SIST ISO 12640-2:2005

INTERNATIONAL ISO
STANDARD 12640-2
First edition
2004-07-15

Graphic technology — Prepress digital
data exchange —
Part 2:
XYZ/sRGB encoded standard colour
image data (XYZ/SCID)
Technologie graphique — Échange de données numériques de
préimpression —
Partie 2: Données d'images en couleur normalisées codées XYZ/sRGB
(XYZ/SCID)




Reference number
ISO 12640-2:2004(E)
©
ISO 2004

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ISO 12640-2:2004(E)
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ISO 12640-2:2004(E)
Contents Page
Foreword. iv
Introduction . v
1 Scope. 1
2 Normative references . 1
3 Terms and definitions. 1
4 Image data. 2
5 Data description and definition . 2
5.1 Data set definition . 2
5.2 Relationship between CIE XYZ and sRGB colour data. 3
5.3 Natural images . 4
5.4 Synthetic images. 7
6 Electronic data . 12
6.1 Image data characteristics. 12
6.2 File structure . 12
Annex A (normative) Standard colour image digital data — Guidance for use. 14
Annex B (normative) Check-sum data . 16
Annex C (informative) Typical TIFF file headers used for image data . 18
Annex D (informative) Text insertion. 22
Annex E (informative) Image evaluation and reproduction. 23
Bibliography . 25

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ISO 12640-2:2004(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.
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 12640-2 was prepared by Technical Committee ISO/TC 130, Graphic technology.
ISO 12640 consists of the following parts, under the general title Graphic technology — Prepress digital data
exchange:
 Part 1: CMYK standard colour image data (CMYK/SCID)
 Part 2: XYZ/sRGB encoded standard colour image data (XYZ/SCID)
Part 1 was published in 1997 with the number ISO 12640 and is in the process of being renumbered.
A Part 3, under the title of CIELAB standard colour image data (CIELAB/SCID), is in preparation.

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ISO 12640-2:2004(E)
Introduction
The technical content of this International Standard was initiated by the Image Processing Technology
Standard Committee in Japan as input to, and in coordination with, ISO/TC 130, WG 2.
0.1 The need for standard XYZ/sRGB digital test images
The existing Standard Colour Image Data (CMYK/SCID, ISO 12640) is defined in terms of CMYK dot
percentages and as such is mainly applicable to printing applications. If attempts are made to apply it to other
systems such as monitors, CMY printers, etc., the following problems arise.
 The image data, being expressed in terms of CMYK dot percentages, have no simple relationship to
colorimetric values.
 The image data have a bit depth of only 8-bits, often causing inaccurate colour conversions.
 The image data are output-referred to a CMYK printing device; additional colour rendering may be
required to create image data suitable for other devices.
In order to solve these problems a set of image data has been prepared that
 is expressed as sRGB encoded ITU-R BT.709-3 RGB primary based tristimulus values, and
 is output-referred to the standard sRGB display and viewing conditions defined in IEC 61966-2-1.
Furthermore, this part of ISO 12640 also provides 16-bit CIE XYZ image data that correspond to the display
produced CIE XYZ tristimulus values for the sRGB image data, with a display white point chromaticity
equivalent to that of CIE Illuminant D .
65
Because they exist as consistent and high quality image data sets, images of this part of ISO 12640 are
expected to be widely used for the following:
 evaluating the colour reproduction capability of imaging systems and output devices;
 evaluating the coding technologies necessary for the storage and transmission of high-definition image
data, etc.
0.2 Characteristics of test images
The performance of any colour reproduction system will normally be evaluated both subjectively (by viewing
the final output image) and objectively (by measurement of control elements). This requirement dictates that
the test images include both natural scenes (pictures) and synthetic images (computer graphics, a business
graph, a colour chart and a colour vignette).
Because the results of subjective image evaluation are strongly affected by the image content, it was
important to ensure that the natural images were of high quality and contained diverse subject matter.
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ISO 12640-2:2004(E)
0.3 Development of digital test images
A survey was conducted of all TC130 member countries to identify desirable image content and to solicit
submission of suitable images for consideration. The image set that resulted consists of eight natural and
seven synthetic images. The natural images include flesh tones, images with detail in the extreme highlights
or shadows, neutral colours, brown and wood tone colours which are often difficult to reproduce, memory
colours, complicated geometric shapes, fine detail, and highlight and shadow vignettes. The synthetic images
selected were generated electronically and include computer graphics, a business graph, a colour chart and a
series of colour vignettes.
All of the images consist of pixel interleaved data with the data origin at the upper left of the image, as viewed
normally, and organized by rows. The file formats of the RGB images are compliant with TIFF 6.0 format.
TIFF 6.0 does not define a method for storing XYZ colourspace. The XYZ images set the TIFF Photometric
tag to 2 (RGB), which allows TIFF readers to open the TIFF file; however, the image will not be displayed
correctly. The images can be imported and manipulated as necessary by a wide variety of commonly used
imaging software packages, on platforms in general use in the industry. See Annex C for details of the TIFF
header.

