SIST-TS ISO/TS 18621-11:2021
(Main)Image quality evaluation methods for printed matter - Part 11: Colour gamut analysis
Image quality evaluation methods for printed matter - Part 11: Colour gamut analysis
This document defines procedures to measure and compare the colour gamuts of RGB and CMYK printing processes.
It is not applicable to other printing processes.
Titre manque — Partie 11: Titre manque
Metode ocenjevanja kakovosti slike za tiskovine - 11. del: Analiza barvne lestvice
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SLOVENSKI STANDARD
SIST-TS ISO/TS 18621-11:2021
01-julij-2021
Metode ocenjevanja kakovosti slike za tiskovine - 11. del: Analiza barvne lestvice
Image quality evaluation methods for printed matter - Part 11: Colour gamut analysis
Ta slovenski standard je istoveten z: ISO/TS 18621-11:2019
ICS:
37.100.10 Reprodukcijska oprema Reproduction equipment
SIST-TS ISO/TS 18621-11:2021 en
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.
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SIST-TS ISO/TS 18621-11:2021
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SIST-TS ISO/TS 18621-11:2021
TECHNICAL ISO/TS
SPECIFICATION 18621-11
First edition
2019-12
Image quality evaluation methods for
printed matter —
Part 11:
Colour gamut analysis
Reference number
ISO/TS 18621-11:2019(E)
©
ISO 2019
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SIST-TS ISO/TS 18621-11:2021
ISO/TS 18621-11:2019(E)
COPYRIGHT PROTECTED DOCUMENT
© ISO 2019
All rights reserved. Unless otherwise specified, or required in the context of its implementation, no part of this publication may
be reproduced or utilized otherwise in any form or by any means, electronic or mechanical, including photocopying, or posting
on the internet or an intranet, without prior written permission. Permission can be requested from either ISO at the address
below or ISO’s member body in the country of the requester.
ISO copyright office
CP 401 • Ch. de Blandonnet 8
CH-1214 Vernier, Geneva
Phone: +41 22 749 01 11
Fax: +41 22 749 09 47
Email: copyright@iso.org
Website: www.iso.org
Published in Switzerland
ii © ISO 2019 – All rights reserved
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SIST-TS ISO/TS 18621-11:2021
ISO/TS 18621-11:2019(E)
Contents Page
Foreword .iv
Introduction .v
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Describing a colour gamut . 2
4.1 General . 2
4.2 Requirements of a gamut boundary description . 2
4.3 Device gamut and usable gamut . 3
4.4 Procedures for describing a colour gamut . 3
4.4.1 General. 3
4.4.2 Procedure for describing the colour gamut of a reproduction system
based on its ICC profile . 3
4.4.3 Procedure for describing the device gamut of a reproduction system
based on its characterization model . 5
4.4.4 Procedure for describing the device gamut of a reproduction system
based on measurement of a printed gamut target . 5
4.4.5 Procedure for describing the device gamut of a reproduction system
based on characterization data . 5
5 Computing the volume of a colour reproduction gamut . 5
5.1 General . 5
5.2 Volume of a single gamut . 5
5.2.1 Volume calculation . 5
5.2.2 Verifying the volume calculation. 6
5.3 Volume of the intersection of two gamuts . 7
5.3.1 General. 7
5.3.2 Determining if a coordinate is inside or outside a gamut . 7
6 Comparing colour gamuts . 8
6.1 General . 8
6.2 GCI . 8
6.3 Gamut coverage . 8
6.4 Out-of-gamut . 8
7 Encoding and communicating a colour gamut description . 8
Annex A (informative) Images for use in determining the gamut boundary of RGB and
CMYK printing processes .10
Annex B (informative) Gamut volumes for a set of reference profiles .11
Annex C (informative) Errors in triangulation .12
Annex D (normative) Media-relative colour gamuts .14
Bibliography .15
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ISO/TS 18621-11:2019(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.
The procedures used to develop this document and those intended for its further maintenance are
described in the ISO/IEC Directives, Part 1. In particular, the different approval criteria needed for the
different types of ISO documents should be noted. This document was drafted in accordance with the
editorial rules of the ISO/IEC Directives, Part 2 (see www .iso .org/ directives).
