Multimedia systems and equipment - Colour measurement and management - Part 2-2: Colour management - Extended RGB colour space - scRGB

IEC 61966-2-2:2003 is applicable to the encoding, editing and communication of relative scene radiance, wide dynamic range, extended colour gamut, and extended bit precision RGB colours as a colour space used in computer systems and similar applications by defining encoding transformations. Primaries and white point values of the colour space defined in this standard are identical to CIE chromaticities for ITU-R BT.709-5 reference primaries and CIE standard illuminant D65 as its white point. The scRGB colour space is an extension of sRGB and it is considered compatible with sRGB. This bilingual version (2013-02) corresponds to the monolingual English version, published in 2003-01.

Mesure et gestion de la couleur dans les systèmes et appareils multimédia - Partie 2-2: Gestion de la couleur - Espace chromatique RVB étendu - scRVB

La CEI 61966-2-2:2003 s'applique au codage, à l'édition et à la communication des couleurs RVB de radiance de scènes relatives, large plage dynamique, gamme de couleurs étendue et précision en bits étendue, en tant qu'espace colorimétrique utilisé dans les systèmes informatiques et les applications similaires par définition des transformations de codage. Les valeurs des primaires et du point blanc de l'espace colorimétrique définies dans la présente norme sont identiques aux chromaticités CIE pour les primaires de référence UIT-R BT.709-5 et pour l'illuminant normalisé CIE D65 comme son point blanc. L'espace colorimétrique scRVB est une extension de sRVB et il est considéré comme compatible avec sRVB. La présente version bilingue (2013-02) correspond à la version anglaise monolingue publiée en 2003-01.

General Information

Status
Published
Publication Date
22-Jan-2003
Current Stage
PPUB - Publication issued
Start Date
28-Feb-2003
Completion Date
23-Jan-2003
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IEC 61966-2-2
®

Edition 1.0 2003-01
INTERNATIONAL
STANDARD
NORME
INTERNATIONALE
colour
inside


Multimedia systems and equipment – Colour measurement and management –
Part 2-2: Colour management – Extended RGB colour space – scRGB

Mesure et gestion de la couleur dans les systèmes et appareils multimédia –
Partie 2-2: Gestion de la couleur – Espace chromatique RVB étendu – scRVB

IEC 61966-2-2:2003

---------------------- Page: 1 ----------------------
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IEC 61966-2-2

®


Edition 1.0 2003-01




INTERNATIONAL



STANDARD




NORME



INTERNATIONALE
colour

inside










Multimedia systems and equipment – Colour measurement and management –

Part 2-2: Colour management – Extended RGB colour space – scRGB




Mesure et gestion de la couleur dans les systèmes et appareils multimédia –

Partie 2-2: Gestion de la couleur – Espace chromatique RVB étendu – scRVB
















INTERNATIONAL

ELECTROTECHNICAL

COMMISSION


COMMISSION

ELECTROTECHNIQUE

PRICE CODE
INTERNATIONALE

CODE PRIX P


ICS 33.160.60; 37.080 ISBN 978-2-83220-631-7



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® Registered trademark of the International Electrotechnical Commission
Marque déposée de la Commission Electrotechnique Internationale

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– 2 – 61966-2-2  IEC:2003
CONTENTS
FOREWORD . 3
INTRODUCTION . 5

1 Scope . 6
2 Normative references. 6
3 Definitions . 6
4 Encoding characteristics . 7
4.1 General . 7
4.2 Transformation from CIE 1931 XYZ values to 16-bit scRGB values
( R , G , B ) . 7
scRGB scRGB scRGB
(16) (16) (16)
4.3 Transformation from 16-bit scRGB values
( R , G , B ) to CIE 1931 XYZ values . 7
scRGB scRGB scRGB
(16) (16) (16)

Annex A (informative) Simple transformation between 8-bit sRGB and 16-bit scRGB
values . 8
Annex B (informative) Non-linear encoding for scRGB: scRGB-nl and its YCC
Transformation: scYCC-nl . 10
Annex C (informative) scRGB background information . 12

Bibliography . 16

Figure C.1 – Example workflow using scRGB . 15

Table B.1 – Quantization relationships using scRGB . 11

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61966-2-2  IEC:2003 – 3 –
INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________

