SIST-TS ISO/TS 22028-2:2011

TECHNICAL ISO/TS
SPECIFICATION 22028-2
First edition
2006-08-15

Photography and graphic technology —
Extended colour encodings for digital
image storage, manipulation and
interchange —
Part 2:
Reference output medium metric RGB
colour image encoding (ROMM RGB)
Photographie et technologie graphique — Codages par couleurs
étendues pour stockage, manipulation et échange d'image
numérique —
Partie 2: Codage d'image en couleurs RVB par référence de sortie par
voie métrique




Reference number
ISO/TS 22028-2:2006(E)
©
ISO 2006

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ISO/TS 22028-2:2006(E)
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ISO/TS 22028-2:2006(E)
Contents Page
Foreword. iv
Introduction . v
1 Scope . 1
2 Normative references . 1
3 Terms and definitions. 1
4 Requirements . 5
4.1 General. 5
4.2 Reference viewing environment. 5
4.3 Reference medium. 6
4.4 ROMM RGB colour image encoding. 7
4.5 Inverse ROMM RGB transformation . 9
Annex A (informative) Selection of ROMM RGB colour encoding . 11
Annex B (informative) Conversion between ROMM RGB and video RGB . 15
Bibliography . 17

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ISO/TS 22028-2:2006(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 normative 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 22028-2 was prepared by Technical Committee ISO/TC 42, Photography, in collaboration with
Technical Committee ISO/TC 130, Graphic technology.
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) [Technical
Specification]
⎯ Part 3: Reference input medium metric RGB colour image encoding (RIMM RGB) [Technical
Specification]
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ISO/TS 22028-2:2006(E)
Introduction
This part of ISO 22028 has been developed in order to meet the industry need for a complete,
fully-documented, publicly-available definition of a wide-primary output-referred extended gamut
red-green-blue (RGB) colour image encoding. 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 standard RGB (sRGB) colour encoding, or require the use of
negative RGB colorimetry co-ordinates, 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 can 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 can 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 Reference output medium metric RGB (ROMM RGB) colour image encoding specified in this part of
ISO 22028 meets the needs of these types of applications.
This part of ISO 22028 is prepared as a Technical Specification 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 ROMM RGB, this Technical Specification can be
revised as an International Standard.

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TECHNICAL SPECIFICATION ISO/TS 22028-2:2006(E)

