ISO 22028-1:2004

INTERNATIONAL ISO
STANDARD 22028-1
First edition
2004-03-15
Photography and graphic technology —
Extended colour encodings for digital
image storage, manipulation and
interchange —
Part 1:
Architecture and requirements
Photographie et technologie graphique — Codages par couleurs
étendues pour stockage, manipulation et échange d'image numérique —
Partie 1: Architecture et exigences

Reference number
©
ISO 2004
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Contents Page
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Image-state-based digital imaging architecture . 7
4.1 General . 7
4.2 Scene-referred colour encodings . 8
4.3 Picture-referred colour encodings . 9
4.4 Colour-rendering transforms . 10
4.5 Colour re-rendering transforms . 10
4.6 Film rendering and unrendering transforms . 11
5 Requirements for specifying a colour encoding . 11
5.1 Colour encoding hierarchy . 11
5.2 Information needed to define a colour space . 12
5.3 Information needed to define a colour space encoding . 17
5.4 Information needed to define a colour image encoding . 18
Annex A (informative) Example system workflows . 23
Annex B (informative) Characteristics of existing colour encodings . 33
Annex C (informative) Criteria for selection of colour encoding . 40
Bibliography . 46
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ISO 2004 – All rights reserved 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.
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.
ISO 22028-1 was prepared by Technical Committee ISO/TC 42, Photography, in collaboration with ISO/TC 130,
Graphic technology, and the International Commission on Illumination (CIE).
ISO 22028 consists of the following parts, under the general title Photography and graphic technology —
Extended colour encodings for digital image storage, manipulation and interchange:
— Part 1: Architecture and requirements
The following parts are under preparation:
— Part 2: Reference output medium metric RGB colour image encoding (ROMM RGB)
— Part 3: Reference input medium metric RGB colour image encoding (RIMM RGB)
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iv ISO 2004 – All rights reserved

Introduction
Modern digital imaging systems serve a variety of consumer and commercial applications. Depending on the
application, differing priorities will apply to such system attributes as image quality, interoperability, simplicity of
system architecture and computations, and the flexibility for optimally using images for a variety of purposes.
Trade-offs among these attributes are application-dependent.
A fundamental choice for any imaging system architecture is how to represent images numerically, in what
colour space and with what digital encoding. In some applications, a single colour encoding designed to be
compatible with the prevalent mode of image viewing by the end-user may suffice. Since both multimedia and
internet-based imaging rely heavily on the viewing of images on a softcopy display, the use of sRGB as a colour
encoding makes sense for those applications. However, because the colour gamut of sRGB does not
encompass the colour gamuts of many common input and output devices, a system architecture that depends
exclusively on the use of sRGB would compromise colour reproduction accuracy unacceptably for some
applications.
Colour management systems, such as that defined by the International Color Consortium (ICC), provide a
mechanism for transforming between various device-dependent and device-independent colour encodings
through the use of colour profiles that are used to define transformations between the various colour encodings
and a standard colour space known as the profile connection space (PCS). (The ICC.1:2001-12 specification
defines two different PCS variations; one for colorimetric intent profiles, and one for perceptual intent profiles.)
The ICC PCS is intended to be a colour space to be used for connecting together different colour profiles, and
as such has a colour gamut large enough to encompass most common input and output devices and media.
However, the ICC PCS was not designed to be used as a colour encoding for the storage, transmission or
editing of digital images. Additionally, since ICC colour management is primarily designed to work with colour
images in a picture-referred image state, it does not provide any explicit mechanism for the representation and
manipulation of image data corresponding to other image states.
There are many different applications in the fields of digital photography and graphic technology that involve
editing, storage and interchange of digital images in a variety of image states and colour encodings. In order to
clearly communicate colour image information within and between these applications, it is necessary to
unambiguously describe the meaning of the colour values used to encode digital images. The colour encoding
definitions need to not only include a specification of the relationship between the digital code values and
corresponding physical colour values, but they also need to clearly specify any other information needed to
unambiguously interpret the colour values. Accordingly, there is a need to identify what information is required
when defining a colour encoding in order to ensure that digital image data can be clearly communicated
between various applications.
This part of ISO 22028 addresses this need by specifying a set of requirements to be met by colour encodings
defined for various digital imaging applications. This part of ISO 22028 also describes a reference image-state-
based digital imaging architecture that is flexible enough to support a wide variety of applications and
workflows. This image-state-based digital imaging architecture can be used to classify colour encodings into a
number of different image states. However, this part of ISO 22028 does not specify any particular workflow(s)
that need to be used for any particular digital imaging applications.
