ISO 22028-4:2023

Photography and graphic technology — Extended colour encodings for digital image storage, manipulation and interchange — Part 4: European Colour Initiative RGB colour image encoding [eciRGB (2008)]

This document defines an extended colour-gamut output-referred RGB colour image encoding designated as European Colour Initiative RGB [eciRGB (2008)]. Digital images encoded using eciRGB (2008) can be manipulated, stored, transmitted, displayed, or printed by digital still picture imaging systems. Two precision levels are defined, using 8 bits/channel and 16 bits/channel.

Photographie et technologie graphique — Codages par couleurs étendues pour stockage, manipulation et échange d'image numérique — Partie 4: Codage d'image en couleurs RGB par initiative de couleur européenne [eciRGB(2008)]

Fotografija in grafična tehnologija - Razširjeno barvno kodiranje za shranjevanje, izmenjavo in ravnanje z digitalnimi slikami - 4. del: RGB po Evropski barvni pobudi za barvno kodiranje slik [eciRGB (2008)]

Ta dokument določa razširjeno kodiranje barvne slike z barvno lestvico RGB, imenovano evropska barvna pobuda RGB [eciRGB (2008)]. Digitalne slike, kodirane z eciRGB (2008), se lahko obdelujejo, shranjujejo, prenašajo, prikazujejo ali tiskajo z digitalnimi sistemi za slikanje nepremičnih slik. Opredeljeni sta dve stopnji natančnosti, in sicer z uporabo 8-bitov/kanal in 16-bitov/kanal.

General Information

• Status: Published
• Publication Date: 06-Sep-2023
• ICS: 37.040.99 - Other standards related to photography
• ICS: 37.100.01 - Graphic technology in general
• Technical Committee: ISO/TC 42 - Photography
• Drafting Committee: ISO/TC 42 - Photography
• Current Stage: 6060 - International Standard published
• Start Date: 07-Sep-2023
• Due Date: 09-Aug-2024
• Completion Date: 07-Sep-2023
• Ref Project: SIST ISO 22028-4:2024 - Photography and graphic technology — Extended colour encodings for digital image storage, manipulation and interchange — Part 4: European Colour Initiative RGB colour image encoding [eciRGB (2008)]

Relations

• Revises: ISO/TS 22028-4:2012 - Photography and graphic technology — Extended colour encodings for digital image storage, manipulation and interchange — Part 4: European Colour Initiative RGB colour image encoding [eciRGB (2008)]
• Effective Date: 06-Jun-2022

Standards Content (Sample)

SLOVENSKI STANDARD
01-marec-2024
Nadomešča:
SIST-TS ISO/TS 22028-4:2014
Fotografija in grafična tehnologija - Razširjeno barvno kodiranje za shranjevanje,
izmenjavo in ravnanje z digitalnimi slikami - 4. del: RGB po Evropski barvni pobudi
za barvno kodiranje slik [eciRGB (2008)]
Photography and graphic technology — Extended colour encodings for digital image
storage, manipulation and interchange — Part 4: European Colour Initiative RGB colour
image encoding [eciRGB (2008)]
Photographie et technologie graphique — Codages par couleurs étendues pour
stockage, manipulation et échange d'image numérique — Partie 4: Codage d'image en
couleurs RGB par initiative de couleur européenne [eciRGB(2008)]
Ta slovenski standard je istoveten z: ISO 22028-4:2023
ICS:
01.070 Barvno kodiranje Colour coding
35.040.30 Kodiranje grafičnih in Coding of graphical and
fotografskih informacij photographical information
37.100.01 Grafična tehnologija na Graphic technology in
splošno general
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.

