ISO/TS 22028-4:2012

TECHNICAL ISO/TS
SPECIFICATION 22028-4
First edition
2012-11-01
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/TS 22028-4:2012(E)
©
ISO 2012

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ISO/TS 22028-4:2012(E)

COPYRIGHT PROTECTED DOCUMENT
© ISO 2012
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Published in Switzerland
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ISO/TS 22028-4:2012(E)

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
4.3 Reference display . 6
4.4 eciRGB (2008) colour image encoding . 7
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
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ISO/TS 22028-4:2012(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.
International Standards are drafted in accordance with the rules given in the ISO/IEC Directives, Part 2.
The main task of technical committees is to prepare International Standards. Draft International
Standards adopted by the technical committees are circulated to the member bodies for voting.
Publication as an International Standard requires approval by at least 75 % of the member bodies
casting a vote.
In other circumstances, particularly when there is an urgent market requirement for such documents, a
technical committee may decide to publish other types of document:
— an ISO Publicly Available Specification (ISO/PAS) represents an agreement between technical
experts in an ISO working group and is accepted for publication if it is approved by more than 50 %
of the members of the parent committee casting a vote;
— an ISO Technical Specification (ISO/TS) represents an agreement between the members of a
technical committee and is accepted for publication if it is approved by 2/3 of the members of the
committee casting a vote.
An ISO/PAS or ISO/TS is reviewed after three years in order to decide whether it will be confirmed for
a further three years, revised to become an International Standard, or withdrawn. If the ISO/PAS or
ISO/TS is confirmed, it is reviewed again after a further three years, at which time it must either be
transformed into an International Standard or be withdrawn.
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.
ISO/TS 22028-4 was prepared by Technical Committee ISO/TC 42, Photography.
ISO/TS 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) [Technical Specification]
— Part 4: European Colour Initiative RGB colour image encoding [eciRGB (2008)] [Technical Specification]
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ISO/TS 22028-4:2012(E)