There is also a need for the specification of standard extended-gamut colour encodings that can be used in the
context of this architecture to preserve the full range of colour information at every stage of the workflow, from
the initial image capture through to the final step of producing a softcopy or hardcopy reproduction. It is
anticipated that subsequent parts of this multi-part standard will define at least one scene-referred extended-
gamut colour encoding and at least one output-referred extended-gamut colour encoding.
The International Organization for Standardization (ISO) draws attention to the fact that it is claimed that
compliance with this document may involve the use of a patent concerning colour management given in
Clauses 4 and 5.4.3.
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ISO 2004 – All rights reserved v

ISO takes no position concerning the evidence, validity and scope of this patent right.
The holder of this patent right has assured ISO that he/she is willing to negotiate licences under reasonable and
non-discriminatory terms and conditions with applicants throughout the world. In this respect, the statement of
the holder of this patent right is registered with ISO. Information may be obtained from:
Microsoft Corporation
One Microsoft Way, Redmond, WA 98052.
Attention is drawn to the possibility that some of the elements of this document may be the subject of patent
rights other than those identified above. ISO shall not be held responsible for identifying any or all such patent
rights.
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vi ISO 2004 – All rights reserved

INTERNATIONAL STANDARD ISO 22028-1:2004(E)
Photography and graphic technology — Extended colour
encodings for digital image storage, manipulation and
interchange —
Part 1:
Architecture and requirements
1Scope
This part of ISO 22028 specifies a set of requirements to be met by any extended-gamut colour encoding that
is to be used for digital photography and/or graphic technology applications involving digital image storage,
manipulation and/or interchange. This part of ISO 22028 is applicable to pictorial digital images that originate
from an original scene, as well as digital images with content such as text, line art, vector graphics and other
forms of original artwork. This part of ISO 22028 also describes a reference image-state-based digital imaging
architecture, encompassing many common workflows, that can be used to classify extended colour encodings
into a number of different image states. However, this part of ISO 22028 does not specify any particular
workflow(s) that are to be used for digital photography and/or graphic technology applications.
2 Normative references
The following 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/CIE 10527, CIE standard colorimetric observers
3 Terms and definitions
For the purposes of this document, the following terms and definitions apply.
3.1
absolute colorimetric coordinates
tristimulus values, or other colorimetric coordinates derived from a tristimulus values, where the numerical
values correspond to the magnitude of the physical stimulus
EXAMPLE When CIE 1931 standard colour-matching functions are used, the Y-coordinate value corresponds to the
luminance, not the luminance factor (or some scaled value thereof).
3.2
adapted white
colour stimulus that an observer who is adapted to the viewing environment would judge to be perfectly
achromatic and to have a luminance factor of unity; i.e. absolute colorimetric coordinates that an observer
would consider to be a perfect white diffuser
NOTE The adapted white may vary within a scene.
3.3
additive RGB colour space
colorimetric colour space having three colour primaries (generally red, green and blue) such that CIE XYZ
tristimulus values can be determined from the RGB colour space values by forming a weighted combination of
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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.
3.4
adopted white
spectral radiance distribution as seen by an image capture or measurement device and converted to colour
signals that are considered to be perfectly achromatic and to have an observer adaptive luminance factor of
unity; i.e. colour signals that are considered to correspond to a perfect white diffuser
NOTE 1 The adopted white may vary within a scene.
NOTE2 No assumptions should be made concerning the relation between the adapted or adopted white and
measurements of near perfectly reflecting diffusers in a scene, because measurements of such diffusers will depend on the
illumination and viewing geometry, and other elements in the scene that may affect perception. It is easy to arrange
conditions for which a near perfectly reflecting diffuser will appear to be grey or coloured.
3.5
colorimetric colour space
colour space having an exact and simple relationship to CIE colorimetric values
NOTE 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).
3.6
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 may be used.
3.7
colour encoding
generic term for a quantized digital encoding of a colour space, encompassing both colour space encodings
and colour image encodings
3.8
colour gamut
solid in a colour space, consisting of all those colours that are either: present in a specific scene, artwork,
photograph, photomechanical, or other reproduction; or capable of being created using a particular output
device and/or medium
3.9
colour image encoding
digital encoding of the colour values for a digital image, including the specification of a colour space encoding,
together with any information necessary to properly interpret the colour values such as the image state, the
intended image viewing environment and the reference medium
NOTE 1 In some cases the intended image viewing environment will be explicitly defined for the colour image encoding. In
other cases, the intended image viewing environment may be specified on an image-by-image basis using metadata
associated with the digital image.