INTERNATIONAL ISO
STANDARD 22028-4
First edition
2023-09
Photography and graphic
technology — Extended colour
encodings for digital image storage,
manipulation and interchange —
Part 4:
European Colour Initiative RGB colour
image encoding [eciRGB (2008)]
Photographie et technologie graphique — Codages par couleurs
étendues pour stockage, manipulation et échange d'image
numérique —
Partie 4: Codage d'image en couleurs RGB par initiative de couleur
européenne [eciRGB(2008)]
Reference number
© ISO 2023
All rights reserved. Unless otherwise specified, or required in the context of its implementation, no part of this publication may
be reproduced or utilized otherwise in any form or by any means, electronic or mechanical, including photocopying, or posting on
the internet or an intranet, without prior written permission. Permission can be requested from either ISO at the address below
or ISO’s member body in the country of the requester.
ISO copyright office
CP 401 • Ch. de Blandonnet 8
CH-1214 Vernier, Geneva
Phone: [phone]
Email: [e-mail]
Website: www.iso.org
Published in Switzerland
ii
Contents Page
Foreword .iv
Introduction .v
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Requirements . 4
4.1 General . 4
4.2 Reference viewing environment . 5
4.2.1 General . 5
4.2.2 Ambient illumination . 5
4.2.3 Reference display surround . 5
4.2.4 Image size and viewing distance . 5
4.2.5 Glare. 5
4.2.6 Measurements . 6
4.3 Reference display . 6
4.3.1 Contrast ratio . 6
4.3.2 Reference display white point and luminance . 6
4.3.3 Reference display black point and luminance . 6
4.4 eciRGB (2008) colour image encoding . 7
4.4.1 General . 7
4.4.2 Colour space chromaticities and luminance . 7
4.4.3 Colour space encodings . 7
4.4.4 Image state . 7
4.4.5 Normalized and absolute XYZ tristimulus values . 8
4.4.6 Encoding an image in 24-bit eciRGB (2008) colour image encoding . 8
4.4.7 Decoding 24-bit eciRGB (2008) to XYZ (D50) values . 9
Annex A (informative) The eciRGB (2008) ICC profile considerations .11
Annex B (informative) Practical tolerances for viewing eciRGB (2008) encoded images .12
Annex C (informative) Comparison of primaries .15
Bibliography .17
iii
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).
ISO draws attention to the possibility that the implementation of this document may involve the use
of (a) patent(s). ISO takes no position concerning the evidence, validity or applicability of any claimed
patent rights in respect thereof. As of the date of publication of this document, ISO had not received
notice of (a) patent(s) which may be required to implement this document. However, implementers are
cautioned that this may not represent the latest information, which may be obtained from the patent
database available at www.iso.org/patents. ISO shall not be held responsible for identifying any or all
such patent rights.
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 Technical Committee ISO/TC 42 Photography.
This first edition cancels and replaces the first edition (ISO/TS 22028-4:2012), which has been
technically revised.
The main changes are as follows:
— this document is released as an International Standard (prior edition was a Technical Specification);
— editorial revisions have been made.
A list of all parts in the ISO 22028 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
Introduction
This document has been developed in order to meet the industry need for a complete, fully documented,
publicly available definition of an output-referred extended gamut RGB colour image encoding which is
optimized for an 8-bit encoding and the conversion of RGB images into offset print colour spaces. Since
users have also asked for a 16-bit encoding it has been added to this document as well. This colour image
encoding provides a way to represent output-referred images that does not limit the colour gamut to
those colours capable of being displayed on a CRT monitor, such as that represented by the sRGB colour
encoding, or require the use of negative RGB colorimetry coordinates, such as with extended sRGB
colour encodings like bg-sRGB.
An extended colour-gamut colour encoding is particularly desirable for professional photography
applications. For example, colours used for company logos may be outside a monitor gamut and would
therefore need to be clipped or compressed to a less saturated colour. Similarly, scanned photographic
prints that are to be duplicated may contain colours outside a monitor RGB colour-gamut. By using a
standard output-referred extended gamut colour image encoding, images containing such colours can
be stored, interchanged, manipulated, and later printed, without limiting or distorting the colours of
the final output.
The European Colour Initiative (ECI) RGB colour image encoding [eciRGB (2008)] specified in this
document meets the needs of these types of applications.
The primaries of eciRGB (2008) (see Annex C, Figures C.1 and C.2) are between Reference Output
Medium Metric RGB (ROMM RGB) and sRGB, thereby providing a larger gamut than sRGB, together with
lower quantization errors than ROMM RGB. The tone curve has an encoding linear to the L* axis defined
in the CIE 1976 (L*, a*, b*) colour space (CIELAB 1976).
This document has been prepared to provide sufficient documentation, consistent with the definitions
of ISO 22028-1, to allow the imaging community adequate opportunity for implementation and
evaluation of this colour image encoding. Sufficient implementation of, and practical experience in the
use of, eciRGB (2008), has led to a revision of the former Technical Specification and its conversion into
an International Standard.
The European Colour Initiative owns the copyright on the name eciRGB (2008) and has granted ISO the
irrevocable non-exclusive right to use the name for the purpose of this document. A colour encoding
named eciRGB was initiated by ECI in 2004. A second version of this encoding with a modified tonal
curve was defined in 2008. Because of its importance to the European photographers and graphic arts
industry, this document was prepared in order to fully define eciRGB according to ISO 22028-1.
v
INTERNATIONAL STANDARD ISO 22028-4:2023(E)
Photography and graphic technology — Extended colour
encodings for digital image storage, manipulation and
interchange —
Part 4:
