WO2006085094A1 - Improvements in and relating to identifier investigation - Google Patents

Abstract

A method of extracting data from a representation of an identifier, such as a fingerprint is provided. The method includes selecting a plurality of features, such as ridge ends or bifurcations, in the representation of an identifier, considering the positions of those features and generating a reference feature, such a s a centre, from the positions of the plurality of features. The method then links one or more of the features to the reference feature and/or links one or more of the features to one or more of the other features in the plurality of features. From the result, the method extracts data including information on one or more of: one or more of the plurality of features; the reference feature; one or more of the links between a feature and the reference feature; one or more of the links between a feature and another feature.

Description

IMPROVEMENTS IN AND RELATING TO IDENTIFIER INVESTIGATION

This invention concerns improvements in and relating to identifier investigation, particularly, but not exclusively, in relation to the comparison of biometric identifiers or markers, such as prints from a known source with biometric identifiers or markers, such as prints from and unknown source. The invention is applicable to fingerprints, palm prints and a wide variety of other prints or marks, including retina images.

It is useful to be able to capture, process and compare identifiers with a view to obtaining useful information as a result. In the context of fingerprints, the useful result may be evidence to support a person having been at a crime scene.

Problems exist with present methods in terms of the robustness of the data they extract, amongst other problems.

The present invention has amongst its potential aims to extract data form a representation of an identifier in a more robust and useful manner.

According to a first aspect of the present invention we provide a method of extracting data from a representation of an identifier, the method including: selecting a plurality of features in the representation of an identifier; considering the positions of the plurality of features; generating a reference feature from the positions of the plurality of features; linking one or more of the features to the reference feature and/or linking one or more of the features to one or more of the other features in the plurality of features; the extracted data including information on one or more of: one or more of the plurality of features; the reference feature; one or more of the links between a feature and the reference feature; one or more of the links between a feature and another feature. According to a second aspect of the present invention we provide a method of comparing a first representation of an identifier with a second representation of an identifier, the method including: selecting a plurality of features in at least one of the first representation of an identifier and the second representation of an identifier; considering the position of one or more of the plurality of features; generating a reference feature from the considered positions of the plurality of features; linking one or more of the features to the reference feature and/or linking one or more of the features to one or more other features in the plurality of features; extracting data from the representation of the identifier, the extracted data including information on one or more of: one or more of the plurality of features; the reference feature; one or more of the links between a feature and the reference feature; one or more of the links between a feature and another feature; using the extracted data to compare the first representation with the second representation.

The first and/or second aspect of the invention may include features, options or possibilities from amongst the following.

The representation of the identifier may have been captured. The representation may be captured from a crime scene and/or an item and/or a location and/or a person. The representation may have been captured by scanning and/or photography.

The method may process an already processed representation of an identifier. The already processed representation may have been processed to convert a colour and/or shaded representation into a black and white representation. The already processed representation may have been processed using Gabor filters.

The method may process a representation of an identifier which has been altered in format. The alteration in format may involve converting the representation into a skeletonised format. The alteration in format may involve converting the representation into a format in which the representation is formed of components, preferably linked data element sets. The alteration may convert the representation into a representation formed of single pixel wide lines. The processing may have involved cleaning the representation, particularly according to one or more of the techniques provided in applicant's PCT patent application no GB2005/003968 and/or UK patent application number 0502893.1 of 11 February 2004 and/or UK patent application number 0422785.6 of 14 October 2004. The processing may have involved healing the representation, particularly according to one or more of the techniques provided in applicant's PCT patent application no GB2005/003968 and/or UK patent application number 0502893.1 of 11 February 2004 and/or UK patent application number 0422785.6 of 14 October 2004.. The processing may have involved cleaning of the representation followed by healing of the representation. The processing may have involved cleaning of the representation followed by healing of the representation. The processed representation may be subjected to one or more further steps. One or more further steps in which the processed representation is placed in a form for comparison may be provided. The form for comparison may particularly be that set out in detail in applicant's PCT patent application no GB2005/003945 and/or UK patent application number 054502902.0 of 11 February 2004 and/or UK patent application number 0422785.6 of 14 October 2004. The form for comparison may allow the representation to be compared with one or more other representations. The one or more other representations may have been processed according to the present invention. The method of comparison may particularly be that set out in applicant's PCT patent application no GB2005/003961 and/or UK patent application number 0502900.4 of 11 February 2004 and/or UK patent application number 0422784.9 filed 14 October 2004. The comparison may provide an indication of the likelihood of the representation and other representation coming from the same source.

The identifier may be a biometric identifier or other form of marking. The identifier may be a fingerprint, palm print, ear print, retina image or a part of any of these.

The representation of the identifier may be obtained direct or after processing of the type provided above. The selecting of a plurality of features may involve selecting a feature and then selecting one or more further features. The selection of the one or more further features may be made from features present in the representation. The selecting of the one or more further features may be made by selecting the features closest to the feature. Preferably one or more further features which are close to the first selected feature maybe selected. The one or more further features selected may be the features within a given distance of the feature. The distance may be increased until the number of further features reaches a desired number. The one or more further features maybe selected by connecting features in the representation together to form triangles, for instance using Delauney triangulation. Preferably this step is following by selecting a triangle to provide three of the features, for instance, a feature and two further features. This step may be followed by the selection of an adjoining triangle, for instance, at random. Preferably the further triangle includes a further feature. One of more further adjoining triangles may be selected. Preferably triangles are selected until the number of features in the series reaches a desired number.

The selecting of further features may continue until a desired number of features are in the plurality of features. The plurality of features may number three to twenty, more preferably three to sixteen and ideally three to twelve. Preferably all of the features of the plurality of features are features present in the representation.

The selecting of a plurality of features may start at a location in the representation. The location may be at an edge of the representation. The location may be at a corner of the representation. Other locations are possible, including a location which is equidistant from two or more corners and/or two or more edges of the representation.

One or more of the plurality of features may be a ridge end. One or more of the plurality of features may be a bifurcation. One or more of the plurality of features may be another form of minutia.

Preferably the positions of all of the plurality of features are considered. The position(s) of the plurality of features considered may be considered relative to a reference system. The position(s) may be considered relative to a first axis and a second axis, for instance an X axis and a Y axis. The position(s) may be consider in terms of distances, but may be considered in terms of coordinates.

Preferably the position of the reference feature is generated from the considered position(s) of the plurality of features. The reference feature may be a centre or centroid. The reference feature is preferably the centre or centroid of the plurality of features considered and/or all of the plurality of features. The reference feature may be generated by calculating a mean of the plurality of features considered, for instance the mean of their positions, ideally the mean of the coordinates.

