SRGB: Difference between revisions

sRGB

Standard RGB
sRGB colors situated at calculated position in CIE 1931 chromaticity diagram. Luminance ${\displaystyle Y}$ set so that ${\displaystyle R+G+B=1}$ to avoid bright lines toward primaries' complementary colours.
Abbreviation	sRGB
Native name	Standard RGB IEC 61966-2-1:1999
Status	Published
First published	October 18, 1999; 24 years ago (1999-10-18)[1]
Organization	IEC[1]
Committee	TC/SC: TC 100/TA 2[1]
Domain	Color space, color model
Website	webstore.iec.ch/publication/6169

sRGB is a standard^[2] RGB (red, green, blue) color space that HP and Microsoft created cooperatively in 1996 to use on monitors, printers, and the Web. It was subsequently standardized by the IEC as IEC 61966-2-1:1999.^[1] sRGB is the current defined standard colorspace for the web, and it is usually the assumed colorspace for images that are neither tagged for a colorspace nor have an embedded color profile.

sRGB uses the same color primaries and white point as ITU-R BT.709, the standard for HDTV^[3]. However sRGB does use the BT.709 transfer function (aka TRC, sometimes referred to as gamma). Instead the sRGB TRC was created for computer processing convenience, as well as being compatible with the era's CRT displays. An associated viewing environment is designed to match typical home and office viewing conditions. sRGB essentially codifies the display specifications for the Windows-based computers & monitors in use at that time.

sRGB includes a number of variants such as Y′_sCb′_sCr′_s, which is a standard luma-chroma-chroma color space extension to sRGB, referenced in Amendment 1 of IEC standard 61966-2-1 "Default RGB colour space - sRGB"^[4].

sRGB defines the chromaticities of the red, green, and blue primaries, the colors where one of the three channels is nonzero and the other two are zero. The gamut of chromaticities that can be represented in sRGB is the color triangle defined by these primaries. As with any RGB color space, for non-negative values of R, G, and B it is not possible to represent colors outside this triangle, which is well inside the range of colors visible to a human with normal trichromatic vision.

The primaries come from HDTV (Rec. 709), which in turn is based on Color TV (Rec. 601). These values reflect the approximate color of consumer CRT phosphors.

sRGB also defines a nonlinear transfer function between the intensity of these primaries and the actual number stored. The curve is similar to the gamma response of a CRT display. This nonlinear conversion means that sRGB is a reasonably efficient use of the values in an integer-based image file to display human-discernible light levels.

Unlike most other RGB color spaces, the sRGB gamma cannot be expressed as a single numerical value. The overall gamma is approximately 2.2, consisting of a linear (gamma 1.0) section near black, and a non-linear section elsewhere involving a 2.4 exponent and a gamma (slope of log output versus log input) changing from 1.0 through about 2.3. The purpose of the linear section is so the curve does not have an infinite slope at zero, which could cause numerical problems.

The sRGB component values ${\displaystyle R_{\mathrm {srgb} }}$, ${\displaystyle G_{\mathrm {srgb} }}$, ${\displaystyle B_{\mathrm {srgb} }}$ are in the range 0 to 1 (values in the range of 0 to 255 should be divided by 255.0).

${\displaystyle C_{\mathrm {linear} }={\begin{cases}{\frac {C_{\mathrm {srgb} }}{12.92}},&C_{\mathrm {srgb} }\leq 0.04045\\\left({\frac {C_{\mathrm {srgb} }+0.055}{1.055}}\right)^{2.4},&C_{\mathrm {srgb} }>0.04045\end{cases}}}$

• where ${\displaystyle C}$ is ${\displaystyle R}$, ${\displaystyle G}$, or ${\displaystyle B}$.

These gamma-expanded values (sometimes called "linear values" or "linear-light values") are multiplied by a matrix to obtain CIE XYZ:

${\displaystyle {\begin{bmatrix}X_{D65}\\Y_{D65}\\Z_{D65}\end{bmatrix}}={\begin{bmatrix}0.4124&0.3576&0.1805\\0.2126&0.7152&0.0722\\0.0193&0.1192&0.9505\end{bmatrix}}{\begin{bmatrix}R_{\mathrm {linear} }\\G_{\mathrm {linear} }\\B_{\mathrm {linear} }\end{bmatrix}}}$

This is actually the matrix for BT.709 primaries, not just for sRGB, the second row is BT.709-2 matrix coefficients.

The CIE XYZ values must be scaled so that the Y of D65 ("white") is 1.0 (X, Y, Z = 0.9505, 1.0000, 1.0890). This is usually true but some color spaces use 100 or other values (such as in CIELAB, when using specified white points).

