ISO/TR 17321-2:2012

TECHNICAL ISO/TR
REPORT 17321-2
First edition
2012-10-15
Graphic technology and
photography — Colour
characterization of digital still
cameras (DSCs) —
Part 2:
Considerations for determining scene
analysis transforms
Technologie graphique et photographie — Caractérisation de la
couleur des appareils photonumériques —
Partie 2: Considérations pour déterminer les transformations
d’analyse de scène
Reference number
©
ISO 2012
© ISO 2012
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ii © ISO 2012 – All rights reserved

Contents Page
Foreword .iv
Introduction .v
1 Scope . 1
2 Definitions . 1
3 Goals . 4
4 Fundamental colour-related DSC characteristics . 5
4.1 Camera gain . 5
4.2 Camera dark current . 5
4.3 Focal plane opto-electronic conversion function (FP OECF) . 6
4.4 Camera opto-electronic conversion function (Camera OECF) . 6
4.5 Camera flare average percent. 6
4.6 Camera spectral sensitivities (including non-removable optical elements) . 6
4.7 Removable optical element spectral transmittances . 6
5 Scene analysis transform parameters . 7
5.1 Determined using spectral measurements . 7
5.2 Determined using test targets . 8
6 Scene analysis transform determination using spectral measurements .9
7 Scene analysis transform determination using test targets .10
7.1 Procedure .10
7.2 Test target characteristics .12
7.3 Capturing images of test targets .12
8 Applying the scene analysis transform and encoding .13
9 Considerations for selecting scene analysis transforms .14
10 Suggested metadata for scene-referred colour encodings .14
10.1 Camera characteristics .14
10.2 Colour encoding characteristics .14
10.3 Image specific characteristics .14
Annex A (informative) Example calculation results .15
Bibliography .17
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 exceptional circumstances, when a technical committee has collected data of a different kind from
that which is normally published as an International Standard (“state of the art”, for example), it may
decide by a simple majority vote of its participating members to publish a Technical Report. A Technical
Report is entirely informative in nature and does not have to be reviewed until the data it provides are
considered to be no longer valid or useful.
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/TR 17321-2 was prepared by Technical Committee ISO/TC 42, Photography.
ISO 17321 consists of the following parts, under the general title Graphic technology and photography —
Colour characterization of digital still cameras (DSCs):
— Part 1: Stimuli, metrology and test procedures
— Part 2: Considerations for determining scene analysis transforms
iv © ISO 2012 – All rights reserved

Introduction
Digital still cameras (DSCs) have become the predominant means of photographic image capture, but
the nature of the image data produced by different cameras, or even by the same camera operating in
different modes, is quite variable. This variability can cause problems in workflows, miscommunications
and interoperability issues.
This Technical Report provides information about methods for determining scene analysis transforms,
which are transforms that convert raw image data to scene-referred image data. This information is
provided in the form of a Technical Report because there are a number of choices to be made when
determining scene analysis transforms. These choices are influenced by the subject matter being
photographed (including the scene illumination), the scene adopted white and the adopted white of
the scene-referred colour encoding to be used, aesthetic choices regarding scene analysis colour error
minimization, and other considerations. It is not possible to provide more specific recommendations
because the spectral responses of DSC colour analysis channels do not, in general, match those of a
typical human observer, such as defined by a CIE standard colourimetric observer. Nor do the responses
of different DSCs ordinarily match each other. This Technical Report outlines considerations relevant to
the determination of scene analysis transforms based on the minimization of errors in specified colour
spaces. The DSC characterization data obtained using ISO 17321-1 serve as the raw DSC image data.
Good understanding of this Technical Report requires that the three fundamental modes of DSC
operation be distinguished: the raw mode, the scene-referred mode, and the output-referred mode. When
operating in the raw mode, a DSC records image data that is most closely related to the sensor response.
Some types of processing may have been performed, such as dark current subtraction, defect removal
and colour filter array interpolation, but neither a scene analysis nor a colour rendering transform
has been applied. Any encoding transform typically consists of only a non-linearity to better align the
quantization intervals with the image noise characteristics, and possibly some form of compression.
When operating in the scene-referred mode, a DSC records image data that represents an estimate of
the scene or focal plane image relative colourimetry, typically with white balancing to the encoding
adopted white. The image data has not undergone colour rendering for some anticipated output medium
and viewing conditions. In order to produce output-referred images intended for reproduction, it is
necessary to either apply a colour rendering transform directly to the scene-referred images, or convert
them to a working colour space where the desired colour rendering is applied. Camera controls or raw
processing software can offer some aesthetic choices when converting to scene-referred, but the results
of such choices need to be viewed through the intended colour rendering transform in order to see their
effect on the final output. The image data are encoded prior to applying the colour rendering transform,
and are therefore not an encoding of the intended output colourimetry. At present, few DSCs offer an
in-camera scene-referred mode, although some camera raw processing applications have this capability.
When operating in the output-referred mode, the DSC controls are set to achieve the desired output
directly, thereby incorporating the colour rendering, and in many cases a reference output device
encoding (such as for a CRT monitor) in the image file. When operating in this mode the DSC encodes the
colourimetry of the intended output on the reference medium, not scene-referred colourimetry. Also,
the output-referred colourimetry can be in different encodings, with different reference media, and in
some cases will need to be colour re-rendered and/or re-encoded to produce different reproductions.
The information provided in this Technical Report is intended to help camera and raw processing
software manufacturers, professional photographers and colour measurement applications to determine,
communicate about, and select DSC scene analysis transforms. However, it will often not be practical for
end users to determine scene analysis transforms themselves. In addition to the requirement for raw
DSC image data, relatively sophisticated and expensive measurement equipment is required to obtain
chart patch spectral reflectance or radiance, illumination source spectral power, and DSC spectral
sensitivity (as described in ISO 17321-1).
device
Exposing Light
device
dependent
independent
Image
Image
representation in
representation in
A/D Converter Transform
raw digital camera
Sensor scene-referred
response
colour encoding
coordinates
device
device
Colour rendering
dependent
independent
Image Image
representation in representation in
Transform
output-referred printer device
colour encoding coordinates
Figure 1 — Generic image workflow for digital photography
vi © ISO 2012 – All rights reserved

TECHNICAL REPORT ISO/TR 17321-2:2012(E)
Graphic technology and photography — Colour
characterization of digital still cameras (DSCs) —
Part 2:
Considerations for determining scene analysis transforms
1 Scope
This Technical Report provides information about methods for determining scene analysis transforms
based on the minimization of errors in estimated scene or focal-plane colourimetry, including
corresponding colourimetry. These transforms are limited in applicability to raw DSC image data.
This Technical Report concerns only the creation and encoding of scene-referred and focal-plane-referred
image data. It does not address the encoding of output-referred image data. It also does not provide
information relating to the specification of metadata items describing intended artistic adjustments,
colour rendering and viewing.
This Technical Report does not address how to choose adopted whit
...