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. Details of
any patent rights identified during the development of the document will be in the Introduction and/or
on the ISO list of patent declarations received (see www .iso .org/ patents).
Any trade name used in this document is information given for the convenience of users and does not
constitute an endorsement.
For an explanation of the voluntary nature of standards, the meaning of ISO specific terms and
expressions related to conformity assessment, as well as information about ISO's adherence to the
World Trade Organization (WTO) principles in the Technical Barriers to Trade (TBT) see www .iso .org/
iso/ foreword .html.
This document was prepared by Technical Committee ISO/TC 130, Graphic technology.
Any feedback or questions on this document should be directed to the user’s national standards body. A
complete listing of these bodies can be found at www .iso .org/ members .html.
iv © ISO 2019 – All rights reserved
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SIST-TS ISO/TS 18621-11:2021
ISO/TS 18621-11:2019(E)
Introduction
The colour gamut that can be achieved by a reproduction system is an important attribute. It enables
users to compare the colour reproduction capabilities of different printing systems and to determine
whether one system can simulate all the colours available in another. This document describes
procedures to define and compare colour gamuts.
Given a set of coordinates known to lie on the surface of a colour gamut, the volume of the gamut can
be determined by segmenting the gamut into a series of tetrahedra, computing the volume of each
tetrahedron and summing the results. For a reproduction process with three colour components, a
colour will lie on the surface if it satisfies the condition that at least one component has a value of 0
or 1, where 1 represents the maximum amount of the colour component. However, printing processes
usually have four or more colour components (e.g. Cyan, Magenta, Yellow and Black in four-colour
process printing), and determining which coordinates lie on the gamut boundary cannot be done solely
from the relative amounts of the colour components. For CMYK processes, in almost all cases, the Black
colorant extends the gamut below the gamut vertex at each hue angle. This makes it possible to identify
a set of coordinates which are expected to lie on the gamut surface from the relative colorant amounts.
For processes with more than four colour components, some knowledge of the colorimetry of a sample
of colours from the colour data encoding is needed in order to determine which colours lie on the
boundary.
For these reasons, coordinates on the surface of the gamut of RGB and CMYK printing processes can be
determined by printing a test chart with suitable colorant combinations, and measuring the colours;
while for other printing processes, it is necessary to model the colorant-to-colorimetry relationship in
order to identify colours on the gamut boundary.
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SIST-TS ISO/TS 18621-11:2021
TECHNICAL SPECIFICATION ISO/TS 18621-11:2019(E)
Image quality evaluation methods for printed matter —
Part 11:
Colour gamut analysis
1 Scope
This document defines procedures to measure and compare the colour gamuts of RGB and CMYK
printing processes.
It is not applicable to other printing processes.
2 Normative references
The following documents are referred to in the text in such a way that some or all of their content
constitutes requirements 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, Image technology colour management — Architecture, profile format and data structure —
Part 1: Based on ICC.1:2010
ISO 12642-1, Graphic technology — Input data for characterization of four-colour process printing —
Part 1: Initial data set
ISO 13655, Graphic technology — Spectral measurement and colorimetric computation for graphic
arts images
3 Terms and definitions
For the purposes of this document, the following terms and definitions apply.
ISO and IEC maintain terminological databases for use in standardization at the following addresses:
— ISO Online browsing platform: available at https:// www .iso .org/ obp
— IEC Electropedia: available at http:// www .electropedia .org/
3.1
colour gamut
range of colours that can be reproduced by an output device on a given medium, represented in a CIE-
based colour space
Note 1 to entry: The CIE colour space for representation of colour gamuts is normally CIELAB.
3.2
gamut vertex
coordinate in a CIE-based colour space which represents a point on a colour gamut surface and which is
used in defining the surface of the gamut
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3.3
gamut face
planar sub-division of the colour gamut surface formed by three or more coplanar gamut face edges
Note 1 to entry: The colour gamut of most output devices can be described in terms of a set of gamut faces that
completely enclose all the colours that can be reproduced by the device, with no gaps or overlaps.
