ISO/TS 21830:2018

SLOVENSKI STANDARD
01-april-2020
Barvno upravljanje slikovne tehnologije - Kompenzacija črne točke za n-
barvne ICC-profile
Image technology colour management - Black point compensation for n-colour ICC
profiles
Technologie de l'image - BPC pour profil à n couleurs
Ta slovenski standard je istoveten z: ISO/TS 21830:2018
ICS:
17.180.20 Barve in merjenje svetlobe Colours and measurement of
light
37.100.01 Grafična tehnologija na Graphic technology in
splošno general
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.

TECHNICAL ISO/TS
SPECIFICATION 21830
First edition
2018-09
Image technology colour
management — Black point
compensation for n-colour ICC profiles
Technologie de l'image — BPC pour profil à n couleurs
Reference number
©
ISO 2018
© ISO 2018
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ii © ISO 2018 – All rights reserved

Contents Page
Foreword .iv
Introduction .v
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Extension of black point compensation to n-colour ICC profiles . 1
4.1 Constraints . 1
4.2 Computation. 2
4.2.1 General. 2
4.2.2 Computing the SourceBlackPoint . 3
4.2.3 InitialLAB Calculation . 3
Bibliography . 4
Foreword
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iv © ISO 2018 – All rights reserved

Introduction
The xCLR ICC profiles that are used in digital printing applications are often CMYK ICC profiles
extended with red, orange, green, blue and/or violet colourants. Hence there is a need to specify
black point compensation (BPC) for a well-defined class of xCLR ICC profiles, where xCLR refers to a
device-dependent colour space defined in ISO 15076-1 and ICC.1:2001-04, specified for 3 to 15 device
colourants wherein the value of x is a hexadecimal digit within the range 3 to F inclusive. Such xCLR ICC
profiles are also commonly referred to as n-colour profiles.
To guarantee continuity of the black point compensation procedure between CMYK devices and
printing devices with extended colourant sets, xCLR ICC profiles follow the constraints and calculation
of output-capable CMYK ICC profiles wherever possible, as specified in ISO 18619:2015.
Limiting xCLR ICC profiles to CMYK plus combinations from the set of red, orange, green, blue and
violet colourants is a logical extension of the CMYK colourant set to enhance the printing gamut as
applied in digital print.  Some of the key additional assumptions which will likely result in predictive
and expected behaviour for black point compensation calculations include:
— in a similar fashion to most CMYK colourants, the colourants chosen for use with the xCLR ICC
profiles should result in a colour gamut featuring a large range of neutral colours;
— the physical colourants should be sufficiently transparent with well-saturated CMY primaries in
order to keep the gamut shell well-formed and permit the black point compensation algorithm to
work correctly;
— a well-behaving forward model can be constructed near the darkest neutral, with a well-defined
darkest colour, thus guaranteeing a good approximation using curve-fitting as defined by
ISO 18619:2015, 4.2.5.5.
In the case of 4CLR ICC profiles, which are constrained by ISO 15076-1 as well as ICC.1:2001-04 to not
refer to CMYK device-dependent colour spaces, the 4CLR colour space should behave similarly to CMYK
device-dependent colour spaces, and conforms to the requirements defined by the key assumptions
given above for other xCLR ICC profiles.
An additional class of xCLR ICC profiles for consideration by this document are 3CLR and CMY ICC
profiles. For 3CLR ICC profiles, the colourants should be CMY-like in the sense of being chromatic
colourants with widely-spaced hue angles (as distinct from achromatic colourants such as grey or
black). As a result, BPC for 3CLR ICC profiles is defined by this document, and follows the same approach
as for CMY ICC profiles.
The BPC method described in this document does not give meaningful results for most 2CLR ICC
profiles, hence these types of ICC profiles are excluded from this document.
In addition, this document extends the BPC method for the ICC v4 profile types with the device-
dependent colour spaces described above for corresponding ICC v2 profile
...