ISO 22028-1:2016

INTERNATIONAL ISO
STANDARD 22028-1
Second edition
2016-03-15
Photography and graphic
technology — Extended colour
encodings for digital image storage,
manipulation and interchange —
Part 1:
Architecture and requirements
Photographie et technologie graphique — Codages par couleurs
étendues pour stockage, manipulation et échange d’image
numérique —
Partie 1: Architecture et exigences
Reference number
ISO 22028-1:2016(E)
©
ISO 2016

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ISO 22028-1:2016(E)

COPYRIGHT PROTECTED DOCUMENT
© ISO 2016, Published in Switzerland
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ISO 22028-1:2016(E)

Contents Page
Foreword .iv
Introduction .v
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Image-state-based digital imaging architecture . 8
4.1 General . 8
4.2 Scene-referred colour encodings . 9
4.3 Picture-referred colour encodings .10
4.3.1 General.10
4.3.2 Original-referred colour encodings .10
4.3.3 Output-referred colour encodings .10
4.4 Colour-rendering transforms .11
4.5 Colour re-rendering transforms .11
4.6 Film rendering and unrendering transforms .12
5 Requirements for specifying a colour encoding .12
5.1 Colour encoding hierarchy .12
5.2 Information needed to define a colour space .13
5.2.1 General.13
5.2.2 Colourimetric colour spaces .13
5.2.3 Colour appearance colour spaces.15
5.2.4 Device-dependent colour spaces .16
5.3 Information needed to define a colour space encoding .17
5.3.1 General.17
5.3.2 Colour space.17
5.3.3 Digital encoding method .18
5.3.4 Set of valid colour values .19
5.4 Information needed to define a colour image encoding .19
5.4.1 General.19
5.4.2 Colour space encoding .20
5.4.3 Image state .20
5.4.4 Reference image viewing environment .21
5.4.5 Set of valid colour values .22
5.4.6 Reference imaging medium .23
Annex A (informative) Example system workflows .24
Annex B (informative) Characteristics of existing colour encodings .34
Annex C (informative) Criteria for selection of colour encoding .46
Bibliography .52
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ISO 22028-1:2016(E)

Foreword
ISO (the International Organization for Standardization) is a worldwide federation of national standards
bodies (ISO member bodies). The work of preparing International Standards is normally carried out
through ISO technical committees. Each member body interested in a subject for which a technical
committee has been established has the right to be represented on that committee. International
organizations, governmental and non-governmental, in liaison with ISO, also take part in the work.
ISO collaborates closely with the International Electrotechnical Commission (IEC) on all matters of
electrotechnical standardization.
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 ISO documents 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 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).
Any trade name used in this document is information given for the convenience of users and does not
constitute an endorsement.
For an explanation on the meaning of ISO specific terms and expressions related to conformity
assessment, as well as information about ISO’s adherence to the WTO principles in the Technical
Barriers to Trade (TBT) see the following URL: Foreword - Supplementary information
The committee responsible for this document is ISO/TC 42, Photography.
This second edition cancels and replaces the first edition (ISO 22028-1:2004), of which it constitutes
a minor revision with changes in Annex B and Bibliography. It also incorporates the Technical
Corrigendum ISO 22028-1:2004/Cor. 1:2007.
ISO 22028 consists of the following parts, under the general title Photography and graphic technology —
Extended colour encodings for digital image storage, manipulation and interchange:
— Part 1: Architecture and requirements
— Part 2: Reference output medium metric RGB colour image encoding (ROMM RGB)
— Part 3: Reference input medium metric RGB colour image encoding (RIMM RGB)
— Part 4: European Colour Initiative RGB colour image encoding [eciRGB (2008)]
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ISO 22028-1:2016(E)