Preferably the linking of one or more of the features to the reference feature and the linking of one or more of the features to one or more other features in the plurality of features is provided. Preferably one or more of the plurality of features are linked to at least two other features in the plurality of features. More preferably two or more of the plurality of features are linked to at least two other of the plurality of features. Ideally all of the plurality of features are linked to at least two of the other features in the plurality. Preferably, in respect of one or more of the features, the feature is linked to the reference feature and linked to two other features in the plurality of features. Preferably each of the features in the plurality of features is so linked. Preferably a feature is linked to the two features closest to it.

Preferably the linking is provided by straight lines.

Preferably the linking of the plurality of features to each other by lines forms a polygon, particularly with respect to the perimeter profile of the plurality of features. Preferably the linking of two or more of the features to the reference feature forms one or more triangles.. Preferably the linking of the centre feature to the plurality of other selected features and the linking of the other selected features to other selected features defines one or more triangles. The link is preferably in the form of a line. The line is preferably a straight line.

The data extracted from the representation of the identifier preferably includes information on two or more of, and preferably on all of: one or more of the plurality of features; the reference feature; one or more of the links between a feature and the reference feature; one or more of the links between a feature and another feature. The data extracted from the representation of the identifier may include information on the surface area defined by one or more of the polygons formed by the links. The polygon may be a polygon defined by the features and links there between. The polygon may be a polygon defined by involving two or more features and the reference feature and the links there between. The data extracted from the representation of the identifier may include information on the region of the identifier applying to one or more of the features. The data extracted from the representation of the identifier may include information on the general pattern of the representation.

The information on one or more of the plurality of features may include information on the type of feature. The type may be the minutia forming the feature, such as ridge end and/or bifurcation and/or other. Preferably such information is provided for each feature. The information on or more of the plurality of features may include information on the direction of the feature. The direction maybe defined relative to the representation and/or image thereof. Preferably such information is provided for each feature. The information on one or more of the plurality of features may include information on the position of the feature. Preferably such information is provided for each feature.

The information on the reference feature may include information on its position.

The information on one or more of the links between a feature and the reference feature may include information on the distance between the feature and reference feature. Preferably such information is provided for each link. The information on the one or more links between a feature and the reference feature may include information on the direction of the link. Preferably such information is provided for each link.

The information on one or more of the links between a feature and another feature may include information on the distance between the feature and another feature. Preferably such information is provided for each link.

Preferably the extracted data for a representation is subsequently be expressed as a vector.

Preferably the extracted data is compared with extracted data of an equivalent type from the other representation, so as to compare the first representation with the second representation. The results of the comparison may be presented as a likelihood ratio. The likelihood ratio may be the quotient of two probabilities, the numerator being the probability the two representations considering the hypothesis that the vectors originate from two representations of the same identifier, the denominator being the probability of the two representations considering the hypothesis that the vectors originate from representations of different identifiers.

Alternatively or additionally, the extracted data may be compared by using a method of comparison as set out in applicant's PCT patent application no GB2005/003961 and/or UK patent application number 0502900.4 of 11 February 2004 and/or UK patent application number 0422784.9 filed 14 October 2004. The comparison may provide an indication of the likelihood of the representation and other representation coming from the same source.

Various embodiments of the invention will now be described, byway of example only, and with reference to the accompanying figures in which: -

Figure 1 is a schematic overview of the stages, and within them steps, involved in the comparison of a print from an unknown source with a print from a known source;

Figure 2a is a schematic illustration of a part of a basic skeletonised print;

Figure 2b is a schematic illustration of the print of Figure 2a after cleaning and healing;

Figure 3 is a schematic illustration of the generation of representation data for the print of Figure 2b;

Figure 4 is a schematic illustration of a part of a print potentially requiring cleaning;

Figure 5 is a schematic illustration of the neighbourhood approach to cleaning according to the present invention;

Figure 6 is a schematic illustration of a part of a print potentially requiring healing;

Figure 7 is a schematic illustration of the neighbourhood approach to direction determination, particularly useful in healing; Figure 8 is a schematic illustration of the application of a triangle to part of a print as part of the data extraction;

Figure 9 is a schematic illustration of the application of a series of triangle to part of a print according to a further approach to the data extraction;

Figure 10 is a schematic illustration of the application of Delauney triangulation applied to the same part of a print as considered in Figure 9;

Figure 11 is a representation of a probability distribution for variation in prints from the same finger and a probability distribution for variation in prints between different fingers;

Figure 12 shows the distributions of Figure 9 in use to provide a likelihood ratio for a match between known and unknown prints;

Figure 13a illustrates minutia and direction information from a mark and a suspect;

Figure 13b illustrates the presentation of the direction information in a format for comparison;

Figure 13c illustrates the information of Figure 13b being compared; and

Figure 14 is a Bayesian network representation;

Background

A variety of situations call for the comparison of markers, including biometric markers. Such situations include a fingerprint, palm print or other such marking, whose source is known, being compared with a fingerprint, palm print or other such marking, whose source is unknown. Improvements in this process to increase speed and/or reliability of operation are desirable.

In the context of forensic science in particular, the consideration of the unknown source fingerprint may require the consideration of a partial print or print produced in less than ideal conditions. The pressure applied when making the mark, substrate and subsequent recovery process can all impact upon the amount and clarity of information available.

Process overview

The overall process of the comparison is represented schematically in Figure 1. After the recovery of the fingerprint and its representation, which may be achieved in one or more of the conventional manners, a representation of the fingerprint is captured. This may be achieved by the consideration of a photograph or other representation of a fingerprint which has been recovered.

In the next stage, the representation is enhanced. The representation is processed to represent it as a purely black and white representation. Thus any colour or shading is removed. This makes subsequent steps easier to operate. The preferred approach is to use Gabor filters for this purpose, but other possibilities exist.

Following on from this part of the stage, the enhanced representation is converted into a format more readily processed. This skeletonisation includes a number of steps. The basic skeletonisation is readily achieved, for instance using a function within the Matlab software (available from The MathWorks Inc). A section of the basic skeleton achieved in this way is illustrated in Figure 2a. The problem with this basic skeleton is that the ridges 20 often feature relatively short side ridges 22, "hairs", which complicate the pattern and are not a true representation of the fingerprint. Breaks 24 and other features may also be present which are not a true representation of the fingerprint. To counter these issues, the basic skeleton is subjected to a cleaning step and healing step as part of the skeletonisation. The operation of these steps are described in more detail below and gives a clean healed representation, Figure 2b.

