# IEC 61966-2-1:1999/AMD1:2003

(Main)## Amendment 1 - Multimedia systems and equipment - Colour measurement and management - Part 2-1: Colour management - Default RGB colour space - sRGB

## Amendment 1 - Multimedia systems and equipment - Colour measurement and management - Part 2-1: Colour management - Default RGB colour space - sRGB

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INTERNATIONAL IEC

STANDARD

61966-2-1

1999

AMENDMENT 1

2003-01

Amendment 1

Multimedia systems and equipment –

Colour measurement and management –

Part 2-1:

Colour management –

Default RGB colour space - sRGB

Amendement 1

Mesure et gestion de la couleur dans les systèmes

et appareils multimédia –

Partie 2-1:

Gestion de la couleur –

Espace chromatique RVB par défaut - sRVB

IEC 2003 Droits de reproduction réservés Copyright - all rights reserved

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Telephone: +41 22 919 02 11 Telefax: +41 22 919 03 00 E-mail: inmail@iec.ch Web: www.iec.ch

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– 2 – 61966-2–1 Amend. 1 IEC:2003(E)

FOREWORD

This amendment 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.

The text of this amendment is based on the following documents:

FDIS Report on voting

100/555A/FDIS 100/625/RVD

Full information on the voting for the approval of this amendment can be found in the report on

voting indicated in the above table.

It is published as a double logo standard.

In the ISO the Standard has been approved by 10 P-members out of 10 having cast the vote.

_____________

Page 5

CONTENTS

Add the titles of Annexes F, G and H as follows:

Annex F (normative) Default YCC encoding transformation for a standard luma-chroma-

chroma colour space: sYCC

Annex G (informative) Extended gamut encoding for sRGB: bg-sRGB and its YCC

transformation: bg-sYCC

Annex H (informative) CIELAB (L*a*b*) transformation

Page 49

Add the following new Annexes F, G and H after Annex E:

---------------------- Page: 2 ----------------------

61966-2–1 Amend. 1 IEC:2003(E) – 3 –

Annex F

(normative)

Default YCC encoding transformation for a

standard luma-chroma-chroma colour space: sYCC

The method of digitization in this annex is designed to complement current sRGB-based colour

management strategies by explicitly standardizing a default transformation between sRGB and

a standard luma-chroma-chroma colour space (sYCC). Application and hardware developers

who want to support various colour compression schemes based on luma-chroma-chroma

spaces can utilize this annex. Since this sYCC colour space is a simple extension of the sRGB

colour space as defined in this standard, the same reference conditions are shared by both

colour spaces.

F.1 General

The encoding transformations between sYCC values and CIE 1931 XYZ values provide

unambiguous methods to represent optimum image colorimetry when viewed on a hypothetical

reference display that is capable of producing all colours defined by sYCC encoding, in the

reference viewing conditions by the reference observer. Non-linear floating point sR′G′B′

represent the appearance of the image as displayed on the reference display in the reference

viewing condition described in Clause 4 of this standard.

F.2 Transformation from sYCC values ( Y , Cb , Cr ) to CIE 1931 XYZ

sYCC sYCC sYCC

values

The non-linear sY′C ′C′ values can be computed using the following relationship:

b r

′

Y =()Y − KDC(WDC − KDC)

sYCC sYCC

′ (F.1)

Cb =()Cb − Offset Range

sYCC sYCC

Cr′ =()Cr − Offset Range

sYCC sYCC

For 24-bit encoding (8-bit/channel), WDC = 255, KDC = 0, Range = 255, and Offset = 128, and

the relationship is defined as;

′

Y = (Y − 0)()255 − 0 = Y 255

sYCC sYCC sYCC

(8) (8)

Cb′ =()Cb −128 255 (F.2)

sYCC sYCC

(8)

′

Cr =()Cr −128 255

sYCC sYCC

(8)

24-bit encoding (8-bit/channel) shall be the default sYCC encoding bit depth. Other bit depths

may be unsupported for general use.