Photography and graphic technology — Extended colour
encodings for digital image storage, manipulation and
interchange —
Part 2:
Reference output medium metric RGB colour image encoding
(ROMM RGB)
1 Scope
This Technical Specification defines a family of extended colour-gamut output-referred RGB colour image
encodings designated as Reference output medium metric RGB (ROMM RGB). Digital images encoded using
ROMM RGB can be manipulated, stored, transmitted, displayed, or printed by digital still picture imaging
systems. Three precision levels are defined using 8-, 12- 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 15076-1:2005, Image technology colour management — Architecture, profile format and data structure —
Part 1: Based on ICC.1:2004-10
ISO 22028-1:2004, Photography and graphic technology — Extended colour encodings for digital image
storage, manipulation and interchange — Part 1: Architecture and requirements
ISO/CIE 10527:1991, CIE standard colorimetric observers
CIE Publication 15, Colorimetery
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
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
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ISO/TS 22028-2:2006(E)
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.
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
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), 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
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.
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ISO/TS 22028-2:2006(E)
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;
⎯ applying preference adjustments.
3.9
colour space
geometric representation of colours in space, usually of three dimensions
[CIE Publication 17.4:1987, 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
continuous colour space values
real-valued, unbounded colour space values that have not been encoded using a digital encoding method
3.13
extended gamut
colour gamut extending outside that of the standard sRGB CRT display as defined by IEC 61966-2-1
3.14
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.
3.15
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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ISO/TS 22028-2:2006(E)
3.16
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.
3.17
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 17.4:1987, 845-04-69]
3.18
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.
3.19
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
3.20
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 is image data in an output-referred image state.
NOTE 2 Output referred image data is 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 is frequently considered to be the starting point for the colour re-rendering
performed by a printer designed to receive sRGB image data.
3.21
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 17.4:1987, 845-03-22]
3.22
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 Publication 122, the veiling glare of a CRT display is referred to as ambient flare.
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ISO/TS 22028-2:2006(E)
3.23
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
Reference output medium metric RGB (ROMM RGB) is an extended gamut RGB colour image encoding for
representing the colorimetry of output-referred image data in an output-referred image state on a reference
medium. The output-referred image data has the intended colour appearance when viewed 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.
Three different precision levels are defined, and shall be identified as ROMM8 RGB, ROMM12 RGB and
ROMM16 RGB, for 8-, 12- and 16-bits/channel (24-, 36- and 48-bits/pixel) representations, respectively.
The image colorimetry shall be based on flareless (or instrument flare corrected) colorimetric measurements
as described in CIE Publication No. 15 using the CIE 1931 standard colorimetric observer defined in
ISO/CIE 10527.
Flareless colorimetric measurements should be considered equivalent to those obtained from real reflection
media measured using the 0/45 geometry specified in ISO 13655 (without polarizing means). Therefore,
colorimetric quantities referred to in this part of ISO 22028 should be considered to include a level of flare
typical of such measurements.
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
The reference viewing environment shall be such that the adapted white has the chromaticity values of
CIE Standard Illuminant D (x = 0,345 7, y = 0,358 5).
50 0 0
2
The absolute luminance level of the adapted white in the reference viewing environment shall be 160 cd/m .
NOTE 1 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.
NOTE 2 The luminance of the adapting field can be assumed to be 20 % of the luminance of the adapted white.
The reference viewing environment shall be characterized by an “average” surround. This means that the area
immediately surrounding the image border shall be assumed to be a uniform grey having the chromaticity
values of CIE Standard Illuminant D (x = 0,345 7, y = 0,358 5) and a luminance factor of 0,2 relative to the
50 0 0
adapted white.
The reference viewing environment shall be assumed to have a level of viewing flare that is 0,75 % of the
adapted white with the chromaticity values of CIE Standard Illuminant D (x = 0,345 7, y = 0,358 5).
50 0 0
NOTE 3 If the actual output viewing environment differs significantly from that specified here, appropriate
transformations will probably be necessary to determine the corresponding colorimetry that would produce the intended
colour appearance in the reference viewing environment. However, for actual viewing environments similar to the
reference viewing environment, it probably will not be necessary to make such adjustments. The reference viewing
environment was selected to make such adjustments unnecessary for many practical applications.
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ISO/TS 22028-2:2006(E)
4.3 Reference medium