Once the enhanced representation of the recovered fingerprint has been processed to give a clean and healed representation, the data from it to be compared with the other print can be considered. To do this involves first the extraction of representation data which accurately reflects the configuration of the fingerprint present, but which is suitable for use in the comparison process. The extraction of representation data stage is explained in more detail below, but basically involves the use of one of a number of possible techniques.

The first of the possible techniques, see Figure 3, involves defining the position of features 30 (such as ridge ends 32 or bifurcation points 34), forming an array of triangles 36 with the features 30 defining the apex of those triangles 36 and using this and other representation data in the comparison stage. In a second technique, developed by the applicant, the positions of features are defined and the positions of a group of these are considered to define a centre. The centre defines one apex of the triangles, with adjoining features defining the other apexes.

To facilitate the comparison stage, the representation data extracted is formatted before it is used in the comparison stage. This basically involves presenting the information characteristic of the triangles, quadrilaterals or other polygons being considered when the data is extracted in a format mathematically coded for use in the comparison stage. Further details of the format are described below.

Now that the fingerprint has been expressed as representation data, it can be compared with the other fingerprint(s). The comparison stage is based on different representation data being compared to that previously suggested. Additionally, in making the comparison, the technique goes further than indicating that the known and unknown source prints came from the same source or that they did not. Instead, an expression of the likelihood that they came from the same source is generated. In the preferred forms, one or both of the two different models (a data driven approach and a model driven approach) both described in more detail below are used.

Having provided an overview of the entire process, the stages and steps in them will now be discussed in more detail.

Cleaning and healing steps of the skeletonisation stage

Some existing attempts at interpreting the basic skeleton to give an improved version have been made.

In the situation illustrated in Figure 4, the basic skeleton suggests that a ridge island 40 is present, as well as a short ridge 41 which as a result gives a bifurcation point 43 and ridge end 44.

The existing interpretation considers the length of the ridge island 40. If the length is equal to or greater than a predetermined length value then it is deemed a true ridge island and is left. If the length is less than the predetermined length then the ridge island is discarded. In a similar manner, the length from the bifurcation point 43 to the ridge end 44 is considered. Again if it is equal to or greater than the predetermined length it is kept as a ridge with its attendant features. If it is shorter than the predetermined length it is discarded. This approach is slow in terms of its processing as the length in all cases is measured by starting at the feature and then advancing pixel by pixel until the end is reached. The speed is a major issue as there are a lot of such features need to be considered within a print.

The new approach now described has amongst its aims to provide a reliable, faster means for handling such a situation. Instead of advancing pixel by pixel, the new approach illustrated in Figure 5 considers the print in a series of sections or neighbourhoods. Thus a neighbourhood definition, box 50, is applied to part of the print. Features within that neighbourhood 50 are then quickly established by considering any pixel which is only connected to one other. This points to features 51 and 52 which represent ridge ends within the neighbourhood 50. The start point for the data set forming a feature is then determined relative to the neighbourhood 50. hi the case of feature 51 this is the bifurcation feature 53. In the case of feature 52 this is the neighbourhood boundary crossing 54. Thus feature 51 is part of data set A extending between feature 53 and feature 51. Feature 52 is a part of separate data set, data set B, extending between crossing 54 and feature 52. All data sets formed by a feature at both ends, with both features being within the neighbourhood 50 are discarded as being too short to be true features. AU data sets formed by a feature at one end and a crossing at the other are kept as far as the cleaning of that neighbourhood is concerned. Thus feature 51 and its attendant data set are discarded (including the bifurcation feature 53) and feature 52 is kept by this cleaning for this neighbourhood 50.

When further neighbourhoods are considered, it may of course be that the feature 52 is itself part of a data set with the features both within that neighbourhood, where upon it too will be discarded. If, however, it is the end of a ridge of significant length then for all neighbourhoods considered its data set will start with the feature and end with a crossing and so be kept.

This approach can be used to address all ridge ends and attendant bifurcation features within the print to be cleaned.

As well as addressing "extra" data by cleaning, the present invention also addresses the type of situation illustrated in Figure 6 where the basic skeleton shows a first ridge end 60 and a second 61, generally opposing one another, but with a gap 62 between them. Is this a single ridge which needs healing by adding data to join the two ends together? Or is this truly two ridge ends? Not only is it desirable to address this type of situation, but it also must be done in a way which does not detract from the accuracy of the subsequent process, and in particular the generation of the representative data which follows. This is particularly important in the case where the "direction" is a part of the representative data generated, as proposed for the embodiment of the invention detailed below.

To ensure that the "direction" information is not impaired it must be accurately determined and maintained. The pixel by pixel approach of the type used above for cleaning, suggests taking a feature and then moved pixel by pixel away from it for a given length. A projected line between the feature and the pixel the right length away then gives the angle. Again the pixel by pixel approach is labourious and time consuming.

The approach of the present invention is illustrated in Figure 7 and is again based on the neighbourhood approach. A neighbourhood 70 is defined relative to a part of the print. In this case, the part of the print includes a ridge end 71 and bifurcation 72. Also present are points where the ridges cross the boundaries of the neighbourhood, crossings 73, 74, 75, 76. Again the crossings and features define a series of data sets. In this case, ridge end 71 and crossing 73 define data set W; bifurcation 72 and crossing 74 define data set X; bifurcation 72 and crossing 75 define data set Y; and bifurcation 72 and crossing 76 define data set Z.

The direction of data set W is defined by a line drawn between ridge end 71 and crossing 73. A similar determination can be made for the direction of the other data sets.

Once the directions for data sets have been obtained, the type of situation shown in Figure 6 is addressed by considering the direction of the ridge ending in first ridge end 60 and the direction of the ridge ending in second ridge end 61. If the two directions are the same, within the bounds of a limited range, and the separation is small (for instance, the gap falls with the neighbourhood) then the gap is healed and the two ridge ends 60, 61 disappear as features as far as further consideration is required. If the separation is too large and/or if the directions do not match, then no healing occurs and the ridge ends 60, 61 are accepted as genuine.

The approach taken in the present invention allows faster processing of the cleaning and healing stage, in a manner which is accurate and is not to the detriment of subsequent stages and steps. Extraction of representation data

Preferably after the above mentioned processing, the necessary data from it to be compared with the other print can be extracted in a way which accurately reflects the configuration of the fingerprint present, but which is suitable for use in the comparison process.