MULTIMEDIA SYSTEMS AND EQUIPMENT –
COLOUR MEASUREMENT AND MANAGEMENT –

Part 2-2: Colour management –
Extended RGB colour space – scRGB


FOREWORD
1) The IEC (International Electrotechnical Commission) is a worldwide organization for standardization comprising
all national electrotechnical committees (IEC National Committees). The object of the IEC is to promote
international co-operation on all questions concerning standardization in the electrical and electronic fields. To
this end and in addition to other activities, the IEC publishes International Standards. Their preparation is
entrusted to technical committees; any IEC National Committee interested in the subject dealt with may
participate in this preparatory work. International, governmental and non-governmental organizations liaising
with the IEC also participate in this preparation. The IEC collaborates closely with the International Organization
for Standardization (ISO) in accordance with conditions determined by agreement between the two
organizations.
2) The formal decisions or agreements of the IEC on technical matters express, as nearly as possible, an
international consensus of opinion on the relevant subjects since each technical committee has representation
from all interested National Committees.
3) The documents produced have the form of recommendations for international use and are published in the form
of standards, technical specifications, technical reports or guides and they are accepted by the National
Committees in that sense.
4) In order to promote international unification, IEC National Committees undertake to apply IEC International
Standards transparently to the maximum extent possible in their national and regional standards. Any
divergence between the IEC Standard and the corresponding national or regional standard shall be clearly
indicated in the latter.
5) The IEC provides no marking procedure to indicate its approval and cannot be rendered responsible for any
equipment declared to be in conformity with one of its standards.

The International Electrotechnical Commission (IEC) draws attention to the fact that it is claimed that compliance
with this document may involve the use of a patent concerning encoding of colour management given in Clause 4.
The IEC takes no position concerning the evidence, validity and scope of this patent right.
The holder of this patent right has assured the IEC that he 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 IEC. Information may be obtained from:
Eastman Kodak Company
343 State Street
Rochester
New York 14650
USA
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. The IEC shall not be held responsible for identifying any or all such patent rights.
International Standard IEC 61966 has been prepared by Technical Area 2: Colour
measurement and management, of IEC technical committee 100: Audio, video and multimedia
systems and equipment and ISO TC 42: Photography.
This bilingual version (2013-03) corresponds to the monolingual English version, published in
2003-01.

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– 4 – 61966-2-2  IEC:2003
The text of this standard is based on the following documents:
FDIS Report on voting
100/556A/FDIS 100/626/RVD

Full information on the voting for the approval of this standard can be found in the report on
voting indicated in the above table.
The French version of this standard has not been voted upon.
This publication has been drafted in accordance with the ISO/IEC Directives, Part 2.
IEC 61966 consists of the following parts, under the general title Multimedia systems and
equipment – Colour measurement and management:
Part 2-1: Colour management – Default RGB colour space – sRGB
Part 2-2: Colour management – Extended RGB colour space – scRGB
Part 3: Equipment using cathode ray tubes
Part 4: Equipment using liquid crystal display panels
Part 5: Equipment using plasma display panels
Part 7-1: Colour printers – Reflective prints – RGB inputs
Part 8: Multimedia colour scanners
Part 9: Digital cameras
It is published as a double logo standard.
In the ISO the Standard has been approved by 9 P-members out of 10 having cast the vote.
The committee has decided that the contents of this publication will remain unchanged
until 2007. At this date, the publication will be
• reconfirmed;
• withdrawn;
• replaced by a revised edition, or
• amended.
The contents of the corrigendum of August 2003 have been included in this copy.

IMPORTANT – The 'colour inside' logo on the cover page of this publication indicates
that it contains colours which are considered to be useful for the correct understanding
of its contents. Users should therefore print this document using a colour printer.

---------------------- Page: 6 ----------------------
61966-2-2  IEC:2003 – 5 –
INTRODUCTION
The IEC 61966 standards are a series of methods and parameters for colour measurements
and management for use in multimedia systems and equipment applicable to the assessment
of colour reproduction.
The method of digitization in this part is designed to provide high bit precision, large colour
gamut and extended dynamic range that is linear with respect to scene radiance. Based on
IEC 61966-2-1 (sRGB), this colour space is well suited to meet the needs of the multimedia,
gaming and computer graphics applications. This standard provides a robust solution to these
needs. The white point and colour primaries of the scRGB solution are directly inherited from
the IEC 61966-2-1 (sRGB) standard. The encoding transformations provide all of the necessary
information to encode an image.