Introduction
Modern digital imaging systems serve a variety of consumer and commercial applications. Depending
on the application, differing priorities will apply to such system attributes as image quality,
interoperability, simplicity of system architecture and computations, and the flexibility for optimally
using images for a variety of purposes. Trade-offs among these attributes are application-dependent.
A fundamental choice for any imaging system architecture is how to represent images numerically,
in what colour space and with what digital encoding. In some applications, a single colour encoding
designed to be compatible with the prevalent mode of image viewing by the end user can suffice.
Since both multimedia and Internet-based imaging rely heavily on the viewing of images on a softcopy
display, the use of sRGB as a colour encoding makes sense for those applications. However, because
the colour gamut of sRGB does not encompass the colour gamuts of many common input and output
devices, a system architecture that depends exclusively on the use of sRGB would compromise colour
reproduction accuracy unacceptably for some applications.
Colour management systems, such as that defined by the International Colour Consortium (ICC),
provide a mechanism for transforming between various device-dependent and device-independent
colour encodings through the use of colour profiles that are used to define transformations between
the various colour encodings and a standard colour space known as the profile connection space (PCS).
(The ICC.1:2001-12 specification defines two different PCS variations; one for colourimetric intent
profiles and one for perceptual intent profiles.) The ICC PCS is intended to be a colour space to be
used for connecting together different colour profiles and as such has a colour gamut large enough to
encompass most common input and output devices and media. However, the ICC PCS was not designed
to be used as a colour encoding for the storage, transmission or editing of digital images. Additionally,
since ICC colour management is primarily designed to work with colour images in a picture-referred
image state, it does not provide any explicit mechanism for the representation and manipulation of
image data corresponding to other image states.
There are many different applications in the fields of digital photography and graphic technology
that involve editing, storage and interchange of digital images in a variety of image states and colour
encodings. In order to clearly communicate colour image information within and between these
applications, it is necessary to unambiguously describe the meaning of the colour values used to
encode digital images. The colour encoding definitions need to not only include a specification of the
relationship between the digital code values and corresponding physical colour values but they also
need to clearly specify any other information needed to unambiguously interpret the colour values.
Accordingly, there is a need to identify what information is required when defining a colour encoding
in order to ensure that digital image data can be clearly communicated between various applications.
This part of ISO 22028 addresses this need by specifying a set of requirements to be met by colour
encodings defined for various digital imaging applications. This part of ISO 22028 also describes
a reference image-state-based digital imaging architecture that is flexible enough to support a wide
variety of applications and workflows. This image-state-based digital imaging architecture can be used
to classify colour encodings into a number of different image states. However, this part of ISO 22028
does not specify any particular workflow(s) that needs to be used for any particular digital imaging
applications.
There is also a need for the specification of standard extended-gamut colour encodings that can be
used in the context of this architecture to preserve the full range of colour information at every stage
of the workflow, from the initial image capture through to the final step of producing a softcopy or
hardcopy reproduction. It is anticipated that subsequent parts of this multi-part standard will define
at least one scene-referred extended-gamut colour encoding and at least one output-referred extended-
gamut colour encoding.
The International Organization for Standardization (ISO) draws attention to the fact that it is
claimed that compliance with this part of ISO 22028 can involve the use of a patent concerning colour
management given in Clause 4 and 5.4.3.
ISO takes no position concerning the evidence, validity and scope of this patent right.
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ISO 22028-1:2016(E)

The holder of this patent right has assured ISO that he/she is willing to negotiate licences under
reasonable and non-discriminatory terms and conditions with applicants throughout the world. In
this respect, the statement of the holder of this patent right is registered with ISO. Information can be
obtained from:
Microsoft Corporation
One Microsoft Way, Redmond, WA 98052.
Attention is drawn to the possibility that some of the elements of this document may be the subject of
patent rights other than those identified above. ISO shall not be held responsible for identifying any or
all such patent rights.
ISO (www.iso.org/patents) and IEC (http://patents.iec.ch) maintain on-line databases of patents
relevant to their standards. Users are encouraged to consult the databases for the most up to date
information concerning patents.
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INTERNATIONAL STANDARD ISO 22028-1:2016(E)
Photography and graphic technology — Extended colour
encodings for digital image storage, manipulation and
interchange —
Part 1:
Architecture and requirements
1 Scope
This part of ISO 22028 specifies a set of requirements to be met by any extended-gamut colour encoding
that is to be used for digital photography and/or graphic technology applications involving digital
image storage, manipulation and/or interchange. This part of ISO 22028 is applicable to pictorial digital
images that originate from an original scene, as well as digital images with content such as text, line art,
vector graphics and other forms of original artwork. This part of ISO 22028 also describes a reference
image-state-based digital imaging architecture, encompassing many common workflows, that can be
used to classify extended colour encodings into a number of different image states. However, this part
of ISO 22028 does not specify any particular workflow(s) that are to be used for digital photography
and/or graphic technology applications.
2 Normative references
The following documents, in whole or in part, are normatively referenced in this document and are
indispensable for its application. For dated references, only the edition cited applies. For undated
references, the latest edition of the referenced document (including any amendments) applies.
ISO 11664-1, Colorimetry — Part 1: CIE standard colorimetric observers
3 Terms and definitions
For the purposes of this document, the following terms and definitions apply.
3.1
absolute colorimetric coordinates
tristimulus values, or other colorimetric coordinates derived from a tristimulus values, where the
numerical values correspond to the magnitude of the physical stimulus
EXAMPLE When CIE 1931 standard colour-matching functions are used, the Y-coordinate value corresponds
to the luminance, not the luminance factor (or some scaled value thereof).
3.2
adapted white
colour stimulus that an observer who is adapted to the viewing environment would judge to be
perfectly achromatic and to have a luminance factor of unity, i.e. absolute colorimetric coordinates that
an observer would consider to be a perfect white diffuser
Note 1 to entry: The adapted white may vary within a scene.
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ISO 22028-1:2016(E)

