sYCC

sYCC

IEC 61966-2-1 Default YCC encoding transformation for a standard luma-chroma-chroma colour space
Abbreviation	sRGB
Status	Published
Year started	1996
First published	October 18, 1999 (1999-10-18)
Latest version	61966-2–1 Amend. 1 2003
Organization	IEC
Committee	TC/SC: TC 100/TA 2
Base standards	IEC 61966 Colour Measurement and Management in Multimedia Systems and Equipment
Domain	Color space, color model
Website	webstore.iec.ch/publication/6169

sYCC is a standard numerical encoding of colors, similar to the CIE YCbCr encoding, It uses three coordinates: a luma value $Y$, that is roughly proportional to perceived brighness of the color, and two chroma values $C_{b}$ and $C_{r}$, which are roughly the "blueness" and "redness" of the hue. Each coordinate is represented by an integer with some number $N$ of bits, which is interpreted as either unsigned (for $Y$) or signed (for $C_{b}$ and $C_{r}$).

The space is defined by Annex F of the International Electrotechnical Commission (IEC) standard 61966-2-1 Amendment 1 (2003), as a linear transformation of the non-linear sRGB color space defined by the same document.

The official conversion from sYCC to sRGB may result in negative R, G, or B values; meaning that not all sYCC triplets represent colors that can be displayed on a computer screen, printed, or even perceived by the human eye.

sYCC definition

The three unsigned integers $Y,C_{b},C_{r}$ of an sYCC encoded color represent fractional coordinates $Y',C_{b}',C_{r}'$ according to the formulas

$Y'=Y/M$

$C_{b}'=(C_{b}-Z)/M$

$C'_{r}=(C_{b}-Z)/M$

where the scale factor $M=2^{N}-1$ is the maximum unsigned $N$-bit integer, and the offset $Z$ is $2^{N-1}$ (as in the usual two's complement representation of signed integers). Conversely, the encoded integer values are given by

$Y={\mbox{round}}(MY'$

$C_{b}={\mbox{round}}(Z+MC_{b}')$

$C_{r}={\mbox{round}}(Z+MC_{r}')$

with the resulting values clipped to the range $0..M$.

In particular, for $N=8$ (which is the normal bit size), one gets $M=255$ and $Z=128$. Thus the fractional luma $Y'$ ranges from 0 to 1, while the fractional chroma coordinates range from $-128/255\approx -0.50196...$ to $+127/255\approx +0.498039...$.

The standard specifies that these fractional values $Y',C_{b}',C_{r}'$ are related to the non-linear fractional sRGB coordinates $R',G',B'$ by a linear transformation, described by the matrix product

${\begin{bmatrix}Y'\\C_{b}'\\C_{r}'\end{bmatrix}}={\begin{bmatrix}+0.2990&+0.5870&+0.1140\\-0.1687&-0.3313&+0.5000\\+0.5000&-0.4817&-0.0813\end{bmatrix}}{\begin{bmatrix}R'\\G'\\B'\end{bmatrix}}$

This correspondence is the same as the RGB to YCC mapping specified by the old TV standard ITU-R BT.601-5, except that the coefficients of $Y'$ are here defined with four decimal digits instead of just three.

The non-linear fractional sRGB coordinates $R',G',B'$ can be computed from the fractional sYCC coordinates $Y',C_{b}',C_{r}'$ by inverting the above matrix. The standard gives the approximation

${\begin{bmatrix}R'\\G'\\B'\end{bmatrix}}={\begin{bmatrix}+1.0000&0.0000&+1.4020\\+1.0000&-0.3441&-0.7141\\+1.0000&+1.7720&0.0000\end{bmatrix}}{\begin{bmatrix}Y'\\C_{b}'\\C_{r}'\end{bmatrix}}$

which is expected to be accurate enough for $N=8$ bits per component. For bit sizes greater than 8, the standard recommends using a more accurate inverse. It states that the following matrix with 6 decimal digits is accurate enough for $N=16$:

${\begin{bmatrix}R'\\G'\\B'\end{bmatrix}}={\begin{bmatrix}+1.000000&+0.000037&+1.401988\\+1.000000&-0.344113&-0.714104\\+1.000000&+1.771978&+0.000135\end{bmatrix}}{\begin{bmatrix}Y'\\C_{b}'\\C_{r}'\end{bmatrix}}$

The same standard specifies the relation between the non-linear fractional coordinates $R',G',B'$ and the CIE 1931 XYZ coordinates. The connection entails the transfer function ("gamma corection") that maps $R',G',B'$ to the linear R, G, B coordinates, and then a 3D linear transformation that relates these to the CIE $X,Y,Z$.

Since the linear transformation from sRGB to sYCC is defined in terms of non-linear (gamma-encoded) values ($R',G',B'$), rather than the linear ones ($R,G,B$), the $Y'$ component of sYCC is not the CIE Y coordinate, not even a function of it alone. That is, two colors with the same CIE Y value may have different sYCC $Y'$ values, and vice-versa.

Particular values

The integer encoded sYCC triplet $(0,0,0)$ represents the color black whereas $(255,0,0)$ is white (more precisely, the CIE illuminant D65). More generally, triplets $(Y,0,0)$, for $Y$ in 0..255, represent shades of gray.

Note that the 8-bit integer sYCC triplet $(Y,Cb,Cr)=(0,255,255)$ has fractional coordinates $(Y',Cb',Cr')\approx (0.0,0.5,0.5)$, which, according to these matrices, has fractional non-linear sRGB coordinate $G'\approx -0.5\times (0.3341+0.7141)\approx -0.528$, and therefore is not realizable or perceivable. Similarly, the sYCC triplet $(0,0,0)$ has $R'\approx -0.701$ and $B'\approx -0.886$.