ISO/IEC 39794-6:2021

INTERNATIONAL ISO/IEC
STANDARD 39794-6
First edition
2021-03
Information technology — Extensible
biometric data interchange formats —
Part 6:
Iris image data
Technologies de l'information — Formats d'échange de données
biométriques extensibles —
Partie 6: Données d'image de l'iris
Reference number
©
ISO/IEC 2021
© ISO/IEC 2021
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ii © ISO/IEC 2021 – All rights reserved

Contents Page
Foreword .
...................................................................................................................................................................................................................................................................iv
Introduction .v
1 Scope . 1
2 Normative references . 2
3 Terms and definitions . 2
4 Symbols and abbreviated terms . 3
5 Conformance . 4
6 Iris image content specification . 4
6.1 General . 4
6.2 Uncropped iris image . 5
6.3 VGA iris image . 6
6.4 Cropped iris image . 6
6.5 Cropped and masked iris image . 7
6.5.1 General. 7
6.5.2 Masking of the sclera . 7
6.5.3 Masking of the eyelids . 7
6.5.4 Mask transition blurring . 8
7 Abstract data elements . 9
7.1 Purpose and overall structure . 9
7.2 Version block .11
7.3 Representation block .11
7.3.1 General.11
7.3.2 Eye label .11
7.3.3 Iris image kind .11
7.3.4 Bit depth .11
7.3.5 Image data format.12
7.3.6 Horizontal orientation .13
7.3.7 Vertical orientation .13
7.3.8 Compression history .13
7.3.9 Capture date/time block .14
7.3.10 Iris image data .14
7.3.11 Range .14
7.3.12 Capture device block .14
7.3.13 Quality blocks .15
7.3.14 Roll angle block .15
7.3.15 Localization block .16
7.3.16 PAD data block .17
8 Encoding .17
8.1 Tagged binary encoding .17
8.2 XML encoding .17
9 Registered BDB format identifiers .17
Annex A (normative) Formal specifications .19
Annex B (informative) Encoding examples .26
Annex C (normative) Conformance testing methodology .27
Annex D (informative) Iris image capture .33
Bibliography .37
© ISO/IEC 2021 – All rights reserved iii

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.
The procedures used to develop this document and those intended for its further maintenance are
described in the ISO/IEC Directives, Part 1. In particular, the different approval criteria needed for
the different types of document should be noted. This document was drafted in accordance with the
editorial rules of the ISO/IEC Directives, Part 2 (see www .iso .org/ directives).
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. Details of any patent rights identified during the development of the document will be in the
Introduction and/or on the ISO list of patent declarations received (see www .iso .org/ patents) or the IEC
list of patent declarations received (see patents.iec.ch).
Any trade name used in this document is information given for the convenience of users and does not
constitute an endorsement.
For an explanation of the voluntary nature of standards, the meaning of ISO specific terms and
expressions related to conformity assessment, as well as information about ISO's adherence to the
World Trade Organization (WTO) principles in the Technical Barriers to Trade (TBT), see www .iso .org/
iso/ foreword .html.
This document was prepared by Joint Technical Committee ISO/IEC JTC 1, Information technology,
Subcommittee SC 37, Biometrics.
A list of all parts in the ISO/IEC 39794 series can be found on the ISO website.
Any feedback or questions on this document should be directed to the user’s national standards body. A
complete listing of these bodies can be found at www .iso .org/ members .html.
iv © ISO/IEC 2021 – All rights reserved