Note 2 to entry: In this document, gamut faces are defined as having three gamut vertices.
3.4
gamut face edge
line connecting two adjacent vertices of a gamut face
Note 1 to entry: In a continuous gamut surface, each gamut face edge is shared by two gamut faces.
3.5
characterization model
mathematical model that converts between coordinates in a device colour encoding and a CIE-based
colour space
3.6
device gamut
range of colours that corresponds to all possible combinations of colour channels of the device within
the device data encoding, when printed on a substrate
Note 1 to entry: Ink space is an alternative term for device gamut.
3.7
usable gamut
subset of the device gamut that corresponds to the set of combinations of colour channels of the device
in practical use, when reproduced on an output medium
Note 1 to entry: The usable gamut of an output device is normally smaller than the device gamut owing to
practical limitations in the combinations of colour channels. Most CMYK devices cannot produce a print in which
all channels are set to the maximum. The usable gamut is applicable when the gamut to be determined is that of
the system when used as part of a reproduction workflow, using an ICC profile to convert to output channels;
while the device gamut is applicable when the gamut to be determined is that of the reproduction device
independently of the profile and its colour separation method.
Note 2 to entry: In practice, some printers do not allow all possible combinations of ink to be printed, and an ink-
limiting procedure is applied automatically in the printer. Where this is done, this "ink-limited" mode of printing
still should be considered to be the "device gamut".
4 Describing a colour gamut
4.1 General
The colour gamut of a reproduction system is a volume in 3D colour space. It shall be mathematically
described as a closed set of triangular faces on the surface of the gamut which completely encloses the
gamut volume.
4.2 Requirements of a gamut boundary description
Each face shall be defined by three colorimetric coordinates, and the set of faces shall be defined in
such a way that it encloses the volume of the gamut without gaps or overlaps. The surface shall be
encoded as an nx3 array of vertices (in which there are n vertices and each row represents the colour
space coordinates of a gamut vertex) and an mx3 face array of indices into the vertices array (where
there are m faces and each row of the array identifies the three row numbers in the vertex array which
correspond to a gamut face). Each gamut vertex shall be described as a CIELAB L*, a*, b* value computed
from spectral reflectance or tristimulus values according to ISO 13655, and when this is done it shall
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be stated which ISO 13655 measurement mode applies to the data. If the colour space used to describe
the gamut vertices is not CIELAB computed according to ISO 13655, details of the colour space used
(including the CIE colorimetric observer and illuminant) shall be reported as metadata associated with
the gamut description. Where it is desired that the gamut description or comparison is media-relative,
CIELAB L*, a*, b* coordinates shall be scaled as described in Annex D.
In order to satisfy the requirement to enclose the gamut volume without gaps or overlaps, the following
conditions shall be met.
i) The three indices identifying each face shall identify vertices in clockwise order, when viewed from
the exterior of the volume.
ii) Each gamut face edge shall be common to two gamut faces.
CIELAB L*, a*, b* computed according to ISO 13655 is recommended where it is important to be able
to compare colour gamuts or where the gamut is derived from an ICC profile. It is acknowledged that
CIELAB is not perceptually uniform, and that this affects the gamut size. Determining a set of faces that
meet these conditions from an arbitrary set of vertices is non-trivial. For this reason, this document
provides a set of well-spaced coordinates in device space, and an associated triangulation. Full details
of these data are given in Annex A.
4.3 Device gamut and usable gamut
The device gamut can be determined either from the characterization model (usually represented by
an ICC profile) or by direct measurement of colours that lie on the gamut boundary. To compute the
usable gamut, an additional step is required in which the device coordinates are restricted to those
available in the reproduction workflow. If an ICC profile is used to define the usable gamut, CIELAB
gamut surface coordinates in the device gamut can be transformed to device coordinates and then back
to CIELAB coordinates to obtain the usable gamut.
NOTE In some cases, the device or its driver can limit the range of colorant combinations, regardless of
whether an ICC profile is used.
The procedure in 4.4 is recommended for determining the gamut boundary vertex and face arrays. If
a different procedure is used, it shall be stated when communicating the gamut boundary description
which procedure was used to determine the gamut vertices, and whether the device gamut or usable
gamut is described.