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INTERNATIONAL STANDARD ISO 12640-2:2004(E)

Graphic technology — Prepress digital data exchange —
Part 2:
XYZ/sRGB encoded standard colour image data (XYZ/SCID)
1 Scope
This part of ISO 12640 specifies a set of 15 standard colour images (encoded as both 16-bit XYZ and 8-bit
RGB digital data provided in electronic data files) that can be used for the evaluation of changes in image
quality during coding, image processing (including transformation compression and decompression),
displaying on a colour monitor or printing. They can be used for many graphic technology applications such as
research, development, product evaluation, and process control.
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.
IEC 61966-2-1:1999, Multimedia systems and equipment — Colour measurement and management —
Part 2-1: Default RGB colour space - sRGB
ITU-R BT.709-3:1998, Parameter values for the HDTV standards for production and international programme
exchange
TIFF, Revision 6.0 Final, Aldus Corporation (now Adobe Systems Incorporated), June 3, 1992
3 Terms and definitions
For the purposes of this document, the following terms and definitions apply.
3.1
check sum
sum of the digits in a file that can be used to check if a file has been transferred properly
NOTE Often, only the least significant bits are summed.
3.2
colour sequence
order in which the colours are stored in a data file
3.3
colour space
geometric representation of colours in space, usually of three dimensions
[CIE Publication 17.4, definition 845-03-25]
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ISO 12640-2:2004(E)
NOTE This part of ISO 12640 is based on two colour spaces, XYZ and sRGB. The relationship between XYZ and
sRGB is given in 5.2.
3.4
global colour change
change to the colours in an image applied consistently to all parts of the image
NOTE This is in contrast to a local colour change where selected spatial areas of an image are changed separately
from the rest of the image area.
3.5
orientation
origin and direction of the first line of data, with respect to the image content, as viewed by the end user
NOTE The codes used to specify orientation are contained in ISO 12639.
3.6
pixel
smallest element of an image that can be uniquely processed, and is defined by its spatial coordinates and
encoded with colour values
NOTE If a pixel is the result of interpolation, then it shall be noted as such.
3.7
pixel colour value
numeric values associated with each of the pixels
3.8
pixel interleaving
colour data organized such that the XYZ or RGB colour values for one pixel of each colour space are followed
by the same sequence of colour values for the next pixel
NOTE 1 The specific order of colours is determined by the ColorSequence (3.2) tag as defined in ISO 12639.
NOTE 2 Other forms of data interleaving are line and plane.
3.9
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, definition 845-03-22]
4 Image data
This part of ISO 12640 consists of colour image data, encoded as 16-bit XYZ data and 8-bit sRGB data, for
8 natural images and 7 synthetic images. The image characteristics of these data are described in Clause 5
and the data structure in Clause 6. The image data itself is contained in thirty data files that are included in