Introduction
The purpose of this document is to define an International Standard for the exchange of iris image
extensible information. This document contains a specific definition of iris image record attribute data
elements, record’s tagged binary and XML encoding extensible formats for storing and transmitting the
iris image and certain attribute data elements, and conformance criteria.
Currently, the exchange of iris information between equipment from different vendors can be achieved
using images of the eye. While some applications can successfully operate with full size uncompressed
rectilinear images, there are others for which this is expensive in terms of storage and bandwidth. This
document therefore also defines compact representations.
Biometric data interchange formats enable the interoperability of different biometric systems. The
first generation of biometric data interchange formats was published between 2005 and 2007 in the
first edition of the ISO/IEC 19794 series. From 2011 onwards, the second generation of biometric data
interchange formats has been published in the second edition of the established parts and the first
edition of some new parts of ISO/IEC 19794. In the second generation of biometric data interchange
formats, new useful data elements such as those related to biometric sample quality have been added,
the header data structures have been harmonized across all parts of the ISO/IEC 19794 series, and an
XML encoding has been added in addition to the binary encoding.
In anticipation of the future need for additional data elements and in order to avoid future compatibility
issues, ISO/IEC JTC 1/SC 37 has developed the ISO/IEC 39794 series as a third generation of biometric
data interchange formats, defining extensible biometric data interchange formats capable of including
future extensions in a defined way. Extensible specifications in ASN.1 (Abstract Syntax Notation One)
and the Distinguished Encoding Rules of ASN.1 form the basis for encoding biometric data in binary
tag-length-value formats. XML schema definitions form the basis for encoding biometric data in XML
(Extensible Markup Language).
Annex A specifies the ASN.1 schema and XML schema of the formal structure description to which
tagged binary encoded and XML encoded iris image extensible records are to conform (respectively).
Annex B provides sample iris image extensible record encodings. Annex C includes normative assertions
for testing conformance of iris image extensible records. Finally, Annex D gives recommendations on
iris image capture.
© ISO/IEC 2021 – All rights reserved v

INTERNATIONAL STANDARD ISO/IEC 39794-6:2021(E)
Information technology — Extensible biometric data
interchange formats —
Part 6:
Iris image data
1 Scope
This document specifies:
— generic extensible data interchange formats for the representation of iris image data: a tagged
binary data format based on an extensible specification in ASN.1 and a textual data format based on
an XML schema definition that are both capable of holding the same information,
— examples of data record contents,
— application specific requirements, recommendations, and best practices in data acquisition, and
— conformance test assertions and conformance test procedures applicable to this document.
The iris image information is stored as:
— an array of intensity values optionally compressed with ISO/IEC 15948 or ISO/IEC 15444-1, or
— an array of intensity values optionally compressed with ISO/IEC 15948 or ISO/IEC 15444-1 that can
be cropped around the iris, with the iris at the centre, and which can incorporate region-of-interest
masking of non-iris regions.
This document also specifies elements of conformance testing methodology, test assertions, and test
procedures, as applicable to this document.
It establishes:
— test assertions pertaining to the structure of the iris image data format, as specified in Clauses 6, 7,
8 and 9 of this document,
— test assertions pertaining to internal consistency by checking the types of values that may be
contained within each field, and
— semantic test assertions.
The conformance testing methodology specified in this document does not establish:
— tests of other characteristics of biometric products or other types of testing of biometric products
(e.g. acceptance, performance, robustness, security), or
— tests of conformance of systems that do not produce data records conforming to the requirements
of this document.
This document does not establish:
— requirements on the optical specifications of cameras, or
— requirements on photometric properties of iris images, or
— requirements on enrolment processes, workflow and use of iris equipment.
© ISO/IEC 2021 – All rights reserved 1