4.4 Procedures for describing a colour gamut
4.4.1 General
One of the following procedures shall be used to describe the colour gamut of a reproduction system.
NOTE 1 In most cases, results obtained from these procedures, using the set of well-spaced coordinates in
[1]
device space described in this document, give very similar results . Certain factors affect the reproducibility of
the gamut description, such as when the black in a toner-based printer results in a lower L* value than any of the
other colorant combinations.
NOTE 2 An ICC profile is a convenient means of converting data between the device data encoding and
the corresponding colorimetry, and it defines the colour gamut available in a workflow based on ICC profile
conversions. Other methods of obtaining colorimetric values for coordinates on the gamut surface, such as direct
measurement or a characterization model, is also used.
4.4.2 Procedure for describing the colour gamut of a reproduction system based on its ICC profile
The following procedure can be used to compute the faces and vertices of a gamut boundary description
from an ICC profile for the reproduction system. The procedure is applicable to RGB and CMYK devices.
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Where used, the ICC profile shall be created according to ISO 15076-1, from characterization data
representing the printing process whose gamut is to be described. This method estimates the gamut of
the device represented by the profile, and depending on the accuracy of the AToB1 tag and the BToA1
tag of the profile this estimate may or may not itself be accurate. The accuracy of AToB1 and BToA1 tags
shall be reported.
To maintain accuracy, the precision of data used for both device coordinates and CIELAB coordinates
shall be 16 bits or greater.
1) Generate an image whose pixels represent a set of device coordinates on the gamut boundary of the
encoding. The image should be arranged so that the ratio of the relative colorant amounts varies in
the horizontal direction, and the total colorant amount varies in the vertical direction. The white
point and black point are repeated across the first and last rows in the coordinate array. Annex A
gives details of images for this purpose for RGB and CMYK reproduction systems.
2) Convert the image in step 1) to CIELAB using an ICC profile for the reproduction medium, selecting
the ICC-Absolute Colorimetric rendering intent.
The values calculated following step 2) are the gamut vertices of the device gamut for RGB and
CMYK systems. These are also the usable gamut of an RGB reproduction system.
3) To obtain the usable gamut of a reproduction system, convert the CIELAB coordinates back to
device coordinates and then back to PCS CIELAB coordinates, in both cases using the ICC-Absolute
Colorimetric rendering intent. This step is necessary to ensure that only colorant values that are
permitted by the colour separation model are represented in the gamut description.
4) The CIELAB coordinates for each patch from step 3) are read row-wise and arranged as an m × n × 3
array to form the vertex array where m is the number of columns in the test image and n is the
number of rows.
5) To construct the face array for this data, start with the upper left device coordinate and move
clockwise to the two coordinates in the next row, as shown in Figure 1. The first row of the faces
list is therefore [1, m+2, m+1]. The next row in the faces list is [1, 2, m+2]. Continue to move through
the device coordinates until the face list is fully populated with one row per face.
Figure 1 — Triangulation of gamut target image
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4.4.3 Procedure for describing the device gamut of a reproduction system based on its
characterization model
The following procedure can be used to compute the faces and vertices of a gamut boundary description
of a reproduction system using its characterization model.
1) From the device data in the test chart described in step 1) of 4.4.2, compute CIELAB values for each
colour patch using the characterization model.
2) Follow steps 4) to 5) from 4.4.2 to obtain the face and vertex list.
4.4.4 Procedure for describing the device gamut of a reproduction system based on
measurement of a printed gamut target
The following procedure can be used to derive the faces and vertices of a gamut boundary description
of a reproduction system using computations based on direct measurement of printed specimens.
1) Print the test chart described in 4.4.2 1) on the printer without colour management and measure
the printed patches.
NOTE In order to determine the printer gamut independently on any ICC profile, colour management is
not applied.
2) Follow steps 4) to 5) from 4.4.2 to obtain the face and vertex list.
4.4.5 Procedure for describing the device gamut of a reproduction system based on
characterization data
The following procedure can be used to compute the faces and vertices of a gamut boundary description
of a reproduction system from characterization data.