4.3.1 Reference medium white point
ROMM RGB shall be an encoding of the colours of an image on a reference medium having a reference
medium white point with the chromaticity values of CIE Standard Illuminant D (x = 0,345 7, y = 0,358 5)
50 0 0
and a luminance factor of F = 0,89 relative to the adapted white. Accordingly, the reference medium white
W
point tristimulus values are X = F X = 85,81, Y = F Y = 89,00 and Z = F Z = 73,42, where
W W 0 W W 0 W W 0
X = 96,42, Y = 100,00 and Z = 82,49 are the tristimulus values of the adapted white. These tristimulus
0 0 0
values are based on the CIE 1931 standard colorimetric observer defined in ISO/CIE 10527.
NOTE 1 The luminance factor of 0,89 corresponds to a visual density of 0,050 6.
2
NOTE 2 The luminance factor of 0,89 corresponds to a reference medium white point luminance of 142 cd/m .
4.3.2 Reference medium black point
The reference medium shall have a reference medium black point with the chromaticity values of
CIE Standard Illuminant D (x = 0,345 7, y = 0,358 5) and a luminance factor of F = 0,003 091 1 relative to
50 0 0 K
the adapted white. Accordingly, the reference medium black point tristimulus values are X = F X = 0,298 0,
K K 0
Y = F Y = 0,309 1 and Z = F Z = 0,255 0.
K K 0 K K 0
NOTE 1 The luminance factor of 0,003 091 1 corresponds to a visual density of 2,509 9.
2
NOTE 2 The luminance factor of 0,003 091 1 corresponds to a reference medium black point luminance of 0,495 cd/m .
4.3.3 Reference medium primaries
The x-y chromaticity values for the ROMM RGB 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/CIE 10527. Rationale for the choice of these primaries is given in Annex A.
The colour space white point, corresponding to equal amounts of the three RGB primaries, shall have the x-y
chromaticity values of CIE Standard Illuminant D given as given in Table 1.
50
Table 1 — CIE chromaticities for reference medium primaries and white point
CIE chromaticities
Reference medium primaries
and white point
a a
x y u' v'
Red 0,734 7 0,265 3 0,623 4 0,506 5
Green 0,159 6 0,840 4 0,050 0 0,592 5
Blue 0,036 6 0,000 1 0,050 0 0,000 3
White point 0,345 7 0,358 5 0,209 2 0,488 1
a
The u'-v' chromaticity values for the RGB primaries and colour space white point given in this table can be derived from the x-y
chromaticity values and are provided for information purposes.
4.3.4 Reference medium rendering intent
ROMM RGB is an encoding of the intended colour appearance of an output-referred image rendered to the
reference output medium dynamic range and viewing environment. Images encoded in ROMM RGB may
contain colours that are outside the colour gamut for an actual output medium. Therefore, in order to display
an image encoded in ROMM RGB on an actual output medium, the colour values shall be mapped to those
that can be produced on that medium. All such mappings should be designed to retain the encoded colour
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ISO/TS 22028-2:2006(E)
appearance as closely as possible. If the actual viewing environment is significantly different from the
reference viewing environment, the colour mapping from ROMM RGB to the actual output medium should
include appropriate colour appearance transformations to determine the corresponding colorimetry that will
maintain the colour appearance of the ROMM RGB colour values in the reference viewing environment.
Image colorimetry encoded as ROMM RGB shall not contain colours outside the spectrum locus, or colours
that are outside the gamut bounds of the ICC PCS as defined in ISO 15076-1.
Consideration should be given to the gamut limitations of real media in the process of performing colour
rendering to produce image colorimetry for encoding as ROMM RGB. For example, if the encoded image
colorimetry contains many colours which are far outside of the colour gamut for most real output media, the
colour mapping from ROMM RGB to an actual output media will be forced to significantly distort these colours,
which can lead to inconsistent results.
NOTE In some cases, the colour appearance encoded in ROMM RGB might not be ideally suited for display on a
particular real output media in a particular viewing environment. For example, the visual appearance of a high-quality
transparency, brightly illuminated with a dark surround, will typically be different from that of a high-quality reflection print
of the same scene viewed using a moderate illumination level. Therefore, in some applications, it can be desirable to alter
the colour appearance description provided by the ROMM RGB encoding accordingly to produce the optimal image for the
intended output media and viewing environment.
4.4 ROMM RGB colour image encoding
4.4.1 Encoding principles
ROMM RGB colour image encoding values shall be determined from tristimulus values of an image on the
reference medium using a matrix transformation (see 4.4.3) followed by a colour component transfer function
(see 4.4.4) and a digital encoding for one of three different bit-depths (see 4.4.5). The image tristimulus values
shall be those that produce the intended colour appearance when viewed in the reference viewing
environment.
NOTE Images intended to be viewed in other viewing environments, or on a medium different from the reference
medium, can be encoded in ROMM RGB by first determining the corresponding tristimulus values that are expected to
produce the intended colour appearance on the reference medium when viewed in the reference viewing environment.
The corresponding tristimulus values can be determined by using an appropriate colour appearance transformation to
account for the differences between the viewing conditions. Additionally, it might be necessary to account for differences in
the media characteristics.
4.4.2 Tristimulus value normalization
The image tristimulus values shall be normalized such that the normalized Y tristimulus value of the reference
medium white point is 1,0 and the normalized Y tristimulus value of the reference medium black point is 0,0.
()XX− X
KW
X =
N
...