European Colour Initiative RGB colour image encoding
[eciRGB (2008)]
1 Scope
This document defines an extended colour-gamut output-referred RGB colour image encoding
designated as European Colour Initiative RGB [eciRGB (2008)]. Digital images encoded using eciRGB
(2008) can be manipulated, stored, transmitted, displayed, or printed by digital still picture imaging
systems. Two precision levels are defined, using 8 bits/channel and 16 bits/channel.
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 3664:2009, Graphic technology and photography — Viewing conditions
ISO/CIE 11664-1, Colorimetry — Part 1: CIE standard colorimetric observers
ISO 22028-1:2016, Photography and graphic technology — Extended colour encodings for digital image
storage, manipulation and interchange — Part 1: Architecture and requirements
CIE Publication 15, Colorimetry
3 Terms and definitions
For the purposes of this document, the following terms and definitions apply.
ISO and IEC maintain terminology databases for use in standardization at the following addresses:
— ISO Online browsing platform: available at https:// www .iso .org/ obp
— IEC Electropedia: available at https:// www .electropedia .org/
3.1
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 can vary within a scene.
3.2
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.
3.3
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 can be used.
3.4
colour encoding
generic term for a quantized digital encoding of a colour space, encompassing both colour space
encodings and colour image encodings
3.5
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
3.6
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 can 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.
3.7
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 (3.5) of the reproduction; and applying preference adjustments.
3.8
colour space
geometric representation of colours in space, usually of three dimensions
[SOURCE: CIE Publication 17.4, 845-03-25]
3.9
colour space encoding
digital encoding of a colour space (3.8), including the specification of a digital encoding method, and a
colour space (3.8) value range
Note 1 to entry: Multiple colour space encodings can 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 e-sRGB are different colour space encodings based on a particular RGB colour space.)
3.10
colour space white point
colour stimulus to which colour space (3.8) values are normalized
Note 1 to entry: It is not necessary that the colour space white point correspond to the assumed adapted white
point and/or the reference medium white point for a colour image encoding.
3.11
extended gamut
colour gamut (3.5) extending outside that of the standard sRGB CRT display as defined by IEC 61966-2-1
3.12
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 (3.5) capability
Note 1 to entry: The term “gamut mapping” is somewhat more restrictive than the term “colour rendering”
because gamut mapping is performed on colorimetry 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.
3.13
ICC profile
International Colour Consortium file format, used to store transforms from one colour encoding (3.4) to
another, e.g. from device colour coordinates to profile connection space, as part of a colour management
system
Note 1 to entry: See Annex A for more information.
3.14
image state
attribute of a colour image encoding (3.6) indicating the rendering state of the image data
Note 1 to entry: The primary image states defined in this document are the scene-referred image state, the
original-referred image state and the output-referred image state. See Annex A for more information.
3.15
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]
3.16
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.
3.17
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
3.18
output-referred image state
image state (3.14) associated with image data that represents the colour-space coordinates of the
elements of an image that has undergone colour rendering (3.7) appropriate for a specified real or
virtual output device and viewing conditions
Note 1 to entry: When the phrase “output-referred” is used as a qualifier to an object, it implies that the object is
in an output-referred image state. For example, output-referred image data are image data in an output-referred
image state.
Note 2 to entry: Output-referred image data are referred to the specified output device and viewing conditions. A
single scene can be colour rendered to a variety of output-referred representations depending on the anticipated
output viewing conditions, media limitations, and/or artistic intents.
Note 3 to entry: Output-referred image data can become the starting point for a subsequent reproduction process.
For example, sRGB output-referred image data are frequently considered to be the starting point for the colour
re-rendering performed by a printer designed to receive sRGB image data.
3.19
tristimulus values
amounts of the three reference colour stimuli, in a given trichromatic system, required to match the
colour of the stimulus considered
[SOURCE: CIE Publication 17.4, 845-03-22]
3.20
veiling glare
light, reflected from an imaging medium, that has not been modulated by the means used to produce
the image
Note 1 to entry: Veiling glare lightens and reduces the contrast of the darker parts of an image.
Note 2 to entry: In CIE 122, the veiling glare of a CRT display is referred to as ambient flare.
3.21
viewing flare
veiling glare (3.20) that is observed in a viewing environment but not accounted for in radiometric
measurements made using a prescribed measurement geometry
Note 1 to entry: The viewing flare is expressed as a percentage of the luminance of adapted white.
4 Requirements
4.1 General
European Colour Initiative RGB [eciRGB (2008)] is an extended gamut RGB colour image encoding for
representing the colorimetry of display-referred image data in a display-referred image. Scene referred
images can be converted into eciRGB (2008) using colour rendering. The output-referred image data
has the intended colour appearance when viewed on a reference colour monitor in a reference viewing
environment. The image colorimetry is encoded in terms of an additive RGB colour space associated
with a hypothetical additive colour device having a specified set of primaries, no cross-talk between
the
...

