61966-2-1

100/PT61966(PL)34 For IEC use only 1998-05-28 INTERNATIONAL ELECTROTECHNICAL COMMISSION TECHNICAL COMMITTEE NO. 100: AUDIO, VIDEO AND MULTIMEDIA SYSTEMS AND EQUIPMENT Project Team 61966: Colour Measurement and Management in Multimedia Systems and Equipment --------------------------------------------- IEC/4WD 61966-2-1: Colour Measurement and Management in Multimedia Systems and Equipment - Part 2-1: Default RGB Colour Space - sRGB This document, the 4WD for 2CD of IEC 61966-2-1, was prepared taking into account the following documents: z 100/67/CD: Colour measurement and management in multimedia systems and equipment - Part 2.1: Default RGB colour space - sRGB z 100/PT61966(PL)28: Report from the 4th physical meeting in Derby, UK, on 1998-05- 14/15 z 100/PT61966(PL)32: Annex to Compilation of Comments on 100/67/CD: sRGB This document supersedes the 3WD 100/PT61966(PL)16 and will be sent to the Secretariat for its consideration to publish as a committee draft for vote (CDV). When an International Standard will be established, the stable period of three years is suggested. 1966 2.1  IEC: 1998 – 18 – INTERNATIONAL ELECTROTECHNICAL COMMISSION COLOUR MANAGEMENT IN MULTIMEDIA SYSTEMS - Part 2: Colour Management, Part 2.1: DEFAULT RGB COLOUR SPACE - sRGB TABLE OF CONTENTS 1 GENERAL ................................................................................................................. 33 1.1 Introduction ...................................................................................................... 33 1.2 Scope .............................................................................................................. 44 1.3 Normative References ...................................................................................... 44 1.4 Definitions ........................................................................................................ 55 2 REFERENCE CONDITIONS ...................................................................................... 66 2.1 Reference Display Conditions........................................................................... 66 2.2 Reference Viewing Conditions .......................................................................... 67 2.3 Reference Observer Conditions ........................................................................ 67 3 ENCODING CHARACTERISTICS .............................................................................. 77 3.1 Introduction ...................................................................................................... 77 3.2 Transformation from RGB values to 1931 CIE XYZ values ................................ 77 3.3 Transformation from 1931 CIE XYZ values to RGB values ................................ 78 1966 2.1  IEC: 1998 – 28 – INTERNATIONAL ELECTROTECHNICAL COMMISSION COLOUR MANAGEMENT IN MULTIMEDIA SYSTEMS - Part 2: Colour Management, Part 2.1: DEFAULT RGB COLOUR SPACE – sRGB FOREWORD 1) The IEC (International Electrotechnical Commission) is a worldwide organisation for standardisation comprising all national electrotechnical committees (IEC National Committees). The object of the IEC is to promote international co-operation on all questions concerning standardisation in the electrical and electronic fields. To this end and in addition to other activities, the IEC publishes International Standards. Their preparation is entrusted to technical committees; any IEC National Committee interested in the subject dealt with may participate in this preparatory work. International, governmental and non-governmental organisations liaising with the IEC also participate in this preparation. The IEC collaborates closely with the International Organisation for Standardisation (ISO) in accordance with conditions determined by agreement between the two organisations. 2) The formal decisions or agreements of the IEC on technical matters express, as nearly as possible, an international consensus of opinion on the relevant subjects since each technical committee has representation from all interested National Committees. 3) The documents produced have the form of recommendations for international use and are published in the form of standards, technical reports or guides and they are accepted by the National Committees in that sense. 4) In order to promote international unification, IEC National Committees undertake to apply IEC International Standards transparently to the maximum extent possible in their national and regional standards. Any divergence between the IEC Standard and the corresponding national or regional standard shall be clearly indicated in the latter. 5) The IEC provides no marking procedure to indicate its approval and cannot be rendered responsible for any equipment declared to be in conformity with one of its standards. 6) Attention is drawn to the possibility that some of the elements of this International Standard may be the subject of patent rights. The IEC shall not be held responsible for identifying any or all such patent rights. International Standard IEC 61966 has been prepared by project team 61966: Colour measurement and management in multimedia systems and equipment, of IEC technical committee TC100: Audio, Video and Multimedia Systems and Equipment. The text of this standard is based on FDIS Report on voting XXX/FDIS XXX/RVD Full information on the voting for the approval of this standard can be found in the report on voting indicated in the above table. 