ISO/IEC 19794-8:2006

INTERNATIONAL ISO/IEC
STANDARD 19794-8
First edition
2006-10-01

Information technology — Biometric data
interchange formats —
Part 8:
Finger pattern skeletal data
Technologies de l'information — Formats d'échange de données
biométriques —
Partie 8: Données des structures du squelette de l'empreinte




Reference number
ISO/IEC 19794-8:2006(E)
©
ISO/IEC 2006

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ISO/IEC 19794-8:2006(E)
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ISO/IEC 19794-8:2006(E)




Contents Page


Foreword . iv

Introduction. v
1 Scope.1

2 Conformance.1

3 Normative references .1
4 Terms and definitions.1

5 Abbreviated terms .5

6 Determination of finger pattern skeletal data .5
6.1 Minutia .5

6.2 Encoding the skeleton ridge line by a direction code.8
6.3 Skeleton line neighbourhood index.13

7 Finger pattern skeletal data record format .14
7.1 Introduction.14

7.2 Record organization .14
7.3 Record header.14

7.4 Single finger record format .16
7.5 Extended data .18

7.5.1 Common extended data fields .18
7.5.2 Ridge count data format.19
7.5 .3 Core and delta data format.21
7.5.4 Zonal quality data .23
7.5.5 Sweat pore position data.24

7.5.6 Finger pattern skeleton structural data.25
7.6 Pattern record format summary.26

8 Finger pattern skeletal data card format .28
8.1 Normal size finger pattern skeletal format.28
8.2 Compact size finger pattern skeletal format.28
8.3 Finger pattern skeletal data block .29

8.4 The x or y coordinate extension for compact card format .29
8.5 Usage of additional features for the card format.30

8.6 Comparison parameters and card capabilities.30
8.7 Pattern card format summary .31

9 CBEFF format owner and format types.32

Annex A (informative) Examples for finger pattern skeletal data .33
A.1 Virtual continuation .34

A.2 High resolution mode .35
A.3 Bifurcation.36
A.4 Skeleton line neighbourhood index.37
A.5 Quality map .39

Annex B (informative) Example data record.40
B.1 Data.40

B.2 Example data format diagrams.42
B.3 Raw data for the finger pattern skeletal record format.43
B.4 Raw data for the compact size finger pattern skeletal card format.43
Bibliography.44


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ISO/IEC 19794-8:2006(E)



Foreword

ISO (the International Organization for Standardization) and IEC (the International Electrotechnical
Commission) form the specialized system for worldwide standardization. National bodies that are members of

ISO or IEC participate in the development of International Standards through technical committees
established by the respective organization to deal with particular fields of technical activity. ISO and IEC

technical committees collaborate in fields of mutual interest. Other international organizations, governmental
and non-governmental, in liaison with ISO and IEC, also take part in the work. In the field of information

technology, ISO and IEC have established a joint technical committee, ISO/IEC JTC 1.
International Standards are drafted in accordance with the rules given in the ISO/IEC Directives, Part 2.
The main task of the joint technical committee is to prepare International Standards. Draft International

Standards adopted by the joint technical committee are circulated to national bodies for voting. Publication as
an International Standard requires approval by at least 75 % of the national bodies casting a vote.

Attention is drawn to the possibility that some of the elements of this document may be the subject of patent

rights. ISO and IEC shall not be held responsible for identifying any or all such patent rights.

ISO/IEC 19794-8 was prepared by Joint Technical Committee ISO/IEC JTC 1, Information technology,
Subcommittee SC 37, Biometrics.

ISO/IEC 19794 consists of the following parts, under the general title Information technology — Biometric data

interchange formats:

 Part 1: Framework
 Part 2: Finger minutiae data
 Part 3: Finger pattern spectral data
 Part 4: Finger image data

 Part 5: Face image data

 Part 6: Iris image data

 Part 7: Signature/sign time series data

 Part 8: Finger pattern skeletal data

 Part 9: Vascular image data

 Part 10: Hand geometry silhouette data

 Part 11: Signature/sign processed dynamic data







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ISO/IEC 19794-8:2006(E)




