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sRGB

sRGB is a standard 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 International Electrotechnical Commission (IEC) as IEC 61966-2-1:1999. 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
Standard RGB
sRGB colors situated at calculated position inCIE 1931 chromaticity diagram. Luminance Y {\displaystyle Y} set so that R + G + B = 1 {\displaystyle R+G+B=1} to avoid bright lines toward primaries' complementary colours.
Native name

Contents

Chromaticity Red Green Blue White point
x 0.6400 0.3000 0.1500 0.3127
y 0.3300 0.6000 0.0600 0.3290
Y 0.2126 0.7152 0.0722 1.0000

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 (ITU-R BT.709), which are somewhat different from those for older color TV systems (ITU-R BT.601). These values were chosen to reflect the approximate color of consumer CRT phosphors at the time of its design. Since flat-panel displays at the time were generally designed to emulate CRT characteristics, the values also reflected prevailing practice for other display devices as well.

On an sRGB display, each solid bar should look as bright as the surrounding striped dither. (Note: must be viewed at original, 100% size)

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.

From sRGB to CIE XYZ

The sRGB component values R s r g b {\displaystyle R_{\mathrm {srgb} }} , G s r g b {\displaystyle G_{\mathrm {srgb} }} , B s r g b {\displaystyle B_{\mathrm {srgb} }} are in the range 0 to 1. When represented digitally as 8-bit numbers, these color component values are in the range of 0 to 255, and should be divided (in a floating point representation) by 255 to convert to the range of 0 to 1.

C l i n e a r = { C s r g b 12.92 , C s r g b 0.04045 ( C s r g b + 0.055 1.055 ) 2.4 , C s r g b > 0.04045 {\displaystyle C_{\mathrm {linear} }={\begin{cases}{\dfrac {C_{\mathrm {srgb} }}{12.92}},&C_{\mathrm {srgb} }\leq 0.04045\\[5mu]\left({\dfrac {C_{\mathrm {srgb} }+0.055}{1.055}}\right)^{\!2.4},&C_{\mathrm {srgb} }>0.04045\end{cases}}}

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

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

[ X D 65 Y D 65 Z D 65 ] = [ 0.4124 0.3576 0.1805 0.2126 0.7152 0.0722 0.0193 0.1192 0.9505 ] [ R linear G linear B linear ] {\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_{\text{linear}}\\G_{\text{linear}}\\B_{\text{linear}}\end{bmatrix}}}

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

From CIE XYZ to sRGB

The CIE XYZ values must be scaled so that the Y of D65 ("white") is 1.0 (X = 0.9505, Y = 1.0000, Z = 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, which corrected small rounding errors in the original publication by sRGB's creators, and assume the 2° standard colorimetric observer for CIE XYZ.)

[ R linear G linear B linear ] = [ + 3.2406 1.5372 0.4986 0.9689 + 1.8758 + 0.0415 + 0.0557 0.2040 + 1.0570 ] [ X D 65 Y D 65 Z D 65 ] {\displaystyle {\begin{bmatrix}R_{\text{linear}}\\G_{\text{linear}}\\B_{\text{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:

C sRGB = { 12.92 C linear , C linear 0.0031308 1.055 C linear 1 / 2.4 0.055 , C linear > 0.0031308 {\displaystyle C_{\text{sRGB}}={\begin{cases}12.92C_{\text{linear}},&C_{\text{linear}}\leq 0.0031308\\[5mu]1.055C_{\text{linear}}^{1/2.4}-0.055,&C_{\text{linear}}>0.0031308\end{cases}}}

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

sYCC extended-gamut transformation

Amendment 1 to IEC 61966-2-1:1999, approved in 2003, includes the definition of a Y′Cb′Cr′ color representation called sYCC, which is based on sRGB except for supporting negative values of the R, G and B components. Although the RGB color primaries are based on BT.709, the equations for transformation from sRGB to sYCC and vice versa are based on BT.601 instead of BT.709.

