A Standard Default Color Space for the Internet - sRGB

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  A Standard Default Color Space for theInternet - sRGB Michael Stokes (Hewlett-Packard), Matthew Anderson (Microsoft),Srinivasan Chandrasekar (Microsoft), Ricardo Motta (Hewlett-Packard)Version 1.10, November 5, 1996 Introduction Hewlett-Packard and Microsoft propose the addition of support for a standard color space, sRGB, withinthe Microsoft operating systems, HP products, the Internet, and all other interested vendors. The aim of thiscolor space is to complement the current color management strategies by enabling a third method of handlingcolor in the operating systems, device drivers and the Internet that utilizes a simple and robust deviceindependent color definition. This will provide good quality and backward compatibility with minimumtransmission and system overhead. Based on a calibrated colorimetric RGB color space well suited toCathode Ray Tube (CRT) monitors, television, scanners, digital cameras, and printing systems, such a spacecan be supported with minimum cost to software and hardware vendors. Our intent here is to promote itsadoption by showing the benefits of supporting a standard color space, and the suitability of the standardcolor space, sRGB, we are proposing. We will describe some of the system issues and propose amethodology for to implement support for sRGB and color management on the World Wide Web. Part 1 : History and Background of sRGBcolor space A Perceived Need Recently the International Color Consortium has proposed breakthrough solutions to problems incommunicating color in open systems. Yet the ICC profile format does not provide a complete solution for allsituations.Currently, the ICC has one means of tracking and ensuring that a color is correctly mapped from the input tothe output color space. This is done by attaching a profile for the input color space to the image in question.This is appropriate for high end users. However, there are a broad range of users that do not require this levelof flexibility and control. Additionally, most existing file formats do not, and may never support color profileembedding, and finally, there are a broad range of uses actually discourage people from appending any extradata to their files. A common standard RGB color space addresses these issues and is useful and necessary.We expect application developers and users that do not want the overhead of embedding profiles withdocuments or images to convert them to a common color space and store them in that format. Currently thereis a plethora of RGB monitor color spaces attempting to fill this void with little guidance or attempts atstandards. There is a need to merge the many standard and non-standard RGB monitor spaces into a singlestandard RGB color space. Such a standard could dramatically improve the color fidelity in the desktop  environment. For example, if operating system vendors provide support for a standard RGB color space, theinput and output device vendors that support this standard color space could easily and confidentlycommunicate color without further color management overhead in the most common situations. The threemajor factors of this RGB space are the colorimetric RGB definition, the equivalent gamma value of 2.2 andthe well-defined viewing conditions, along with a number of secondary details necessary to enable the clear and unambiguous communication of color. Colorimetric RGB The dichotomy between the device dependent (e.g. amounts of ink expressed in CMYK or digitized videovoltages expressed in RGB) and device independent color spaces (such as CIELAB or CIEXYZ) hascreated a performance burden on applications that have attempted to avoid device color spaces. This is primarily due to the complexity of the color transforms they need to perform to return the colors to devicedependent color spaces. This situation is worsened by a reliability gap between the complexity and variety of the transforms, making it hard to ensure that the system is properly configured.To address these concerns and serve the needs of PC and Web based color imaging systems, we propose acolorimetric RGB specification that is based on the average performance of personal computer displays. Thissolution is supported by the following observations:Most computer monitors are similar in their key color characteristics - the phosphor chromaticities(primaries) and transfer functionRGB spaces are native to displays, scanners and digital cameras, which are the devices with thehighest performance constraintsRGB spaces can be made device independent in a straightforward way. They can also describe color gamuts that are large enough for all but a small number of applications.This combination of factors makes a colorimetric RGB space well suited for wide adoption since it can bothdescribe the colors in an unambiguous way and be the native space for actual hardware devices. This, manyreaders will recognize, describes in a roundabout way what has been the practice in color television for some45 years. This proven methodology provides excellent performance where it is needed the most, the fastdisplay of images in CRT monitors. Gamma and the desired CRT gamma of 2.2 For computer software and hardware designers the most significant aspect of the proposed space is the 2.2CRT gamma. Because gamma correction tends to be a topic surrounded by confusion, it is worthwhilespending a few paragraphs discussing it. Definitions of gamma We start this discussion by defining four separate aspects of gamma.1. viewing gamma  - the overall system gamma that we want to obtain and is typically computed bymultiplying the camera gamma by the display gamma as shown below.(0.1)  2. camera gamma  - the characteristic of the image sensor or video camera standard transfer function3. CRT gamma  - the gamma of the physical CRT.4.  LUT gamma  - the gamma of the frame buffer lookup table5. display gamma  - the display system gamma downstream of the frame buffer which is typicallycomputed by multiplying the CRT gamma by the LUT gamma as shown below.