2 Normative references
The following documents are referred to in the text in such a way that some or all of their content
constitutes requirements 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 2382-37, Information technology — Vocabulary — Part 37: Biometrics
ISO/IEC 8824-1, Information technology — Abstract Syntax Notation One (ASN.1): Specification of basic
notation — Part 1
ISO/IEC 8825-1, Information technology — ASN.1 encoding rules: Specification of Basic Encoding Rules
(BER), Canonical Encoding Rules (CER) and Distinguished Encoding Rules (DER) — Part 1
ISO/IEC 15444-1, Information technology — JPEG 2000 image coding system — Part 1: Core coding system
ISO/IEC 15948, Information technology — Computer graphics and image processing — Portable Network
Graphics (PNG): Functional specification
ISO/IEC 39794-1, Information technology — Extensible biometric data interchange formats — Part 1:
Framework
W3C Recommendation, XML Schema Part 1: Structures Second Edition, 28 October 2004, http:// www .w3
.org/ TR/ xmlschema -1/
W3C Recommendation, XML Schema Part 2: Datatypes Second Edition, 28 October 2004, http:// www .w3
.org/ TR/ xmlschema -2/
3 Terms and definitions
For the purposes of this document, the terms and definitions given in ISO/IEC 2382-37 and
ISO/IEC 39794-1 and the following apply.
ISO and IEC maintain terminological databases for use in standardization at the following addresses:
— ISO Online browsing platform: available at https:// www .iso .org/ obp
— IEC Electropedia: available at http:// www .electropedia .org/
3.1
defocus
image impairment due to the position of the iris (3.4) along the optical axis of the camera away from
the plane or surface of best focus, generally resulting in reduced sharpness (blur) and reduced contrast
3.2
depth of field
distance range relative to the entrance aperture of a capture device over which the iris (3.4) image has
greater than a specified quality with respect to focus
3.3
greyscale
continuous-tone image that has one component, which is pixel intensity
3.4
iris
coloured annular structure in the front portion of the eye comprised of muscular and connective tissue
and pigmented layers, that defines the pupil (3.9) and controls its size
3.5
iris centre
centre of a circle modelling the boundary between iris (3.4) and sclera (3.12)
2 © ISO/IEC 2021 – All rights reserved

3.6
iris radius
radius of a circle modelling the boundary between iris (3.4) and sclera (3.12)
3.7
margin
distance in an image from the iris-sclera border, when modelled as a circle, to the closest image border,
expressed in pixels
Note 1 to entry: Throughout this document, margins are defined in terms of the iris radius, R (3.6). When written
as an ordered pair, the order is (horizontal, vertical).
EXAMPLE (0,6R, 0,2R) indicates that for an iris radius of R, there shall be margins of image data 0,6·R to the
right and left of the iris (3.4) and 0,2·R above and below the iris.
3.8
Modulation Transfer Function
MTF
ratio of the image modulation to the object modulation as a function of spatial frequency (3.14)
3.9
pupil
optical opening in the centre of the eye that serves as a variable light aperture and defines the inner
boundary of the iris (3.4)
3.10
pupil centre
average of coordinates of all the pixels lying on the boundary of the pupil (3.9) and the iris (3.4)
3.11
round
mathematical function applied to a number x such that round(x) is the integer that is closest in value to x
3.12
sclera
generally white wall of the eye peripheral to the iris (3.4)
3.13
segmentation
process of determining, within an image containing an iris (3.4), the boundaries between areas
containing visible iris tissue and those that do not
Note 1 to entry: This process is preceded by localization of the iris, and typically followed by cropping or masking
regions that are not iris tissue.
3.14
spatial frequency
measure of the spatial period of a sinusoidal intensity pattern in space, in units of cycles/degree or of
cycles/mm at a given target range
3.15
spatial sampling rate
number of picture elements (pixels) per unit distance in the object plane or per unit angle in the
imaging system
4 Symbols and abbreviated terms
For the purposes of this document, the abbreviations given in ISO/IEC 39794-1 and the following apply.
© ISO/IEC 2021 – All rights reserved 3