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– 6 – 61966-2-2  IEC:2003
MULTIMEDIA SYSTEMS AND EQUIPMENT –
COLOUR MEASUREMENT AND MANAGEMENT –

Part 2-2: Colour management –
Extended RGB colour space – scRGB



1 Scope
This part of IEC 61966 is applicable to the encoding, editing and communication of relative
scene radiance, wide dynamic range, extended colour gamut, and extended bit precision RGB
colours as a colour space used in computer systems and similar applications by defining
encoding transformations. Primaries and white point values of the colour space defined in this
standard are identical to CIE chromaticities for ITU-R BT.709-5 reference primaries and CIE
standard illuminant D65 as its white point. The scRGB colour space is an extension of sRGB
and it is considered compatible with sRGB.
Additional transformations, such as white point adaptation methods, are beyond the scope of
this standard. The appropriate CIE recommendations should be referred to for guidelines
in this area.
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 60050(845):1987, International Electrotechnical Vocabulary (IEV) – Chapter 845: Lighting
3 Definitions
For the purposes of this document, the following definitions apply. Definitions of illuminance,
radiance, tristimulus, and other relating lighting terms are defined in IEC 60050(845).
3.1
output referred colour space
a colour space that represents the colorimetry of an output device with specified viewing
conditions
3.2
wide dynamic range colour space
a colour space whose encoding encompasses values below black and above white
3.3
luma
luminance signal as defined by SMPTE/EG28: 1993
NOTE Video systems approximate the lightness response of vision by computing a luma component Y′ as a
weighted sum of nonlinear R′G′B′ primary components: Each RGB signal is, comparable to the 1/3 power function
with an offset defined by L*. Luma is often incorrectly referred to as luminance.

---------------------- Page: 8 ----------------------
61966-2-2  IEC:2003 – 7 –
4 Encoding characteristics
4.1 General
The encoding transformations provide unambiguous methods to transform between CIE 1931
XYZ tristimulus values and 16-bit values for each channel of scRGB. The CIE 1931 XYZ
values are scaled so that the sRGB black point to white point luminance is 0,0 to 1,0, not 0,0
to 100,0. Y-tristimulus values less than 0,0 in CIE 1931 XYZ space represent values below
black. Y-tristimulus values greater than 1,0 represent values brighter than relative white.
The scRGB components that range from 0 to 16 384 encompass all visible surface colours
(from −0,5 to 1,5). The range from 12 288 to 65 535 is used to encode an extended specular
range of colours (from larger than 1,0 to 7,4999).
4.2 Transformation from CIE 1931 XYZ values to 16-bit scRGB values
( R , G , B )
scRGB scRGB scRGB
(16) (16) (16)
The relationship is defined as follows:
R 3,240625 −1,537 208 − 0,498629 X
    
scRGB
    
G = − 0,968931 1,875756 0,041518 Y (1)
scRGB
    
    
B 0,055710 − 0,204021 1,056996 Z
 scRGB   
R = round[(R × 8192,0)+ 4 096]
scRGB scRGB
(16)
and: [( ) ] (2)
G = round G × 8192,0 + 4 096
scRGB scRGB
(16)
B = round[(B × 8192,0)+ 4 096]
scRGB scRGB
(16)
4.3 Transformation from 16-bit scRGB values ( R , G , B )
scRGB scRGB scRGB
(16) (16) (16)
to CIE 1931 XYZ values
The relationship is defined as follows:
( )
R = R ÷ 8192,0 − 0,5
scRGB scRGB
(16)
G = (G ÷ 8192,0)− 0,5 (3)
scRGB scRGB
(16)
B = (B ÷ 8192,0)− 0,5
scRGB scRGB
(16)
X 0,412 4 0,357 6 0,180 5 R
    
scRGB
    
and Y = 0,212 6 0,715 2 0,072 2 G (4)
scRGB
    
    
Z 0,019 3 0,119 2 0,950 5 B
scRGB
    

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– 8 – 61966-2-2  IEC:2003
Annex A
(informative)