The first step in the calculation of sRGB from CIE XYZ is a linear transformation, which may be carried out by a matrix multiplication. (The numerical values below match those in the official sRGB specification,^[1][5] which corrected small rounding errors in the original publication^[2] by sRGB's creators, and assume the 2° standard colorimetric observer for CIE XYZ.^[2])

${\displaystyle {\begin{bmatrix}R_{\mathrm {linear} }\\G_{\mathrm {linear} }\\B_{\mathrm {linear} }\end{bmatrix}}={\begin{bmatrix}+3.2406&-1.5372&-0.4986\\-0.9689&+1.8758&+0.0415\\+0.0557&-0.2040&+1.0570\end{bmatrix}}{\begin{bmatrix}X_{D65}\\Y_{D65}\\Z_{D65}\end{bmatrix}}}$

These linear RGB values are not the final result; gamma correction must still be applied. The following formula transforms the linear values into sRGB:

${\displaystyle C_{\mathrm {srgb} }={\begin{cases}12.92C_{\mathrm {linear} },&C_{\mathrm {linear} }\leq 0.0031308\\1.055C_{\mathrm {linear} }^{1/2.4}-0.055,&C_{\mathrm {linear} }>0.0031308\end{cases}}}$

• where ${\displaystyle C}$ is ${\displaystyle R}$, ${\displaystyle G}$, or ${\displaystyle B}$.

These gamma-compressed values (sometimes called "non-linear values") are usually clipped to the 0 to 1 range. This clipping can be done before or after the gamma calculation, or done as part of converting to 8 bits. If values in the range 0 to 255 are required, e.g. for video display or 8-bit graphics, the usual technique is to multiply by 255 and round to an integer.

Amendment 1 to IEC 61966-2-1:1999 describes how to apply the gamma correction to negative values, by applying −f(−x) when x is negative (and f is the sRGB↔linear functions described above), as part of the YCbCr definition. This is also used by scRGB.

Amendment 1 also recommends a higher-precision XYZ to RGB matrix using 7 decimal points, to more accurately invert the RGB to XYZ matrix (which remains at the precision shown above):

${\displaystyle {\begin{bmatrix}R_{\mathrm {linear} }\\G_{\mathrm {linear} }\\B_{\mathrm {linear} }\end{bmatrix}}={\begin{bmatrix}+3.2406255&-1.5372080&-0.4986286\\-0.9689307&+1.8757561&+0.0415175\\+0.0557101&-0.2040211&+1.0569959\end{bmatrix}}{\begin{bmatrix}X_{D65}\\Y_{D65}\\Z_{D65}\end{bmatrix}}}$.^[6]

sRGB is based on a gamma of 2.2, which the standard defines as the EOTF for the display, yet the above transforms show an exponent of 2.4. This is because the net effect of the piecewise decomposition changes the instantaneous gamma at each point in the range: it goes from gamma = 1 at zero to a gamma near 2.4 at maximum intensity, with a median value being close to 2.2. The transformation was designed to approximate a gamma of about 2.2, but with a linear portion near zero to avoid having an infinite slope at K = 0, which can cause computational problems.

Parameterizing the piecewise formulae for ${\displaystyle C_{\mathrm {linear} }}$ using ${\displaystyle K_{0}}$ for the 0.04045, ${\displaystyle \phi }$ for the 12.92, and ${\displaystyle a}$ for the 0.055, the continuity condition at the break point is

${\displaystyle \left({\frac {K_{0}+a}{1+a}}\right)^{\gamma }={\frac {K_{0}}{\phi }}.}$

Solving with ${\displaystyle \gamma =2.4}$ and the standard value ${\displaystyle \phi =12.92}$ yields two solutions, ${\displaystyle K_{0}}$ ≈ ${\displaystyle 0.0381548}$ or ${\displaystyle 0.0404482}$. The IEC 61966-2-1 standard uses the rounded value ${\displaystyle K_{0}=0.04045}$, which yields ${\displaystyle \beta ={\frac {K_{0}}{\phi }}\approx 0.0031308}$. However, if we impose the condition that the slopes match as well then we must have

${\displaystyle \gamma \left({\frac {K_{0}+a}{1+a}}\right)^{\gamma -1}\left({\frac {1}{1+a}}\right)={\frac {1}{\phi }}.}$

We now have two equations. If we take the two unknowns to be ${\displaystyle K_{0}}$ and ${\displaystyle \phi }$ then we can solve to give

${\displaystyle K_{0}={\frac {a}{\gamma -1}}}$,

${\displaystyle \phi ={\frac {(1+a)^{\gamma }(\gamma -1)^{\gamma -1}}{(a^{\gamma -1})(\gamma ^{\gamma })}}.}$