It is possible to fix coordinate axes to the representation and define the features/directions taken relative to that. However, this leads to problems when considering the impact of rotation and a high degree of interrelationship being present between data.

Instead of this approach, with reference to Figure 8, one approach of the present invention will now be explained. Within the illustration, a first bifurcation feature 80, second 81 and ridge end 83 are present. These form nodes which are then joined to one another so that a triangle is formed. Extrapolation of this process to a larger number of minutia features gives a large number of triangles. A print can typically be represented by 50 to 70 such triangles. The Delauney triangulation approach is preferred.

Whilst this one approach is suitable for use in the new mathematical coding of the information extracted set out below, the use of Delauney triangulation does not extract the data in the most robust way.

In the alternative approach, developed by the applicant, an entirely new approach is taken. Referring to Figure 9 a series of features 120a through 1201 are identified within a representation 122. A number of approaches can be used to identify the features to include in a series. Firstly, it is possible to identify all features in the representation and join features together to form triangles (for instance, using Delauney triangulation). Having done so, one of the triangles is selected and this provides the first three features of the series. One of the adjoining triangles to the first triangle is then selected at random and this provides a further feature for the series. Another triangle adjoining the pair is then selected randomly and so on until the desired number of features are in the series, hi a second approach, a feature is selected (for instance, at random) and all features within a given radius of the first feature are included in the series. The radius is gradually increased until the series includes the desired number of features.

Having established the series of features, the position of each of these features is considered and used to define a centre 124. Preferably, and as illustrated in this embodiment this is done by considering the X and Y position of each of the features and obtaining a mean for each. The mean X position and mean Y position define the centre 124 for that group of features

120a through 1201. Other approaches to the determination of the centre are perfectly useable. Instead of defining triangles with features at each apex, the new approach uses the centre 124 as one of the apexes for each of the triangles. The other two apexes for first triangle 126 are formed by features 120a and 120b. The next triangle 128 is formed by centre 124, feature 120b and 120c. Other triangles are formed in a similar way, preferably moving around the centre 124 in sequence. The set of triangles formed in this approach is unique, simple and easy to describe data set. The approach is more robust than the Delauney triangulation described previously, particularly in relation to distortion. Furthermore, the improvement is achieved without massively increasing the amount of data that needs to be stored and/or the computing power needed to process it. For comparison purposes, Figure 10 illustrates the Delauney triangulation approach applied to the same set of features.

Either the first, Delauney triangulation, based approach or the second, radial triangulation, approach extract data which is suitable for formatting according to the preferred approach of the present process.

Format of representative data

Having considered the print in one of the above mentioned ways to extract the representative data, the data must be suitably mathematically coded to allow the comparison process and here a different approach is taken to that considered before. The approach presents the extracted data in vector form, and so allows easy comparison between expressions of different representations.

Particularly with reference to the first approach, for a given triangle, a number of pieces of information are taken and used to form a feature vector. The information is: the type of the minutia feature each node represents (three pieces of information in total); the relative direction of the minutia features (three pieces of information in total); and the distances between the nodes (three pieces of information in total). Thus the feature vector is formed of nine pieces of information. The type of minutia can be either ridge end or bifurcation. The direction, a number between 0 and 2π radians, is calculated relative to the orientation, a number between 0 and π radians, of the opposing segment of the triangle as reference and so the parameters of the triangle are independent from the image.

In particular the feature vector may be expressed as:

FV=[GP, Reg, (T₁, A₁, D_U2, T₂, A₂, D_2ι3, T₃, A₃, D₃JJ where

GP is the general pattern of the fingerprint;

Reg is the region of the fingerprint the triangle is in;

T₁ is the type of minutia 1;

A₁ is the direction of the minutia at location 1 relative to the direction of the opposing side of the triangle;

D₁₂ is the length of the triangle side between minutia 1 and minutia 2;

T₂ is the type of minutia 2;

A₂ is the direction of the minutia at location 2 relative to the direction of the opposing side of the triangle;

D₂₃ is the length of the triangle side between minutia 2 and minutia 3;

T₃ is the type of minutia 3;

A₃ is the direction of the minutia at location 3 relative to the direction of the opposing side of the triangle;

Z)_{i ;} is the length of the triangle side between minutia 3 and minutia 1.

To avoid the same feature vector representing two symmetrical triangles, the features are recorded for all the triangles in the same order (either clockwise or anticlockwise). A rule of starting with the furthest feature to the left is used, but other such rules could be applied.

As each triangle considered is independent of the others and is also independent of the print image this addresses the problem of rotational issues in the comparison.

Advantageously the second data extraction approach described above is also suited to be mathematically coded using the vector format and so allow comparison with data extracted from other representations. The pieces of information used to foπn the feature vector in this case are: the general pattern of the fingerprint; the type of minutia; the direction of the minutia relative to the image; the radius of the minutia from the centre or centroid; the length of the polygon side between a minutia and the minutia next to it; the surface area of the triangle defined by the mmutia, the minutia next to it and the centroid

In particular the vector may be expressed as:

FV=[GP, (T₁, A₁, R₁, L₁₂, SJ, ,{T_k, A_k, R_k L_kk+1, SJ, ,{T_N, A_N, R_N, L_NJ, SJJ where

GP is the general pattern of the fingerprint;

T_k is the type of mmutia /;

A_k is the direction of mmutia k relative to the image;

L_kk+1 is the length of the polygon side between mmutia k and mmutia k+1;

S₁ is the surface area of the triangle defined by mmutia k, k+1 and the centroid; and

R_k is the radius between the centroid and the mmutia k.

When compared with the expression of the vector set out above in the context of the approach taken for the first data extraction approach, it should be noted that region of the fingerprint is no longer considered. The set of features can extend across region boundaries and so it is potentially not appropriate to consider one region m the vector. The region could still be considered, however, and the expression set out below is a suitable one in that context, with the region designated Reg and the other symbols having the meanings outlined above. Note a separate region is possible for each mmutia.

FV=[GP, (T₁, A₁, R₁, Re_gl, L₁₂, S₁), . ,{T_h A_k, R_k, Re_gk> L_kk+1, SJ (T_N, A_N, R_N, Reg_N, L₁₁₁, SJ]

Using the types of format described above, it is possible to present the data extracted from the representations in a format particularly useful to the comparison stage.

Comparison Approaches

A number of different approaches to the comparison between a feature vector of the above mentioned type which represent the print from an unknown source with the a feature vector which represent the print from the known source are possible. A match/not match result may simply be stated. However, substantial benefits exist in making the comparison in such a way that a measure of the strength of a match can be stated.

