Method for processing an image acquired by a fingerprint sensor, to discriminate between fingers and traces

Abstract

A method for processing an image acquired by a fingerprint sensor, comprising: determining (200) a peak value (R) associated with a pixel of the image and a valley value (V) associated with the pixel; calculating (202) a normalized dynamic range (Dn) associated with the pixel as a ratio between a deviation between the peak value (R) associated with the pixel and the valley value (V) associated with the pixel, and a reference value being a linear combination of the peak value (R) associated with the pixel and the valley value (V) associated with the pixel; comparing (204) the normalized dynamic range associated with the pixel and a dynamic range threshold (T1); and generating a result associated with the pixel, the result associated with the pixel indicating that the pixel shows a finger in view of the fingerprint sensor only if the normalized dynamic range (Dn) is greater than the dynamic range threshold (T1). Figure for abstract: Fig. 3

Description

Title of the invention: Method for processing an image acquired by a fingerprint sensor, to discriminate between fingers and traces Technical field

[0001] The present invention relates to a method for processing an image acquired by a fingerprint sensor. STATE OF THE ART

[0002] Some sensors used to acquire fingerprint images are sensitive to the following parameters: skin type of the person (light skin, dark skin), humidity of the person's fingers, lighting conditions of the skin (ambient light and / or internal light of the sensor used). As a result, the fingerprints obtained in an image acquired by such sensors have a large variability.

[0003] Furthermore, a finger placed on a surface in view of the sensor may leave a trace, after this finger is no longer in view of the sensor. This trace is likely to have peaks and valleys in the same way as a dermatoglyph. Consequently, an image acquired by a sensor may show a finger in view of the sensor, but also traces that do not correspond to a finger in view of the sensor.

[0004] Such traces are also subject to variability, and may be difficult to distinguish from actual fingerprints in view. SUMMARY

[0005] An aim of the invention is to achieve more precise discrimination, in an image acquired by a fingerprint sensor, between fingers in view of the sensor and other traces.

[0006] To this end, according to a first aspect, a method is proposed for processing an image acquired by a fingerprint sensor, the method comprising the following steps implemented for at least one pixel of the image: • determining a peak value associated with the pixel and a valley value associated with the pixel, wherein the peak value is a maximum pixel value of the image in a predefined neighborhood of the pixel, and the valley value is a minimum pixel value of the image in a predefined neighborhood of the pixel; • calculate a normalized dynamics associated with the pixel as a ratio between a deviation between the peak value associated with the pixel and the valley value associated with the pixel, and a reference value being a linear combination of the peak value R associated with the pixel and the valley value V associated with the pixel ; • compare the normalized dynamics associated with the pixel and a dynamic threshold; and • generate a result associated with the pixel, the result associated with the pixel indicating that the pixel shows a finger in view of the fingerprint sensor only if the normalized dynamic range is greater than the dynamic range threshold.

[0007] The method according to the first aspect may also comprise the following optional features, taken alone or in combination whenever this makes technical sense.

[0008] Preferably, the reference value is proportional to the peak value associated with the pixel or proportional to the valley value associated with the pixel.

[0009] Preferably, the method according to the first aspect further comprises: • compare the peak value associated with the pixel or the valley value associated with the pixel with a value threshold, • in which the result associated with the pixel indicates that the pixel shows a fingerprint only if the following conditions are met: • the normalized dynamic range is greater than the dynamic range threshold, and • the peak value or valley value is greater than the value threshold.

[0010] Preferably, the method according to the first aspect further comprises: • identify an area of ​​interest in the image showing a candidate object likely to be a fingerprint, • repeating the steps of determining, calculating, comparing and generating a result for each pixel of the area of ​​interest, so as to generate a mask comprising a plurality of results respectively associated with the pixels of the area of ​​interest, • generating a consolidated result associated with the area of ​​interest from the mask, wherein the consolidated result indicates that the candidate object is a fingerprint only if the plurality of results respectively associated with the pixels of the area of ​​interest comprises a majority of results indicating pixels showing a finger in view of the fingerprint sensor.

[0011] Preferably, the method according to the first aspect further comprises: • repeating the steps of determining, calculating, comparing and generating a result for different pixels of the image, so as to generate a mask comprising a plurality of results respectively associated with the different pixels, • in the mask, identification of a group of results respectively associated with adjacent pixels of the image, the group of results indicating that the adjacent pixels associated with it all show a finger in view of the fingerprint sensor, • provided that the result group has a number of results less than a predefined number, adjustment in the result group mask so as to indicate that none of the adjacent pixels show a finger in view of the fingerprint sensor.