The amendment describes how to apply the gamma correction to negative values, by applyingf(−x) whenx is negative (andf is the sRGB↔linear functions described above), as part of the Y′Cb′Cr′ definition. This is also used by scRGB.

The amendment also recommends a higher-precision XYZ to sRGB matrix using seven decimal points, to more accurately invert the sRGB to XYZ matrix (which remains at the precision shown above):

[ R linear G linear B linear ] = [ + 3.2406255 1.5372080 0.4986286 0.9689307 + 1.8757561 + 0.0415175 + 0.0557101 0.2040211 + 1.0569959 ] [ X D 65 Y D 65 Z D 65 ] {\displaystyle {\begin{bmatrix}R_{\text{linear}}\\G_{\text{linear}}\\B_{\text{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}}} .

Additionally, the amendment contains an extended-gamut encoding for sRGB called bg-sRGB and a YCC transformation for it called bg-sYCC.

x axis: encoded value
Left y axis: effective local gamma
Right y axis: intensity
Plot of the sRGB intensities versus sRGB numerical values (red), and this function's slope in log-log space (blue), which is the effective gamma at each point. Below a compressed value of 0.04045 or a linear intensity of 0.00313, the curve is linear so the gamma is 1. Behind the red curve is a dashed black curve showing an exact gamma = 2.2 power law.

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 C l i n e a r {\displaystyle C_{\mathrm {linear} }} using K 0 {\displaystyle K_{0}} for the 0.04045, ϕ {\displaystyle \phi } for the 12.92, and a {\displaystyle a} for the 0.055, the continuity condition at the break point is

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

Solving with γ = 2.4 {\displaystyle \gamma =2.4} and the standard value ϕ = 12.92 {\displaystyle \phi =12.92} yields two solutions, K 0 {\displaystyle K_{0}} 0.0381548 {\displaystyle 0.0381548} or 0.0404482 {\displaystyle 0.0404482} . The IEC 61966-2-1 standard uses the rounded value K 0 = 0.04045 {\displaystyle K_{0}=0.04045} , which yields β = K 0 ϕ 0.0031308 {\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

γ ( K 0 + a 1 + a ) γ 1 ( 1 1 + a ) = 1 ϕ . {\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 K 0 {\displaystyle K_{0}} and ϕ {\displaystyle \phi } then we can solve to give

K 0 = a γ 1 {\displaystyle K_{0}={\frac {a}{\gamma -1}}} ,
ϕ = ( 1 + a ) γ ( γ 1 ) γ 1 ( a γ 1 ) ( γ γ ) . {\displaystyle \phi ={\frac {(1+a)^{\gamma }(\gamma -1)^{\gamma -1}}{(a^{\gamma -1})(\gamma ^{\gamma })}}.}

Substituting a = 0.055 {\displaystyle a=0.055} and γ = 2.4 {\displaystyle \gamma =2.4} gives K 0 = 11 280 0.0392857 {\displaystyle K_{0}={\frac {11}{280}}\approx 0.0392857} and ϕ 12.9232102 {\displaystyle \phi \approx 12.9232102} , with the corresponding linear-domain threshold at β 0.00303993 {\displaystyle \beta \approx 0.00303993} . These values, rounded to K 0 = 0.03928 {\displaystyle K_{0}=0.03928} , ϕ = 12.92321 {\displaystyle \phi =12.92321} and β = 0.00304 {\displaystyle \beta =0.00304} , sometimes describe sRGB conversion. Draft publications by sRGB's creators rounded to K 0 = 0.03928 {\displaystyle K_{0}=0.03928} and ϕ = 12.92 {\displaystyle \phi =12.92} , hence β 0.00304025 {\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. For the standard, the rounded value ϕ = 12.92 {\displaystyle \phi =12.92} was kept and the K 0 {\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.