Likelihood ratio approach

One general type of approach that can be taken, which allows the comparison to be expressed in terms of a measure of the strength of the match is through the use of a likelihood ratio.

The likelihood ratio is the quotient of two probabilities, one being that of two feature vectors conditioned on their being from the same source, the other two feature vectors being conditioned on their being from different sources. Feature vectors obtained according to the first data extraction approach and/or second extraction approach described above can be compared in this way, the differences being in the data represented in the feature vectors rather than in the comparison stage itself. hi each case, therefore, the approach can be derived from the expression:

Pr(Jv, Jv _m \ Hp)

LR= Pr(fv, Jv _m \ Hd)

Where the feature vector fv contains the information extracted from the representation and formatted. The addition of the subscript s to this abbreviation denotes that a feature vector comes from the suspect, and the addition of the subscript m denotes that a feature vector originates from the crime. The symbol fv_s then denotes a feature vector from the known source or suspect, and fv_m denoted the feature vector originated from an unknown source from the crime scene. For modelling purposes it is useful to classify a feature vector into discrete quantities (which may include general pattern, region, type, and other data) and continuous quantities (which may include the distances between minutiae, relative directions and other data).

The preferred forms for the quotient in the context of the first approach and second approach are discussed in more detail below in the context of their use in the data driven approach to the comparison stage. Within the general concept of a likelihood ratio approach, a number of ways of implementing such an approach exist. One such approach which allows the comparison to be expressed in terms of a measure of the strength of the match is through the use of a data driven approach.

Data driven approach

In general terms, the data driven approach involves the consideration of a quotient defined by a numerator which considers the variation in the data which is extracted from different representations of the same fingerprint and by a denominator which considers the variation in the data which is extracted from representations of different fingerprints. The output of the quotient is a likelihood ratio.

In order to quantify the likelihood ratio, the feature vector for the first representation, the crime scene, and the feature vector for the second representation, the suspect are obtained, as described above. The difference between the two vectors is effectively the distance between the two vectors. Once the distance has been obtained it is compared with two different probability distributions obtained from two different databases.

In the first instance, the probability distribution for these distances is estimated from a database of prints taken from the same finger. A large number of pairings of prints are taken from the database and the distance between them is obtained. This involves a similar approach to that described above. Each of the prints has data extracted from it and that data is formatted as a feature vector. The differences between the two feature vectors give the distance between that pairing. Repeating this process for a large number of pairings gives a range of distances with different frequencies of occurrence. A probability distribution reflecting the variation between prints of the same figure is thus obtained.

Ideally, the database would be obtained from a number of prints taken from the same finger of the suspect. However, the approach can still be applied where the prints are taken from the same finger, but that finger is someone's other than the suspect. This database needs to reflect how a print (more particularly the resulting triangles and their respective feature vectors) from the same finger changes with pressure and substrate. This database is formed from a significant number of sets of information, each set being a large number of prints taken from the same finger under the full range of conditions encountered in practice. The database is populated by the identification, by an operator, of corresponding triangles in several applications of the same finger. Alternatively, a smaller set of prints can be processed as described above, distortion functions can then be calculated. The prefer method is thin plate splines, but other methods exist. The distortion function can then be applied to other prints to simulate further sets of data.

In the second instance, the probability distribution for these distances is estimated from a database of prints taken from different fingers. Again a large number of pairings of prints are taken from the database and the distance between them obtained. The extraction of data, formatting as a feature vector, calculation of the distance using the two feature vectors and determination of the distribution is performed in the same way, but uses the different database.

This different database needs to reflect how a print (more particularly the resulting triangles and their respective feature vectors) from a number of different fingers varies between fingers and, potentially, with various pressures and substrates involved. Again , the database is populated by the identification, by an operator, of triangles in the various representations obtained from the different fingers of different persons.

Having established the manner in which the databases and probability distributions are obtained, the comparison of a crime scene print against a suspect print is considered further.

The numerator may thus be thought of as considering a first representation obtained from a crime scene or an item linked to a crime, against a second representation from a suspect through an approach involving: taking and/or generating a number of example representations of the second representation; considering the example representations as a number of triangles; considering the value of the feature vector for a given triangle in respect of each of the example representations; obtaining the feature vector value of the first representation; forming a probability distribution of the frequency of the cross-differences of different feature vector values for a given triangle between example representations; comparing the difference of the feature vector value of the first representation and the feature vector value of the second representation with the probability distribution. The denominator may thus be thought of as considering the second representation obtained from a suspect against a series of representations taken from a population through an approach involving: talcing or generating a number of example representations of representations taken from a population; considering the example representations as a number of triangles; considering the values of the feature vectors in respect of each of the example representations; forming a probability distribution of the frequency of differences between the feature vector of the first representation and the different feature vector values from the example representations; obtaining the feature vector value of the second representation; comparing the difference between the feature vector value of the first representation and the feature vector value of the second representation with the probability distribution.

Applying the data driven approach, and in the context of the first data extraction approach (Delauney triangulation), and after some algebraic operations, a probability for the numerator of the likelihood ratio is computed using the following formula: -

Num = ∑{Pr(d(β'_s,cA_n,c)\^_s,dβ^; _m,φH_py. for all>_s>rf andft,,_hcl such thatβ_sjφ_{m d}}

where fv means feature vector, c means continuous, d means discrete, m means mark and s means suspect and therefore: fv_{m c} : continuous data of the feature vector from the mark fv_{m d} : discrete data of the feature vector from the mark jv_{s c} : discrete data of the feature vector from the suspect fv_{s d} : discrete data of the feature vector from the suspect d{fv_sc,Jv_m^) is the distance measured between the continuous data of the two feature vectors from the mark and the suspect

H_p is the prosecution hypothesis, that is the two feature vectors originate from the same source.

Notice that, conditioning on H_p, suggests Jv _sc and/v_{m c} become measurements extracted from the same finger of the same person. The subscript in the summation symbol means that the probabilities in the right-hand-side of equation are added up for all the cases where the values of the discrete quantities of the features vectors coincide, hi some occasions some or all of the discrete variables are present in the fingermark. For these cases the index of the summation is replaced by values of the quantities that are not present. The summation symbol is removed when all discrete quantities are present in the fingermark.