this part of ISO 12640. File names correspond to the image names as described in 5.3 and 5.4.
5 Data description and definition
5.1 Data set definition
Each set of standard colour image data consists of eight natural (photographed) images and seven synthetic
images created digitally on a computer. The natural images are identified as N1 to N8, and each of them also
has a descriptive name derived from the picture content (e.g. woman with glass). The synthetic images,
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ISO 12640-2:2004(E)
identified as S1 to S7, consist of computer graphics images, a business graph, a colour chart and a series of
colour vignettes.
The sRGB images are identified by ISO-sRGB in small type in order to distinguish them from the XYZ images
which have the designation ISO-XYZ. The co-ordinates of the text insertion are provided in Annex D.
5.2 Relationship between CIE XYZ and sRGB colour data
5.2.1 Characteristics of standard output-referred sRGB image data
In order to assure maximum interoperability among a large number of imaging devices, sRGB image data are
output-referred to the hypothetical sRGB display and viewing conditions specified in IEC 61966-2-1. Whatever
colour processing is required to produce the desired image appearance on the sRGB display in the sRGB
viewing conditions shall be performed prior to encoding sRGB image data. The sRGB image data should be
considered to convey the desired sRGB appearance. Subsequent colour rendering may be performed to
produce a somewhat different appearance as necessitated by different media capabilities and viewing
conditions, but in general such colour rendering should not automatically alter the appearance in a substantial
way, or consider that the sRGB image data is “unfinished”. Exceptions to this rule include editing and
manipulation of image data by a user, and the processing of arbitrary RGB image data which may not be
sRGB.
5.2.2 Relationship between XYZ data and sRGB data
The encoding transformations between CIE 1931 XYZ tristimulus values and sRGB digital values are
specified in IEC 61966-2-1, and are provided below. These transformations define how XYZ tristimulus values
shall be calculated from 8-bit sRGB values. The colorimetric values so determined shall be those of the
intended image colorimetry when viewed on the reference display, in the reference viewing conditions, by the
standard observer.
The relationships are defined as follows:
′
R =R 255  (1)
sRGB 8bit
′
G = G 255 (2)
sRGB 8bit
′
B = B 255 (3)
sRGB 8bit
where
R is the code value for sRGB R in 8-bit encoding;
8bit
G is the code value for sRGB G in 8-bit encoding;
8bit
B is the code value for sRGB B in 8-bit encoding;
8bit
R' is the sRGB R image value;
sRGB
G' is the sRGB G image value;
sRGB
B' is the sRGB B image value.
sRGB
2,4
If R' u 0,040 45, then R = R'12,92 ; else R R=+()' 0,055 1,055 .
sRGB sRGB sRGB sRGB
sRGB
2,4
If G' u 0,040 45, then G = G'12,92 ; else G =()G ' + 0,055 1,055 .
sRGB sRGB sRGB sRGB
sRGB
2,4
If B' u 0,040 45, then B = B'12,92 ; else B =()B' + 0,055 1,055 .
sRGB sRGB sRGB sRGB
sRGB
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ISO 12640-2:2004(E)
and
X 0,,,412 4 0 357 6 0 180 5R
  