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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.
3.10
colour-matching functions
tristimulus values of monochromatic stimuli of equal radiant power
[CIE Publication 17.4, 845-03-23]
3.11
colour rendering
mapping of image data representing the colour-space coordinates of the elements of a scene to output-referred
image data representing the colour-space coordinates of the elements of a reproduction
NOTE Colour rendering generally consists of one or more of the following: compensating for differences in the input and
output viewing conditions, tone scale and gamut mapping to map the scene colours onto the dynamic range and colour
gamut of the reproduction, and applying preference adjustments.
3.12
colour re-rendering
mapping of picture-referred image data appropriate for one specified real or virtual imaging medium and
viewing conditions to picture-referred image data appropriate for a different real or virtual imaging medium
and/or viewing conditions
NOTE Colour re-rendering generally consists of one or more of the following: compensating for differences in the viewing
conditions, compensating for differences in the dynamic range and/or colour gamut of the imaging media, and applying
preference adjustments.
3.13
colour space
geometric representation of colours in space, usually of three dimensions
[CIE Publication 17.4, 845-03-25]
3.14
colour space encoding
digital encoding of a colour space, including the specification of a digital encoding method, and a colour space
value range
NOTE Multiple colour space encodings may be defined based on a single colour space where the different colour space
encodings have different digital encoding methods and/or colour space value ranges. (For example, 8-bit sRGB and 10-bit
e-sRGB are different colour space encodings based on a particular additive RGB colour space.)
3.15
colour space white point
colour stimulus to which colour space values are normalized
NOTE The colour space white point may or may not correspond to the assumed adapted white point and/or the reference
medium white point for a colour image encoding.
3.16
continuous colour space value
real-valued, unbounded colour space value that has not been encoded using a digital encoding method
3.17
device-dependent colour space
colour space defined by the characteristics of a real or idealized imaging device
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NOTE Device-dependent colour spaces having a simple functional relationship to CIE colorimetry can also be categorized
as colorimetric colour spaces. For example, additive RGB colour spaces corresponding to real or idealized CRT displays can
be treated as colorimetric colour spaces.
3.18
digital imaging system
system that records and/or produces images using digital data
3.19
extended gamut
colour gamut extending outside that of the standard sRGB CRT display as defined by IEC 61966-2-1
3.20
film rendering transform
mapping of image data representing measurements of a photographic negative to output-referred image data
representing the colour-space coordinates of the elements of a reproduction
3.21
film unrendering transform
mapping of image data representing measurements of a photographic negative to scene-referred image data
representing estimates of the colour-space coordinates of the elements of the original scene
3.22
gamut mapping
mapping of the colour-space coordinates of the elements of a source image to colour-space coordinates of the
elements of a reproduction to compensate for differences in the source and output medium colour gamut
capability
NOTE 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.
3.23
hardcopy
representation of an image on a substrate which is self-sustaining and reasonably permanent
[ISO 3664]
3.24
ICC profile
International Color Consortium's file format, used to store transforms from one colour encoding to another, e.g.
from device colour coordinates to profile connection space, as part of a colour management system
3.25
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.
3.26
International Color Consortium profile connection space (ICC PCS)
standard colour image encoding defined by the International Color Consortium providing a standard connection
point for combining ICC profiles
NOTE The ICC.1:2001 specification defines two variations of the PCS, an original-referred variation for colorimetric intent
profiles, and an output-referred variation for perceptual intent profiles.
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3.27
luminance factor
ratio of the luminance of the surface element in the given direction to that of a perfect reflecting or transmitting
diffuser identically illuminated.
[CIE Publication 17.4, 845-04-69]
3.28
luminance ratio
ratio of the maximum luminance to the minimum luminance that is either: present in a specific scene, artwork,
photograph, photomechanical, or other reproduction; or is capable of being created using a particular output
device and medium
3.29
medium black point
neutral colour with the lowest luminance that can be produced by an imaging medium in normal use, measured
using the specified measurement geometry
NOTE It is generally desirable to specify a medium black point that has the same chromaticity as the medium white point.
3.30
medium white point
neutral colour with the highest luminance that can be produced by an imaging medium in normal use, measured
using the specified measurement geometry
3.31
metadata
data associated with a digital image aside from the pixel values that comprise the digital image
NOTE Metadata are typically stored as tags in the digital image file.