1966 2.1  IEC: 1998 – 38 – INTERNATIONAL ELECTROTECHNICAL COMMISSION COLOUR MANAGEMENT IN MULTIMEDIA SYSTEMS - Part 2: Colour Management, Part 2.1: DEFAULT RGB COLOUR SPACE – sRGB 1 GENERAL 1.1 Introduction The method of digitisation in this part is designed to complement current colour management strategies by enabling a method of handling colour in the operating systems, device drivers and the Internet that utilises a simple and robust device independent colour definition. This will provide good quality and backward compatibility with minimum transmission and system overhead. Based on a calibrated colorimetric RGB colour space well suited to cathode ray tube (CRT) displays, flat panel displays, television, scanners, digital cameras, and printing systems, such a space can be supported with minimum cost to software and hardware vendors. The intent is to promote its adoption by showing the benefits of supporting a standard colour space, and the suitability of this standard colour space, sRGB. Recently the International Colour Consortium has proposed breakthrough solutions to problems in communicating colour in open systems. Yet the ICC profile format does not provide a complete solution for all situations. Currently, the ICC has one means of tracking and ensuring that a colour is correctly mapped from the input to the output colour space. This is done by attaching a profile for the input colour 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 level of flexibility and control in an embedded profile mechanism. Instead it is possible to create a single, standard default colour space definition that can be processed as an implicit ICC sRGB profile. Additionally, most existing file formats do not, and may never, support colour profile embedding, and finally, there are a broad range of uses that actually discourage people from appending any extra data to their files. A common standard RGB colour space addresses these issues and is useful and necessary. This approach maintains the advantage of a clear relationship with ICC colour management systems while minimising software processes and support requirements. Application developers and users who do not want the overhead of embedding profiles with documents or images should convert them to a common colour space for storage. Currently there is a plethora of RGB CRT-based colour spaces attempting to fill this void with little guidance or attempts at standardisation. There is a need to merge the many standard and non- standard RGB display spaces into a single standard RGB colour space. This standard dramatically improves the colour fidelity in the desktop environment by meeting this need. For example, if operating system vendors provide support for this standard RGB colour space, the input and output device vendors that support this standard colour space could easily and confidently communicate colour without further colour management overhead in the most common situations. The three major factors of this RGB space are the colorimetric RGB definition, the simple exponent value of 2.2, and the well-defined viewing conditions, along with a number of secondary details necessary to enable the clear and unambiguous communication of colour. The dichotomy between the device dependent (e.g. amounts of ink expressed in CMYK or digitised video voltages expressed in RGB) and device independent colour spaces (such as CIELAB or CIEXYZ) has created a performance burden on applications that have attempted to avoid device colour spaces. This is primarily due to the complexity of the colour transforms they 1966 2.1  IEC: 1998 – 48 – need to perform to return the colours to device dependent colour 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. This standard addresses these concerns, serves the needs of PC and Web based colour imaging systems is based on the average performance of personal computer displays. This solution is supported by the following observations: • Most computer displays are similar in their key colour characteristics — the phosphor chromaticities (primaries) and transfer function • RGB spaces are native to displays, scanners and digital cameras, which are the devices with the highest performance constraints • RGB spaces can be made device independent in a straightforward way. They can also describe colour 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 both describe the colours in an unambiguous way and be the native space for actual hardware devices. This, many readers will recognise, describes in a roundabout way what has been the practice in colour television for some 45 years. This proven methodology provides excellent performance where it is needed the most, the rapid display of images in CRT displays. There are two parts to the proposed standard described in this standard: the encoding transformations and the reference conditions. The encoding transformations provide all of the necessary information to encode an image for optimum display in the reference conditions. If actual conditions differ from reference conditions, additional rendering transformations may be required. 