SRGB color space
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- The title of this article is incorrect because of technical limitations. The correct title is sRGB color space.
sRGB color space, or standard RGB (Red Green Blue), is an RGB color space created cooperatively by Hewlett-Packard and Microsoft Corporation. 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 PNG.
The relative luminance of an sRGB image is defined by the following function, after scaling the values in the image to the 0-1 range:
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SRGB_plot.png
Image:sRGB_plot.png
<math> v / 12.92 \qquad \quad \ \mbox{ for } v < 0.04045<math>
<math>\left ( \frac{v + 0.055}{1.055} \right ) ^{2.4}\ \mbox{for } v \ge 0.04045<math>
The linear portion is designed so that the function is invertible without an infinite slope at zero. There is no need to replicate it in actual hardware as this is hidden by reflected internal and ambient light on even the highest-contrast devices.
sRGB also defines the red, green, and blue primaries as colors where one of the three channels is at the maximum value and the other 2 are at zero. All other values produce the color obtained by mulitplying each primary by the value from the above curve and adding them together. sRGB has been criticised for poor placement of these primary colors. If you restrict the indexes to the 0 to 1 range you are unable to address outside the gamut (the triangle produced by them), which is well inside the set of visible colors to a human. However the values of these primaries is of far less importance than the above luminance curve in reproducing an image as well as possible.
It is important to realize that sRGB is designed to match what current (in 1996) CRT monitors do. Vast amounts of software, both professional and personal computer software, assume an 8-bit image file placed unchanged into a 8-bit/channel display buffer will display correctly. For this reason you might assume any 8-bit file found on the Internet is in the sRGB color space (in the absence of embedded profiles or other information). Also all non-CRT hardware, such as LCD screens, digital cameras, and printers, although they don't naturally produce an sRGB curve, are built with compensating circuitry or software so that in the end the also obey this standard, though this is less true in professional equipment. For this reason you might also assume almost any other image file I/O you find with 8 bits per channel is in sRGB.
There is a debate over color spaces that may (to outsiders) be surprising in its forcefulness. sRGB is frequently criticised by publishing professionals because of its narrow color gamut, which means that some colors that are visible, even some colors that can be reproduced in CMYK cannot be represented by sRGB. See RGB color space for the view that Adobe RGB is a preferred standard.
Reference
A Standard Default Color Space for the Internet - sRGB (http://www.w3.org/Graphics/Color/sRGB)