Simple transformation between 8-bit sRGB and 16-bit scRGB values

A.1 General
This annex describes a simple transformation between 8-bit sRGB and 16-bit scRGB. While
more complicated and intelligent tonal rendering should be applied for the scRGB images to
obtain the most preferred images, this transformation is targeted to real-time display
transformations for quick and easy previewing. Other transformations that focus on other
requirements are possible. If such other transformations are intended to exchange with other
devices or applications, these transformations should be described within the application
documentation or file format as appropriate.
A.2 Transformation from 16-bit scRGB values ( R , G , B )
scRGB scRGB scRGB
(16) (16) (16)
to 8-bit sRGB values ( R , G , B )
sRGB sRGB sRGB
(8) (8) (8)
The relationship is defined as follows:
R = (R ÷ 8192)− 0,5
scRGB scRGB
(16)
G = (G ÷ 8192)− 0,5 (A.1)
scRGB scRGB
(16)
B = (B ÷ 8192)− 0,5
scRGB scRGB
(16)
( )
If R ,G , B < 0 R ,G , B ≤ 4095
scRGB scRGB scRGB scRGB scRGB scRGB
(16) (16) (16)
R = 0
sRGB
(8)
G = 0 (A.2)
sRGB
(8)
B = 0
sRGB
(8)
else if
0 ≤ R ,G , B < 0,018 (4096 ≤ R ,G , B ≤ 4243)
scRGB scRGB scRGB scRGB scRGB scRGB
(16) (16) (16)
R = round[(4,500× R )× 255]
sRGB scRGB
(8)
G = round[(4,500× G )× 255] (A.3)
sRGB scRGB
(8)
B = round[(4,500× B )× 255]
sRGB scRGB
( )
8
else if
0,018 ≤ R ,G , B ≤ 1,0 (4244 ≤ R ,G , B ≤ 12288)
scRGB scRGB scRGB scRGB scRGB scRGB
(16) (16) (16)

---------------------- Page: 10 ----------------------
61966-2-2  IEC:2003 – 9 –
(0,45)
R = round[((1,099× R )− 0,099)× 255]
sRGB scRGB
(8)
(0,45)
G = round[((1,099× G )− 0,099)× 255] (A.4)
sRGB scRGB
(8)
(0,45)
B = round[((1,099× B )− 0,099)× 255]
sRGB scRGB
(8)
R = 255
sRGB
(8)
else G = 255 (A.5)
sRGB
(8)
B = 255
sRGB
(8)

A.3 Transformation from 8-bit sRGB values ( R , G , B )
sRGB sRGB sRGB
(8) (8) (8)
to 16-bit scRGB values ( R , G , B )
scRGB scRGB scRGB
(16) (16) (16)
The relationship is defined as follows:
If 0 ≤ R ,G , B < 21
sRGB sRGB sRGB
(8) (8) (8)
R = round(7,139× R + 4096)
scRGB sRGB
(16) (8)
( ) (A.6)
G = round 7,139× G + 4096
scRGB sRGB
(16) (8)
B = round(7,139× B + 4096)
scRGB sRGB
(16) (8)
1,0 
 
 
0,45
 
(R + 25,245) 
 sRGB 
(8)
R = round × 8192 + 4096
  
scRGB
(16)
280,245
 
  
 
1,0
 
 
 
0,45
(G + 25,245)  
 
 
sRGB
(8)
else G = round × 8192 + 4096 (A.7)
  
scRGB
(16)
280,245
 
  
 
1,0 
 
 
0,45
(B + 25,245)  
 
 sRGB 
(8)
B = round × 8192 + 4096
 
scRGB  
(16) 280,245
 
  
 

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– 10 – 61966-2-2  IEC:2003
Annex B
(informative)

Non-linear encoding for scRGB: scRGB-nl
and its YCC Transformation: scYCC-nl

B.1 General
This annex describes non-linear encoding for scRGB: scRGB-nl and its YCC transformation:
scYCC-nl. Applications and hardware developers who want to support various colour
compression schemes based on luma-chroma-chroma spaces can utilise this standard. This
transformation is targeted for compression and storage, and is not targeted for displaying
images.
B.2 Non-linear encoding in 12-bit
The relationship is defined as follows:
If R , G , B ≥ 0,003 130 8
scRGB scRGB scRGB
(1,0 2,4)