1) Select the characterization data set which represents the device, using a test chart such as that
described in ISO 12642-1.
2) Use the alpha shapes method [2, 3, 4] to determine the set of faces which connect the coordinates in
step 2).
An alpha shapes radius of 40 is recommended. Since the radius depends on sampling size and
distribution, other values may be optimal for a given data set. Alpha shapes can generate an error
depending on the chosen radius.
NOTE The face list returned by the alpha shapes method will be different from that obtained by the
procedure in 4.4.3 and 4.4.4, but the volume calculated from these data according to Clause 5. below has been
found to be in good agreement. See Reference [1] for more details.
5 Computing the volume of a colour reproduction gamut
5.1 General
The volume of colour reproduction gamuts shall be defined as follows.
5.2 Volume of a single gamut
5.2.1 Volume calculation
The gamut volume shall be calculated as shown below.
1) Define a point at the approximate centre of the gamut volume, whose CIELAB coordinates are the
average of the coordinates of the white and black point of the gamut.
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2) Add this point to the set of points defining each gamut face to form a set of tetrahedra spanning the
gamut volume.
3) For each tetrahedron in turn, let the four vertices be p , p , and p (corresponding to the planar
1 2 3
face on the gamut boundary) and p (the approximate gamut centroid). If each vertex p , etc is
4 1
represented by its CIELAB L*, a* and b* coordinates, the (signed) volume of each tetrahedron can
be computed using the scalar triple product shown in Formula (1):
ab⋅×c
()
V = (1)
6
where
a = p – p ;
1 4
b = p – p ;
2 4
c = p – p .
3 4
The ‘·’ and ‘ × ’ symbols denote dot product and cross product respectively.
4) Sum the volumes of all the tetrahedra to obtain the total gamut volume.
NOTE If the scalar triple product is negative for a given tetrahedron, the winding order of the planar face is
incorrect. (See Annex C for more details.) By retaining signed values in Formula (1), a more accurate estimate of
the total volume is obtained than if the absolute volumes of each tetrahedron are summed.
The volume shall be reported as the number of "cubic CIELAB units", i.e. the number of hexahedra with
sides of 1,0 in the dimensions of CIELAB L*, a* and b*.
5.2.2 Verifying the volume calculation
If the conditions in 4.2 are met, the procedure in 5.2.1 will result in an accurate estimate of the gamut
volume. An additional check can be performed by computing the solid angle of each tetrahedron and
summing them. If the faces of the gamut surface correctly enclose the volume, the resulting value will
be equal to the solid angle of a sphere, i.e. 4π steradians.
NOTE A method of calculating the solid angle of a tetrahedron is given in Reference [5].
For guidance, the gamut volumes calculated from a set of reference profiles is provided in Annex B. This
can be used to verify the correctness of the procedure used.
In practice it can be difficult to satisfy the conditions in 4.2 for every face in a gamut. It is recommended
that the total solid angle is reported with the estimated volume. The volume associated with the number
of incorrectly-oriented faces should also be calculated and reported.
When the target image method is used to calculate the gamut surface the gamut volume shall be
reported as:
Gamut volume = 123456 (32)
where the figure in parentheses indicates the maximum error in the calculation. If this value is greater
than 1 % of the total volume, an alternative method for calculation of the gamut surface should be used.
See Annex C for details.
Differences in the CMM used to perform the profile transforms describe above, especially in the LUT
interpolation methods used, may give rise to small differences in volume estimates. Differ
...
TECHNICAL ISO/TS
SPECIFICATION 18621-11
First edition
2019-12
Image quality evaluation methods for
printed matter —
Part 11:
Colour gamut analysis
Reference number
ISO/TS 18621-11:2019(E)
©
ISO 2019
---------------------- Page: 1 ----------------------
ISO/TS 18621-11:2019(E)
COPYRIGHT PROTECTED DOCUMENT
© ISO 2019
All rights reserved. Unless otherwise specified, or required in the context of its implementation, no part of this publication may
be reproduced or utilized otherwise in any form or by any means, electronic or mechanical, including photocopying, or posting
on the internet or an intranet, without prior written permission. Permission can be requested from either ISO at the address
below or ISO’s member body in the country of the requester.