3.32
original-referred image state
image state associated with image data that represents the colour-space coordinates of the elements of a two-
dimensional hardcopy or softcopy image, typically produced by scanning artwork, photographic transparencies
or prints, or photomechanical or other reproductions
NOTE 1 When the phrase “original-referred” is used as a qualifier to an object, it implies that the object is in an original-
referred image state. For example, original-referred image data is image data in an original-referred image state.
NOTE 2 Original-referred image data are related to the colour-space coordinates of the original, typically measured
according to ISO 13655, and do not include any additional veiling glare or other flare.
NOTE 3 The characteristics of original-referred image data that most generally distinguish them from scene-referred image
data are that they refer to a two-dimensional surface, and the illumination incident on the two-dimensional surface is
assumed to be uniform (or the image data corrected for any non-uniformity in the illumination).
NOTE 4 There are classes of originals that produce original-referred image data with different characteristics. Examples
include various types of artwork, photographic prints, photographic transparencies, emissive displays, etc. When selecting a
colour re-rendering algorithm, it is usually necessary to know the class of the original in order to determine the appropriate
colour re-rendering to be applied. For example, a colorimetric intent is generally applied to artwork, while different perceptual
algorithms are applied to produce photographic prints from transparencies, or newsprint reproductions from photographic
prints. In some cases the assumed viewing conditions are also different between the original classes, such as between
photographic prints and transparencies, and will usually be considered in well-designed systems.
NOTE 5 In a few cases, it may be desirable to introduce slight colorimetric errors in the production of original-referred
image data, for example to make the gamut of the original more closely fit the colour space, or because of the way the image
data were captured (such as a Status A densitometry-based scanner).
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3.33
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 may 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.
3.34
picture-referred image state
image state associated with image data that represents the colour-space coordinates of the elements of a
hardcopy or softcopy image, encompassing both original-referred image data and output-referred image data
NOTE 1 When the phrase “picture-referred” is used as a qualifier to an object, it implies that the object is in a picture-
referred image state. For example, picture-referred image data are image data in a picture-referred image state.
NOTE 2 Picture-referred image data will generally be colour-rendered for a specific real or virtual imaging medium and
viewing condition.
NOTE 3 Picture-referred image data can include image data that do not originate from an original scene, such as text, line
art, vector graphics and other forms of original artwork.
3.35
scene
spectral radiances of a view of the natural world as measured from a specified vantage point in space and at a
specified time
NOTE A scene may correspond to an actual view of the natural world or to a computer-generated virtual scene simulating
such a view.
3.36
scene-referred image state
image state associated with image data that represents estimates of the colour-space coordinates of the
elements of a scene
NOTE 1 When the phrase “scene-referred” is used as a qualifier to an object, it implies that the object is in a scene-referred
image state. For example, scene-referred image data are image data in a scene-referred image state.
NOTE 2 Scene-referred image data can be determined from raw DSC image data before colour-rendering is performed.
Generally, DSCs do not write scene-referred image data in image files, but some may do so in a special mode intended for
this purpose. Typically, DSCs write standard output-referred image data where colour-rendering has already been
performed.
NOTE 3 Scene-referred image data typically represent relative scene colorimetry estimates. Absolute scene colorimetry
estimates may be calculated using a scaling factor. The scaling factor can be derived from additional information such as the
image OECF, FNumber or ApertureValue, and ExposureTime or ShutterSpeedValue tags.
NOTE 4 Scene-referred image data may contain inaccuracies due to the dynamic range limitations of the capture device,
noise from various sources, quantization, optical blurring and flare that are not corrected for, and colour analysis errors due
to capture device metamerism. In some cases, these sources of inaccuracy can be significant.
NOTE 5 The transformation from raw DSC image data to scene-referred image data depends on the relative adopted
whites selected for the scene and the colour space used to encode the image data. If the chosen scene adopted white is
inappropriate, additional errors will be introduced into the scene-referred image data. These errors may be correctable if the
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transformation used to produce the scene-referred image data is known, and the colour encoding used for the incorrect
scene-referred image data has adequate precision and dynamic range.
NOTE 6 The scene may correspond to an actual view of the natural world, or may be a computer-generated virtual scene
simulating such a view. It may also correspond to a modified scene determined by applying modifications to an original
scene to produce some different desired scene. Any such scene modifications should leave the image in a scene-referred
image state, and should be done in the context of an expected colour-rendering transform.
