SRGB color space

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CIE 1931 xy chromaticity diagram showing the gamut of the sRGB color space and location of the primaries. The D65 white point is shown in the center.

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 that point. Below a compressed value of 0.04045 or a linear intensity of 0.00313, the curve is linear so the gamma is 1.

sRGB color space, a standard RGB (Red Green Blue) color space, was created cooperatively by Hewlett-Packard and Microsoft Corporation for use on monitors, printers, and the Internet. It has been endorsed by the W3C, Exif, Intel, Pantone, Corel, and many other industry players. It is also well accepted by Open Source software such as the GIMP, and is used in proprietary and open graphics file formats such as SVG.

sRGB is intended as a common color space for the creation of images for viewing on the Internet and World Wide Web (WWW), the resultant color space chosen using a gamma of 2.2, the average response to linear voltage levels of CRT displays at that time. A common misconception is that Macintosh (and SGI) hardware had different response, but this was due to a hardware lookup table that has been removed in OS X^{[citation needed]} so that a Macintosh naturally obeys the sRGB standard. The gamma was chosen to match the usage of hardware by Windows and Unix workstations.

It is important to realize that sRGB was designed to match what current (in 1996) CRT monitors do, not to be an ideal color space. Vast amounts of software, both professional and personal computer software, assumed an 8-bit image file placed unchanged into an 8-bit/channel display buffer will display with these colors and intensities. Modern non-CRT hardware, such as LCD, digital cameras, and printers, although they don't naturally produce an sRGB curve, have been built with compensating circuitry or software so that in the end they also obey this standard (this is somewhat less true for high-end professional equipment). For this reason you can assume (in the absence of embedded profiles or any other information) that any 8-bit image file, and any 8-bit image API or device interface, is in the sRGB color space.

[edit] Overview

sRGB defines the chromaticities of the red, green, and blue primaries, the colors where one of the three channels is at the maximum value and the other two are at zero. In CIE xy chromaticity coordinates red is at [0.6400, 0.3300], green is at [0.3000, 0.6000], and blue is at [0.1500, 0.0600], and the white point is the D65 white point at [0.3127,0.3290]. As with any RGB color space, for non-negative values of R, G, and B it is not possible to represent colors outside the color triangle define by the primaries, the chromaticity gamut, which is well inside the range of colors visible to a human.

sRGB also defines a nonlinear transformation between the intensity of these primaries and the actual number stored. The curve is similar to the gamma response of a CRT display. It is more important to replicate this curve than the primaries to get correct display of an sRGB image. This nonlinear conversion means that sRGB is a reasonably efficient use of the values in an integer-based image file to display human-discernable light levels.

sRGB is usually avoided by 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. See RGB color space for the view that Adobe RGB is a preferred colorspace for publishing.

[edit] Specification of the transformation

[edit] The forward transformation (CIE xyY or CIE XYZ to sRGB)

^[1]Calculation of sRGB tristimulus values from CIE xyY chromaticity coordinates first requires transformation to CIE XYZ tristimulus values.

Now the XYZ values can be transformed to linear RGB values using a matrix. These linear values are not the final result.

R_linear, G_linear and B_linear are in in the range [0,1]. sRGB was designed to reflect a typical real-world monitor with a gamma of 2.2, and the following formula transforms the linear values into sRGB. Let C_linear be R_linear, G_linear, or B_linear, and C_srgb be R_srgb,G_srgb or B_srgb:

• If then
• If then
• 

These gamma corrected values are in the range 0 to 1. 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.

[edit] The reverse transformation

Again the sRGB component values R_srgb, G_srgb, B_srgb are in the range 0 to 1. (A range of 0 to 255 can simply be divided by 255).

where

for

otherwise

[edit] Theory of the transformation

The non linear part of the transformation was designed to approximate a gamma of about 2.2, but without having a zero slope at K=0, which can cause numerical problems. This is approximately true for the sRGB transformation. The condition that g(K) match at some K₀ is:

where the standard value of φ = 12.92 which was used above, yields K₀=0.04045... and this is the transformation used. If we impose the condition that the slope match as well then we must have:

We now have two equations. If we take the two unknowns to be K₀ and φ then we can solve to give K₀=0.03928... and φ=12.9232.... These values are sometimes used for the sRGB specification, although they are not standard.

[edit] Usage

As the recommended color space for the Internet sRGB should be used for editing and saving all images intended for publication to the WWW, however due to the somewhat limited gamut available compared to other color spaces, images intended for professional printing, e.g., prepress output, should where possible, choose another color space such as Adobe RGB (1998), which allows for a wider gamut. Another alternative that is becoming possible on modern hardware is to work with sRGB primaries but allow negative and greater than one intensities, which makes the gamut infinite).

Images intended for the Internet and taken/created in one of the other color spaces may be converted to sRGB when editing using a suitable editing program, e.g., Paint Shop Pro or Adobe Photoshop, etc., although the original non-sRGB file should be saved and the conversion to sRGB done on a copy, as some loss of image information occurs when converting to the narrower color space and it is advisable to retain the original unaltered non-sRGB image file for possible later non-Web use.

Due to the use of sRGB on the Internet many low- to medium-end consumer digital cameras and scanners use sRGB as the default or only available working color space and, used in conjunction with an inkjet printer, produces what is regarded by many as results quite satisfactory for home use.

The two dominant programming interfaces for 3D graphics, OpenGL and Direct3D, have both incorporated sRGB. OpenGL 2.1 incorproates sRGB textures which were first introduced by the EXT_texture_sRGB extension. OpenGL's EXT_framebuffer_sRGB extension supports rendering into framebuffers assuming either a linear or sRGB color space. DirectX 9 supports sRGB textures and rendering into sRGB surfaces using Direct3D.

[edit] References

• 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.
• The fourth working draft of IEC 61966-2-1 is available online, but is not the complete standard.

1. ^ sRGB description with XYZ conversion formula

[edit] External links

• International Color Consortium
• Basic sRGB Math
• A Standard Default Color Space for the Internet - sRGB
• OpenGL extension for sRGB textures