Fingerprint enrolment algorithm

1. A method of enrolling a fingerprint comprising:

receiving a first fingerprint image strip, a second fingerprint image strip and a third fingerprint image strip, each fingerprint image strip representing a portion of the fingerprint and the second and third fingerprint image strips overlapping the first fingerprint image strip,

wherein the fingerprint image strips are input using a swipe-type fingerprint sensor having a rectangular scan area where a width of a long side thereof is less than 10 mm;

extracting minutiae from each of the first, second and third fingerprint image strips;

matching the extracted minutiae of the first, second and third fingerprint image strips to determine relative positions of the fingerprint image strips;

combining the minutiae of the first, second and third fingerprint image strips based on their relative positions to produce a combined set of minutiae representing a larger portion of the fingerprint; and

determining that the fingerprint has been successfully enrolled if a width of the larger portion of the fingerprint is greater than a predetermined minimum width;

wherein matching the extracted minutiae of at least two of the first, second and third fingerprint image strips comprises:

determining a first local neighbourhood for each extracted minutiae of one of the fingerprint image strips and for each extracted minutiae of another of the fingerprint image strips, wherein each first local neighbourhood includes at least one extracted minutia neighbouring the respective extracted minutia;

comparing the first local neighbourhoods of the extracted minutiae of the one of the fingerprint image strips with the first local neighbourhoods of the extracted minutiae of the other of the fingerprint image strips to determine matched minutiae appearing in both the extracted minutiae of the one of the fingerprint image strips and the extracted minutiae of the other of the fingerprint image strips;

determining a second local neighbourhood for each unmatched extracted minutiae of the one of the fingerprint image strips and in the other of the fingerprint image strips, wherein each extracted minutia that is not a matched minutia is an unmatched extracted minutia, and wherein each second local neighbourhood includes at least one matched minutia neighbouring the respective unmatched extracted minutia;

comparing the second local neighbourhoods of the unmatched extracted minutiae of the one of the fingerprint image strips with the second local neighbourhoods of the unmatched extracted minutiae of the other of the fingerprint image strips to determine further matched minutiae appearing in both the extracted minutiae of the one of the fingerprint image strips and the extracted minutiae of the other of the fingerprint image strips, wherein the relative position of fingerprint image strips is determined also based on the further matched minutia.

2. A method as claimed in claim 1 , further comprising:

if the width of the larger portion of the fingerprint is not greater than the predetermined minimum width, receiving subsequent fingerprint image strips until the width of the larger portion of the fingerprint is greater than the predetermined minimum width.

3. A method as claimed in claim 1 , wherein receiving the first, second and third fingerprint image strips, wherein the second and third fingerprint image strips overlap the first fingerprint image strip, comprises:

receiving an initial set of first, second and third fingerprint image strips;

determining whether two of the initial set of fingerprint image strips overlap another of the initial set of fingerprint image strips; and

if so, using the initial set of fingerprint image strips as the first, second and third fingerprint image strips; and

if not, receiving further fingerprint image portions until two of the fingerprint image strips overlap another of the fingerprint image strips, and using those fingerprint image strips as the first, second and third fingerprint image strips.

4. A method as claimed in claim 1 , wherein each of the fingerprint image strips has a width of less than 10 mm.

5. A method as claimed in claim 1 , wherein the predetermined minimum width for the larger portion is at least 12 mm or at least twice a width of the fingerprint image strips.

6. A method as claimed in claim 1 , wherein each of the fingerprint image strips has the width and a length, the width being narrower than the length and the lengths of each of the fingerprint image strips being substantially parallel to one another.

7. A method as claimed in claim 6 , wherein the lengths of each of the fingerprint image strips is substantially parallel to a longitudinal direction of the finger.

8. A method as claimed in claim 1 , wherein when the width of the larger portion of the fingerprint is not greater than a predetermined minimum width, the method comprises requesting input of further fingerprint image strips.