3.37
softcopy
representation of an image produced using a device capable of directly representing different digital images in
succession and in a non-permanent form
EXAMPLE The most common example is a monitor.
[ISO 3664]
3.38
standard original-referred colour encoding
a colour encoding for original-referred image data defined and documented by an authorized standards body or
industry consortium
3.39
standard output-referred colour image encoding
a colour image encoding for output-referred image data defined and documented by an authorized standards
body or industry consortium
3.40
standard scene-referred colour image encoding
a colour image encoding for scene-referred image data defined and documented by an authorized standards
body or industry consortium
3.41
tristimulus value
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]
3.42
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.
3.43
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 Image-state-based digital imaging architecture
4.1 General
The architecture of a digital imaging system can be described, on the one hand, as the sum of its components
and how those components are interconnected and, on the other hand, as the functions of those components
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and how they interact with each other as an integrated system. One important aspect of a digital imaging
architecture is how the digital image data is encoded as it progresses through the system workflow from image
capture/creation, through image processing/storage/interchange, and finally to output on one or more output
devices.
The need for various colour encodings and the rationale for their specifications can be best understood in the
context of the particular industry and workflow for which they are intended. The digital photography and graphic
technology industries are very diverse and often complex. However, their core activities can be represented by
a fairly simple model where images are classified according to their image state. As shown in Figure 1, this
model consists of a generic digital imaging architecture that can be used to describe the workflows for many
different applications. Examples showing how a number of typical workflows can be described in the context of
this architecture are given in Annex A.
This image-state-based digital imaging architecture, and the associated terminology, facilitates a common
framework for classifying different colour encodings, and describing imaging chains for many diverse types of
digital imaging systems. Any colour image encodings that are defined in this multi-part standard shall be
described within the context of this architecture. In particular, the colour image encoding shall be identified with
an appropriate image state. This part of ISO 22028 does not specify any workflows that should be used for any
particular applications to transform image data to/from the identified image states.
The digital imaging architecture shows examples of where different types of devices may fit within typical
workflows utilizing colour image encodings compliant with this part of ISO 22028. It is not intended to constrain
the workflows for any particular applications to those shown in Figure 1. For example, raw digital camera
captures may be processed directly to an output-referred colour encoding without stopping in a scene-referred
colour encoding.
Workflows associated with particular applications may include additional colour encodings that may correspond
to image states different than the standard image states defined in this image-state-based digital imaging
architecture. For example, it may be useful to define a colour encoding for representing colour negative scans,
or an intermediate colour encoding for partially colour-rendered images. While such colour encodings may be
valuable internally to particular applications, they should generally not be used for image interchange in an open
system environment unless all components in the system are enabled to properly interpret/use the image data
and/or provision is made for communicating a transformation of the image data to one of the standard image
states.
The image state diagram shown in Figure 1 shows that most colour encodings can broadly be categorized into
scene-referred or picture-referred image states.
4.2 Scene-referred colour encodings
Scene-referred colour encodings are representations of the estimated colour-space coordinates of the
elements of an original scene, where a scene is defined to be the spectral radiances of a view of the natural
world as measured from a specified vantage point in space and at a specified time.
EXAMPLE Scene-referred image data may be represented in many different ways including encoding scene colour values
using a CIE colour space such as CIE XYZ or CIELAB, or in terms of the response of an idealized scene capture device
such as RIMM RGB.
Scene-referred image data may correspond to an actual view of the natural world, or to a computer-generated
virtual scene simulating such a view. It may also correspond to a modified scene determined by applying
modifications to an original scene. For example, such modifications could include removing haze from the
captured image, or allowing a user to manually adjust the exposure/white balance. It could also include more
complex operations such as using a “dodge-and-burn” algorithm to correct over-exposed regions of a back-lit
scene. (This can be viewed as being analogous to “re-lighting” the scene.) Scene modifications could also
include applying desired changes to the scene such as simulating a “night” scene, making grass greener to
make it look healthier, or making the sky bluer to make it look clearer. Any such scene modifications should
leave the image in a scene-referred image state, and should be done in the context of the expected
colour-rendering transform. For example, typical colour-rendering transforms will include a boost in the chroma
of the image. Any boost in colourfulness of the scene (e.g. making the grass greener) should be done with the
knowledge that there will be an additional chroma boost during colour-rendering. Consequently, the
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Figure 1 — Image state diagram showing relationship between various types of colour encodings
colour-rendering transform should be included in any image preview path that is used to provide subjective
feedback to a user during the scene-editing process. Image modifications that change the image state of the
image (e.g. tone scale and gamut-mapping operations that colour-render an image to map the scene colours
onto the dynamic range and colour gamut of a certain output medium) are inappropriate for determining a
modified scene.