JPEG2000 Joint Photographic Experts Group enhanced compression standard for images, as
defined in ISO/IEC 15444
PGM Netpbm greyscale image format
PPM Netpbm colour image format
PNG Portable Network Graphics lossless compression standard for images, as defined in
ISO/IEC 15948
VGA Video Graphics Array image format, having width 640 pixels and height 480 pixels
5 Conformance
A BDB conforms to this document if it satisfies all of the requirements related to:
a) its data structure, data values and the relationships between its data elements as specified
throughout Clauses 6, 7, 8 and Annex A of this document, and
b) the relationship between its data values and the input biometric data from which the biometric
data record was generated as specified throughout Clauses 6, 7, 8 and Annex A of this document.
A system that produces biometric data records is conformant to this document if all biometric data
records that it outputs conform to this document (as defined in points a) to b) above) as claimed in the
ICS associated with that system. A system does not need to be capable of producing biometric data
records that cover all possible aspects of this document, but only those that are claimed to be supported
by the system in the ICS. The test for output record conformance shall be conducted in accordance with
the normative content of Annex C.
A system that uses biometric data records is conformant to this document if it can read, and use for the
purpose intended by that system, all biometric data records that conform to this document (as defined
in points a) to b) above) as claimed in the ICS associated with that system. A system does not need to be
capable of using biometric data records that cover all possible aspects of this document, but only those
that are claimed to be supported by the system in an ICS.
A binary BDB conforms to this document if it satisfies the format requirements with respect to its
structure, with respect to relations among its fields, and with respect to relations between its fields
and the underlying input that are specified within A.1.
An XML document conforms to this document if it satisfies the format requirements with respect to its
structure, with respect to relations among its fields, and with respect to relations between its fields
and the underlying input that are specified within A.2.
6 Iris image content specification
6.1 General
This clause establishes requirements on the semantic content of the images that are allowed by this
document. These requirements relate to the geometric structure, pre-processing, compression protocol,
format and dimensions of the image data. (Guidance on iris image capture is given in Annex D.) Image
data may be uncompressed or compressed. If uncompressed, then it shall be encoded using PGM or PPM
[3]
image format . All uncompressed raw images shall have an 8 bit pixel depth. Images with a pixel depth
other than 8 bits shall be encoded using PNG or JPEG2000.
The remaining subclauses of Clause 6 group these requirements according to the type of image.
As shown in Table 1, four image types are defined according to a hierarchy inherited from an
4 © ISO/IEC 2021 – All rights reserved

unconstrained abstract basic iris image. The associated abstract values are provided in subclause 7.3.3.
The requirements of Clause 7 establish the encoding specifications for the image.
NOTE The specifications of image types, compression protocols, formats and cropping dimensions in this
[6]
first edition of this document have been determined by the NIST Interoperable Iris Exchange (IREX-1) study
(2009), which was commissioned for this purpose.
Table 1 — Hierarchy of iris image types
Margins (R:
Width and
Data
iris radius) Data size Compression
Iris Cen-
height
FORMAT NAME encoding
tring
Hori- Vertical kB mode
method
pixels
zontal
IMAGE_TYPE_UNCROPPED no ≥ 0,6R ≥ 0,2R unspecified variable none PGM or
PPM
variable lossless PNG or
JPEG2000
variable lossy JPEG2000
IMAGE_TYPE_VGA no ≥ 0,6R ≥ 0,2R W = 640 307,2 none PGM or
PPM
H = 480
t y pically lossless PNG or
70-140 JPEG2000
variable lossy JPEG2000
IMAGE_TYPE_CROPPED yes =0,6R =0,2R unspecified variable none n/a
t y pically lossless PNG or
40-70 JPEG2000
t y pically lossy (see JPEG2000
8-24 (com- NOTE 4)
pact)
IMAGE_TYPE_CROPPED_ yes =0,6R =0,2R unspecified variable none n/a
AND_MASKED
t y pically lossless PNG or
20-50 JPEG2000
t y pically lossy JPEG2000
2-6
(compact)
NOTE 1 The application of lossy compression to IMAGE_TYPE_UNCROPPED images is not recommended for
images with spatial sampling rate below 10 pixels/mm.
NOTE 2 Typical data sizes for IMAGE_TYPE_CROPPED and IMAGE_TYPE_CROPPED_AND_MASKED assume an
iris of approximately 120 pixels radius. Other sizes are listed as variable to reflect variations in spatial sam-
pling rate and in iris size.
NOTE 3 The use of cropping, masking, or lossy compression can degrade iris recognition accuracy.
NOTE 4 For applications of 1:1 comparison, the compressed IMAGE_TYPE_CROPPED data size can be as low
as 3,5 kB.
6.2 Uncropped iris image
An uncropped iris image shall contain a raster scan image of a single eye. An example is shown in
Figure 1. For an iris radius of R, there shall be margins of image data at least 0,2R above and below the
iris, and at least 0,6R to the right and left of the iris. These margins of image data shall be acquired from
© ISO/IEC 2021 – All rights reserved 5

