Proposal for a Standard Default Color Space for the Internet—sRGB Matthew Anderson,* Ricardo Motta,† Srinivasan Chandrasekar,* Michael Stokes† *Microsoft, Redmond, Washington †Hewlett-Packard, Palo Alto, California Introduction could easily and confidently communicate color without further color management overhead in the most common HP and Microsoft propose the addition of support for a stan- situations. The two major factors of this RGB space are the dard color space, sRGB, within the Microsoft OS’s, HP colorimetric RGB definition and the gamma value of 2.2, products and the Internet. The aim of this color space is to along with a number of secondary details necessary to en- complement the current color management strategies by able the clear and unambiguous communication of color. enabling a third method of handling color in the OS’s and the Internet that utilizes a simple and robust device inde- Colorimetric RGB pendent color definition that will provide good quality and The dichotomy between the device dependent (e.g. backwards compatibility with minimum transmission and amounts of ink expressed in CMYK or digitized video volt- system overhead. Based on a colorimetric RGB color space ages expressed in RGB) and device independent color well suited to Cathode Ray Tube (CRT) monitors, televi- spaces (such as CIELAB or CIEXYZ) has created a per- sion, scanners, digital cameras, and printing systems, such formance burden on the system because of the complexity a space can be supported with minimum cost to software of the transforms, and a reliability gap since the complex- and hardware vendors. Our intent here is to promote its ity and variety of the transforms make it hard to ensure that adoption by showing the benefits of supporting a standard the system is properly configured. color space, the suitability of the standard color space we To address these concerns and serve the needs of PC are proposing, describe some of the system issues and pro- and Web based color imaging systems, we propose a colo- pose a methodology for its implementation on the Web. rimetric RGB specification that is based on the average per- formance of personal computer displays. This solution is Part 1—Definition of the sRGB Color Space supported by the following observations: Recently the International Color Consortium has proposed • Most computer monitors are similar in their key color and is positioned to provide breakthrough solutions to prob- characteristics—the phosphor chromaticities (prima- lems in communicating color in open systems. Yet the ICC ries) and transfer function profile format does not provide a complete solution for all • RGB spaces are native to displays, scanners and digi- situations. tal cameras, which are the devices with the highest per- Currently, the ICC has one means of tracking and en- formance constraints suring that a color is correctly mapped from the input to • RGB spaces can be made device independent in a the output color space. This is done by attaching a profile straightforward way. They can also describe color gam- for the input color space to the image in question. This is uts that are large enough for all but a small number of appropriate for high end users. However, there are a broad applications. range of users that do not require this level of color quality, a broad range of file formats that will never support color This combination of factors makes a colorimetric RGB profile embedding, and a broad range of uses that discour- space well suited for wide adoption since it can both de- age people from appending any extra data to their files. It scribe the colors in a unambiguous way and be the native is at this level that a common standard RGB color space space for actual hardware devices. This, many readers will becomes useful and necessary. recognize, describes in a roundabout way what has been We expect applications and users that do not want the the practice in color television for some 45 years. This overhead of embedding profiles with documents or images proven methodology provides highest performance where to convert them to a common color space and store them in it is needed the most, the fast display of images in CRT that format. Currently there is a plethora of RGB monitor monitors. color spaces filling this void. There is a need to merge the many standard and non-standard RGB monitor spaces into Gamma 2.2 a single standard RGB color space. Such a standard could For computer software and hardware designers the dramatically improve the color fidelity in the desktop en- most significant aspect of the proposed space is the 2.2 vironment. For example, if operating system vendors pro- gamma. Because gamma correction tends to be a topic sur- vide support for a standard RGB color space, the input and rounded by confusion, it is worthwhile spending a few para- output device vendors that support this standard color space graphs discussing it. 198—Recent Progress in Color Management and Communications The non-linearity of the electro-optical radiation transfer scribed by a gamma of 1.8 because despite having 2.2 function of CRTs is often expressed by the exponent gamma. gamma display systems, the color were dithered into the 4 This transfer function describes how much visible radiant bit frame buffers, resulting in a flattening of the system energy (cd/m2) results from voltages applied to the CRT transfer function. This happens because screen dithering electron-gun. Because most of the other characteristics of mixes colors linearly in the eye, making it less dependent CRT based computer monitors are linear (including DACs of the CRT non-linearity. Since currently most Windows and video amplifiers) the resulting transfer function has PC support 16 or 24 bit color modes, 2.2 gamma is now the the same gamma value determining its non-linearity. average. The second systematic deviation happens when the gamma I = A (k1D + k2) (1) graphics system in the computer hardware or software im- poses its own gamma correction. This is done for a variety Where k1 and k2 are the system gain and offset, D is the of reasons, but is usually an attempt to compromise be- normalized pixel value, A is the maximum luminance of tween image display and graphics/image processing per- the CRT and I is the resulting luminance. formance (most computer graphic rendering assumes linear The key point that we wish to convey here is that radiation space, e.g., transparency operations, and so does gamma is dependent only on the electron gun design, and image processing, e.g., scaling and filtering). The gamma the vast majority of monitors and TV sets in use today are correcting of image data can be described by applying an based on designs that result, on average, in the value 2.2 exponent (g2) to the image data as follows : for gamma. Most of the variation between computer moni- tors and between TV sets are due to the differences in sys- I = A (D1/g2)gamma (3) tem gain and offsets (k1 and k2), which are partially under control of the user in the form of contrast and brightness and the apparent system transfer function becomes: knobs. Unfortunately, the actual set-up is often not know, gamma but the best CRT performance happens when the system I = AD( /g2) (4) offset puts the dark parts of the images at the CRT cut-off, i.e. the black (pixel value 0) parts of the CRT image are For the Macintosh the apparent exponent (gamma/g2) is just about to emit light. Under these conditions equation 1 around 1.8 and for SGI workstations around 1.7. above becomes In summary, there has been some concern with the choice a 2.2 gamma as opposed to a 1.8 gamma. We feel I = ADgamma (2) that there are many reasons to support this choice, includ- ing compatibility with a large legacy of images (Photo CD, and the monitor has the widest-dynamic range. The simpli- many Unix workstations, PC’s with 256+ colors and their fied form in equation 2 is what is usually found in the com- desktop color schemes and icons, several ultra-large image puter literature. collections, analog television and CCIR 601 images), it is The gamma value 2.2 was the standard for television also a better fit with Weber’s fraction, it is compatible with encoding before the advent of color TVs and was formal- numerous standards (TIFF/EP, EXIF, digital TV, HDTV, ized in 1953. It is well matched to the eye’s own non-lin- analog video, and PhotoCD), it is closer to native CRTs earity and it helps minimize transmission noise in the dark gamma and it consistent with a larger market of displays. areas. Because all television sets have to display content generated with this encoding, it was very important for all Video Camera Encoding and ITU-R BT.709 CRT designs to conform to it. Only recently has the com- Compatibility puter monitor market become as large as the TV market. There has been some confusion in previous versions As a result, most computer monitors have a native 2.2 of this proposal with respect to compatibility with the ITU- gamma, with widest variations being in the set-up and R BT.709 standard and with respect to the 709 recommen- screen reflectivity (older and less expensive display can dation in general. The ITU-R BT.709 standard specifically reflect up to 20% of the ambient light). These factors typi- describes the encoding transfer function for a video cam- cally can not be characterized a priori since they might era that when viewed on a “standard” monitor will pro- change in the course of the day (ambient light) and at the duce excellent image quality.