CIE 1931 xy chromaticity diagram showing the gamut of the sRGB color space (the triangle). The outer curved boundary is the spectral (or monochromatic) locus, with wavelengths shown in nanometers (labeled in blue). This image is drawn using sRGB, so colors outside the triangle cannot be accurately colored and have been interpolated. The D65 white point is shown in the center, and the Planckian locus is shown with color temperatures labeled in kelvins. D65 is not an ideal 6504-kelvin black body because it is based on atmospheric filtered daylight.
Parameter Value
Screen luminance level 80 cd/m2
Illuminant white point x = 0.3127, y = 0.3290 (D65)
Image surround reflectance 20% (~medium gray)
Encoding ambient illuminance level 64 lux
Encoding ambient white point x = 0.3457, y = 0.3585 (D50)
Encoding viewing flare 1.0%
Typical ambient illuminance level 200 lux
Typical ambient white point x = 0.3457, y = 0.3585 (D50)
Typical viewing flare 5.0%

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. 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.

Comparison of some RGB and CMYK colour gamuts on a CIE 1931 xy chromaticity diagram

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, in part because color spaces with a larger gamut need a higher bit depth to maintain a low color error rate (∆E). An ICC profile or a LUT may be used to convert sRGB to other color spaces. ICC profiles for sRGB are widely distributed, and the ICC distributes several variants of sRGB profiles, 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. Even when parametric curves are used, software will often reduce to a run-time LUT for efficient processing.

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, added to the core in OpenGL 2.1) and rendering into sRGB gamma encoded framebuffers (first introduced with EXT_framebuffer_sRGB extension, 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.

  1. "IEC 61966-2-1:1999". IEC Webstore. International Electrotechnical Commission. Retrieved3 March 2017.
  2. Michael Stokes; Matthew Anderson; Srinivasan Chandrasekar; Ricardo Motta (November 5, 1996). "A Standard Default Color Space for the Internet – sRGB, Version 1.10".
  3. Charles A. Poynton (2003). Digital Video and HDTV: Algorithms and Interfaces. Morgan Kaufmann. ISBN 1-55860-792-7.
  4. "IEC 61966-2-1:1999 Multimedia systems and equipment – Colour measurement and management – Part 2-1: Colour management – Default RGB colour space – sRGB: Amendment 1". International Electrotechnical Commission. 2003.
  5. "How to interpret the sRGB color space"(PDF). color.org. Retrieved17 October 2017.
  6. Phil Green & Lindsay W. MacDonald (2002). Colour Engineering: Achieving Device Independent Colour. John Wiley and Sons. ISBN 0-471-48688-4.
  7. Jon Y. Hardeberg (2001). Acquisition and Reproduction of Color Images: Colorimetric and Multispectral Approaches. Universal-Publishers.com. ISBN 1-58112-135-0.
  8. Rodney, Andrew (2005). Color Management for Photographers. Focal Press. p. 121. ISBN 978-0-240-80649-5. Why Calibrate Monitor to D65 When Light Booth is D50
  9. sRGB profiles, ICC
  10. "EXT_texture_sRGB". 24 January 2007. Retrieved12 May 2020.
  11. "EXT_framebuffer_sRGB". 17 September 2010. Retrieved12 May 2020.
  12. "GPU Gems 3: Chapter 24. The Importance of Being Linear, section 24.4.1". NVIDIA Corporation. Retrieved3 March 2017.