the actual object being imaged, not synthesized values. It is not assumed that the iris is centred within
the image.
If uncropped image data is compressed, then ideally it should be compressed losslessly. PNG shall not be
used in its interlaced mode. If JPEG2000 is used, image data shall be stored in JPEG2000 format.
The uncropped iris image type shall be identified in the iris record by assigning the abstract value
uncropped to the iris image type element in subclause 7.3.3, as defined in Table 3.
6.3 VGA iris image
A VGA iris image is a special case of the uncropped iris image; the image width shall be 640 pixels and
the image height shall be 480 pixels. An example is shown in Figure 1. Additional constraints of margins
and container are inherited from the uncropped image type in subclause 6.2.
If images are compressed, then images shall be compressed in accordance with either PNG or JPEG2000
for lossless compression, or JPEG2000 for lossy compression.
The VGA iris image type shall be identified in the iris record by assigning the abstract value vGA to the
iris image type element in 7.3.3, as defined in Table 3.
Figure 1 — Example of uncropped iris image or VGA iris image
6.4 Cropped iris image
A cropped version of a rectilinear iris image may be instantiated. This supports moderately compact
storage. It requires a coarse localization of the iris.
The cropped rectilinear image shall contain an iris centred relative to the geometric centre of the raster
representation. An example is shown in Figure 2.
The crop region shall be sized such that a margin 0,6R pixels wide is included on both the right and left
sides of the iris, where R is an estimate of the iris radius. Margins above and below the iris shall include
0,2R pixels. Margin pixels shall represent actual sensor readings, not substitute values.
Parts of the iris estimated to have been cropped during capture (i.e. absent in the input image) shall
be replaced with pixels of value 0. Note that records with partially or fully missing iris data should not
ordinarily be generated; instead, the defect should be detected and another capture attempted.
6 © ISO/IEC 2021 – All rights reserved

The cropped iris image type inherits all of the normative requirements of the uncropped iris image
type in 6.2 with respect to compression.
The cropped iris image type shall be identified in the iris record by assigning the abstract value cropped
to the iris image type element in 7.3.3, as defined in Table 3.
Figure 2 — Example of cropped iris image
6.5 Cropped and masked iris image
6.5.1 General
A cropped rectilinear image may be masked to produce a highly compressible image. This masking
operation involves pixels in three regions: the upper and lower eyelids, and the sclera. A mask shall
consist of a single grey value assigned to a four-connected region of pixels. Examples are shown in
[4]
Figure 3. The utility of this approach has been documented in the academic literature .
In the cropped and masked iris image type, the image regions outside of the iris itself shall be masked
with specified below uniform pixel values in order to increase compressibility and to ensure that coding
bytes are allocated maximally to the iris texture itself.
When upper and/or lower eyelids are detected within the cropped image, then pixels in these eyelid
regions and beyond shall be replaced with the value 128, such that normal methods for detecting and
fitting such eyelid boundaries in unmasked images may continue to function with the cropped and
masked iris image type. Note that none, one or both of the upper or lower eyelids may occlude the iris
(see Figure 3). In all these cases, the pixels in the sclera shall be replaced uniformly as specified in 6.5.2
with the value 200, and if any eyelid regions are detected, pixels in those regions and beyond shall be
replaced with the value 128 as specified in 6.5.3.
The cropped and masked iris image type inherits all of the normative requirements of the cropped iris
image type in subclause 6.4 with respect to compression.
The cropped and masked iris image type shall be identified in the iris record by assigning the abstract
value croppedAndMasked to the iris image type element in 7.3.3, as defined in Table 3.
NOTE Masking serves compressibility only; the presence of a mask grey value is not a reliable segmentation
indicator. When an image is compressed, the mask value can be altered by the compression algorithm.
6.5.2 Masking of the sclera
The pixels in the sclera region shall be substituted with a fixed mask value of 200. The sclera mask shall
extend to the first and last columns unless the extremes of the upper and lower eyelids meet inside the
left or right image boundary.
6.5.3 Masking of the eyelids
The pixels in the upper and lower eyelid regions shall be substituted with a fixed mask value of 128.
© ISO/IEC 2021 – All rights reserved 7