INTERNATIONAL ISO
STANDARD 22028-4
First edition
2023-09
Photography and graphic
technology — Extended colour
encodings for digital image storage,
manipulation and interchange —
Part 4:
European Colour Initiative RGB colour
image encoding [eciRGB (2008)]
Photographie et technologie graphique — Codages par couleurs
étendues pour stockage, manipulation et échange d'image
numérique —
Partie 4: Codage d'image en couleurs RGB par initiative de couleur
européenne [eciRGB(2008)]
Reference number
© ISO 2023
All rights reserved. Unless otherwise specified, or required in the context of its implementation, no part of this publication may
be reproduced or utilized otherwise in any form or by any means, electronic or mechanical, including photocopying, or posting on
the internet or an intranet, without prior written permission. Permission can be requested from either ISO at the address below
or ISO’s member body in the country of the requester.
ISO copyright office
CP 401 • Ch. de Blandonnet 8
CH-1214 Vernier, Geneva
Phone: [phone]
Email: [e-mail]
Website: www.iso.org
Published in Switzerland
ii
Contents Page
Foreword .iv
Introduction .v
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Requirements . 4
4.1 General . 4
4.2 Reference viewing environment . 5
4.2.1 General . 5
4.2.2 Ambient illumination . 5
4.2.3 Reference display surround . 5
4.2.4 Image size and viewing distance . 5
4.2.5 Glare. 5
4.2.6 Measurements . 6
4.3 Reference display . 6
4.3.1 Contrast ratio . 6
4.3.2 Reference display white point and luminance . 6
4.3.3 Reference display black point and luminance . 6
4.4 eciRGB (2008) colour image encoding . 7
4.4.1 General . 7
4.4.2 Colour space chromaticities and luminance . 7
4.4.3 Colour space encodings . 7
4.4.4 Image state . 7
4.4.5 Normalized and absolute XYZ tristimulus values . 8
4.4.6 Encoding an image in 24-bit eciRGB (2008) colour image encoding . 8
4.4.7 Decoding 24-bit eciRGB (2008) to XYZ (D50) values . 9
Annex A (informative) The eciRGB (2008) ICC profile considerations .11
Annex B (informative) Practical tolerances for viewing eciRGB (2008) encoded images .12
Annex C (informative) Comparison of primaries .15
Bibliography .17
iii
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).
ISO draws attention to the possibility that the implementation of this document may involve the use
of (a) patent(s). ISO takes no position concerning the evidence, validity or applicability of any claimed
patent rights in respect thereof. As of the date of publication of this document, ISO had not received
notice of (a) patent(s) which may be required to implement this document. However, implementers are
cautioned that this may not represent the latest information, which may be obtained from the patent
database available at www.iso.org/patents. ISO shall not be held responsible for identifying any or all
such patent rights.
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 Technical Committee ISO/TC 42 Photography.
This first edition cancels and replaces the first edition (ISO/TS 22028-4:2012), which has been
technically revised.
The main changes are as follows:
— this document is released as an International Standard (prior edition was a Technical Specification);
— editorial revisions have been made.
A list of all parts in the ISO 22028 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
Introduction
This document has been developed in order to meet the industry need for a complete, fully documented,
publicly available definition of an output-referred extended gamut RGB colour image encoding which is
optimized for an 8-bit encoding and the conversion of RGB images into offset print colour spaces. Since
users have also asked for a 16-bit encoding it has been added to this document as well. This colour image
encoding provides a way to represent output-referred images that does not limit the colour gamut to
those colours capable of being displayed on a CRT monitor, such as that represented by the sRGB colour
encoding, or require the use of negative RGB colorimetry coordinates, such as with extended sRGB
colour encodings like bg-sRGB.
An extended colour-gamut colour encoding is particularly desirable for professional photography
applications. For example, colours used for company logos may be outside a monitor gamut and would
therefore need to be clipped or compressed to a less saturated colour. Similarly, scanned photographic
prints that are to be duplicated may contain colours outside a monitor RGB colour-gamut. By using a