ISO copyright office
CP 401 • Ch. de Blandonnet 8
CH-1214 Vernier, Geneva
Phone: +41 22 749 01 11
Fax: +41 22 749 09 47
Email: copyright@iso.org
Website: www.iso.org
Published in Switzerland
ii © ISO 2019 – All rights reserved
---------------------- Page: 2 ----------------------
ISO/TS 18621-11:2019(E)
Contents Page
Foreword .iv
Introduction .v
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Describing a colour gamut . 2
4.1 General . 2
4.2 Requirements of a gamut boundary description . 2
4.3 Device gamut and usable gamut . 3
4.4 Procedures for describing a colour gamut . 3
4.4.1 General. 3
4.4.2 Procedure for describing the colour gamut of a reproduction system
based on its ICC profile . 3
4.4.3 Procedure for describing the device gamut of a reproduction system
based on its characterization model . 5
4.4.4 Procedure for describing the device gamut of a reproduction system
based on measurement of a printed gamut target . 5
4.4.5 Procedure for describing the device gamut of a reproduction system
based on characterization data . 5
5 Computing the volume of a colour reproduction gamut . 5
5.1 General . 5
5.2 Volume of a single gamut . 5
5.2.1 Volume calculation . 5
5.2.2 Verifying the volume calculation. 6
5.3 Volume of the intersection of two gamuts . 7
5.3.1 General. 7
5.3.2 Determining if a coordinate is inside or outside a gamut . 7
6 Comparing colour gamuts . 8
6.1 General . 8
6.2 GCI . 8
6.3 Gamut coverage . 8
6.4 Out-of-gamut . 8
7 Encoding and communicating a colour gamut description . 8
Annex A (informative) Images for use in determining the gamut boundary of RGB and
CMYK printing processes .10
Annex B (informative) Gamut volumes for a set of reference profiles .11
Annex C (informative) Errors in triangulation .12
Annex D (normative) Media-relative colour gamuts .14
Bibliography .15
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ISO/TS 18621-11:2019(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.
The procedures used to develop this document and those intended for its further maintenance are
described in the ISO/IEC Directives, Part 1. In particular, the different approval criteria needed for the
different types of ISO documents should be noted. This document was drafted in accordance with the
editorial rules of the ISO/IEC Directives, Part 2 (see www .iso .org/ directives).
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. Details of
any patent rights identified during the development of the document will be in the Introduction and/or
on the ISO list of patent declarations received (see www .iso .org/ patents).
Any trade name used in this document is information given for the convenience of users and does not
constitute an endorsement.
For an explanation of the voluntary nature of standards, the meaning of ISO specific terms and
expressions related to conformity assessment, as well as information about ISO's adherence to the
World Trade Organization (WTO) principles in the Technical Barriers to Trade (TBT) see www .iso .org/
iso/ foreword .html.
This document was prepared by Technical Committee ISO/TC 130, Graphic technology.
Any feedback or questions on this document should be directed to the user’s national standards body. A
complete listing of these bodies can be found at www .iso .org/ members .html.
iv © ISO 2019 – All rights reserved
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ISO/TS 18621-11:2019(E)
Introduction
The colour gamut that can be achieved by a reproduction system is an important attribute. It enables
users to compare the colour reproduction capabilities of different printing systems and to determine
whether one system can simulate all the colours available in another. This document describes
procedures to define and compare colour gamuts.
Given a set of coordinates known to lie on the surface of a colour gamut, the volume of the gamut can
be determined by segmenting the gamut into a series of tetrahedra, computing the volume of each
tetrahedron and summing the results. For a reproduction process with three colour components, a
colour will lie on the surface if it satisfies the condition that at least one component has a value of 0
or 1, where 1 represents the maximum amount of the colour component. However, printing processes
usually have four or more colour components (e.g. Cyan, Magenta, Yellow and Black in four-colour
process printing), and determining which coordinates lie on the gamut boundary cannot be done solely
from the relative amounts of the colour components. For CMYK processes, in almost all cases, the Black
colorant extends the gamut below the gamut vertex at each hue angle. This makes it possible to identify
a set of coordinates which are expected to lie on the gamut surface from the relative colorant amounts.