The upper eyelid mask shall extend to the first (top) row of the image. The upper eyelid mask shall
extend to the leftmost and rightmost columns of the image. The lower eyelid mask shall extend to
the last (bottom) row of the image. The lower eyelid mask shall extend to the leftmost and rightmost
columns of the image.
(a) Iris occluded by both (b) Iris occluded by the upper
eyelids eyelid and not the lower eyelid
(c) Iris occluded by the (d) Iris is not occluded by eyelids
lower eyelid and not the
upper eyelid
Figure 3 — Examples of cropped and masked iris images
6.5.4 Mask transition blurring
The transitions from iris and sclera regions to the eyelid mask regions, and from the iris to the sclera mask
regions, shall be locally smoothed to minimize the boundary’s impact on the compression coding budget.
The method shall be as follows: After the eyelid mask and the sclera mask values have replaced the
original image pixel values, the borders of these mask regions shall be smoothed by low-pass filtering.
Each image pixel, whose centred 7 × 7 neighbourhood contains at least one mask pixel, shall be replaced
8 © ISO/IEC 2021 – All rights reserved

by a weighted sum of a 7 × 7 binomial kernel. The coefficients of this kernel, K, are defined by the outer
product:
T
KU=× 16/()46 4 U
where
[1,6 15 20 15,6,1]T
U =
The border-smoothing pixel values shall be computed after the masking operation but before further
pixel replacement begins. In the case of pixels that belong to both the iris-sclera mask transition and
the iris or sclera to eyelid transition neighbourhoods, the replacement values used shall be those of the
eyelid border-smoothing operation.
7 Abstract data elements
7.1 Purpose and overall structure
This clause describes the contents of data elements defined in this document. These descriptions are
independent of the encoding of the data elements.
The presence of data elements is specified in Annex A.
For an explanation of the XML schema definition see A.2. A tagged binary encoding in ASN.1 is
described in A.1.
Each BDB shall pertain to a single subject and shall contain one or more images of a human iris. The
organization of the record format is as follows:
— A version number containing information about the version used for encoding. See 7.2.
— A representation element for each iris representation. See 7.3.
The structure of the abstract data elements is additionally informatively described in Figure 4.
Naming conventions for ASN.1 modules, types and components in the ISO/IEC 39794 series and definition
extensions in ASN.1 are specified within the common framework standard ISO/IEC 39794-1:2019,
Clause 9.
Naming conventions for XML schema definitions, types and elements in the ISO/IEC 39794
series and definition extensions in XML are specified within the common framework standard
ISO/IEC 39794-1:2019, Clause 10.
© ISO/IEC 2021 – All rights reserved 9

Key
in the upper left of a
box, denotes that this
element is defined in
subclause 7.n.n.
Figure 4 — Iris image data block
The figure is not automatically generated and should only be viewed as a high level overview of the
structure.
10 © ISO/IEC 2021 – All rights reserved