The expression d(Jv_sc ,β',_lhC) denotes a distance between the continuous quantities of the feature vectors for the prints. The continuous quantities in a feature vector are the length of the triatigle sides and minutia direction relative to the opposite side of the triangle. There are a number of distance measures that can be used but the distance measure describe below is preferred. This distance measure is computed by first subtracting term by term. The result is a vector containing nine quantities. This is then normalised to ensure that the length and angle are given equal weighting. By taking the sum of the squares of the distances from all the feature vectors considered in this way a single value is obtained. hi such a case, and after some algebraic operations, a probability for the denominator of the likelihood ratio is computed using the following formula,

Den = ∑{Pr(^,_cΛ,,c)R,>_m,Λ)Pr(/v_m,_rfrø: for all Jv_Sιd and/i^ such that β_s,_dφ,,J

where

Jv means feature vector, c means continuous, d means discrete, m means mark and s means suspect, and therefore:

Jv_{m c} : continuous data of the feature vector from the mark

Jv_{m d} : discrete data of the feature vector from the mark Jv_{s c} : discrete data of the feature vector from the suspect fv_{s d} : discrete data of the feature vector from the suspect d(fv_sc,β>_{m c}) is the distance measured between the continuous data of the two feature vectors from the mark and the suspect

H_d is the defence hypothesis, that is the two feature vectors originate from different sources.

Several distance measures exist but the one described above is preferred. The subscript in the summation symbol means that the probabilities in the right-hand-side of this equation are added up for all the cases where the values of the discrete quantities of the features vectors coincide. In some occasions some or all of the discrete variables are present in the fingermark. For these cases the index of the summation is replaced by values of the quantities that are not present. The summation symbol is removed when all discrete quantities are present in the fingermark.

Conditioning on H_d, that is "the prints originated from different sources", the features vectors come from different fingers of different people. The probability distribution for distances d(fv_{s c} , fv_{m c} ) can be estimated from a reference database of fingerprints. This database needs to reflect how much variability there is in respect of all prints (again more particularly the resulting triangles and their feature vectors) between different sources. This database can readily be formed by taking existing records of different source fingerprints and analysing them in the above mentioned way.

The second factor Pr(/v_{m d}|H_rf) is a probability distribution of discrete variables including general pattern. A probability distribution for general pattern was computed based on frequencies compiled by the FBI for the National Crime Information Centre in 1993. These data can be found on http://liome.att.net/~dennatoglyphics/mfre/. A probability distribution for the remaining discrete variables can be estimated from a reference database using a number of methods. A probability tree is preferred because it can more efficiently code the asymmetry of this distribution, for example, the number of regions depends on the general pattern. Again applying the data driven approach, and in the context of the second data extraction approach (radial triangulation), a probability for the numerator of the likelihood ratio is computed using the following formula:

Num = Pr(d(f\>_sβJ \H_p) where d(fi'_sfo,,_ι) i^{s me} distance measured between discrete and continuous data of the two feature vectors from the mark and suspect;

H is the prosecution hypothesis, that is the two vectors originate from the same source.

The probability for the numerator is computed using the following formula:

Den = Pr(d(fv_sfvJ \H_d) where

H_d is the defence hypothesis, that is the two vectors originate from different sources.

In each case, similar approaches to those detailed above can be used to generate the relevant probability distributions.

In the second approach, it is possible to measure the distance between feature vectors in the above described manner of the first data extraction approach in respect of each orientation of the polygon in the mark and suspect representations. However, the large number of minutia which may now be being considered in a feature vector (for instance 12) would mean that there are very many rotations (for instance 12 rotations) of the feature vector which must be considered, compared with the more practical three of the first approach. The use of a greater number of minutia is desirable as this increases the discriminating power of the process. Investigations to date suggest that by the time 12 minutia are being considered, there is little or no overlap between the within finger distribution and between finger distributions illustrated in Figure 11. In a modification, therefore, a feature vector is first considered against another feature vector in terms of only part of the information it contains. In particular, the information apart from the minutia direction can be compared. In the comparison, the data set included in one of the vectors is fixed in orientation and the data set included in the other vector with which it is being compared is rotated. If the data set relates to three minutia then three rotations would be considered, if it related to twelve then twelve rotations would be used. The extent of the fit at each position is considered and the best fit rotation obtained. This leads to the association of minutiae pairs across both feature vectors.

In respect of the best fit rotation, in each case, the process then goes on to compare the remaining data in each set, the minutia direction. To achieve this, the minutiae directions are made independent of the orientation of the print on the image. The approach taken on direction is described with reference to Figurel3a through 13c. hi Figure 13a, a mark set of minutia 200 and a suspect set of minutia 202 are being considered against one another. Each set is formed of four minutia, 204a, 204b, 204c, 204d and 206a, 206b, 206c, 206d respectively. The allocation of the minutia reference numerals reflects the suggested best match between the two sets arising from the consideration of the minutia type, length of the polygon sides between minutia, surface of the polygon defined by the minutia and centroid. Each of the minutia has an associated direction 208a, 208b, 208c, 208d and 210a, 210b, 210c, 21Od respectively. For the mark set 200 and the suspect set 202, a circle 212, 214 of radius one is taken. To the mark circle 212 is added a radius 216 for each of the minutia directions, see Figure 13b. To the suspect circle 214 is added a radius 218 from each of the minutia directions, Figure 13b. Rotation of one of the circles relative to the other allows the orientation of the minutia to be brought into agreement, according to the set of the pairs of minutiae that were determined before, Figure 13 c, and allows the extent of the match in terms of the minutiae directions for each pair of minutiae to be considered, hi the illustrated case there is extensive agreement between the two circles and hence between the two marks in respect of the data being considered. hi effect, the match between the polygons is being considered in terms of the minutia type, distance between minutia, radius between the minutia and the centroid, surface area of the triangle defined between the minutia and the centroid and minutia direction. All of these considerations serve to compliment one another in the comparison process. One or more maybe omitted, however, and a practical comparison be carried out.

The comparison provides a distance which can be considered against the two distributions in the manner previously described with reference to Figures 11 and 12 below. Various means can be used for computing the distance, including algorithms (such as Euclidean, Pearson, Manhattan etc) or using neural networks.

Assessing a comparison using the data driven approaches

Having extracted the data, formatted it in feature vector form and compared two feature vectors to obtain the distance between them, that distance is compared with the two probability distributions obtained from the two databases to give the assessment of match between the first and second representation.

In Figure 11, the distribution for prints from the same finger is shown, S, and shows good correspondence between examples apart from in cases of extreme distortion or lack of clarity. Almost the entire distribution is close to the vertical axis. Also shown is the distribution for prints from the fingers of different individuals, D. This shows a significant spread from a low number of extremely different cases, to an average of very different and with a number of little different cases. The distribution is spread widely across the horizontal axis.