(0.2)These definitions have been kindly provided by the World Wide Web Consortium and are included in thePNG file format specification available at http://www.w3.org/pub/WWW/TR/REC-png-multi.html. Thesedefinitions do not   describe the individual gamma parameter in equation 0.4 below. Instead, they describe theresulting power parameter of the appropriate transfer function when fit by a power function. It is extremelyimportant to keep this distinction clear or else one implicitly assumes equations 0.4 and 0.5 are equivalent andthe system black level is truly 0.0 and the system gain is 1.0. Viewing Gamma The reason that a viewing gamma of 1.125 is used instead of 1.0 is to compensate for the viewingenvironment conditions, including ambient illumination and flare. Historically, viewing gammas of 1.5 have been used for viewing projected slides in a dark room and viewing gammas of 1.25 have been used for viewing monitors in a very dim room. This very dim room value of 1.25 has been used extensively in televisionsystems and assumes a ambient luminance level of approximately 15 lux. The current proposal assumes anencoding ambient luminance level of 64 lux which is more representative of a dim room in viewing computer generated imagery. Such a system assumes a viewing gamma of 1.125 and is thus consistent with the 709standard described below. While we believe that the typical office or home viewing environment actually hasan ambient luminance level around 200 lux, we found it impractical to attempt to account for the resultinglarge levels of flare that resulted. In addition, recent work by the ISO JTAG2 standards committee supportsthe ambient luminance level of 64 lux.If the viewing condition is different from the standard, then the decoding process must compensate. This can be done by modifying the gamma values in equation 1.2 below by the appropriate factor. If one does modifythe gamma values in equation 1.2 below, extreme care must be taken to avoid quantization errors whenworking with 24 bit images and high viewing flare levels.The ITU-R BT.709 transfer function in combination with its target monitor is attempting to achieve a viewinggamma of 1.125 by incorrectly assuming a CRT gamma of 2.5 and an LUT gamma of 1.0/2.222 as shown inthe equation below. The justification of a viewing gamma value of 1.125 is described below in the section onviewing environment compensation.(0.3) Using the actual power function fit value for the 709 transfer function of 1.0/1.956 and maintaining the displaygamma of 1.125, we can solve for the ideal target monitor gamma of 2.2. This is consistent with the CRTgamma value proposed in this paper. Camera Gamma  The camera gamma 1.0/2.2 was the standard for television camera encoding before the advent of color TVsand was formalized in 1953 with the NTSC broadcast television standards. More recently ITU-R BT.709has been adopted internationally and contains camera gamma of 1.0/1.956. The actual exponent factor in the709 transfer function is 1.0/2.222. Despite the fact that the exponent of the 709 function is 1.0/2.222, theactual 709 encoding transfer function is closer to a CRT gamma of 1.0/1.956 than 1.0/2.222. This is due tothe large offset of 0.099 in the transfer function equation. This is well matched to the eye's own non-linearityand it helps minimize transmission noise in the dark areas.Broadcast television camera gamma standards and the ITU-R BT.709 standard in particular defines thetransformation of real world CIEXYZ tristimulus values into a target RGB monitor space. This is essentially acomposite of two transformations; one from real world CIEXYZ tristimulus values into standard monitor CIEXYZ tristimulus values and one from these standard monitor CIEXYZ tristimulus values into standardmonitor rgb values. The resulting image is not an exact appearance match of the srcinal scene, but instead isa preferred reproduction of the srcinal scene that is consistent with the limitations of a monitor.Because all television sets have to display content generated with this encoding, it was very important for allCRT gamma designs to conform to it. Only recently has the computer monitor market become as large as theTV market. As a result, most computer monitors still perform optimally with imagery using with a cameragamma value of approximately 1.0/1.956 CRT Gamma The non-linearity of the electro-optical radiation transfer function of CRTs is often expressed by amathematical power function exponent parameter called gamma. This transfer function describes how muchvisible radiant energy (cd/m 2 ) results from voltages applied to the CRT electron-gun. Because most of theother characteristics of CRT based computer monitors are linear (including DACs and video amplifiers) theresulting transfer function has the same gamma value determining its non-linearity.(0.4) Where k  1  and k  2  are the system gain and offset, D is the normalized pixel value, A is the maximum luminanceof the CRT and I is the resulting luminance. This equation and a thorough analysis of the CRT characteristicsand history are well described in An Analytical Model for the Colorimetric Characterization of Color CRTs  by Ricardo Motta, Rochester Institute of Technology, 1991.The key point that we wish to convey here is that gamma component of the CRT gamma is dependent onlyon the electron gun design, and the vast majority of monitors and TV sets in use today are based on designsthat result, on average, in the value 2.2 for gamma component of the CRT gamma and a 2.2 overall CRTgamma value when typical system gain and offsets are optimally set. Most of the variation between computer monitors and between TV sets are due to the differences in system gain and offsets (k  1  and k  2 ), which are partially under control of the user in the form of contrast and brightness knobs. Unfortunately, the actual set-up is often not known, but the best CRT performance happens when the system 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 just about to emitlight. Under these conditions equation 0.4 above becomes>(0.5)and the monitor has the widest-dynamic range. Unfortunately, this is not the common condition. Unfortunatelythe simplified form in equation 0.5 is what is usually found in the computer literature.
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