[0012] Preferably, the method according to the first aspect further comprises: • repeating the steps of determining, calculating, comparing and generating a result for different pixels of the image, so as to generate a mask comprising a plurality of results respectively associated with the different pixels, • in the mask, identification of a group of results respectively associated with adjacent pixels of the image, the group of results indicating that the adjacent pixels associated with it all show a finger in view of the fingerprint sensor; • provided that the group of results has a number of results greater than a predefined number, identification, in the mask, of complementary results respectively associated with complementary pixels of the image forming with the adjacent pixels an area of ​​the image of predefined shape, for example ovoid; • adjustment in the mask so that the complementary results indicate that the complementary pixels show a finger in view of the fingerprint sensor.

[0013] Preferably, the fingerprint sensor is a contact sensor, preferably a direct line-of-sight contact.

[0014] A second aspect of the present disclosure is a computer program product comprising program code instructions for executing the steps of the method according to the first aspect, when this program is executed by an image processing module.

[0015] A third aspect of the present disclosure is a memory readable by an image processing module storing instructions executable by an image processing module for executing the steps of the method according to the first aspect.

[0016] A fourth aspect of the present disclosure is a device comprising a fingerprint sensor and an image processing module configured to process an image acquired by the fingerprint sensor, in accordance with the method according to the first aspect. DESCRIPTION OF FIGURES

[0017] Other characteristics, aims and advantages of the invention will emerge from the following description, which is purely illustrative and non-limiting, and which must be read in conjunction with the appended drawings in which:

[0018] [Fig.l] schematically illustrates a device according to one embodiment of the invention.

[0019] [Fig.2] is a flowchart of steps of an image processing method according to one embodiment of the invention.

[0020] [Fig.3] is a flowchart detailing the sub-steps of a step of the method of [Fig.2], according to one embodiment.

[0021] [Fig.4] schematically represents an image involved in the implementation of the method of [Fig.2].

[0022] [Fig.5] schematically represents a detection mask generated during the implementation of the method of [Fig.2].

[0023] Throughout the figures, similar elements bear identical references. DETAILED DESCRIPTION OF THE INVENTION Device

[0024] With reference to [Fig.l], a device 1 comprises a fingerprint sensor 2 and an image processing module 4.

[0025] The fingerprint sensor 2, more simply called sensor 2 in the following, has the function of generating an image showing a fingerprint of the finger of a user of the device 1, when the user places this finger in a predefined acquisition zone in view of the sensor 2. The sensor 2 generates the image from the light that it receives. A part of this light has been returned by the ridges of the fingerprint, and another part of the light that it receives has been returned by the valleys of the fingerprint.

[0026] The sensor 2 is for example a sensor 2 with contact, which assumes that the device 1 comprises an acquisition surface 6 on which the finger is supposed to be placed, when the sensor 2 acquires an image.

[0027] In particular, the sensor 2 may be a direct-view contact sensor 2, for example of the TFT (Thin Film Transistor) type, with a glass substrate, or of the CMOS (Complementary metaloxide Silicon) type, with a silicon substrate. Alternatively, the sensor 2 may be a sensor operating on the principle of total reflection (in English: “frustrated total internal re-flection”, abbreviated to FTIR).

[0028] The image processing module 4, more simply called module 4 hereinafter, has the function of processing an image acquired by the sensor 2.

[0029] Module 4 is for example a dedicated physical component, such as a circuit, for example for example a programmable circuit (FPGA) or a non-programmable circuit (ASIC). Alternatively, the device 1 comprises a processor, and the module 4 is a software component, in other words a computer program comprising code instructions intended to be executed by the processor. The processor may comprise one or more cores (to execute tasks simultaneously).

[0030] The device 1 further comprises a memory 8 suitable for storing the program, images acquired by the sensor 2, or data produced by the module 4. The memory typically comprises a non-volatile memory in which the program is stored, and a volatile memory in which the program can be loaded and for temporarily storing data calculated by the module 4. Image processing method

[0031] We will now detail in the following a method implemented by the image processing module 4 with reference to [Fig.2].

[0032] In a preliminary step, an image was acquired by the sensor 2, then transmitted to the image processing module 4.