3.3
additive RGB colour space
colorimetric colour space having three colour primaries (generally red, green and blue) such that
CIE XYZ tristimulus values can be determined from the RGB colour space values by forming a weighted
combination of the CIE XYZ tristimulus values for the individual colour primaries, where the weights are
proportional to the radiometrically linear colour space values for the corresponding colour primaries
Note 1 to entry: A simple linear 3 × 3 matrix transformation can be used to transform between CIE XYZ
tristimulus values and the radiometrically linear colour space values for an additive RGB colour space.
Note 2 to entry: Additive RGB colour spaces are defined by specifying the CIE chromaticity values for a set of
additive RGB primaries and a colour space white point, together with a colour component transfer function.
3.4
adopted white
spectral radiance distribution as seen by an image capture or measurement device and converted
to colour signals that are considered to be perfectly achromatic and to have an observer adaptive
luminance factor of unity, i.e. colour signals that are considered to correspond to a perfect white diffuser
Note 1 to entry: The adopted white may vary within a scene.
Note 2 to entry: No assumptions should be made concerning the relation between the adapted or adopted white
and measurements of near perfectly reflecting diffusers in a scene, because measurements of such diffusers will
depend on the illumination and viewing geometry, and other elements in the scene that may affect perception.
It is easy to arrange conditions for which a near perfectly reflecting diffuser will appear to be grey or coloured.
3.5
colorimetric colour space
colour space having an exact and simple relationship to CIE colorimetric values
Note 1 to entry: Colourimetric colour spaces include those defined by CIE (e.g. CIE XYZ, CIELAB, CIELUV, etc.),
as well as colour spaces that are simple transformations of those colour spaces (e.g. additive RGB colour spaces).
3.6
colour component transfer function
single variable, monotonic mathematical function applied individually to one or more colour channels
of a colour space
Note 1 to entry: Colour component transfer functions are frequently used to account for the nonlinear response
of a reference device and/or to improve the visual uniformity of a colour space.
Note 2 to entry: Generally, colour component transfer functions will be nonlinear functions such as a power-law
(i.e. “gamma”) function or a logarithmic function. However, in some cases a linear colour component transfer
function may be used.
3.7
colour encoding
generic term for a quantized digital encoding of a colour space, encompassing both colour space
encodings and colour image encodings
3.8
colour gamut
solid in a colour space, consisting of all those colours that are either present in a specific scene, artwork,
photograph, photomechanical, or other reproduction or capable of being created using a particular
output device and/or medium
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ISO 22028-1:2016(E)

3.9
colour image encoding
digital encoding of the colour values for a digital image, including the specification of a colour space
encoding, together with any information necessary to properly interpret the colour values such as the
image state, the intended image viewing environment and the reference medium
Note 1 to entry: In some cases, the intended image viewing environment will be explicitly defined for the colour
image encoding. In other cases, the intended image viewing environment may be specified on an image-by-image
basis using metadata associated with the digital image.
Note 2 to entry: Some colour image encodings will indicate particular reference medium characteristics, such as
a reflection print with a specified density range. In other cases, the reference medium will be not applicable, such
as with a scene-referred colour image encoding, or will be specified using image metadata.
Note 3 to entry: Colour image encodings are not limited to pictorial digital images that originate from an original
scene, but are also applicable to digital images with content such as text, line art, vector graphics and other forms
of original artwork.
3.10
colour-matching functions
tristimulus values of monochromatic stimuli of equal radiant power
[SOURCE: CIE Publication 17.4, 845-03-23]
3.11
colour rendering
mapping of image data representing the colour-space coordinates of the elements of a scene to output-
referred image data representing the colour-space coordinates of the elements of a reproduction
Note 1 to entry: Colour rendering generally consists of one or more of the following: compensating for differences
in the input and output viewing conditions, tone scale and gamut mapping to map the scene colours onto the
dynamic range and colour gamut of the reproduction, and applying preference adjustments.
3.12
colour re-rendering
mapping of picture-referred image data appropriate for one specified real or virtual imaging medium
and viewing conditions to picture-referred image data appropriate for a different real or virtual
imaging medium and/or viewing conditions
Note 1 to entry: Colour re-rendering generally consists of one or more of the following: compensating for
differences in the viewing conditions, compensating for differences in the dynamic range and/or colour gamut of
the imaging media, and applying preference adjustments.
3.13
colour space
geometric representation of colours in space, usually of three dimensions
[SOURCE: CIE Publication 17.4, 845-03-25]
3.14
colour space encoding
digital encoding of a colour space, including the specification of a digital encoding method, and a colour
space value range
Note 1 to entry: Multiple colour space encodings may be defined based on a single colour space where the
different colour space encodings have different digital encoding methods and/or colour space value ranges. (For
example, 8-bit sRGB and 10-bit bg-sRGB are different colour space encodings based on a particular additive RGB
colour space.)
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ISO 22028-1:2016(E)