sRGB

  
Y,= 0 212 6 0,715 2 0,072 2G (4)
sRGB
  
  

Z,0 019 3 0,119 2 0,950 5
B
   sRGB

where R , G and B represent linear sRGB values, and the X, Y and Z values represent those that
sRGB sRGB sRGB
would be measured if the sRGB images were displayed on a hypothetical sRGB display, and the
measurements are conducted in a fashion that eliminates veiling glare and viewing flare.
The CIE Publication 131 XYZ tristimulus values resulting from Equation (4) are relative values scaled from 0,0
to 1,0 (not 0,0 to 100,0 as is sometimes done). Absolute tristimulus values are calculated from the 0,0-to-1,0
scaled relative values by multiplying by 80 (the sRGB display white point luminance).
5.2.3 XYZ image data
The XYZ image data computed in 5.2.2 are converted to 16-bits-per-channel code values (which are
contained in the XYZ image data files) by normalizing them with the corresponding value for the display white
point and multiplying by the data range represented by 16 bits. Thus:
XX=×65 535 (X ) (5)
16bit 65
YY=×65 535 (Y ) (6)
16bit 65
ZZ=×65 535 (Z ) (7)
16bit 65
where
X is the code value for X in 16-bit encoding;
16bit
Y is the code value for Y in 16-bit encoding;
16bit
Z is the code value for Z in 16-bit encoding;
16bit
X, Y and Z are any set of tristimulus values computed in 5.2.2, which are the tristimulus values of
a pixel on the display, excluding internal flare, veiling glare, and viewing flare;
X , Y and Z are the tristimulus values of the display white point.
65 65 65
5.2.4 Image data arrangement
The image data are pixel-interleaved in the order of R then G then B (8-bit), or X then Y then Z (16-bit). The
arrangement of data follows the scanning of each image from the upper left corner to the right, then moving to
the next lower horizontal line.
5.3 Natural images
The characteristics and typical usage for the natural images are provided in Table 1. The descriptive names of
these images are given following the identification code. Figure 1 shows reduced-size reproductions of the
natural images. The natural images have the following characteristics:
 Picture size 4 096 × 3 072 pixels;
NOTE 1 The natural images (4 096 × 3 072 pixels) produce a physical image size of 256 mm by 192 mm when
rendered at 16 pixels/mm.
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ISO 12640-2:2004(E)
 Interleaving pixel interleaving;
 Colour sequence RGB or XYZ;
 Colour values RGB data consisting of three 8-bit values;
XYZ data consisting of three 16-bit values;
 Image data orientation horizontal scanning starting from top left and ending at bottom right.
NOTE 2 Although the original of the image “Field fire” is a painting, it is classified as a natural image.
NOTE 3 The encoding of the data in the headers of the individual files is provided in Annex C, and is in accordance
with the formats specified in ISO 12639.
NOTE 4 The unused light end code values for N1 and N5 limit the tonal range present in these images.
NOTE 5 Clipping at light or dark end of N2, N4 and N8 may impact perceived quality for highly critical observers.
However, such clipping is present in many typical images.
Table 1 — Natural images
ID
Image name Aspect Characteristics
N1 Woman with glass Portrait Close-up image of a woman with a glass; suitable for evaluating the
reproduction of human skin tones
N2 Flowers Landscape Useful for assessing tonal reproduction of highlight tones and contouring
in dark tones
N3 Fishing goods Portrait Low-key image of fishing goods; suitable for evaluating image sharpness
N4 Japanese goods Landscape Image obtained by photographing a collection of Japanese traditional
handicrafts, including many highly saturated colours; suitable for
evaluating colour reproduction capabilities
N5 Field fire Landscape Useful for evaluating the accuracy of colour reproduction for delicate
colours
N6 Pier Landscape Image with complicated geometric shapes; suitable for evaluating the
results of image processing
N7 Threads Landscape Image of woollen yarn, colour pencils and ribbons; suitable for evaluating
the colour gamut of devices
N8 Silver Portrait Image of silverware; suitable for evaluating the tone reproduction of greys,
as well as the reproduction of the lustrous appearance of metallic objects

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ISO 12640-2:2004(E)

N1 — Woman with glass N2 — Flowers

N3 — Fishing goods N4 — Japanese goods

N5 — Field fire N6 — Pier

N7 — Threads N8 — Silver
Figure 1 — Reduced-size reproductions of the natural images
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ISO 12640-2:2004(E)
5.4 Synthetic images
5.4.1 General
The synthetic images consist of computer graphics, a business graph, a colour chart and a series of colour
vignettes. Figure 2 shows reduced-size reproductions of the synthetic images. The interleaving, colour
sequence, colour values and orientation are the same as for the natural images. The image sizes are given in
Table 2.
Table 2 — Synthetic images
Height, Width,
ID Image name Aspect
pixels pixels
S1 Teapot Landscape 360 480
S2 Japanese doll Landscape 1 536 2 048
S3 Cat Landscape 1 536 2 048
S4 Sports Portrait 2 048 1 536
S5 Business graph Landscape 1 536 2 048
S6 Colour chart Landscape 1 332 2 736
S7 Colour vignettes Landscape 2 608 4 256
Images S1 to S5 were originally defined as 8-bit sRGB data by the way that they were produced. The 16-bit
XYZ data representing them were prepared by applying the sRGB-to-XYZ transform described in 5.2.2 to 8-bit
sRGB data. Images S6 and S7 were defined by first determining equal intervals in CIE L*. For each set of
intervals selected for the various parts of the images, the appropriate L* values (with a* and b* both set equal
to 0) were converted to 16-bit XYZ data, and then to 8-bit sRGB using appropriate normalization and the
XYZ-to-sRGB transform specified in IEC 61966-2-1.
NOTE Images S1 and S2 contain some clipped highlight areas which will be reproduced without detail.
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ISO 12640-2:2004(E)