[0033] The image acquired by the sensor 2 is likely to include one or more patterns having peaks and valleys. In the following, two types of patterns are distinguished: • patterns showing dermatoglyphics of fingers which were in view of the sensor 2 when the image was acquired, and which will conventionally be called “fingerprints”; • valley and ridge patterns not related to fingers in view of the sensor 2 when the image was acquired. By convention, these patterns will be called “traces” in the following, to differentiate these patterns from fingerprints. Typically, such traces may have been left on a surface of the device 1 by a finger. In this case, there is a deposit of material (for example, greasy) on the surface, which comes from a finger, but this finger is no longer in view of the sensor 2: it is this deposited material which is seen by the sensor 2, and which could be confused with a real fingerprint, despite the fact that there is no finger in view of the sensor 2.

[0034] In a preprocessing step 100, the module 4 identifies in the image at least one area of ​​interest, each area of ​​interest showing a candidate pattern having peaks and valleys. At this stage, we speak of a “candidate” pattern, because the module 4 does not yet know whether a peak and valley pattern is a fingerprint (therefore relating to a finger in view of the sensor 2) or a trace (not relating to a finger in view of the sensor 2).

[0035] This identification can typically be carried out by segmenting the image on a spatial frequency criterion or a level criterion. During this seg mentation, at least one area of ​​the low-frequency image is considered to be a background of the image not constituting an area of ​​interest, and at least one area of ​​the high-frequency image is considered to be an area of ​​interest. Indeed, a peak-and-valley pattern is a pattern with high spatial frequencies.

[0036] In a step 102, the module generates a fingerprint detection mask associated with the image.

[0037] With reference to [Fig. 3], step 102 comprises the following sub-steps applied to a pixel of the image acquired by the sensor 2, this pixel being located in an area of ​​interest identified in the pre-processing step 100.

[0038] In a step 200, the module 4 determines a peak value R associated with the pixel and a valley value V associated with the pixel.

[0039] The peak value R is a maximum pixel value of the image in a first predefined neighborhood of the pixel. The pixel of maximum value is the pixel that received the most light among the pixels of the first neighborhood. The peak value R is indicative of an amount of light received by the sensor 2 in the pixel, after being returned by peaks shown in the image.

[0040] Furthermore, the valley value V associated with the pixel, the valley value V being a minimum pixel value of the image in a second predefined neighborhood of the pixel. The pixel of minimum value is the pixel that received the least light among the pixels of the second neighborhood. The valley value V is indicative of an amount of light received by the sensor 2 at this pixel, after being returned by valleys shown in the image.

[0041] The first neighborhood or the second neighborhood is typically a set of connected pixels forming a rectangle or a square, for example centered on the pixel considered. Preferably, the first neighborhood or the second neighborhood has a height or a width in number of pixels corresponding to three times the average inter-peak distance of an adult human finger. This dimension is adjusted taking into account the resolution of the sensor 2.

[0042] The first neighborhood and the second neighborhood may be identical or different.

[0043] In a step 202, the module 4 calculates a normalized dynamic Dn associated with the pixel as a ratio between: • a dynamic D associated with the pixel, constituting a difference between the peak value R associated with the pixel and the valley value V associated with the pixel, and • a reference value associated with the pixel, the reference value associated with the pixel being a linear combination of the peak value R associated with the pixel and the valley value V associated with the pixel.

[0044] The normalized dynamic Dn associated with the pixel thus has the following general form: [00451 Du - D - R'v LJ one — aR+pV - aH+pv

[0046] It should be noted that the reference value associated with the pixel can be proportional to the valley value V associated with the pixel or to the peak value R associated with the pixel. In this case, one of the weights a or / 1 is zero.

[0047] In particular embodiments, the reference value associated with the pixel is the valley value V associated with the pixel or is the peak value R associated with the pixel (which implies that one of the two weights a or P is zero, while the other weight is equal to 1).

[0048] In a step 204, the module 4 compares the normalized dynamics with a predefined dynamics threshold T1 (the predefined dynamics threshold T1 is previously stored in the memory 8).

[0049] Module 4 then generates a result associated with the pixel, which can take two values: • a first OK value (also called “positive result” in the following) indicating that the pixel shows a finger in view of the sensor 2 (in other words, it is considered by the module 4 that the candidate pattern of the area of ​​interest including the pixel is a fingerprint); or • a second KO value (also called “negative result” in the following) indicating that the pixel does not show a finger in view of sensor 2 (in other words, it is considered by module 4 that the candidate pattern of the area of ​​interest including the pixel is a simple trace on a surface of the device in view of sensor 2).