Introduction


With the interest of implementing interoperable personal biometric recognition systems, this part of
ISO/IEC 19794 establishes a data interchange format for pattern-based skeletal fingerprint recognition
algorithms. Pattern-based algorithms process sections of biometric images. Pattern-based algorithms have
been shown to work well with the demanding, but commercially driven, fingerprint sensor formats such as
small-area and swipe sensors.
The exchange format defined in this part of ISO/IEC 19794 describes all characteristics of a fingerprint in a
small data record. Thus it allows for the extraction of both spectral information (orientation, frequency, phase,
etc.) and features (minutiae, core, ridge count, etc.). Transformations like translation and rotation can also be
accommodated by the format defined herein.
With this part of ISO/IEC 19794 for pattern-based skeletal representation of fingerprints
 interoperability among fingerprint recognition vendors based on a small data record is allowed;
 proliferation of low-cost commercial fingerprint sensors with limited coverage, dynamic range, or
resolution is supported;
 a data record that can be used to store biometric information on a variety a storage media (including but
not limited to, portable devices and smart cards) is defined;
 adoption of biometrics in applications requiring interoperability is encouraged.
It is recommended that biometric data protection techniques in ANSI/X9 X9.84 or ISO/IEC 15408 are used
to safeguard the biometric data defined herein for confidentiality, integrity and availability.
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INTERNATIONAL STANDARD ISO/IEC 19794-8:2006(E)

Information technology — Biometric data interchange
formats —
Part 8:
Finger pattern skeletal data
1 Scope
This part of ISO/IEC 19794 specifies the interchange format for the exchange of pattern-based skeletal
fingerprint recognition data. The data format is generic, in that it may be applied and used in a wide range of
application areas where automated fingerprint recognition is involved.

2 Conformance
A system conforms to this part of ISO/IEC 19794 if it satisfies the mandatory requirements herein for
extraction and description of the skeleton described in Clause 6 and the generation of the data record as
described in Clause 7.
Since any finger skeletal data extraction and comparison algorithm supporting the described finger skeletal
data interchange formats may be used, interoperability testing is of extreme importance, especially for
environments in which components of different manufacturers interact.
3 Normative references
The following referenced documents are indispensable for the application of this document. For dated
references, only the edition cited applies. For undated references, the latest edition of the referenced
document (including any amendments) applies.
ISO/IEC 7816-6:2004, Identification cards — Integrated circuit cards — Part 6: Interindustry data elements for
interchange
ISO/IEC 7816-11:2004, Identification cards — Integrated circuits cards — Part 11: Personal verification through
biometric methods
ISO/IEC 19784-1:2006, Information technology — Biometric application programming interface — Part 1:
BioAPI specification
4 Terms and definitions
For the purposes of this document, the following terms and definitions apply.
4.1
biometrics
automated recognition of individuals based on their behavioural and biological characteristics
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4.2
biometric algorithm
sequence of instructions that tell a biometric system how to solve a particular problem
NOTE An algorithm will have a finite number of steps and is typically used by the biometric engine (i.e. the biometric
system software) to compute whether a biometric sample and template are a match.
4.3
biometric data
biometric sample at any stage of processing, biometric reference, biometric feature or biometric property
4.4
biometric information template
constructed data object in a card containing information needed by the outside world for a verification process
NOTE See ISO/IEC 7816-11.
4.5
biometric reference
one or more stored biometric samples, biometric templates or biometric models attributed to a subject and
used for comparison
EXAMPLES Face image on a passport; fingerprint minutiae template on a national ID card; Gaussian mixture model,
for speaker recognition, in a database.
4.6
biometric sample
analog or digital representation of biometric characteristics prior to feature extraction process and obtained
from a biometric device
4.7
biometric system
automated system capable of
1. capturing a biometric sample from a subject;
2. extracting a biometric feature from that sample;
3. comparing the biometric feature with that contained in the biometric reference;
4. deciding how well they match; and
5. indicating whether or not an identification or verification of identity has been achieved
4.8
biometric template
set of stored biometric features comparable directly to biometric features of a presented biometric sample
NOTE 1 A biometric reference consisting of an image, or other captured biometric sample in its original, enhanced or
compressed form, is not a biometric template.
NOTE 2 The biometric features are not considered to be a biometric template unless they are stored for reference.
4.9
bit-depth
number of bits used to represent a data element
4.10
capture
method of taking a biometric sample from the subject
4.11
cell
rectangular region defined by a uniform and non-overlapping division of the image
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4.12
closed-set identification
biometric application that ranks the biometric references in the enrolment database in order of decreasing
similarity against a presented biometric sample
4.13
comparison
estimation, calculation or measurement of similarity or dissimilarity between biometric sample(s) and biometric
reference(s)
4.14
core
1)
singular point in the fingerprint, where the curvature of the ridges reaches a maximum
NOTE For simplicity, the core can be considered as a U-turn, sometimes enclosing a few ridge endings. It serves as
an approximation of the centre of the fingerprint image.
4.15
delta
1)
structure where three fields of parallel ridge lines meet
NOTE   From Danuta Z. Loesch, “Quantative dermatoglyphics – classification, genetics, and pathology”, Oxford
Monographs on Medical Genetics No. 10, Oxford University Press 1983, ISBN 0-19-261305-7, page 7.
4.16
dimension
number of pixels in an acquired biometric sample in either the x- or y- direction
4.17
enrolment
process of creating and storing, for an individual, a data record associated with an individual and including
biometric reference(s) and, typically, non-biometric data
4.18
friction ridge
structure on the skin of the fingers and toes, the palms and soles of the feet, which makes contact with an
incident surface under normal touch
NOTE On the fingers, the unique patterns formed by the friction ridges make up fingerprints.
4.19
identification
biometric system function that performs a one-to-many search
NOTE An identification function may be used to verify a claim of enrolment in an enrolment database without a
specified biometric reference identifier.
4.20
latent
fingerprint collected from an intermediate surface, rather than directly via a live capture from the finger itself
4.21
live capture
process of capturing a biometric sample by an interaction between a subject and a biometric system