7.2 Version block
Abstract values: The abstract values for the version block are defined in ISO/IEC 39794-1.
Contents: The generation number of this document shall be 3. The year shall be the year of
the publication of this document. See ISO/IEC 39794-1.
7.3 Representation block
7.3.1 General
This data element is the container for all the data associated with the iris image, except for the version
block information.
7.3.2 Eye label
Abstract values: unknown, rightIris, leftIris
Contents: This element refers to the subject's own eyes label. See Table 2 for a description of
the abstract values.
Table 2 — Abstract values for Eye Label
Abstract value Description
unknown It is unknown if the image is the sub-
ject’s right or left eye.
rightIris It is an image of the subject’s right eye.
leftIris It is an image of the subject’s left eye.
7.3.3 Iris image kind
Abstract values: uncropped, vGA, cropped, croppedAndMasked
Contents: This element refers to the kind of iris image. The image shall conform to the nor-
mative requirements of the subclauses cited in Table 3.
Table 3 — Image kinds and their requirements
Image kind Abstract value Description Governing subclauses
1 uncropped An uncropped rectilinear iris image. 6.2
2 vGA A rectilinear iris image in VGA 6.3
(640 × 480) format.
3 cropped A cropped, centred, iris image with (0,6R 6.4
0,2R) margins.
7 croppedAndMasked A cropped and region-of-interest masked, 6.5 with subclauses
centred, iris image with (0,6R 0,2R) 6.5.1, 6.5.2, 6.5.3, 6.5.4
margins.
7.3.4 Bit depth
Abstract values: An integer 8 to 24.
Contents: This element refers to the bit depth in bits per pixel. Images having more than
8 bits per pixel shall be encoded using PNG or JPEG2000.
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7.3.5 Image data format
Abstract values: pgm, ppm, png, jpeg2000Lossless, jpeg2000Lossy
Contents: This element refers to image data format. See Table 4 for a description of the ab-
stract values.
In the event that a greyscale iris image is encoded in the Netpbm portable greyscale binary image
format (PGM), the format definition is as follows:
1) A "magic number" = “P5” for identifying the file type followed by
2) Any Whitespace (blanks, TABs, CRs, LFs).
3) A width, formatted as ASCII characters in decimal.
4) Any Whitespace (blanks, TABs, CRs, LFs).
5) A height, again in ASCII decimal.
6) Any Whitespace (blanks, TABs, CRs, LFs).
7) The maximum grey value (Maxval), again in ASCII decimal. Shall be less than 65536, and more
than zero.
8) A single Whitespace character (usually a newline).
9) A raster of Height rows, in order from top to bottom. Each row consists of Width grey values, in
order from left to right. Each grey value is a number from 0 through Maxval, with 0 being black
and Maxval being white. Each grey value is represented in pure binary by either 1 or 2 bytes. If the
Maxval is less than 256, it is 1 byte. Otherwise, it is 2 bytes. The most significant byte is first.
A PGM encoded greyscale iris image shall be encoded in a P5 format.
In the event that a colour iris image is encoded in the Netpbm portable colour binary image format
(PPM), the format definition is as follows:
1) A "magic number" = “P6” for identifying the file type followed by
2) Any Whitespace (blanks, TABs, CRs, LFs).
3) A width, formatted as ASCII characters in decimal.
4) Any Whitespace (blanks, TABs, CRs, LFs).
5) A height, again in ASCII decimal.
6) Any Whitespace (blanks, TABs, CRs, LFs).
7) The maximum channel value (Maxval), again in ASCII decimal. Shall be less than 256, and more
than zero.
8) A single Whitespace character (usually a newline).
9) A raster of Height rows, in order from top to bottom. Each row consists of Width pixel values, in
order from left to right. Each pixel value is represented by 1 number for red, 1 number for green
and 1 number for blue, each from 0 through Maxval; thus each pixel value is represented in pure
binary by 3 bytes.
A PPM encoded colour iris image shall be encoded in a P6 format.
12 © ISO/IEC 2021 – All rights reserved