[0050] The result associated with the pixel can thus be a boolean. For example, OK=1 and KO=0.

[0051] The value taken by the result depends on the comparison of step 204. In a manner In general, the result generated by module 4 is positive only if Dn > TL. Thus, this condition Dn > Tl is necessary for the result to be positive (OK). If this condition is not met, the result generated by module 4 is negative.

[0052] In a simple embodiment of implementation, this condition is sufficient: thus, module 4 relies solely on this condition to generate a positive result.

[0053] In a particularly advantageous embodiment, this condition based on the normalized dynamics Dn is not sufficient: an additional condition must be fulfilled for the positive result to be generated, this additional condition being based on the peak value R or the valley value V associated with the pixel.

[0054] Thus, in an optional step 206, the module 4 can compare the peak value R with a peak threshold and / or compare the valley value V with a valley threshold. The positive result OK can then be generated only if the two conditions following conditions are met: Dn > Tl and R > T2. If these two conditions are not met, module 4 generates the negative result KO.

[0055] We will see later that the additional test improves the reliability of the results generated by module 4 (reduction in the rate of false alarms and missed finger detections).

[0056] Steps 200 to 206 are repeated for each pixel of the image included in an area of ​​interest. Preferably, these steps are not applied to the other pixels of the image.

[0057] Steps 204 and 206 can be performed in any order.

[0058] As a result of this repetition, the module 4 obtains a plurality of results respectively associated with the pixels of the image located in the area(s) of interest of the image. This plurality of results constitutes the mask for detecting fingerprints in the image, resulting from step 102. This mask provides pixel-by-pixel information, which is in itself advantageous and which allows the mask to be used in a subsequent processing of authentication or identification of an individual whose finger has been imaged.

[0059] Returning to [Fig.2], the module implements the consolidation step 104 of the detection mask, so as to obtain a consolidated mask. The consolidation step 104 may comprise the following sub-steps.

[0060] Module 4 identifies in the mask a group of positive results respectively associated with adjacent pixels of the image. In reality, module 4 can in this sub-step distribute all the positive results obtained into one or more groups of adjacent pixels. Then, module 4 compares the number of positive results of a given group with a first predefined number.

[0061] If the number of positive results in the group is less than the first predefined number, then module 4 adjusts the mask so that the results in the group become negative in the consolidated mask. Otherwise (i.e., if the number of results in the group is not less than the first predefined number), then this adjustment is not implemented.

[0062] This first adjustment of the mask makes it possible to eliminate in the consolidated mask areas of interest that are too small to be usable in subsequent applications such as authentication or identification, and therefore constitute noise. In particular, traces of dust on the sensor 2 can be eliminated by this adjustment.

[0063] Furthermore, module 4 compares the number of results of a given group with a second predefined number. If the number of results of the group is greater than the second predefined number, module 4 identifies in the mask complementary results respectively associated with complementary pixels of the image, these pixels complementary pixels forming with the adjacent pixels of the group considered an area of ​​the image having a predefined shape. Then, module 4 adjusts the mask so that the complementary results are positive in the consolidated mask. This implies that any complementary result which was negative in the mask becomes positive in the consolidated mask.

[0064] This second adjustment makes it possible to “recover” a fingerprint of which only a part of the pixels will have been identified in the initial mask.

[0065] Preferably this predefined shape is an ovoid shape. This shape is advantageous because it is a shape that correctly approximates the shape of a conventional fingerprint.

[0066] A third adjustment that can be made during step 104 consists of assigning to an area of ​​interest that has been determined in step 100 an overall positive OK result only if the pixels of the area of ​​interest are mostly associated with respective positive OK results. Thus, in the event of such a majority, the negative KO results in the area of ​​interest considered become positive results.

[0067] One of the three adjustments proposed above may be selectively implemented during consolidation step 104, and may be combined. In case of combination, the second predefined number will be greater than or equal to the first predefined number.

[0068] Of course, each adjustment can be applied to each identified group of results or to each identified area of ​​interest.

[0069] The consolidated mask thus results from each adjustment made to the detection mask that module 4 had obtained at the end of step 102.