In Figure 12, these distributions are considered against a distance I obtained from the comparison of an unknown source (for instance, crime scene) and known source (for instance, suspect) fingerprint in the manner described above. At this distance, I, the values (Q and R respectively) of the distributions S and D can be taken, dotted lines. The likelihood ratio of a match between the two prints is then Q/R. In the illustrated case, distance I is small and so there is a strong probability of a match. If distance I were great then the value of Q would fall dramatically and the likelihood ratio would fall dramatically as a result. The later approach to the distance measure issue is advantageous as it achieves the result in a single iteration, provides a continuous output and does not require the determination of thresholds.

The databases used to define the two probability distributions preferably reflect the number of minutia being considered in the process. Thus different databases are used where three minutia are being considered, than where twelve minutia are being considered. The manner in which the databases are generated and applied are generally speaking the same, variations in the way the distances are calculated are possible without changing the operation of the database set up and use. Equally, it is possible to form the various databases from a common set of data, but with that data being considered using a different number of minutia to form the database specific to that number of minutia.

The databases maybe generated in advance in respect of the numbers of minutia expected to be considered in practice, for instance 3 to 12, with the relevant databases being used for the number of minutia being considered in a particular case, for instance 6. Pre-generation of the databases avoids any delays whilst the databases are generated. However, it is also possible to have to hand the basic data which can be used to generate the databases and generate the database required in a specific case in response to the number of minutia which need to be considered. Thus, a mark may be best considered using six minutia and the desire to consider this mark would lead to the database being generated for six minutia from the basic database of fingerprint representations by considering that using six minutia. The data set size which needs to be stored would be reduced as a result.

In certain circumstances it is also possible to generate the probability distributions in advance. This can occur, for instance, where the within finger variation is being considered and that is considered on the basis of a single (or several) finger(s) not from the suspect, hi the case of the model based approach, discussed below, it is possible to generate and store both probability distributions in advance.

Significant benefit from this overall approach arise due to: incorporating distortion and clarity in the numerator of the likelihood ratio; introducing the distance measure between the quantities in the feature vector; the use of probability distribution distances between features vectors from the same source and its estimation from a dedicated sets of data of replicates of the same finger; the use of probability distribution for the distances between print of different sources and its estimation from a reference database containing prints from different sources.

The description presented here exemplifies the use of this methodology, but the methodology is readily adapted for use in other forms. For instance, the Delauney triangulation foπn could be extended to cover more than three minutiae. Model based approach

Within the general concept of a likelihood ratio approach, another approach which allows the comparison to be expressed in terms of a measure of the strength of the match is through the use of a model based approach. hi such an approach, and after some algebraic operations a probability for the numerator of the likelihood ratio is computed using the following formula,

Num = ∑{Pr(fv_mJfv_{s c},β>_{s di}fv_{m d},H_p): for all/v anάβ_m4 such thatft_{s d}=fv_{m d}}

where jy means feature vector, c means continuous, d means discrete, m means mark and s means suspect, and therefore:

Jv_{m c} : continuous data of the feature vector from the mark fv_{m i} : discrete data of the feature vector from the mark jv_sc : discrete data of the feature vector from the suspect fv_sά : discrete data of the feature vector from the suspect d(fv_{s c}, fv_{m c}) is the distance measured between the continuous data of the two feature vectors from the mark and the suspect

H_p is the prosecution hypothesis, that is the two feature vectors originate from the same source;

As noted before, the continuous quantities, when conditioning on/v^ _c andyv_{m c} become measurement of the same finger and person. The subscript in the summation symbol means that the probabilities in the right-hand-side of the equation are added up for all the cases where the values of the discrete quantities of the features vectors coincide. In some occasions some or all of the discrete variables are present in the fingermark. For these cases the index of the summation is replaced by values of the quantities that are not present. The summation symbol is removed when all discrete quantities are present in the fingermark. The probability distribution for/v, _c is computed using a Bayesian network estimated from a database of prints taken from the same finger as described above. Many algorithms exists for estimating the graph and conditional probabilities in a Bayesian networks, but the preferred algorithms are the NPC algorithm for estimating acyclic directed graph, see Steck H., Hofmann, R., and Tresp, V. (1999). Concept for the PRONEL Learning Algorithm, Siemens AG, Munich and/or the EM-algorithm, S. L. Lauritzen (1995). The EM algorithm for graphical association models with missing data. Computational Statistics & Data Analysis, 19:191-201. for estimating the conditional probability distributions. The contents of both documents, particularly in relation to the algorithms they describe are incorporated herein by reference.

Further explanation of the use of Bayesian networks follows below.

The manner in which the first representation is considered against the second representation, through the use of a probability distribution, is as described above, save for the probability distribution being computed using the Bayesian network approach rather than a series of example representations of the second representation.

Using this approach and after some algebraic operations a probability for the denominator of the likelihood ratio is computed using the following formula,

where fv means feature vector, c means continuous, d means discrete, m means mark and s means suspect, and therefore: jy_{m c} : continuous data of the feature vector from the mark fv_m4 : discrete data of the feature vector from the mark fv_s<c : discrete data of the feature vector from the suspect β>_sd : discrete data of the feature vector from the suspect d(fv_SιC,fo_m>c) is the distance measured between the continuous data of the two feature vectors from the mark and the suspect

H_ά is the defence hypothesis, that is the two feature vectors originate from different sources. The subscript in the summation symbol means that the probabilities in the right-hand-side of equation are added up for all the cases where the values of the discrete quantities of the features vectors coincide. In some occasions some or all of the discrete variables are present in the fingermark. For these cases the index of the summation is replaced by values of the quantities that are not present. The summation symbol is removed when all discrete quantities are present in the fingermark.

The probability distribution in the first factor of the right hand side of equation above is computed with a Bayesian network estimated from a database of feature vectors extracted from different sources. There are many methods for estimating Bayesian networks as noted above, but the preferred methods are the NPC-algorithm of Steck et al., 1999 for estimating an acyclic directed graph and/or the EM-algorithm of Lauritzen, 1995 for the conditional probability distributions. There is a Bayesian network for each combination of values of the discrete variables. The second factor ?r(β>_{m d}\H_d) is estimated in the same manner as described for the data-driven approach above.

Again the approach to considering the second representation against the population representations is as detailed above, save for the probability distribution being computed using the Bayesian network approach.