1)
The definitions of core and delta in ISO/IEC 19794-3 and this part of ISO/IEC 19794 are identical. However there is a
different definition in ISO/IEC 19794-2. Although both definitions try to define the same thing, this difference has occurred
for historical reasons.
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4.22
minutia
friction ridge characteristic, occurring at a point where a single friction ridge deviates from an uninterrupted
flow, that is used to individualize a fingerprint
NOTE 1 Deviation may take the form of ending, division, or a more complicated “composite” type.
NOTE 2 The plural of minutia is minutiae.
4.23
one-to-many search
comparison process in which a biometric sample set of one individual is compared against the biometric
references of more than one individual to return a set of comparison scores
NOTE 1 A biometric identification function performs a one-to-many search.
NOTE 2 In the case of a multimodal biometric system, biometric sample and biometric reference in the above definition
comprise individual biometric samples/references of the component modalities.
NOTE 3 The degree of similarity may be specified on the basis of comparison score and/or rank.
4.24
open-set identification
biometric application that determines a possibly empty candidate list by collecting one or more biometric
samples from an individual and searching the enrolment database for similar biometric references
4.25
record
reference and other information about the subject
NOTE E.g. to access permissions.
4.26
resolution
number of pixels (picture elements) per unit distance in the image of the fingerprint
4.27
ridge bifurcation
minutia assigned to the location at which a friction ridge splits into two ridges or, alternatively, where two
separate friction ridges combine into one
4.28
ridge ending
minutia assigned to the location at which a friction ridge terminates or begins
4.29
skeleton
line representation of an object that is one pixel thick through the “middle” of the object and preserves the
topology of the object
4.30
swipe
method of fingerprint collection where the finger is manually moved across a one-dimensional sensor to
produce the two-dimensional image
4.31
sweat pore
minute opening in the dermis, allowing loss of fluid as a part of the temperature control of the body
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4.32
user
client to any biometric vendor
NOTE The user must be differentiated from the end-user (subject) and is responsible for managing and implementing
the biometric application rather than actually interacting with the biometric system.
4.33
valley
area surrounding a friction ridge, which does not make contact with an incident surface under normal touch
4.34
verification
verify
process of comparing a submitted biometric sample against the biometric reference template of a single
enrolee whose identity is being claimed, to determine whether it matches the enrolee’s template
cf. identification
5 Abbreviated terms
For the purposes of this document, the following abbreviated terms apply.
BER Basic Encoding Rules
BIT Biometric Information Template
CBEFF Common Biometric Exchange Formats Framework
DO Data Object
ppcm      pixels per centimetre
6 Determination of finger pattern skeletal data
This ISO/IEC standard for finger pattern interchange data is based on the skeleton representation of friction
ridges. Since the result of different skeleton generation algorithms will differ at a maximum of about a quarter
of the ridge width this will have no impact on interoperability. In order to get a robust skeleton of the ridges a
noise reduction and regularization may take place on the raw image. The direction encoding of the skeleton
line elements is included in the interchange data record. The start and endpoints of the skeleton ridgelines are
included as real or virtual minutiae, and the line from start to endpoint is encoded by successive direction
changes. In the following first the minutiae characteristics and then the encoding definition for one skeleton
line is described.
6.1 Minutia
Minutiae are points located at the places in the fingerprint image where friction ridges end or split into two
ridges.
6.1.1 Minutia type
Each minutia point has a “type” associated with it. There are two major types of minutia: a “ridge ending”
represented by the 2-bit value 01 and a “ridge bifurcation” or split point represented by 2-bit value 10. Points
with three or more intersecting ridges (trifurcations, etc.) will be treated as a “ridge bifurcation” type.
Ridge skeletons require the use of both real and "virtual" minutiae. Virtual minutiae are points on the
fingerprint image where a real ridge ending or a bifurcation does not exist, but a point is required to finish, or
continue, a skeleton ridgeline. Virtual minutiae have thus two types: virtual endings and virtual continuations.
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 Virtual endings are necessary to describe skeleton lines ending at the image boundary or at border lines
to those areas where there is insufficient image quality to determine ridges and real minutiae points (see
Figure A.3). They are also needed to finish the encoding of a closed loop (Table A.1). Virtual endings
have been assigned the 2-bit value 00.
 In rare cases a skeleton line description will require the insertion of a virtual minutia point on a ridgeline.
For example, such points will be required to begin an encoding of a closed loop for which no real minutiae
exist, as well as to describe ridges with high curvature at a sufficient accuracy (see note about maximal
curvature in 6.2.4). These are called “virtual continuation” and have been assigned the 2-bitvalue 11
(Table A1).
6.1.2 Minutia location and coordinate system
The coordinate system used to express the position of the minutiae points of a fingerprint shall be a Cartesian
coordinate system. Points shall be represented by their x and y coordinates, where x increases to the right
and x increases downward (opposite of the pointing direction of the finger), when viewing on a latent print of
the finger (see Figure 1). Note that this is in agreement with most imaging and image processing use. When
viewing on the finger, x increases from right to left as shown in Figure 1. All x and y values are non-negative.
For the skeletal pattern record format, the resolution is specified in the record header, see 7.3.7. For the
skeletal pattern card format, the resolution of the x and y coordinates of the minutia shall be in metric units.
The granularity is one bit per five hundredth of a millimetre in the normal format and one tenth of a millimetre
in the compact format:
1 unit = 0,05 mm (normal format) or 0,1 mm (compact format).
x x
y y
finger
latent print