R = 1,055× R − 0,055
scRGB scRGB
(1,0 2,4)

G = 1,055×G − 0,055 (B.1)
scRGB scRGB
(1,0 2,4)

B = 1,055× B − 0,055
scRGB scRGB
If
0,003 130 8 > R , G , B > −0,003 130 8
scRGB scRGB scRGB
R′ = 12,92× R
scRGB scRGB
G′ = 12,92× G (B.2)
scRGB scRGB

B = 12,92× B
scRGB scRGB
If R , G , B ≤ −0,003 130 8
scRGB scRGB scRGB
(1,0 2,4)
R′ = −1,055×(− R ) + 0,055
scRGB scRGB
(1,0 2,4)
G′ = −1,055×(− G ) + 0,055 (B.3)
scRGB scRGB
(1,0 2,4)

B = −1,055×(− B ) + 0,055
scRGB scRGB
12 bit non-linear version of scRGB-nl: R , G , B is defined as:
scRGB-nl scRGB-nl scRGB-nl

R = round(1 280× R +1 024)
scRGB-nl scRGB
G = round(1 280× G′ +1 024) (B.4)
scRGB-nl scRGB
B = round(1 280× B′ +1 024)
scRGB-nl scRGB

---------------------- Page: 12 ----------------------
61966-2-2  IEC:2003 – 11 –
For compression, scRGB-nl is converted to luma-chroma-chroma encoding: scYCC-nl.
Y′ 0,299 0 0,587 0 0,114 0 R′
    
scYCC scRGB
    
Cb′ = − 0,168 7 − 0,331 3 0,500 0 G′ (B.5)
scYCC scRGB
    
    
Cr′ 0,500 0 − 0,418 7 − 0,081 3 B′
 scYCC   scRGB
And quantization for 12 bit non-linear scYCC-nl: Y , Cb , Cr is defined as:
scYCC-nl scYCC-nl scYCC-nl

Y = round(1 280×Y +1 024)
scYCC-nl scYCC
Cb = round(1 280× Cb′ + 2 048) (B.6)
scYCC-nl scYCC
Cr = round(1 280× Cr′ + 2 048)
scYCC-nl scYCC
Note that this quantization leads to the following relationships, where a value of 65 535 in
scRGB is equivalent to 7,499 9 in scRGB and 4 080 in scRGB-nl. This is to ease
(16)
computational implementations.
Table B.1 – Quantization relationships using scRGB
scRGB scRGB scR'G'B scRGB-nl
(16)
−0,603 8 −0,800 0
N/A 0
−0,5 −0,735 4
0 83
−0,25 −0,537 1
2 048 337
4 096 0 0,000 0 1 024
12 288 1 1,000 0 2 304
20 480 2 1,353 3 2 756
28 672 3 1,612 5 3 088
36 864 4 1,824 8 3 360
45 056 5 2,008 0 3 594
53 248 6 2,170 8 3 803
61 440 7 2,318 4 3 992
65 535 7,499 9 2,387 6 4 080
N/A 7,5 2,387 7 4 080
N/A 7,591 3 2,400 0 4 096

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– 12 – 61966-2-2  IEC:2003
Annex C
(informative)