3.15
colour space white point
colour stimulus to which colour space values are normalized
Note 1 to entry: The colour space white point may or may not correspond to the assumed adapted white point
and/or the reference medium white point for a colour image encoding.
3.16
continuous colour space value
real-valued, unbounded colour space value that has not been encoded using a digital encoding method
3.17
device-dependent colour space
colour space defined by the characteristics of a real or idealized imaging device
Note 1 to entry: Device-dependent colour spaces having a simple functional relationship to CIE colourimetry can
also be categorized as colourimetric colour spaces. For example, additive RGB colour spaces corresponding to
real or idealized CRT displays can be treated as colourimetric colour spaces.
3.18
digital imaging system
system that records and/or produces images using digital data
3.19
extended gamut
colour gamut extending outside that of the standard sRGB CRT display as defined by IEC 61966-2-1
3.20
film rendering transform
mapping of image data representing measurements of a photographic negative to output-referred
image data representing the colour-space coordinates of the elements of a reproduction
3.21
film unrendering transform
mapping of image data representing measurements of a photographic negative to scene-referred image
data representing estimates of the colour-space coordinates of the elements of the original scene
3.22
gamut mapping
mapping of the colour-space coordinates of the elements of a source image to colour-space coordinates
of the elements of a reproduction to compensate for differences in the source and output medium colour
gamut capability
Note 1 to entry: The term “gamut mapping” is somewhat more restrictive than the term “colour-rendering”
because gamut mapping is performed on colourimetry that has already been adjusted to compensate for viewing
condition differences and viewer preferences, although these processing operations are frequently combined in
reproduction and preferred reproduction models.
3.23
hardcopy
representation of an image on a substrate which is self-sustaining and reasonably permanent
[SOURCE: ISO 3664:2009: 3.4, modified]
3.24
ICC profile
International Color Consortium”s file format, used to store transforms from one colour encoding
to another, e.g. from device colour coordinates to profile connection space, as part of a colour
management system
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3.25
image state
attribute of a colour image encoding indicating the rendering state of the image data
Note 1 to entry: The primary image states defined in this part of ISO 22028 are the scene-referred image state,
the original-referred image state and the output-referred image state.
3.26
International Color Consortium profile connection space
ICC PCS
standard colour image encoding defined by the International Color Consortium providing a standard
connection point for combining ICC profiles
Note 1 to entry: The ICC.1:2001 specification defines two variations of the PCS, an original-referred variation for
colourimetric intent profiles, and an output-referred variation for perceptual intent profiles.
3.27
luminance factor
ratio of the luminance of the surface element in the given direction to that of a perfect reflecting or
transmitting diffuser identically illuminated
[SOURCE: CIE Publication 17.4, 845-04-69]
3.28
luminance ratio
ratio of the maximum luminance to the minimum luminance that is either present in a specific scene,
artwork, photograph, photomechanical, or other reproduction or is capable of being created using a
particular output device and medium
3.29
medium black point
neutral colour with the lowest luminance that can be produced by an imaging medium in normal use,
measured using the specified measurement geometry
Note 1 to entry: It is generally desirable to specify a medium black point that has the same chromaticity as the
medium white point.
3.30
medium white point
neutral colour with the highest luminance that can be produced by an imaging medium in normal use,
measured using the specified measurement geometry
3.31
metadata
data associated with a digital image aside from the pixel values that comprise the digital image
Note 1 to entry: Metadata are typically stored as tags in the digital image file.
3.32
original-referred image state
image state associated with image data that represents the colour-space coordinates of the elements of
a two-dimension
...