Where other N-bit/channel encoding is supported ( N > 8), the relationship is defined as;

---------------------- Page: 3 ----------------------

– 4 – 61966-2–1 Amend. 1 IEC:2003(E)

N

′

Y = Y (2 −1)

sYCC sYCC

(N)

N −1 N

Cb′ =()Cb − 2 ()2 −1 (F.2′)

sYCC sYCC

(N)

N −1 N

′

Cr =()Cr − 2 ()2 −1

sYCC sYCC

(N)

For 24-bit encoding (8-bit/channel), the non-linear sY′C ′C′ values are transformed to the non-

b r

linear sR′G′B′ values as follows;

R′ 1,000 0 0,000 0 1,402 0 Y ′

sRGB sYCC

′ ′

G = 1,000 0 − 0,344 1 − 0,714 1 Cb (F.3)

sRGB sYCC

′ ′

B 1,000 0 1,772 0 0,000 0 Cr

sRGB sYCC

For N-bit/channel encoding ( N > 8), it is recommended to replace the matrix coefficients in the

equation F.3 with the coefficients of the inverse matrix of the equation F.12 with enough

accuracy decimal points. For example, following matrix with 6 decimal points has enough

accuracy for the case of 16-bit/channel.

R′ 1,000 000 0,000 037 1,401 988 Y ′

sRGB sYCC

′ ′

G = 1,000 000 − 0,344 113 − 0,714 104 Cb (F.3′)

sRGB sYCC

B′ 1,000 000 1,771 978 0,000 135 Cr′

sRGB sYCC

The non-linear sR′G′B′ values are then transformed to CIE 1931 XYZ values as follows:

′ ′ ′

If R ,G ,B < −0,040 45

sRGB sRGB sRGB

2,4

′

()− R + 0,055

sRGB

R = −

sRGB

1,055

2,4

′

()− G + 0,055

sRGB

G = − (F.4)

sRGB

1,055

2,4

′

()− B + 0,055

sRGB

B = −

sRGB

1,055

′ ′ ′

If − 0,040 45 ≤ R ,G , B ≤ 0,040 45 ,

sRGB sRGB sRGB

′

R = R ÷12,92

sRGB sRGB

′

G = G ÷12,92 (F.5)

sRGB sRGB

′

B = B ÷12,92

sRGB sRGB

′ ′ ′

If R ,G , B > 0,040 45 ,

sRGB sRGB sRGB

2,4

′

()R + 0,055

sRGB

R =

sRGB

1,055

2,4

′

()G + 0,055

sRGB

G = (F.6)

sRGB

1,055

2,4

′

()B + 0,055

sRGB

B =

sRGB

1,055

---------------------- Page: 4 ----------------------

61966-2–1 Amend. 1 IEC:2003(E) – 5 –

For 24-bit encoding (8-bit/channel), the linear sRGB values are transformed to CIE 1931 XYZ

values as follows:

X 0,412 4 0,357 6 0,180 5R

sRGB

Y = 0,212 6 0,715 2 0,072 2 G (F.7)

sRGB

Z 0,019 3 0,119 2 0,950 5 B

sRGB

F.3 Transformation from CIE 1931 XYZ values to sYCC values

( Y , Cb , Cr )

sYCC sYCC sYCC

The CIE 1931 XYZ values can be transformed to non-linear sR′G′B′ values as follows

R 3,240 6 −1,537 2 − 0,498 6 X

sRGB

G = − 0,968 9 1,875 8 0,041 5 Y (F.8)

sRGB

B 0,055 7 − 0,204 0 1,057 0 Z

sRGB

For N-bit/channel encoding ( N > 8), it is recommended to replace the matrix coefficients in the

equation F.8 with the coefficients of the inverse matrix of the equation F.7 with enough

accuracy decimal points. For example, following matrix with 7 decimal points has enough

accuracy for the case of 16-bit/channel.

R 3,240 625 5 −1,537 208 0 − 0,498 628 6 X

sRGB

G = − 0,968 930 7 1,875 756 1 0,041 517 5 Y (F.8′)

sRGB

B 0,055 710 1 − 0,204 021 1 1,056 995 9 Z

sRGB

In the sYCC encoding process, negative sRGB tristimulus values, and sRGB tristimulus values

greater than 1,0 are retained.