Introduction
This Technical Specification 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 Technical
Specification 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
international standard meets the needs of these types of applications.
The primaries of eciRGB (2008) 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 Technical Specification 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. It is anticipated that, when there is sufficient implementation
of, and practical experience in the use of, eciRGB (2008), this Technical Specification can be revised as
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 Technical Specification. 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 Technical Specification was prepared in order to fully define eciRGB according to ISO 22028-1.
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TECHNICAL SPECIFICATION ISO/TS 22028-4:2012(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 Technical Specification 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 referenced documents are indispensable for the application of this document. For dated
references, only the edition cited applies. For undated references, the latest edition of the referenced
document (including any amendments) applies.
ISO 3664:2009, Graphic technology and photography — Viewing conditions
ISO 11664-1, Colorimetry — Part 1: CIE standard colorimetric observers
ISO 22028-1, 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.
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 The adapted white can vary within a scene.
3.2
additive RGB colour space
a 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 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.
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NOTE 2 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
colorimetric colour space
a colour space having an exact and simple relationship to CIE colorimetric values
NOTE Colorimetric 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).
3.4
colour component transfer function
single variable, monotonic mathematical function applied individually to one or more colour channels
of a colour space
NOTE 1 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 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.
3.5
colour encoding
a generic term for a quantized digital encoding of a colour space, encompassing both colour space
encodings and colour image encodings
3.6
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
3.7
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 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 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 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.
3.8
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 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.
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3.9
colour space
geometric representation of colours in space, usually of three dimensions
[CIE Publication 17.4, 845-03-25]
3.10
colour space encoding
digital encoding of a colour space, including the specification of a digital encoding method, and a colour
space value range
NOTE 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.)
3.11
colour space white point
colour stimulus to which colour space values are normalized
NOTE 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.
3.12
extended gamut
colour gamut extending outside that of the standard sRGB CRT display as defined by IEC 61966-2-1
3.13
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 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.
3.14
ICC profile
International Colour Consortium 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
3.15
image state
attribute of a colour image encoding indicating the rendering state of the image data
NOTE 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.
3.16
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
[CIE Publication 17.4, 845-04-69]
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3.17
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 It is generally desirable to specify a medium black point that has the same chromaticity as the
medium white point.
3.18
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
3.19
output-referred image state
image state associated with image data that represents the colour-space coordinates of the elements of
an image that has undergone colour rendering appropriate for a specified real or virtual output device
and viewing conditions
NOTE 1 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 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 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.
3.20
tristimulus values
amounts of the three reference colour stimuli, in a given trichromatic system, required to match the
colour of the stimulus considered
[CIE Publication 17.4, 845-03-22]
3.21
veiling glare
light, reflected from an imaging medium, that has not been modulated by the means used to produce
the image
NOTE 1 Veiling glare lightens and reduces the contrast of the darker parts of an image.
NOTE 2 In CIE 122, the veiling glare of a CRT display is referred to as ambient flare.
3.22
viewing flare
veiling glare that is observed in a viewing environment but not accounted for in radiometric measurements
made using a prescribed measurement geometry
NOTE 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. The output-
referred image data has the intended colour appearance when viewed on a reference colour monitor in
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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 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 environment.
The colour image encoding defined in this Technical Specification conforms to the requirements defined
in ISO 22028-1:2004, Clause 5.
4.2 Reference viewing environment
4.2.1 General
The following reference viewing conditions define the reference viewing environment for the eciRGB
(2008) colour image encoding. They are based on conditions for appraisal of images displayed on colour
monitors as specified in ISO 3664:2009, 4.5. These specifications are applicable for images viewed
independently of any form of hardcopy; they are not designed for direct comparison between hardcopy
and softcopy.
4.2.2 Ambient illumination
When measured, with the monitor turned off, at the monitor faceplate, the ambient illumination level
shall be equal to 32 lx. When measured, with the monitor turned off, in any plane between the monitor
and the observer, the ambient illumination level shall be within the range of 16 lx to 64 lx. The ambient
illumination shall have the same chromaticity as the white point of the display.
4.2.3 Reference display surround