For processes with more than four colour components, some knowledge of the colorimetry of a sample
of colours from the colour data encoding is needed in order to determine which colours lie on the
boundary.
For these reasons, coordinates on the surface of the gamut of RGB and CMYK printing processes can be
determined by printing a test chart with suitable colorant combinations, and measuring the colours;
while for other printing processes, it is necessary to model the colorant-to-colorimetry relationship in
order to identify colours on the gamut boundary.
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TECHNICAL SPECIFICATION ISO/TS 18621-11:2019(E)
Image quality evaluation methods for printed matter —
Part 11:
Colour gamut analysis
1 Scope
This document defines procedures to measure and compare the colour gamuts of RGB and CMYK
printing processes.
It is not applicable to other printing processes.
2 Normative references
The following documents are referred to in the text in such a way that some or all of their content
constitutes requirements 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, Image technology colour management — Architecture, profile format and data structure —
Part 1: Based on ICC.1:2010
ISO 12642-1, Graphic technology — Input data for characterization of four-colour process printing —
Part 1: Initial data set
ISO 13655, Graphic technology — Spectral measurement and colorimetric computation for graphic
arts images
3 Terms and definitions
For the purposes of this document, the following terms and definitions apply.
ISO and IEC maintain terminological databases for use in standardization at the following addresses:
— ISO Online browsing platform: available at https:// www .iso .org/ obp
— IEC Electropedia: available at http:// www .electropedia .org/
3.1
colour gamut
range of colours that can be reproduced by an output device on a given medium, represented in a CIE-
based colour space
Note 1 to entry: The CIE colour space for representation of colour gamuts is normally CIELAB.
3.2
gamut vertex
coordinate in a CIE-based colour space which represents a point on a colour gamut surface and which is
used in defining the surface of the gamut
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ISO/TS 18621-11:2019(E)
3.3
gamut face
planar sub-division of the colour gamut surface formed by three or more coplanar gamut face edges
Note 1 to entry: The colour gamut of most output devices can be described in terms of a set of gamut faces that
completely enclose all the colours that can be reproduced by the device, with no gaps or overlaps.
Note 2 to entry: In this document, gamut faces are defined as having three gamut vertices.
3.4
gamut face edge
line connecting two adjacent vertices of a gamut face
Note 1 to entry: In a continuous gamut surface, each gamut face edge is shared by two gamut faces.
3.5
characterization model
mathematical model that converts between coordinates in a device colour encoding and a CIE-based
colour space
3.6
device gamut
range of colours that corresponds to all possible combinations of colour channels of the device within
the device data encoding, when printed on a substrate
Note 1 to entry: Ink space is an alternative term for device gamut.
3.7
usable gamut
subset of the device gamut that corresponds to the set of combinations of colour channels of the device
in practical use, when reproduced on an output medium
Note 1 to entry: The usable gamut of an output device is normally smaller than the device gamut owing to
practical limitations in the combinations of colour channels. Most CMYK devices cannot produce a print in which
all channels are set to the maximum. The usable gamut is applicable when the gamut to be determined is that of
the system when used as part of a reproduction workflow, using an ICC profile to convert to output channels;
while the device gamut is applicable when the gamut to be determined is that of the reproduction device
independently of the profile and its colour separation method.
Note 2 to entry: In practice, some printers do not allow all possible combinations of ink to be printed, and an ink-
limiting procedure is applied automatically in the printer. Where this is done, this "ink-limited" mode of printing
still should be considered to be the "device gamut".
4 Describing a colour gamut
4.1 General
The colour gamut of a reproduction system is a volume in 3D colour space. It shall be mathematically
described as a closed set of triangular faces on the surface of the gamut which completely encloses the
gamut volume.