S1 — Teapot S2 — Japanese doll

S3 — Cat S4 — Sports

S5 — Business graph S6 — Colour chart


S7 — Colour vignettes
Figure 2 — Reduced-size reproductions of the synthetic images
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ISO 12640-2:2004(E)
5.4.2 Computer graphics
Computer graphics are artificially generated pictures. Thus they are characterized by the fact that, in contrast
to natural images, they include large, noise-free, constant-hue areas, and gradation areas in which the hue
and/or lightness changes very smoothly.
S1 and S2 are both three-dimensional computer graphics pictures with shadows, while S3 and S4 are both
two-dimensional computer graphics pictures without shadows.
The four computer graphics pictures have been produced for monitor display, and have a bit depth resolution
of 8 bits for each of the sRGB channels.
 S1, Teapot:
Image S1 is a three-dimensional computer graphic obtained by mapping a watercolour painting onto a
widely used three-dimensional data set named UTAH Teapot. In general, a curved surface is
approximated with polygonal surfaces. However, by the use of a ray tracing method based on Bezier
clipping, the objects are displayed highly accurately. Further, the delicate tones characterizing
watercolours are expressed in terms of the Bezier function and are deemed to have been reproduced
sufficiently well.
 S2, Japanese doll:
Image S2 was produced by mapping the data of an actual photograph of a three-dimensional object onto
a three-dimensional computer graphic. A computer graphics artist two-dimensionally arranged various
data sets, including a primitive, standard three-dimensional data, two-dimensional data, a two-
dimensional computer graphics sketch, etc.
The design intent is to visualize a scene of the new multi-media world by placing side-by-side the mask of
Fukusuke and modern goods such as a personal computer, compact disc, etc. The former represents one
of the traditional characters in Japan, while the latter represent digital appliances.
 S3, Cat:
Image S3 is an example of two-dimensional computer graphics, and has been produced from a digital
image obtained by sampling a photograph. The image data were subjected to domain separation; each
domain was modified, taking into account the characteristics of the human eye and the hue, as well as
the shape of the domain, according to the preference of the computer graphics artist. The image is
characterized by including a wide gamut of colour information.
 S4, Sports:
This image was produced by first sampling an original photograph of a skier, and then subjecting the
digital data to hue and shape processing according to the preference of the computer graphics artist.
The design intent of this image is to express vivacity (sprightliness) by fusing sporting animal spirits with
the fine arts. The image characteristically contains painting-like elements having a broad range of colour
information and well-balanced hues.
5.4.3 Business graph
Image S5 is a business-graph consisting of a bar chart, pie charts, gradation bars and letters. The bar chart
includes seven bars against a neutral background, and each bar consists of five segments, i.e., a primary
colour segment, thinner, middle- and highlight-tone segments of the same colour, a darkened primary colour
segment and a darkened middle-tone colour segment. One of the pie charts is coloured with seven primary
colours, and the other with seven less saturated primary colours, both against a white background. The
business-graph contains two gradation bars, one starting from yellow and ending with orange, and the other
changing from grass green to deep blue.
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ISO 12640-2:2004(E)
5.4.4 Colour chart
5.4.4.1 Layout
Image S6 is a colour chart that consists of colour patches that are all specified to be within the colour space
defined by the ITU-R BT.709-3 RGB primaries, without the use of negative tristimulus values. By using these
patches, the fidelity of colour reproduction of an image output device to the colorimetry of the original image
file may be evaluated objectively by measurement. Image S6 (XYZ) is encoded in 16-bit XYZ, and
image S6 (sRGB) is encoded in 8-bit sRGB. The chart has two sections:
3
 section containing 6 (i.e. 216) tertiary colour patches;
 primary, secondary and grey (tertiary) colour section (77 patches in total).
NOTE The sRGB and XYZ image data encoded can be converted to display-produced image colorimetry using the
transforms specified in 5.2.2 and 5.2.3. However, display-produced colorimetric values do not include any internal flare,
veiling glare, or viewing flare, and therefore do not exactly represent the colorimetry which would be observed if a
hypothetical sRGB display were viewed. When comparing the fidelity of a colour reproduction to that of an original, it is
generally more appropriate to compare viewer-observed colorimetric values.
The fidelity of measured-reproduction colorimetry to original-image colorimetry (display produced or viewer observed)
should not generally be considered as indicative of the quality of the reproduction. To produce optimal quality, it is
frequently necessary to adjust the colorimetry of a reproduction to be different from that directly associated with the image
data in order to account for differences between the sRGB-viewing conditions and the reproduc
...