Substituting ${\displaystyle a=0.055}$ and ${\displaystyle \gamma =2.4}$ gives ${\displaystyle K_{0}={\frac {11}{280}}\approx 0.0392857}$ and ${\displaystyle \phi \approx 12.9232102}$, with the corresponding linear-domain threshold at ${\displaystyle \beta \approx 0.00303993}$. These values, rounded to ${\displaystyle K_{0}=0.03928}$, ${\displaystyle \phi =12.92321}$ and ${\displaystyle \beta =0.00304}$, sometimes describe sRGB conversion.^[7] Draft publications by sRGB's creators^[2] rounded to ${\displaystyle K_{0}=0.03928}$ and ${\displaystyle \phi =12.92}$, hence ${\displaystyle \beta \approx 0.00304025}$, resulting in a small discontinuity in the curve. Some authors adopted these incorrect values, in part because the draft paper was freely available and the official IEC standard is behind a paywall.^[8] For the standard, the rounded value ${\displaystyle \phi =12.92}$ was kept and the ${\displaystyle K_{0}}$ value was recomputed to make the resulting curve continuous, as described above, resulting in a slope discontinuity from 12.92 below the intersection to 12.70 above.

The sRGB specification assumes a dimly lit encoding (creation) environment with an ambient correlated color temperature (CCT) of 5003 K. This differs from the CCT of the illuminant (D65). Using D50 for both would have made the white point of most photographic paper appear excessively blue.^[9] The other parameters, such as the luminance level, are representative of a typical CRT monitor.

For optimal results, the ICC recommends using the encoding viewing environment (i.e., dim, diffuse lighting) rather than the less-stringent typical viewing environment.^[2]

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Due to the standardization of sRGB on the Internet, on computers, and on printers, many low- to medium-end consumer digital cameras and scanners use sRGB as the default (or only available) working color space. However, consumer-level CCDs are typically uncalibrated, meaning that even though the image is being labeled as sRGB, one can't conclude that the image is color-accurate sRGB.

If the color space of an image is unknown and it is an 8-bit image format, sRGB is usually the assumed default, and is a safe choice. An ICC profile or a LUT may be used. ICC profiles for sRGB are widely distributed, and the ICC distributes several variants of sRGB profiles, ^[10] including variants for ICCmax, version 4, and version 2. Version 4 is generally recommended, but version 2 is still commonly used and is the most compatible with other software including browsers. Version 2 of the ICC profile specification does not officially support piecewise parametric curve encoding ("para"), though version 2 does support simple gamma curves. Nevertheless, LUTs are more commonly used as LUTs are computationally more efficient.

In 2020, an sRGB related bug in the Android operating system resulted in some devices crashing and requiring a factory reset. While some reported this was due to an ICC profile, it was actually traced to a rounding error in a hard coded matrix in Java^[11].

As the sRGB gamut meets or exceeds the gamut of a low-end inkjet printer, an sRGB image is often regarded as satisfactory for home printing. sRGB is sometimes avoided by high-end print publishing professionals because its color gamut is not big enough, especially in the blue-green colors, to include all the colors that can be reproduced in CMYK printing. Images intended for professional printing via a fully color-managed workflow (e.g. prepress output) sometimes use another color space such as Adobe RGB (1998), which accommodates a wider gamut. Such images used on the Internet may be converted to sRGB using color management tools that are usually included with software that works in these other color spaces.

The two dominant programming interfaces for 3D graphics, OpenGL and Direct3D, have both incorporated support for the sRGB gamma curve. OpenGL supports textures with sRGB gamma encoded color components (first introduced with EXT_texture_sRGB extension,^[12] added to the core in OpenGL 2.1) and rendering into sRGB gamma encoded framebuffers (first introduced with EXT_framebuffer_sRGB extension,^[13] added to the core in OpenGL 3.0). Correct mipmapping and interpolation of sRGB gamma textures has direct hardware support in texturing units of most modern GPUs (for example nVidia GeForce 8 performs conversion from 8-bit texture to linear values before interpolating those values), and does not have any performance penalty.^[14]

• IEC 61966-2-1:1999 is the official specification of sRGB. It provides viewing environment, encoding, and colorimetric details.
• Amendment A1:2003 to IEC 61966-2-1:1999 describes an analogous sYCC encoding for YCbCr color spaces, an extended-gamut RGB encoding, and a CIELAB transformation.
• sRGB, International Color Consortium
• The fourth working draft of IEC 61966-2-1 is available online, but is not the complete standard. It can be downloaded from www2.units.it.
• Archive copy of sRGB.com, now unavailable, containing much information on the design, principles, and use of sRGB

• International Color Consortium
• A Standard Default Color Space for the Internet – sRGB the early, obsolete draft of the standard at w3.org
• Conversion matrices for RGB vs. XYZ conversion
• Will the Real sRGB Profile Please Stand Up?