The area immediately surrounding the displayed image may affect the local state of adaptation of the
eye upon viewing the image. This surround shall be a neutral gray, of the same chromaticity as the
reference display white point. The surround shall extend at least two degrees from the edge of the image
2
in all directions. Its luminance shall be 20 % of the reference display white point, that is, 32,00 cd/m .
NOTE If the monitor is equipped with a hood, the ambient illumination of the extended surround outside
the hood can be kept higher than the ambient illumination at the monitor faceplate, possibly enabling the use of a
reflective extended surround.
4.2.4 Image size and viewing distance
The normal to the centre of the display faceplate shall be the viewer’s direction of gaze. The viewing
distance shall be equal to the image diagonal, or longest chord.
4.2.5 Glare
The veiling glare in the reference viewing environment shall be included in the display black point,
as would result from measurement of the display from the viewer position in the reference viewing
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environment. Viewing flare that may result in viewing conditions different from the reference viewing
conditions shall not be included.
NOTE When positioning a display in a viewing environment, it is important to arrange the ambient lighting
so that specular reflections off the display faceplate, as seen from the viewer position, are avoided. This can
usually be achieved by placing ambient light sources at an angle of at least 45° relative to the normal to the display
faceplate, which is assumed to be the viewer’s direction of gaze.
4.2.6 Measurements
All illuminance or luminance measurements shall be made with a photometer having the spectral
sensitivity of the CIE standard photopic photometric observer, V(λ), and measuring an area having a
diameter no greater than 1/20 of the shortest linear dimension of the illuminated surface area. All other
tristimulus and chromaticity values shall be measured using the same geometry with a spectroradiometer
or colorimeter based on the CIE 1931 two-degree standard observer. See CIE Publication 15.
Display measurements shall be performed in the reference viewing environment.
The use of telespectroradiometers or telecolorimeters for display measurement from the viewer
position is recommended, as they include allowance for any veiling glare present, and therefore provide
an accurate representation of the colour as perceived by the viewer. Where such instruments are not
available, and measurements are made in contact with the face of the display, the veiling glare should be
measured from the viewer position and used to correct the measurement data obtained.
NOTE Care should be taken when making measurements of displays to ensure that the sampling frequency,
or integration time, of the instrument used is synchronized with the frequency of scanning of the display. If not,
at least 10 measurements should be taken and averaged.
4.3 Reference display
4.3.1 Contrast ratio
The contrast ratio shall be the ratio of reference display white point luminance over reference display
black point luminance, (Y /Y ), which is 320.
W K
4.3.2 Reference display white point and luminance
The reference display white point shall be such that the adapted white has the chromaticity values of CIE
Illuminant D50 (x = 0,345 7, y = 0,358 5).
0 0
2
The absolute luminance level of the adapted white on the reference display shall be 160 cd/m .
NOTE This absolute luminance level is equivalent to that of a perfect white Lambertian reflector illuminated
with 500 lx as specified in ISO 3664 for the practical appraisal of prints.
4.3.3 Reference display black point and luminance
The reference display shall have a reference display black point with the chromaticity values of CIE
Illuminant D50 (x = 0,345 7, y = 0,358 5) and a luminance factor of F = 0,003 125 relative to the
0 0 K
display white. Accordingly, the reference display black point tristimulus values are X = F X = 0,301 3,
K K 0
Y = F Y = 0,312 5, and Z = F Z = 0,257 8.
K K 0 K K 0
2
NOTE The luminance factor of 0,003 125 corresponds to a luminance value of 0,5 cd/m .
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4.4 eciRGB (2008) colour image encoding
4.4.1 General
The eciRGB (2008) colour space is an extended additive RGB colour space defined by a set of additive
primaries, a white point, a black point, and a colour component transfer function.
4.4.2 Colour space chromaticities and luminance
The x-y chromaticity values for the eciRGB primaries shall be as given in Table 1. All chromaticity
values specified in this document shall be based on the CIE 1931 two-degree standard observer
defined in ISO 11664-1.
The colour space white point, corresponding to equal amounts of the three RGB primaries, shall have the
x-y chromaticity values of CIE Illuminant D given as given in Table 1.
50
Table 1 — CIE chromaticities for reference medium primaries and white point
x y u′ v′
Red 0,670 0 0,330 0 0,476 9 0,528 5
Green 0,210 0 0,710 0 0,075 7 0,575 7
Blue 0,140 0 0,080 0 0,152 2 0,195 7
White 0,345 7 0,358 5 0,209 2 0,488 1
The colour space white point shall be equal to the reference display white point.
The colour space black point shall be equal to the reference display black point.
4.4.3 Colour space encodings
The value range for eciRGB (2008) colour space component values shall be [0, 1].
The colour component values shall be encoded using integer or floating-point encodings.
Integer encodings shall be unsigned with 8 bits or 16 bits per component, with the same number of
bits for all three components. R, G, and B are the linear tristimulus values for the conversion from and
to XYZ. These values are transferred into the non-linear values R′, G′, B′ with a component value range
[0, 1] using the transfer function mentioned in 4.4.6.3 . They shall be encoded over the code value range
[0, max. integer value]. R′, G′, B′ code values of 0, 0, 0 shall represent the colour space black point, and
max. integer code values shall represent the colour space white point.
For integer encodings, all code values shall be within the colour space gamut.
Floating-point encodings shall be 32 bit per component using the floating point encoding defined for
the applicable image format. If no such encoding format is defined, then use IEEE 754-1985. In floating-
point encodings, a component value and its encoding value are the same. Code values 0.0, 0.0, 0.0 shall
represent the colour space black point, and code values 1.0, 1.0, 1.0 shall represent the colour space
white point. Component values outside the range [0, 1] are not allowed for floating-point enc
...