target image on the substrate 170Impact point of the laser beam, pixel of the image 175Wave train of pixels of the image 200Image conversion method, exemplary embodiment 205-225Process steps of the method 200 305 digital (source) image in raster graphics 310 pixel sequence of the source image 305 315 pixel of the source image 305 / of the pixel sequence 310 325 "other" pixel,on which an image value of image point 315 is mapped by the distortion rule 330 "Direction of propagation" of the transverse wave resulting from distortion of the image point series 310 400 Intermediate image 405-415 transverse wave-shaped arrangement of the "other" image points 420 Scan path 425, 430 Image points captured during scanning on transverse wave 415 425a, 430 a position-compensated image points from transverse wave 415 435, 440 Image points captured during scanning on transverse wave 410 435a, 440 a position-compensated image points from transverse wave 410 445, 450 Image points captured during scanning on transverse wave 405 445a, 450 a position-compensated image points from transverse wave 405 455Target image, with compensated pixel positions 455aLaser path 460Wave train in target image 465, 165Image of the target image on substrate,470 Image section produced by means of laser marking 470a Enlarged image section 470 500 Pixel of the intermediate image 400 505 Portion of the pixel transferred to the target image 500 510 Portions of the pixel not transferred to the target image 500 600 Verification procedure, exemplary embodiment 605-640 Procedure steps of the verification procedure 600 700 Digital image to be verified 705 Image sectors in the image to be verified 700 710 Selected image sector 715 2D Fourier spectrum of image sector 710 720 2D Fourier reference spectrum of the image sector corresponding to image sector 710 in the original image (here source image 305),

Claims

1. An image conversion method (200) for integration of a security feature into a digital source image (305) in order to generate a target image (455) secured by the integrated security feature, wherein the image conversion method (200) comprises: acquiring (205) source image data which represent a digital source image (305) to be protected by means of the security feature, which digital source image comprises pixels (315) arranged in a grid of straight parallel rows of pixels (310), each having at least one pixel value per pixel (315); generating (210) intermediate image data, which represent an intermediate image (400) that results from the source image (305) by applying a distortion rule, according to which, for each row of pixels (310) of the source image (305), the respective pixel values of pixels (315) of the row of pixels (310) are transferred within the grid along a direction which is angled to the row of pixels (310), in particular perpendicular to the row of pixels (310), to a respective other pixel (325; 500) of the grid which, in relation to the respective pixel (315), is determined or determinable by the distortion rule in such a way that the arrangement (405; 410; 415) of these other pixels (325; 500) in the grid has a transverse waveform; generating (220) target image data representing the target image (455), wherein: the intermediate image (400) is scanned in rows of pixels in order to define for each row of pixels of the intermediate image (400) a sequence of pixels (425, 430, 435, 440, 445, 450) that follow each other according to the scanning, the pixel values of which have resulted from the transfer of corresponding pixel values from the source image (305) in accordance with the distortion rule; each pixel (425, 430, 435, 440, 445, 450) of the sequence is transformed into a respective corresponding pixel (425a, 430a, 435a, 440a, 445a, 450a) of the target image (455) by determining its position in the target image (455) based on its position in the intermediate image (400) by compensating for the distortion suffered by applying the distortion rule when generating the intermediate image (400), so that the arrangement (405; 410; 415) of the respective pixels of each sequence in the target image (455) represents a transverse-wave-shaped wave packet; and the integrated security feature is defined by the wave packets, wherein the arrangement (405; 410; 415) of the respective pixels of each sequence in the target image (455) is determined such that each of two adjacent wave packets are separated from each other by a gap; and wherein the substrate (160) is selected or processed in such a way that it has a color that stands out relative to the average of the colors of the pixels that form the wave packets of the target image (455), which are determined according to the pixel values, in such a way that neighboring wave packets can be visually distinguished.

2. The image conversion method (200) according to claim 1, further including: generating (225) a physical reproduced image (165; 465) of the target image (455) on a surface of a substrate (160), wherein the reproduced image (165; 465) of the target image (455) is generated by serially generating pixels on the substrate (160) by, in order to generate a series of pixels on the substrate (160), which corresponds to a respective sequence of pixels of the target image (455), generating these pixels of the series on the substrate (160) in accordance with the pixel order defined by the sequence of the corresponding pixels of the target image (455).