[0070] As an example, [Fig. 4] schematically represents an image acquired by the sensor 2. This image comprises seven areas of interest showing seven respective candidate patterns M1 to M7: fingerprints M1, M2, M3 and M4 and traces M5, M6 and M7. [Fig. 5] is a representation of a consolidated mask obtained at the end of step 104, applied to the image of [Fig. 4]. The white parts of this mask are those having a KO result, and the black parts are those having an OK result. It can be seen that the three traces M5, M6 and M7 have been eliminated, and that the four groups of pixels forming the black parts are ovoid in shape.

[0071] In a masking step 106, the module applies the consolidated mask to the image, so as to obtain an output image. This application consists, for example, in retaining in the output image any pixel associated with an OK result, and ensuring that all the other pixels of the image have values ​​positioned at a constant value (pixels outside areas of interest and pixels associated with KO results in the consolidated mask). This constant value is, for example, an extreme value (white or black).

[0072] The output image can then be used in a biometric authentication or identification step from the image, known to those skilled in the art. Advantages and comparative results

[0073] An advantage provided by the use of the normalized dynamic range Dn as a criterion for deciding whether a pixel truly shows a finger in view of the sensor 2 (and not a trace), is that this data Dn is little or not dependent on the quantity of light which is received by a finger imaged by the sensor 2.

[0074] To understand this, let us assume that the peak value R and the valley value V associated with a given pixel are proportional to the quantity of light received by a finger (this light combining the light emanating from an internal illumination source of the device 1, and surrounding light coming from outside the device 1), which is true for any sensor 2 with a photon-to-electron conversion element that has a linear response, whether it is total reflection or direct view. We can then assume that there are parameters kl and k2 such that:

[0075] R = kLL

[0076] V = C2. L

[0077] By injecting these terms into the formula giving the normalized dynamics, we have:

[0078] Du -■&¥■■■■ - -MLdZL- - 1 1 L - aR+flV “ akïL+pk2L ~ akl+pk2

[0079] We can clearly see that the term Dn no longer depends on L.

[0080] Ultimately, the normalized dynamic range Dn is a criterion that varies according to the skin (parameters kl and k2) but which does not vary in theory with the light of the finger L. kl rather characterizes the capacity of the finger to return light, or roughly its color, and k2 the capacity of the finger to couple well with the surface of the sensor 2 (dry or wet finger). This is the reason why the finger / trace discrimination carried out by the module 4 using the normalized dynamic range Dn is much more efficient than a finger / trace discrimination which would be based on the dynamic range D (non-normalized, and dependent on L).

[0081] The table below illustrates this performance gain, in an application of the method described above to a device comprising a direct view sensor.

[0082] [Tables 1] Case Pattern Real Finger? RD Dn 1 Dark underfingerprint No 40 24.5 0.613 2 Dark finger #1 Yes 75 27.5 0.367 3 Nominal finger Yes 180 60 0.333 4 Very dark finger #2 Yes 70 20 0.286 5 Very dark finger #3 Yes 60 13 0.217 6 Trace illuminated by light emanating from a source internal to device 1 (lighter part) No 35 6 0.171 7 Dry but bright finger Yes 180 30 0.167 8 Extremely dark finger (Real finger with black marker on it) Yes 50 8 0.160 9 DRY and dark finger #4 Yes 70 10 0.143 10 Trace + light emanating from the internal source brighter background No 73 7 0.096 11 Strong and bright trace No 200 17 0.085 12 Trace illuminated by light emanating from the internal source (dark room) No 63 5 0.079 13 Trace illuminated from the outside No 240 16 0.067 14 Trace without light emanating from a source internal to the device 1 No 160 5 0.031

[0083] The first column lists different candidate patterns appearing in images acquired by a sensor 2, and indicates any particular conditions under which these images were acquired.

[0084] We note that the dynamics D (fourth column) vary greatly, in any case much more than the normalized dynamics Dn (fifth column). Indeed, by choosing here T 1 = 0.1, we only make two discrimination errors between finger and trace (in cases 1 and 6).

[0085] This dynamic D combines particularly well with the peak value R in a two-condition embodiment in which it is necessary to have Dn > Tl and R > T2 to generate a positive result. Indeed, by choosing here Tl=0.1 and T2=80, we manage to no longer make any discrimination errors between finger and trace. Other embodiments

[0086] In the embodiment shown in [Fig.3], the additional test carried out in step 206 involves the peak value R. In other embodiments, it may be provided to generate the OK position result for a pixel, when: • Dn > Tl and V > T3 (2 conditions to be met), or • Dn > Tl and R > T2 and V > T3 (3 conditions to be met).