scRGB background information

C.1 Two basic families of colour spaces: perceptual and radiance
There exist two basic families of colour spaces that almost all other colour spaces can be
categorized into; perceptual colour spaces like sRGB and CIELAB and radiance colour spaces
like scRGB and CIE 1931 XYZ. Each of these basic types of colour space have their
appropriate uses and using one colour space in processes natively intended for the other
results in less than optimal colour quality. For example, computing the perceptual colour
difference is usually done in CIELAB, where colour differences are more uniformly distributed.
Computing perceptual colour differences in CIE 1931 XYZ can easily lead to misleading results.
When comparing different device gamuts, it is also recommended to visualize the gamuts in
the three dimensional CIELAB or CIELUV colour spaces instead of two dimensional xy
chromaticity diagrams. This is because colour spaces and gamuts are inherently three
dimensional, not two dimensional.
C.2 Processes best performed in radiance colour spaces
Just as there are processes best performed in perceptual spaces, there are also processes
best performed in radiance spaces, such as scRGB or CIE 1931 XYZ. This includes processes
that are fundamentally based on linear processing of light, such as transparency blending, anti-
aliasing, convolution, light rendering, etc. In fact, most virtual reality and computer visualization
processes are performed in radiance colour spaces before being converted for display or
output as a final step in the process. This allows for much more efficient and effective
processing. It is already clear that these types of processing are migrating into the consumer
spaces. For example, major multimedia equipment manufacturers have products that
fundamentally rely on radiance processing for their success. In the future, the line between
such realistic effects and processing and traditional imaging that has been based on perceptual
spaces will be very blurred. In order to provide potential and reasonable paths forward for
vendors to understand and investigate the impact of these trends, it is critical to have robust
and standard support for both types of colour spaces, both radiance and perceptual. While one
could theoretically use CIELAB and CIE 1931 XYZ to address these investigations, most
consumer workflows depend upon RGB solutions and thus sRGB and scRGB are well-suited to
address these needs.
C.3 Bit depth benefits to consumer devices
In addition to the linear space, the trends of most consumer devices are toward higher quality
as is evidenced by improvements in resolution by both consumer printers and digital cameras.
This also applies to bit precision as evidenced by consumer scanners output 12 bits and 16 bits
per channel. As digital camera D/As become more affordable, it is clear that even consumer
digital cameras will be able to output 10 bits, 12 bits and possible 14 bits per channel in the
next five years. Some colour printers today can already accept higher bit precision than 8 bits
per channel. This standard enables support for very large colour gamut device
Finally, as discussed above, there are two fundamental colour space types, radiance and
perceptual, each of which has their best uses. In order to provide vendors maximum flexibility
to address their markets in the future, it appears to be advantageous to provide infrastructure
support by these two colour spaces to these vendors.

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61966-2-2  IEC:2003 – 13 –
C.4 Advantages of scRGB
The basic advantage of scRGB is to allow work in a large gamut, wide dynamic range linear
space. For processes which require or desire such a space, the potentially simple 1D LUT
conversion is a small price to pay for the computational simplicity gained.
C.5 Disadvantages of scRGB
The additional bits per channel is the largest disadvantage. The other issues such as signed
encoding and linear gamma can be shown as advantages for particular markets using
appropriate demonstrations.
Since scRGB is a higher bit precision space, it suffers the same performance and memory
limitations that other 16 bit channel colours spaces suffer. This alone will make it inappropriate
for many low end consumer devices. Yet, many high end consumers are willing to accept these
performance and memory issues to obtain the advantages in linear process capabilities, gamut
and dynamic range. There are three potential disadvantages of scRGB; 1) bit precision,
2) linear gamma incompatible with many current consumer practices and 3) signed encoding
incompatible with many current consumer practices. The advantages of linear gamma and
signed encoding have been discussed above that thus, this is simply a matter of each vendor
weighing the advantages and the disadvantages. The increased bit precision allows
improvements in banding, contouring and signal-to-noise issues. These advantages must also
be weighed by each vendor with the disadvantage of increased memory sizes.
C.6 Explanation of negative tristimulus values in scRGB
The colour space, scRGB, allows for negative tristimulus values which provide some significant
advantages. For gamut encapsulation, one might consider display gamuts to be shaped like
apples, with the bulk of the colour gamut in the bright colours due to the additive nature of the
devices and similarly one might consider printer gamuts to be shaped like pears, with the bulk
of the colour gamut in the shadow colours due to the subtractive nature of the devices. While it
is possible to use wide primaries to accommodate the pear shape, this requires very
aggressive matrix transformations for display, which is a disadvantage for many workflows. By
allowing negative tristimulus values to be encoded in an unsigned encoding, this creates a
black region instead of a black point. Having a black region provides for easy encapsulation of
very dark, but saturated colours while maintaining a reasonable use of gamut volume and
coverage. The negative encoding has a second significant advantage in simplifying colour
processing applications.
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