3. The image conversion method (200) according to claim 2, wherein the physical reproduced image (165; 465) of the target image (455) is generated on the substrate (160) using laser inscription, in which the pixels of the reproduced image (165; 465) are sequentially generated on the substrate (160) using a laser beam (130).

4. The image conversion method (200) according to claim 3, wherein the different positions of the series at which the laser beam (130) strikes the substrate (160) to generate the pixels of the reproduced image (165; 465) are controlled by variable deflection of the laser beam (130) in a mirror-based laser galvanometer (120).

5. The image conversion method (200) according to claim 4, wherein at least one mirror (135; 145) of the laser galvanometer (120) is controlled using a control signal for controlling a respective position of a pixel of the reproduced image (165; 465) to be generated on the substrate (160), which control signal is defined as a function of the compensation that has been determined for that pixel of the intermediate image (400) which corresponds to the pixel of the reproduced image (165; 465) to be generated.

6. The image conversion method (200) according to claim 4 or 5, wherein a laser galvanometer (120) is used for variable deflection of the laser beam (130), in which the inertia of at least one of its mirrors (135; 145) used for deflection is so large that, when imaging the wave packets of the target image (455) by generating the pixels of the reproduced image (165; 465) on the substrate (160), deviations between the reproduced image (165; 465) and the target image (455) arise in the event of abrupt changes in direction along the course of the wave packets.

7. The image conversion method (200) according to any one of the preceding claims, wherein the arrangement (405; 410; 415) of the respective pixels of each sequence in the target image (455) is determined so that it represents a transverse-wave-shaped wave packet (460) which is periodic at least in some portions, wherein the arrangement (405; 410; 415) of the respective pixels of each sequence in the target image (455) is preferably determined so that it represents a transverse-wave-shaped wave packet (460) which is sinusoidal at least in some portions.

8. The image conversion method (200) according to any one of the preceding claims, wherein each of the wave packets resulting in a respective sequence has at least two inflection points.

9. The image conversion method (200) according to any one of the preceding claims, wherein the grid of pixels of the source image (305) has rows and columns, and the resolution of the rows is different from the resolution of the columns.

10. The image conversion method (200) according to claim 9, wherein the resolution of the source image (305) in the direction orthogonal to the rows of pixels is at most 70% of the resolution in the direction running along the rows of pixels.

11. The image conversion method (200) according to claim 9 or 10, wherein the resolution of the source image (305) in the direction orthogonal to the rows of pixels is at least 200 pixels per inch or per 2.54 cm, PPI.

12. The image conversion method (200) according to any one of the preceding claims, wherein the pixels of the target image (455) are determined in such a way that their respective extents are the same along and orthogonal to the transverse direction of the wave packets, and / or wherein the distortion rule is defined such that when it is applied to rows of pixels of the source image (305), at least for a subset of the rows of pixels, the wave packet respectively resulting from this has a first waveform in one or more portions of its course and a second waveform different from the first waveform in at least another portion of its course.

13. An image conversion device (105) which is configured to carry out the image conversion method (200) according to any one of claims 1 to 12.

14. A computer program or computer program product, comprising instructions which cause the image conversion device (105) according to claim 13 to carry out the image conversion method (200) according to any one of claims 1 to 12.

15. A digital image (700) with an integrated security feature, obtainable by the image conversion method (200) according to any one of claims 1 to 12 as a target image (455) or reproduced image (165; 465) of the same on a substrate, and preferably wherein the digital image (700) with an integrated security feature has a plurality of mutually parallel rows of pixels, each of which has a transverse-wave-shaped course, wherein adjacent rows of pixels are separated from one another by a gap which, as regards its color, stands out at least in some portions with respect to the rows of pixels separated thereby, wherein the security feature is defined by the wave-shaped course of the rows of pixels and of the gaps between them, and / or wherein the digital image is formed on a document page serving as a substrate (160) for a value document or a security document.