[0087] In another embodiment, step 206 is not implemented.

[0088] It is also recalled that steps 100 and 104, although advantageous, remain optional. In particular, it is possible to repeat the steps of [Fig.3] for each pixel of the image.

[0089] The method advantageously finds application with direct view sensors, but is not limited to this application.

Claims

Claims

1. A method for processing an image acquired by a fingerprint sensor, the method comprising the following steps implemented for at least one pixel of the image: • determining (200) a peak value (R) associated with the pixel and a valley value (V) associated with the pixel, wherein: • the peak value (R) is a maximum pixel value of the image in a predefined neighborhood of the pixel; • the valley value (V) being a minimum pixel value of the image in a predefined neighborhood of the pixel; • calculating (202) a normalized dynamic range (Dn) associated with the pixel as a ratio between: • a deviation between the peak value (R) associated with the pixel and the valley value (V) associated with the pixel, and • a reference value being a linear combination of the peak value (R) associated with the pixel and the valley value (V) associated with the pixel;• compare (204) the normalized dynamic range (Dn) associated with the pixel and a dynamic range threshold (Tl); • generate a result associated with the pixel, the result associated with the pixel indicating that the pixel shows a finger in view of the fingerprint sensor only if the normalized dynamic range (Dn) is greater than the dynamic range threshold (Tl).;

2. Method according to the preceding claim, in which the reference value is proportional to the peak value (R) associated with the pixel or proportional to the valley value (V) associated with the pixel.

3. A method according to any preceding claim, further comprising: • comparing (206) the peak value (R) associated with the pixel or the valley value (V) associated with the pixel with a threshold value (T2), • wherein the result associated with the pixel indicates that the pixel shows a fingerprint only if the conditions following conditions are met: • the normalized dynamics (Dn) is greater than the dynamics threshold (Tl), and • the peak value (R) or the valley value (V) is greater than the value threshold (T2).

4. A method according to any preceding claim, comprising: • identifying an area of ​​interest of the image showing a candidate object likely to be a fingerprint, • repeating the steps of determining (200), calculating (202), comparing (204) and generating a result for each pixel of the area of ​​interest, so as to generate a mask comprising a plurality of results respectively associated with the pixels of the area of ​​interest, • generating a consolidated result associated with the area of ​​interest from the mask, wherein the consolidated result indicates that the candidate object is a fingerprint only if the plurality of results respectively associated with the pixels of the area of ​​interest comprises a majority of results indicating pixels showing a finger in view of the fingerprint sensor.

5. A method according to any one of the preceding claims, • repeating the steps of determining, calculating, comparing and generating a result for different pixels of the image, so as to generate a mask comprising a plurality of results respectively associated with the different pixels, • in the mask, identifying a group of results respectively associated with adjacent pixels of the image, the group of results indicating that the adjacent pixels associated with it all show a finger in view of the fingerprint sensor, • provided that the group of results has a number of results less than a predefined number, adjusting in the mask the group of results so as to indicate that no adjacent pixels does not show a finger in view of the fingerprint sensor.

6. A method according to any one of the preceding claims, comprising: • repeating the steps of determining, calculating, comparing and generating a result for different pixels of the image, so as to generate a mask comprising a plurality of results respectively associated with the different pixels, • in the mask, identifying a group of results respectively associated with adjacent pixels of the image, the group of results indicating that the adjacent pixels associated with it all show a finger in view of the fingerprint sensor; • provided that the group of results has a number of results greater than a predefined number, identifying, in the mask, complementary results respectively associated with complementary pixels of the image forming with the adjacent pixels an area of ​​the image of predefined shape, for example ovoid;• adjustment in the mask so that the complementary results indicate that the complementary pixels show a finger in view of the fingerprint sensor.;

7. A method according to any preceding claim, wherein the fingerprint sensor is a contact sensor, preferably a line-of-sight contact sensor.

8. Computer program product comprising program code instructions for executing the steps of the method according to any one of the preceding claims, when this program is executed by an image processing module (4).

9. Memory (8) readable by an image processing module storing instructions executable by an image processing module (4) for executing the steps of the method according to one of claims 1 to 7.

10. Device (1) comprising: • a fingerprint sensor (2), an image processing module (4) configured to process an image acquired by the fingerprint sensor (2), in accordance with the method according to any one of claims 1 to 7.

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