If R ,G , B < −0,003 130 8

sRGB sRGB sRGB

()1,0 / 2,4

′

R = −1,055 ×()− R + 0,055

sRGB sRGB

()1,0 / 2,4

′ () (F.9)

G = −1,055 × − G + 0,055

sRGB sRGB

()1,0 / 2,4

′

B = −1,055 ×()− B + 0,055

sRGB sRGB

If − 0,003 130 8 ≤ R ,G , B ≤ 0,003 130 8 ,

sRGB sRGB sRGB

′

R = 12,92 × R

sRGB sRGB

G′ = 12,92 × G (F.10)

sRGB sRGB

′

B = 12,92 × B

sRGB sRGB

---------------------- Page: 5 ----------------------

– 6 – 61966-2–1 Amend. 1 IEC:2003(E)

If R ,G ,B > 0,003 130 8 ,

sRGB sRGB sRGB

()1,0 / 2,4

′

R = 1,055 ×()R − 0,055

sRGB sRGB

()1,0 / 2,4

′ () (F.11)

G = 1,055 × G − 0,055

sRGB sRGB

()1,0 / 2,4

′

B = 1,055 ×()B − 0,055

sRGB sRGB

The relationship between non-linear sRGB and sYCC is defined as follows:

Y ′ 0,299 0 0,587 0 0,114 0 R′

sYCC sRGB

′ ′

Cb = − 0,168 7 − 0,331 3 0,500 0 G (F.12)

sYCC sRGB

Cr′ 0,500 0 − 0,418 7 − 0,081 3 B′

sYCC sRGB

NOTE The coefficients in equation F.12 are from ITU-R BT.601-5. The ITU-R BT.601-5 defines Y′ of YCC to the

three decimal place accuracy. An additional decimal place is defined above to be consistent with the other matrix

coefficients defined in this standard.

And quantization for sYCC is defined as;

′

Y = round[]()WDC − KDC × Y + KDC

sYCC sYCC

′

Cb = round[]()Range × Cb + Offset (F.13)

sYCC sYCC

[]()′

Cr = round Range × Cr + Offset

sYCC sYCC

For 24-bit encoding (8-bit/channel), the relationship is defined as:

Y = round[]()255 − 0 × Y ′ + 0 = round[255 × Y ′]

sYCC sYCC sYCC

(8)

′ (F.14)

Cb = round[]()255 × Cb + 128

sYCC sYCC

(8)

′

Cr = round[]()255 × Cr + 128

sYCC sYCC

(8)

For 24-bit encoding, the sYCC values shall be limited to a range from 0 to 255 after equation

(8)

F.14.

24-bit encoding (8-bit/channel) shall be the default sYCC encoding bit depth. Other bit depths

may be unsupported in general use.

Where other N-bit/channel encoding is supported ( N > 8 ), the relationship is defined as;

N

′

Y = round[(2 −1)×Y ]

sYCC sYCC

(N)

N N −1

[]()() ′ (F.14′)

Cb = round 2 −1 × Cb + 2

sYCC sYCC

(N)

N N −1

′

Cr = round[]()()2 −1 × Cr + 2

sYCC sYCC

(N)

For N-bit/channel encoding ( N > 8), the sYCC values shall be limited to a range from 0 to

(N)

N

2 –1 after equation F.14′.

---------------------- Page: 6 ----------------------

61966-2–1 Amend. 1 IEC:2003(E) – 7 –

F.4 Transformation from 8-bit sYCC values ( Y , Cb , Cr )

sYCC sYCC sYCC

(8) (8) (8)

to 8-bit sRGB values ( R , G , B )

sRGB(8) sRGB(8) sRGB(8)

Y ′ = Y 255

sYCC sYCC

(8)

′

Cb =()Cb −128 255 (F.15)

sYCC sYCC

(8)

Cr′ =()Cr −128 255

sYCC sYCC

(8)

′ ′

R 1,000 0 0,000 0 1,402 0 Y

sRGB sYCC

′ ′ (F.16)