Assessing a comparison using the model based approach

Given a feature vector from know source jv_s and from an unknown source /v_ΛI, the numerator is given by the equation and is calculated with a Bayesian network dedicated for modelling distortion. The second factor in the denominator is calculated in the same manner as with the data-driven approach. The first factor is computed using Bayesian networks. A Bayesian network is selected for the combination of values off_{m d} which is then use for computing a probability Vt{fv_{m c}\fv_{m d}J^^). This process is repeated for all values in the index of the summation. The likelihood ratio is then obtained by computing the quotient of the numerator over the denominator.

Significant benefit from this approach arise due to: using Bayesian networks for computing the numerators and denominator of the likelihood ratio; estimating Bayesian networks for the numerator from dedicated databases containing replicates of the same finger and under several distortion conditions; estimating Bayesian networks for the denominator from dedicated databases containing prints from different fingers and people.

The description above is an example of using Bayesian networks for calculating the likelihood ratio, but the invention is not limited to it. Another example is estimating one Bayesian network per general pattern. This invention can also be used for more than three minutiae by defining suitable feature vectors.

As mentioned above, in order to estimate the numerator and denominator in the above likelihood ratio consideration, it is possible to use a Bayesian network representation to specify a probability distribution. For brevity of explaination the concept of a Bayesian network is presented through an example. A Bayesian network is an acyclic directed graph together with conditional probabilities associated to the nodes of the graph. Each node in the graph represents a quantity and the arrows represent dependencies between the quantities. Figure 14 displays an acyclic graph of a Bayesian network representation for the quantities X, Y and Z. This graph contains the information that the joint distribution of X, Y an d Z is given by the equation

p(x,y,z) = p00pO|x)pCΦ) for all x,y^

and so the joint distribution is completely specified within the graph and the conditional probability distributions {p(x): for all x}, {ρ(y/x) for all x and y} and {ρ(z/y) for all z and y}. A detailed presentation on Bayesian networks can be found in a number of books, such as Cowell, R.G., Dawid A.P., Lauritzen S.L. and Spiegelhalter DJ. (1999) "Probabilistic networks and expert systems".

Claims

1. A method of extracting data from a representation of an identifier, the method including: selecting a plurality of features in the representation of an identifier; considering the positions of the plurality of features; generating a reference feature from the positions of the plurality of features; linking one or more of the features to the reference feature and/or linking one or more of the features to one or more of the other features in the plurality of features; the extracted data including information on one or more of: one or more of the plurality of features; the reference feature; one or more of the links between a feature and the reference feature; one or more of the links between a feature and another feature.

2. A method according to claim 1 in which the position of the reference feature is generated from the positions of the plurality of features.

3. A method according to claim 1 or claim 2 in which the reference feature is a centre or centroid.

4. A method according to any preceding claim in which the reference feature is the centre or centroid of the plurality of features considered.

5. A method according to any preceding claim in which the reference feature is generated by calculating a mean of the positions of the plurality of features considered.

6. A method according to any preceding claim in which the linking of one or more of the features to the reference feature and the linking of one or more of the features to one or more other features in the plurality of features is provided.

7. A method according to any preceding claim in which all of the plurality of features are linked to at least two of the other features in the plurality.

8. A method according to any preceding claim in which one or more of the features is linked to the reference feature and linked to two other features in the plurality of features.

9. A method according to any preceding claim in which the linking of the plurality of features to each other by lines forms a polygon with respect to the perimeter profile of the plurality of features.

10. A method according to any preceding claim in which the selecting of a plurality of features involves selecting a feature and then selecting one or more further features, the one or more further features selected being the features within a given distance of the feature.

11. A method according to claim 10 in which the distance is increased until the number of further features reaches a desired number.

12. A method according to any preceding claim in which the one or more further features are selected by connecting features in the representation together to form triangles, followed by selecting a triangle to provide three of the features, followed by the selection of an adjoining triangle at random.

13. A method according to claim 12 in which triangles are selected until the number of features in the series reaches a desired number.

14. A method according to any preceding claim in which the data extracted from the representation of the identifier includes information on two or more of: one or more of the plurality of features; the reference feature; one or more of the links between a feature and the reference feature; one or more of the links between a feature and another feature.

15. A method according to any preceding claim in which the data extracted from the representation of the identifier includes information on the surface area defined by one or more of the polygons formed by the links.

16. A method according to any preceding claim in which the data extracted from the representation of the identifier includes information on the region of the identifier applying to one or more of the features.

17. A method according to any preceding claim in which the data extracted from the representation of the identifier includes information on the general pattern of the representation.

18. A method according to any preceding claim in which the information on one or more of the plurality of features includes information on the type of feature.

19. A method according to any preceding claim in which the information on or more of the plurality of features includes information on the direction of the feature.

20. A method according to any preceding claim in which the information on one or more of the plurality of features includes information on the position of the feature.

21. A method according to claim 1 in which the information on the reference feature includes information on its position.

22. A method according to any preceding claim in which the information on one or more of the links between a feature and the reference feature includes information on the distance between the feature and reference feature.

23. A method according to any preceding claim in which the information on the one or more links between a feature and the reference feature includes information on the direction of the link.

24. A method according to any preceding claim in which the information on one or more of the links between a feature and another feature includes infoπnation on the distance between the feature and another feature.

25. A method according to any preceding claim in which the extracted data for a representation is subsequently expressed as a vector.

26. A method according to any preceding claim in which the extracted data is compared with extracted data of an equivalent type from the other representation, so as to compare the first representation with the second representation.

27. A method according to claim 26 in which the results of the comparison are presented as a likelihood ratio.

28. A method according to claim 27 in which the likelihood ratio is the quotient of two probabilities, the numerator being the probability the two representations considering the hypothesis that the vectors originate from two representations of the same identifier, the denominator being the probability of the two representations considering the hypothesis that the vectors originate from representations of different identifiers.

29. A method of comparing a first representation of an identifier with a second representation of an identifier, the method including: selecting a plurality of features in at least one of the first representation of an identifier and the second representation of an identifier; considering the position of one or more of the plurality of features; generating a reference feature from the considered positions of the plurality of features; linking one or more of the features to the reference feature and/or linking one or more of the features to one or more other features in the plurality of features; extracting data from the representation of the identifier, the extracted data including information on one or more of: one or more of the plurality of features; the reference feature; one or more of the links between a feature and the reference feature; one or more of the links between a feature and another feature; using the extracted data to compare the first representation with the second representation.