standard output-referred extended gamut colour image encoding, images containing such colours can
be stored, interchanged, manipulated, and later printed, without limiting or distorting the colours of
the final output.
The European Colour Initiative (ECI) RGB colour image encoding [eciRGB (2008)] specified in this
document meets the needs of these types of applications.
The primaries of eciRGB (2008) (see Annex C, Figures C.1 and C.2) are between Reference Output
Medium Metric RGB (ROMM RGB) and sRGB, thereby providing a larger gamut than sRGB, together with
lower quantization errors than ROMM RGB. The tone curve has an encoding linear to the L* axis defined
in the CIE 1976 (L*, a*, b*) colour space (CIELAB 1976).
This document has been prepared to provide sufficient documentation, consistent with the definitions
of ISO 22028-1, to allow the imaging community adequate opportunity for implementation and
evaluation of this colour image encoding. Sufficient implementation of, and practical experience in the
use of, eciRGB (2008), has led to a revision of the former Technical Specification and its conversion into
an International Standard.
The European Colour Initiative owns the copyright on the name eciRGB (2008) and has granted ISO the
irrevocable non-exclusive right to use the name for the purpose of this document. A colour encoding
named eciRGB was initiated by ECI in 2004. A second version of this encoding with a modified tonal
curve was defined in 2008. Because of its importance to the European photographers and graphic arts
industry, this document was prepared in order to fully define eciRGB according to ISO 22028-1.
v
INTERNATIONAL STANDARD ISO 22028-4:2023(E)
Photography and graphic technology — Extended colour
encodings for digital image storage, manipulation and
interchange —
Part 4:
European Colour Initiative RGB colour image encoding
[eciRGB (2008)]
1 Scope
This document defines an extended colour-gamut output-referred RGB colour image encoding
designated as European Colour Initiative RGB [eciRGB (2008)]. Digital images encoded using eciRGB
(2008) can be manipulated, stored, transmitted, displayed, or printed by digital still picture imaging
systems. Two precision levels are defined, using 8 bits/channel and 16 bits/channel.
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 3664:2009, Graphic technology and photography — Viewing conditions
ISO/CIE 11664-1, Colorimetry — Part 1: CIE standard colorimetric observers
ISO 22028-1:2016, Photography and graphic technology — Extended colour encodings for digital image
storage, manipulation and interchange — Part 1: Architecture and requirements
CIE Publication 15, Colorimetry
3 Terms and definitions
For the purposes of this document, the following terms and definitions apply.
ISO and IEC maintain terminology databases for use in standardization at the following addresses:
— ISO Online browsing platform: available at https:// www .iso .org/ obp
— IEC Electropedia: available at https:// www .electropedia .org/
3.1
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 can vary within a scene.
3.2
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.
3.3
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 can be used.
3.4
colour encoding
generic term for a quantized digital encoding of a colour space, encompassing both colour space
encodings and colour image encodings
3.5
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
3.6
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 can 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.
3.7
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 (3.5) of the reproduction; and applying preference adjustments.
3.8
colour space
geometric representation of colours in space, usually of three dimensions
[SOURCE: CIE Publication 17.4, 845-03-25]
3.9
colour space encoding
digital encoding of a colour space (3.8), including the specification of a digital encoding method, and a
colour space (3.8) value range
Note 1 to entry: Multiple colour space encodings can 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 e-sRGB are different colour space encodings based on a particular RGB colour space.)
3.10
colour space white point
colour stimulus to which colour space (3.8) values are normalized
Note 1 to entry: It is not necessary that the colour space white point correspond to the assumed adapted white
point and/or the reference medium white point for a colour image encoding.
3.11
extended gamut