G = 1,000 0 − 0,344 1 − 0,714 1 Cb

sRGB sYCC

′ ′

B 1,000 0 1,772 0 0,000 0 Cr

sRGB sYCC

′

R = round(255 × R )

sRGB(8) sRGB

G = round(255 × G′ ) (F.17)

sRGB(8) sRGB

′

B = round(255 × B )

sRGB(8) sRGB

NOTE Since 8 bit sYCC values are not limited by the gamut of 8 bit sRGB values, some kind of mapping is

needed for the colours that contains over-ranged non-linear floating point sR′G′B′ tristimulus values (under 0,0 or

over 1,0), when converting 8 bit sYCC to 8 bit sRGB.

F.5 Transformation from 8-bit sRGB values ( R , G , B )

sRGB(8) sRGB(8) sRGB(8)

to 8-bit sYCC values ( )

Y , Cb , Cr

sYCC sYCC sYCC

(8) (8) (8)

′

R = R 255

sRGB sRGB(8)

′

G = G 255 (F.18)

sRGB sRGB(8)

′

B = B 255

sRGB sRGB(8)

Y ′ 0,299 0 0,587 0 0,114 0 R′

sYCC sRGB

′ ′

Cb = − 0,168 7 − 0,331 3 0,500 0 G (F.19)

sYCC sRGB

Cr′ 0,500 0 − 0,418 7 − 0,081 3 B′

sYCC sRGB

Y = round()255 × Y ′

sYCC sYCC

(8)

′ (F.20)

Cb = round[]()255 × Cb + 128

sYCC sYCC

(8)

′

Cr = round[]()255 × Cr + 128

sYCC sYCC

(8)

---------------------- Page: 7 ----------------------

– 8 – 61966-2–1 Amend. 1 IEC:2003(E)

Annex G

(informative)

Extended gamut encoding for sRGB: bg-sRGB

and its YCC transformation: bg-sYCC

G.1 General

This annex provides equations necessary for extended gamut encoding for sRGB. While the

main body of this standard imply that extended gamut encoding is possible by replacing the

KDC and WDC variables in equations 2 and 11, no clear recommendation is given. This annex

provides such specific recommendations, where for 10 bits, KDC = 384 and WDC = 894

(N −3) (N −9)

(KDC= 3× 2 and WDC= 255× 2 +KDC, for N bits where N > 10). This encoding is

called bg-sRGB, and its YCC transformation is called bg-sYCC.

G.2 Transformation from bg-sRGB values ( R , G , B )

bg-sRGB bg-sRGB bg-sRGB

to CIE 1931 XYZ values

The non-linear floating point sR′G′B′ values can be computed using following relationship

′ ()

R = (R − KDC ) WDC − KDC

sRGB bg-sRGB

′

G =()G − KDC()WDC − KDC (G.1)

sRGB bg-sRGB

′

B =()B − KDC()WDC − KDC

sRGB bg-sRGB

For 30-bit encoding (10-bit/channel), WDC = 894, KDC = 384, and the relationship is defined as;

(R − 384)

bg-sRGB

(10)

′

R =

sRGB

510

()

G − 384

bg-sRGB

(10)

′

G = (G.2)

sRGB

510

()B − 384

bg-sRGB

(10)

B′ =

sRGB

510

30-bit encoding (10-bit/channel) shall be the default bg-sRGB encoding bit depth. Other bit

depths may be unsupported in general use.

Where other N-bit/channel encoding is supported ( N > 10 ), the relationship is defined as;

(N −3)

(R − (3× 2 ))

bg-sRGB

(N)

′

R =

sRGB

(N −9)

255 × 2

(N −3)

()G −()3× 2

bg-sRGB

(N)

G′ = (G.2’)

sRGB

(N −9)

255 × 2

(N −3)

()()

B − 3× 2

bg-sRGB

(N0)

′

B =

sRGB

(N −9)

255× 2

---------------------- Page: 8 ----------------------

61966-2–1 Amend. 1 IEC:2003(E) – 9 –

The non-linear sR′G′B′ values are then transformed to CIE 1931 XYZ values as follows:

′ ′ ′

**...**

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