It should be noted that the image colorimetry of the scene-referred image data may contain inaccuracies due to
the dynamic range limitations of the capture device, noise from various sources, quantization, optical blurring
and flare that are not corrected for, and colour analysis errors due to capture device metamerism. In some
cases, these sources of inaccuracy can be significant.
NOTE Since scene-referred image data have not been colour-rendered, they are usually not ready to be either displayed
or printed.
4.3 Picture-referred colour encodings
4.3.1 General
Picture-referred colour encodings are representations of the colour-space coordinates of a hardcopy or softcopy
image. Picture-referred colour encodings can be further subdivided into original-referred colour encodings and
output-referred colour encodings.
4.3.2 Original-referred colour encodings
Original-referred colour encodings are representative of the colour-space coordinates (or an approximation
thereof) of a two-dimensional hardcopy or softcopy input image. Original-referred image data are generally
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ISO 2004 – All rights reserved 9

produced by scanning artwork, photographic transparencies/prints, or photomechanical reproductions, etc. The
characteristics of original-referred image data are tightly coupled to the characteristics of the original image
source.
NOTE Examples of ways that original-referred image data may be represented include encoding picture colour values
using a CIE colour space such as CIE XYZ or CIELAB, in terms of the response of an idealized measurement device such
as a Status A densitometer, or in terms of device-dependent control signals for a particular image scanning device.
When a scene-capture device, such as a digital camera, is used for the purposes of digitizing a two-dimensional
hardcopy or softcopy image, the resulting image data should generally be treated as original-referred image
data rather than scene-referred image data. In this case, it is usually unnecessary and inappropriate to apply a
colour-rendering transform to the image data for purposes of determining output-referred image data since the
original image has already been colour-rendered. However, it may be desirable to apply a colour re-rendering
transform to account for the differences between the media/viewing condition characteristics of the original
image source and the final output-referred image.
4.3.3 Output-referred colour encodings
Output-referred colour encodings are representative of the colour-space coordinates of image data that are
appropriate for a specified real or virtual output device and viewing conditions. Output-referred colour encodings
are tightly coupled to the characteristics of a particular real or virtual output device and viewing conditions.
Standard output-referred colour encodings are most commonly used for image data intended for open
interchange.
NOTE 1 Examples of ways that output-referred image data may be represented include encoding picture colour values
using a CIE colour space such as CIE XYZ or CIELAB, by a colour encoding derived from CIE colorimetry (e.g. sRGB or
ROMM RGB), or by device-dependent control signals for a particular softcopy or hardcopy output device.
NOTE 2 In some cases, output-referred image data can become the starting point for a subsequent reproduction process.
For example, sRGB standard output-referred image data are frequently considered to be the starting point for many desktop
printing systems. In this case, the printing systems will generally perform a colour re-rendering process to transform the
sRGB colour values to those appropriate for the particular output device and assumed viewing conditions.
4.4 Colour-rendering transforms
A colour-rendering transform is used to transform a scene-referred image to an output-referred image. Colour-
rendering transforms embody the tone and colour reproduction aims of an imaging system, relating the
corresponding scene colorimetry to the desired picture colorimetry.
It should be noted that colour-rendering transforms are usually substantially different from identity transforms
and accomplish several important purposes including compensating for differences in the input and output
viewing conditions, applying tone and gamut mapping to account for the dynamic range and colour gamut of the
output medium, and applying colour adjustments to account for preferences of human observers.
NOTE Colour-rendering transforms are typically proprietary and irreversible.
4.5 Colour re-rendering transforms
For cases where original-referred image data are to be transformed to output-referred image data, a colour
re-rendering transform should generally be used to adjust the image colorimetry when the media and/or
intended viewing condition characteristics are not the same.
EXAMPLE If an original-referred image represents the colorimetry of a photographic transparency intended to be projected
in a dark room, and it is desired to transform the image data to an output-referred colour encoding associated with a
reference reflection print medium and viewing conditions, then the colour re-rendering transform would need to account for
the difference in the intended viewing conditions, as well as the differences in the media dynamic range and colour gamut.
NOTE 1 Colour re-rendering transforms are typically proprietary and can be irreversible depending on the relative dynamic
range and colour gamut of the original-referred and output-referred image data.
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10 ISO 2004 – All rights reserved

Since most real input/outpu
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