Table 4 — Abstract values for image data format
Abstract value Description
pgm Image shall be monochrome and uncompressed using PGM
format as defined in 7.3.5.
ppm Image shall be colour and uncompressed using PPM format as
defined in 7.3.5.
png Image shall be monochrome or colour and compressed using
PNG algorithm as specified in ISO/IEC 15948.
jpeg2000Lossless Image shall be monochrome or colour and compressed lossless-
ly using JPEG2000 algorithm as specified in ISO/IEC 15444-1, in
JPEG2000 lossless file format.
jpeg2000Lossy Image shall be monochrome or colour and compressed using
JPEG2000 algorithm as specified in ISO/IEC 15444-1, in
JPEG2000 lossy file format.
7.3.6 Horizontal orientation
Abstract values: undefined, leftToRight, rightToLeft
Contents: This element refers to the horizontal orientation of the image. See Table 5 for a
description of the abstract values.
Table 5 — Abstract values for horizontal orientation
Abstract value Description
Unknown The horizontal orientation is unknown.
leftToRight Left side of subject’s eye (i.e. nasal side of subject’s left eye, or tempo-
ral side of subject's right eye) is on left side of the image as viewed.
rightToLeft Horizontal orientation is opposite from that described for
leftToRight, i.e. mirrored about a vertical axis.
7.3.7 Vertical orientation
Abstract values: undefined, topToBottom, bottomToTop
Contents: This element refers to the vertical orientation of the image. See Table 6 for a de-
scription of the abstract values.
Table 6 — Abstract values for vertical orientation
Abstract value Description
unknown The vertical orientation is unknown.
topToBottom Superior edge of subject’s eye is at top of image.
bottomToTop Vertical orientation is opposite from that described for
topToBottom, i.e. mirrored about a horizontal axis.
7.3.8 Compression history
Abstract values: undefined, losslessOrNone, lossy
Contents: This element refers to the image compression history and indicates whether lossy or
lossless compression occurred. See Table 7 for a description of the abstract values.
© ISO/IEC 2021 – All rights reserved 13

Table 7 — Abstract values for compression history
Abstract value Description
unknown Compression history is unknown.
losslessOrNone The image was not compressed, or was
losslessly compressed, before being
represented in the current format.
lossy The image was lossy compressed be-
fore being represented in the current
format.
7.3.9 Capture date/time block
Abstract values: The abstract values for the capture date/time block are defined in ISO/IEC 39794-1.
Contents: The capture date/time block element shall indicate when the capture of this rep-
resentation started in Coordinated Universal Time (UTC), as specified in ISO/
IEC 39794-1.
7.3.10 Iris image data
Abstract values: Octet string.
Contents: This data element contains the encoded iris image data.
7.3.11 Range
Abstract values: unassigned, failed, overflow, range (2 to 65533)
Contents: This element specifies an estimate of the distance between the optical centre of
the camera lens and the subject iris, measured in mm, or whether an attempt to
estimate the range has been made, but failed. See Table 8 for a description of the
abstract values.
NOTE The magnification cannot be derived from the range value if the camera can change its focal length,
using a zoom lens or other method.
Table 8 — Abstract values for range
Abstract value Description
unassigned No range estimation attempt.
failed Range estimation attempt has been
made, but failed.
overflow Range overflow (exceeds 2 – 2).
range Distance between the optical centre
of the camera lens and the subject iris,
measured in mm (2 to 2 – 2).
7.3.12 Capture device block
7.3.12.1 Model identifier block
See ISO/IEC 39794-1.
14 © ISO/IEC 2021 – All rights reserved

7.3.12.2 Capture device technology identifier
Abstract values: unknown, cMOS/
...