Standards

  • 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 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

sRGB
sRGB Language Watch Edit sRGB is a standard RGB red green blue color space that HP and Microsoft created cooperatively in 1996 to use on monitors printers and the Web 2 It was subsequently standardized by the International Electrotechnical Commission 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 sRGBStandard RGBsRGB colors situated at calculated position in CIE 1931 chromaticity diagram Luminance Y displaystyle Y set so that R G B 1 displaystyle R G B 1 to avoid bright lines toward primaries complementary colours Native nameStandard RGB IEC 61966 2 1 1999StatusPublishedFirst publishedOctober 18 1999 22 years ago 1999 10 18 1 OrganizationIEC 1 CommitteeTC SC TC 100 TA 2 1 DomainColor space color modelAbbreviationsRGBWebsitewebstore wbr iec wbr ch wbr publication wbr 6169 sRGB uses the same color primaries and white point as ITU R BT 709 the standard for HDTV 3 However sRGB does not use the BT 709 nonlinear transfer function sometimes informally referred to as gamma Instead the sRGB transfer function 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 and monitors in use at that time An amendment of the IEC 61966 2 1 standard document that defines sRGB includes the definition of a number of variants including sYCC which is a Y Cb Cr luma chroma chroma color representation of sRGB colors with an extended range of values in the RGB domain supporting negative values in the RGB domain 4 Contents 1 The sRGB gamut 2 The sRGB transfer function gamma 3 Transformation 3 1 From sRGB to CIE XYZ 3 2 From CIE XYZ to sRGB 3 3 sYCC extended gamut transformation 4 Theory of the transformation 5 Viewing environment 6 Usage 7 References 7 1 Standards 8 External linksThe sRGB gamut EditChromaticity Red Green Blue White pointx 0 6400 0 3000 0 1500 0 3127y 0 3300 0 6000 0 0600 0 3290Y 0 2126 0 7152 0 0722 1 0000 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 ITU R BT 709 which are somewhat different from those for older color TV systems ITU R BT 601 These values were chosen to reflect the approximate color of consumer CRT phosphors at the time of its design Since flat panel displays at the time were generally designed to emulate CRT characteristics the values also reflected prevailing practice for other display devices as well 1 The sRGB transfer function gamma Edit On an sRGB display each solid bar should look as bright as the surrounding striped dither Note must be viewed at original 100 size 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 Transformation EditFrom sRGB to CIE XYZ Edit The sRGB component values R s r g b displaystyle R mathrm srgb G s r g b displaystyle G mathrm srgb B s r g b displaystyle B mathrm srgb are in the range 0 to 1 When represented digitally as 8 bit numbers these color component values are in the range of 0 to 255 and should be divided in a floating point representation by 255 to convert to the range of 0 to 1 C l i n e a r C s r g b 12 92 C s r g b 0 04045 C s r g b 0 055 1 055 2 4 C s r g b gt 0 04045 displaystyle C mathrm linear begin cases dfrac C mathrm srgb 12 92 amp C mathrm srgb leq 0 04045 5mu left dfrac C mathrm srgb 0 055 1 055 right 2 4 amp C mathrm srgb gt 0 04045 end cases where C displaystyle C is R displaystyle R G displaystyle G or B displaystyle B These gamma expanded values sometimes called linear values or linear light values are multiplied by a matrix to obtain CIE XYZ X D 65 Y D 65 Z D 65 0 4124 0 3576 0 1805 0 2126 0 7152 0 0722 0 0193 0 1192 0 9505 R linear G linear B linear displaystyle begin bmatrix X D65 Y D65 Z D65 end bmatrix begin bmatrix 0 4124 amp 0 3576 amp 0 1805 0 2126 amp 0 7152 amp 0 0722 0 0193 amp 0 1192 amp 0 9505 end bmatrix begin bmatrix R text linear G text linear B text linear end bmatrix This is actually the matrix for BT 709 primaries not just for sRGB the second row corresponds to the BT 709 2 luma coefficients From CIE XYZ to sRGB Edit The CIE XYZ values