Figure 1 — Coordinate system
The position of the minutia for a ridge ending shall be defined as the coordinates of the skeleton point with
only one neighbour pixel belonging to the skeleton.
NOTE In some format types of ISO/IEC 19794-2 a ridge ending refers to the point of bifurcation of the valley in front
of the ridge.
The position of the minutia for a ridge bifurcation shall be defined as the point of forking of the skeleton of the
ridge. In other words, the point where three or more ridges intersect is the location of the minutia.
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The position of a virtual ending shall be defined like the position of a real ridge ending.
The position for the minutiae type “virtual continuation” is not evaluated by comparison algorithms, that
analyse minutiae points and angles only. Minutiae of this type are only used for reconstructing the skeleton
but may support subsequent classifications of the reconstructed pattern. One may assign any point on the
skeleton necessary to increase the accuracy of the ridge line description (Table A.1).
6.1.3 Angle conventions
The minutiae angle is measured increasing counter clockwise starting from the horizontal axis to the right. The
angle of a minutia is scaled to fit the bit width of the data field defined in the record header.
The direction of a ridge skeleton endpoint is defined as the angle between the tangent to the ending ridge and
the horizontal axis extending to the right right of the ridge ending point.
A ridge skeleton bifurcation point has three intersection ridges. The two ridges enclosing the ending valley
encompass an acute angle. The direction of a ridge bifurcation is defined as the mean direction of their
tangents. Where each direction is measured as the angle the tangent forms with the horizontal axis to the
right.
The direction of the lines starting or ending at a point with more than three arms (trifurcation, etc.) shall be
defined like the direction of a real ridge ending.
The direction of a virtual ending shall be defined like the direction of a real ridge ending.
The direction for the minutia type “virtual continuation” is not evaluated by comparison algorithms, that analyse
minutiae points and angles only. Minutiae of this type are only used for reconstructing the skeleton but may
support subsequent classifications of the reconstructed pattern. One may assign the mean of the incoming
and outgoing direction or the outgoing direction (Table A.1).
6.1.4 Differences to minutia data in ISO/IEC 19794-2 – finger minutia data
The definition of the minutia position and direction is identical with ISO/IEC 19794-2 card format (Format type
‘0004’ or ‘0006’) with
 minutia placement on a ridge bifurcation encoded as a ridge skeleton bifurcation point, and
 minutia placement on a ridge skeleton endpoint.
To compare minutiae with any other definition, a position and direction correction may be necessary. There
may be performance interoperability differences with the other format types of ISO/IEC 19794-2.
The angular resolution of minutiae in the finger pattern skeletal data record is defined in the header. The
minimal resolution allowed is 16 directions, that is 22,5° per least significant bit. A resolution below the
recommended 64 directions (5,625°)(Table 5: Bit-depth of direction code start and stop direction) may cause a
decrease in match quality for purely minutiae based comparison algorithms. This recommendation
corresponds to the angular resolution of the compact card format in finger minutiae data.
There are no virtual minutiae (type ID 00 and 11) in the finger minutiae data format.
There is no minutia type “other” (type ID 00) in the skeletal pattern data format.
Point with more than three arms (trifurcation, etc.) are not mentioned in the finger minutiae data, so the may
be omitted or encoded as “other”. In the finger pattern skeletal data these structures get the type “bifurcation”.
© ISO/IEC
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