4.2 Requirements of a gamut boundary description
Each face shall be defined by three colorimetric coordinates, and the set of faces shall be defined in
such a way that it encloses the volume of the gamut without gaps or overlaps. The surface shall be
encoded as an nx3 array of vertices (in which there are n vertices and each row represents the colour
space coordinates of a gamut vertex) and an mx3 face array of indices into the vertices array (where
there are m faces and each row of the array identifies the three row numbers in the vertex array which
correspond to a gamut face). Each gamut vertex shall be described as a CIELAB L*, a*, b* value computed
from spectral reflectance or tristimulus values according to ISO 13655, and when this is done it shall
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ISO/TS 18621-11:2019(E)
be stated which ISO 13655 measurement mode applies to the data. If the colour space used to describe
the gamut vertices is not CIELAB computed according to ISO 13655, details of the colour space used
(including the CIE colorimetric observer and illuminant) shall be reported as metadata associated with
the gamut description. Where it is desired that the gamut description or comparison is media-relative,
CIELAB L*, a*, b* coordinates shall be scaled as described in Annex D.
In order to satisfy the requirement to enclose the gamut volume without gaps or overlaps, the following
conditions shall be met.
i) The three indices identifying each face shall identify vertices in clockwise order, when viewed from
the exterior of the volume.
ii) Each gamut face edge shall be common to two gamut faces.
CIELAB L*, a*, b* computed according to ISO 13655 is recommended where it is important to be able
to compare colour gamuts or where the gamut is derived from an ICC profile. It is acknowledged that
CIELAB is not perceptually uniform, and that this affects the gamut size. Determining a set of faces that
meet these conditions from an arbitrary set of vertices is non-trivial. For this reason, this document
provides a set of well-spaced coordinates in device space, and an associated triangulation. Full details
of these data are given in Annex A.
4.3 Device gamut and usable gamut
The device gamut can be determined either from the characterization model (usually represented by
an ICC profile) or by direct measurement of colours that lie on the gamut boundary. To compute the
usable gamut, an additional step is required in which the device coordinates are restricted to those
available in the reproduction workflow. If an ICC profile is used to define the usable gamut, CIELAB
gamut surface coordinates in the device gamut can be transformed to device coordinates and then back
to CIELAB coordinates to obtain the usable gamut.
NOTE In some cases, the device or its driver can limit the range of colorant combinations, regardless of
whether an ICC profile is used.
The procedure in 4.4 is recommended for determining the gamut boundary vertex and face arrays. If
a different procedure is used, it shall be stated when communicating the gamut boundary description
which procedure was used to determine the gamut vertices, and whether the device gamut or usable
gamut is described.
4.4 Procedures for describing a colour gamut
4.4.1 General
One of the following procedures shall be used to describe the colour gamut of a reproduction system.
NOTE 1 In most cases, results obtained from these procedures, using the set of well-spaced coordinates in
[1]
device space described in this document, give very similar results . Certain factors affect the reproducibility of
the gamut description, such as when the black in a toner-based printer results in a lower L* value than any of the
other colorant combinations.
NOTE 2 An ICC profile is a convenient means of converting data between the device data encoding and
the corresponding colorimetry, and it defines the colour gamut available in a workflow based on ICC profile
conversions. Other methods of obtaining colorimetric values for coordinates on the gamut surface, such as direct
measurement or a characterization model, is also used.
4.4.2 Procedure for describing the colour gamut of a reproduction system based on its ICC profile
The following procedure can be used to compute the faces and vertices of a gamut boundary description
from an ICC profile for the reproduction system. The procedure is applicable to RGB and CMYK devices.
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Where used, the ICC profile shall be created according to ISO 15076-1, from characterization data
representing the printing process whose gamut is to be described. This method estimates the gamut of
the device represented by the profile, and depending on the accuracy of the AToB1 tag and the BToA1
tag of the profile this estimate may or may not itself be accurate. The accuracy of AToB1 and BToA1 tags
shall be reported.
To maintain accuracy, the precision of data used for both device coordinates and CIELAB coordinates
shall be 16 bits or greater.
1) Generate an image whose pixels represent a set of device coordinates on the gamut boundary of the
encoding. The image should be arranged so that the ratio of the relative colorant amounts varies in
the horizontal direction, and the total colorant amount varies in the vertical direction. The white
point and black point are repeated across the first and last ro
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