must be scaled so that the Y of D65 white is 1 0 X 0 9505 Y 1 0000 Z 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 R linear G linear B linear 3 2406 1 5372 0 4986 0 9689 1 8758 0 0415 0 0557 0 2040 1 0570 X D 65 Y D 65 Z D 65 displaystyle begin bmatrix R text linear G text linear B text linear end bmatrix begin bmatrix 3 2406 amp 1 5372 amp 0 4986 0 9689 amp 1 8758 amp 0 0415 0 0557 amp 0 2040 amp 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 C sRGB 12 92 C linear C linear 0 0031308 1 055 C linear 1 2 4 0 055 C linear gt 0 0031308 displaystyle C text sRGB begin cases 12 92C text linear amp C text linear leq 0 0031308 5mu 1 055C text linear 1 2 4 0 055 amp C text linear gt 0 0031308 end cases where C displaystyle C is R displaystyle R G displaystyle G or B 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 sYCC extended gamut transformation Edit Amendment 1 to IEC 61966 2 1 1999 approved in 2003 includes the definition of a Y Cb Cr color representation called sYCC which is based on sRGB except for supporting negative values of the R G and B components Although the RGB color primaries are based on BT 709 the equations for transformation from sRGB to sYCC and vice versa are based on BT 601 instead of BT 709 4 The amendment 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 Y Cb Cr definition This is also used by scRGB The amendment also recommends a higher precision XYZ to sRGB matrix using seven decimal points to more accurately invert the sRGB to XYZ matrix which remains at the precision shown above R linear G linear B linear 3 2406255 1 5372080 0 4986286 0 9689307 1 8757561 0 0415175 0 0557101 0 2040211 1 0569959 X D 65 Y D 65 Z D 65 displaystyle begin bmatrix R text linear G text linear B text linear end bmatrix begin bmatrix 3 2406255 amp 1 5372080 amp 0 4986286 0 9689307 amp 1 8757561 amp 0 0415175 0 0557101 amp 0 2040211 amp 1 0569959 end bmatrix begin bmatrix X D65 Y D65 Z D65 end bmatrix 4 Additionally the amendment contains an extended gamut encoding for sRGB called bg sRGB and a YCC transformation for it called bg sYCC 4 Theory of the transformation Edit x axis encoded value Left y axis effective local gamma Right y axis intensity Plot of the sRGB intensities versus sRGB numerical values red and this function s slope in log log space blue which is the effective gamma at each point Below a compressed value of 0 04045 or a linear intensity of 0 00313 the curve is linear so the gamma is 1 Behind the red curve is a dashed black curve showing an exact gamma 2 2 power law 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 C l i n e a r displaystyle C mathrm linear using K 0 displaystyle K 0 for the 0 04045 ϕ displaystyle phi for the 12 92 and a displaystyle a for the 0 055 the continuity condition at the break point is K 0 a 1 a g K 0 ϕ displaystyle left frac K 0 a 1 a right gamma frac K 0 phi Solving with g 2 4 displaystyle gamma 2 4 and the standard value ϕ 12 92 displaystyle phi 12 92 yields two solutions K 0 displaystyle K 0 0 0381548 displaystyle 0 0381548 or 0 0404482 displaystyle 0 0404482 The IEC 61966 2 1 standard uses the rounded value K 0 0 04045 displaystyle K 0 0 04045 which yields b K 0 ϕ 0 0031308 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 g K 0 a 1 a g 1 1 1 a 1 ϕ 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 K 0 displaystyle K 0 and ϕ displaystyle phi then we can solve to give K 0 a g 1 displaystyle K 0 frac a gamma 1 ϕ 1 a g g 1 g 1 a g 1 g g displaystyle phi frac 1 a gamma gamma 1 gamma 1 a gamma 1 gamma gamma Substituting a 0 055 displaystyle a 0 055 and g 2 4 displaystyle gamma 2 4 gives K 0 11 280 0 0392857 displaystyle K 0 frac 11 280 approx 0 0392857 and ϕ 12 9232102 displaystyle phi approx 12 9232102 with the corresponding linear domain threshold at b 0 00303993 displaystyle beta approx 0 00303993 These values rounded to K 0 0 03928 displaystyle K 0 0 03928 ϕ 12 92321 displaystyle phi 12 92321 and b 0 00304 displaystyle beta 0 00304 sometimes describe sRGB conversion 6 Draft publications by