9. A method as claimed in claim 1 , further comprising:

alerting a user when additional fingerprint image strips are required to complete enrolment including when the width of the enrolled portion of the fingerprint is not greater than the predetermined minimum width and/or when three overlapping fingerprint image strips have not been provided.

10. An electronic card comprising a swipe-type fingerprint sensor having a rectangular scan area where a width of a longer side thereof is less than 10 mm for obtaining first, second, and third fingerprint image strips, and a processor for enrolment of a fingerprint, the electronic card being arranged to:

receive data representing the first fingerprint image strip, the second fingerprint image strip and the third fingerprint image strip, each fingerprint image strip representing a portion of the fingerprint and the second and third fingerprint image strips overlapping the first fingerprint image strip;

extract minutiae from each of the first, second and third fingerprint image strips;

match the extracted minutiae of the first, second and third fingerprint image strips to determine relative positions of the fingerprint image strips;

combine the minutiae of the first, second and third fingerprint image strips based on their relative positions to produce a combined set of minutiae representing a larger portion of the fingerprint; and

determine that the fingerprint has been successfully enrolled if a width of the larger portion of the fingerprint is greater than a predetermined minimum width;

wherein the processor is arranged such that matching the extracted minutiae of at least two of the first, second and third fingerprint image strips comprises:

determining a first local neighbourhood for each extracted minutiae of one of the fingerprint image strips and for each extracted minutiae of another of the fingerprint image strips, wherein each first local neighbourhood includes at least one extracted minutia neighbouring the respective extracted minutia;

comparing the first local neighbourhoods of the extracted minutiae of the one of the fingerprint image strips with the first local neighbourhoods of the extracted minutiae of the other of the fingerprint image strips to determine matched minutiae appearing in both the extracted minutiae of the one of the fingerprint image strips and the extracted minutiae of the other of the fingerprint image strips;

determining a second local neighbourhood for each unmatched extracted minutiae of the one of the fingerprint image strips and in the other of the fingerprint image strips, wherein each extracted minutia that is not a matched minutia is an unmatched extracted minutia, and wherein each second local neighbourhood includes at least one matched minutia neighbouring the respective unmatched extracted minutia;

comparing the second local neighbourhoods of the unmatched extracted minutiae of the one of the fingerprint image strips with the second local neighbourhoods of the unmatched extracted minutiae of the other of the fingerprint image strips to determine further matched minutiae appearing in both the extracted minutiae of the one of the fingerprint image strips and the extracted minutiae of the other of the fingerprint image strips, wherein the relative position of fingerprint image strips is determined also based on the further matched minutia.

11. A non-transitory computer program product encoded with computer instructions comprising instructions that when executed will cause a processor to perform a method according to claim 1 .

12. A method as claimed in claim 1 , wherein each of the fingerprint image strips has a width of less than 7 mm.

13. A method according to claim 1 , further comprising, after determining the matched minutiae and before determining the further matched minutiae, filtering the matched minutiae based on the position and/or orientation of each of the matched minutiae within the first and second sets of minutiae, wherein filtering the matched minutiae comprises:

determining a relative position and/or a relative orientation of each matched minutia between the first and second sets of minutiae; and

filtering out matched minutiae having a relative position and/or a relative orientation that is outside of an allowable region,

wherein the allowable region has a predetermined range and is positioned so as to maximise the number of matched minutiae within the range.

14. The electronic card as claimed in claim 10 , wherein the electronic card comprising a memory and a processor.

15. The electronic card as claimed in claim 10 , wherein the fingerprint sensor has a width of less than 7 mm.

16. The electronic card according to claim 10 , wherein the processor is arranged to, after determining the matched minutiae and before determining the further matched minutiae, filter the matched minutiae based on the position and/or orientation of each of the matched minutiae within the first and second sets of minutiae, wherein filtering the matched minutiae comprises:

determining a relative position and/or a relative orientation of each matched minutia between the first and second sets of minutiae; and

filtering out matched minutiae having a relative position and/or a relative orientation that is outside of an allowable region,

wherein the allowable region has a predetermined range and is positioned so as to maximise the number of matched minutiae within the range.