colour gamut (3.5) extending outside that of the standard sRGB CRT display as defined by IEC 61966-2-1
3.12
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 (3.5) capability
Note 1 to entry: The term “gamut mapping” is somewhat more restrictive than the term “colour rendering”
because gamut mapping is performed on colorimetry 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.
3.13
ICC profile
International Colour Consortium file format, used to store transforms from one colour encoding (3.4) to
another, e.g. from device colour coordinates to profile connection space, as part of a colour management
system
Note 1 to entry: See Annex A for more information.
3.14
image state
attribute of a colour image encoding (3.6) indicating the rendering state of the image data
Note 1 to entry: The primary image states defined in this document are the scene-referred image state, the
original-referred image state and the output-referred image state. See Annex A for more information.
3.15
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]
3.16
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.
3.17
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
3.18
output-referred image state
image state (3.14) associated with image data that represents the colour-space coordinates of the
elements of an image that has undergone colour rendering (3.7) appropriate for a specified real or
virtual output device and viewing conditions
Note 1 to entry: When the phrase “output-referred” is used as a qualifier to an object, it implies that the object is
in an output-referred image state. For example, output-referred image data are image data in an output-referred
image state.
Note 2 to entry: Output-referred image data are referred to the specified output device and viewing conditions. A
single scene can be colour rendered to a variety of output-referred representations depending on the anticipated
output viewing conditions, media limitations, and/or artistic intents.
Note 3 to entry: Output-referred image data can become the starting point for a subsequent reproduction process.
For example, sRGB output-referred image data are frequently considered to be the starting point for the colour
re-rendering performed by a printer designed to receive sRGB image data.
3.19
tristimulus values
amounts of the three reference colour stimuli, in a given trichromatic system, required to match the
colour of the stimulus considered
[SOURCE: CIE Publication 17.4, 845-03-22]
3.20
veiling glare
light, reflected from an imaging medium, that has not been modulated by the means used to produce
the image
Note 1 to entry: Veiling glare lightens and reduces the contrast of the darker parts of an image.
Note 2 to entry: In CIE 122, the veiling glare of a CRT display is referred to as ambient flare.
3.21
viewing flare
veiling glare (3.20) that is observed in a viewing environment but not accounted for in radiometric
measurements made using a prescribed measurement geometry
Note 1 to entry: The viewing flare is expressed as a percentage of the luminance of adapted white.
4 Requirements
4.1 General
European Colour Initiative RGB [eciRGB (2008)] is an extended gamut RGB colour image encoding for
representing the colorimetry of display-referred image data in a display-referred image. Scene referred
images can be converted into eciRGB (2008) using colour rendering. The output-referred image data
has the intended colour appearance when viewed on a reference colour monitor in a reference viewing
environment. The image colorimetry is encoded in terms of an additive RGB colour space associated
with a hypothetical additive colour device having a specified set of primaries, no cross-talk between
the colour channels and a luminance dynamic range defined by an associated medium black point and
medium white point.
The encoding can either be in 8 bits/channel or 16 bits/channel (24 bits/pixel or 48 bits/pixel).
The image colorimetry shall be based on flareless (or instrument flare corrected) colorimetric
measurements as described in CIE Publication 15 using the CIE 1931 Standard Colorimetric Observer
defined in ISO/CIE 11664-1.
NOTE The intended colour appearance can be reproduced on a physical device in an actual viewing
environment, only when the actual viewing environment matches the reference viewing environment. See
Annex B for recommended tolerances for viewing eciRGB (2008)-encoded data in an actual viewing environme
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