sRGB s creators 2 rounded to K 0 0 03928 displaystyle K 0 0 03928 and ϕ 12 92 displaystyle phi 12 92 hence b 0 00304025 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 7 For the standard the rounded value ϕ 12 92 displaystyle phi 12 92 was kept and the K 0 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 Viewing environment Edit CIE 1931 xy chromaticity diagram showing the gamut of the sRGB color space the triangle The outer curved boundary is the spectral or monochromatic locus with wavelengths shown in nanometers labeled in blue This image is drawn using sRGB so colors outside the triangle cannot be accurately colored and have been interpolated The D65 white point is shown in the center and the Planckian locus is shown with color temperatures labeled in kelvins D65 is not an ideal 6504 kelvin black body because it is based on atmospheric filtered daylight Parameter ValueScreen luminance level 80 cd m2Illuminant white point x 0 3127 y 0 3290 D65 Image surround reflectance 20 medium gray Encoding ambient illuminance level 64 luxEncoding ambient white point x 0 3457 y 0 3585 D50 Encoding viewing flare 1 0 Typical ambient illuminance level 200 luxTypical ambient white point x 0 3457 y 0 3585 D50 Typical viewing flare 5 0 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 8 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 Usage Edit Comparison of some RGB and CMYK colour gamuts on a CIE 1931 xy chromaticity diagram 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 in part because color spaces with a larger gamut need a higher bit depth to maintain a low color error rate E An ICC profile or a LUT may be used to convert sRGB to other color spaces ICC profiles for sRGB are widely distributed and the ICC distributes several variants of sRGB profiles 9 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 9 Nevertheless LUTs are more commonly used as LUTs are computationally more efficient Even when parametric curves are used software will often reduce to a run time LUT for efficient processing 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 10 added to the core in OpenGL 2 1 and rendering into sRGB gamma encoded framebuffers first introduced with EXT framebuffer sRGB extension 11 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 12 References Edit a b c d e f IEC 61966 2 1 1999 IEC Webstore International Electrotechnical Commission Retrieved 3 March 2017 a b c d e Michael Stokes Matthew Anderson Srinivasan Chandrasekar Ricardo Motta November 5 1996 A Standard Default Color Space for the Internet sRGB Version 1 10 Charles A Poynton 2003 Digital Video and HDTV Algorithms and Interfaces Morgan Kaufmann ISBN 1 55860 792 7 a b c d IEC 61966 2 1 1999 Multimedia systems and equipment Colour measurement and management Part 2 1 Colour management Default RGB colour space sRGB Amendment 1 International Electrotechnical Commission 2003 How to interpret the sRGB color space PDF color org Retrieved 17 October 2017 Phil Green amp Lindsay W MacDonald 2002 Colour Engineering Achieving Device Independent Colour John Wiley and Sons ISBN 0 471 48688 4 Jon Y Hardeberg 2001 Acquisition and Reproduction of Color Images Colorimetric and Multispectral Approaches Universal Publishers com ISBN 1 58112 135 0 Rodney Andrew 2005 Color Management for Photographers Focal Press p 121 ISBN 978 0 240 80649 5 Why Calibrate Monitor to D65 When Light Booth is D50 a b sRGB profiles ICC EXT texture sRGB 24 January 2007 Retrieved 12 May 2020 EXT framebuffer sRGB 17 September 2010 Retrieved 12 May 2020 GPU Gems 3 Chapter 24 The Importance of Being Linear section 24 4 1 NVIDIA Corporation Retrieved 3 March 2017 Standards Edit 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 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 sRGBExternal links EditInternational 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 Retrieved from https en wikipedia org w index php title SRGB amp oldid 1052660776, wikipedia, wiki, book,

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