1.2 Scope The IEC 61966 standards are a series of methods and parameters for colour measurements and management for use in multimedia systems and equipment applicable to the assessment of colour reproduction. This part of IEC 61966 is applicable to the encoding and communication of RGB colours used in computer systems and similar applications by defining encoding transformations for use in defined reference conditions. The encoding transformations are the default RGB colour definition when no other colour space information is available or appropriate. If actual conditions differ from the reference conditions, additional rendering transformations could be required. Such additional rendering transformations are beyond the scope of this standard. 1.3 Normative References The following normative documents contain provisions, which, through reference in this text, constitute provisions of this International Standard. At the time of publication, the editions indicated were valid. All normative documents are subject to revision, and parties to agreements based on this International Standard are encouraged to investigate the possibility of applying the most recent editions of the normative documents indicated below. Members of IEC and ISO maintain registers of currently valid International Standards IEC 60050(845): 1987, International Electrotechnical Vocabulary (IEV) - Chapter 845: Lighting ISO/CIE 10527: 1986, Colorimetry (2nd Ed.) CIE 122: 1996, The relationship between digital and colorimetric data for computer-controlled CRT displays 1966 2.1  IEC: 1998 – 58 – ITU-R BT.709-2: 1995, Parameter Values for the HDTV Standards for Production and International Programme Exchange ISO 3664: 1975, Photography – Illumination conditions for viewing colour transparencies and their reproduction ISO 9358:1994, Optics and optical instruments -- Veiling glare of image forming systems -- Definitions and methods of measurement 1.4 Definitions For the purpose of this International Standard, the following definitions apply. Definitions of illuminance, luminance, tristimulus, and other relating lighting terms are defined in reference IEC 60050(845). Veiling glare is defined in reference ISO 9358. 1.4.1 ambient illuminance level The illuminance level due to lighting in the viewing environment not from the display measured normal from the display faceplate at a typical viewing distance. 1.4.2 ambient white point The coordinate point in the CIE 1931 XYZ colour space defined by ISO/CIE 10527 due to lighting in the viewing environment, not from the display measured normal from the display faceplate. 1.4.3 display illuminant white point The point in the 1931 xy chromaticity diagram defined by ISO/CIE 10527 when the red, green and blue channels are at 100% as measured normal from the display faceplate. 1.4.4 display background The environment of the colour element extending typically for about ten degrees from the edge of the proximal field in all, or most directions. When the proximal field is the same colour as the background, the latter is regarded as extending from the edge of the colour element considered. 1.4.5 display model offset The display model offset measured consistently with CIE 122, representing the black offset level of the display grid voltage. 1.4.6 display input/output characteristic The transfer characteristic relating the normalised input signal and the normalised output luminance as represented by an exponential function. 1.4.7 display luminance level The luminance level of the display measured consistently with CIE 122. 1.4.8 display surround 1966 2.1  IEC: 1998 – 68 – The field outside the background, filling the field of vision. 1.4.9 display proximal field The immediate environment of the colour element considered, extending typically for about two degrees from the edge of the colour element considered in all or most directions. 2 REFERENCE CONDITIONS 2.1 Reference Display Conditions 1. Display luminance level 80 cd/m2 2. Display white point x = 0,3127, y = 0,3290 (D65) 3. Display model Offset (R,G and B) 0,0 4. Display input/output characteristic (R, G, and B) 2,2 The CIE chromaticities for the red, green, and blue ITU-R BT.709-2 reference primaries, and for CIE Standard Illuminant D65, are given in table 1. Table 1 CIE chromaticities for ITU-R BT.709 reference primaries and CIE standard illuminant Red Green Blue D65 x 0,6400 0,3000 0,1500 0,3127 y 0,3300 0,6000 0,0600 0,3290 z 0,0300 0,1000 0,7900 0,3583 The reference display characterisation is based on the characterisation in CIE 122. Relative to this methodology, the reference display is characterised by the equation below where V’sRGB is the input data signal and VsRGB is the output normalized lumiance. ( ) 2,20.0+′= sRGBsRGB VV (1) 2.2 Reference Viewing Conditions Specifications for the reference viewing environments are based on ISO 3664 and are defined as follows: 1. Reference Background for the background as part of the display screen, the background is 20% of the reference display luminance level 2. Reference Surround 20% reflectance of the reference ambient illuminance level 3. Reference Proximal Field 20% of the reflectance of the reference display luminance level 4. Reference Ambient Illuminance Level 64 lx 5. Reference Ambient White Point x = 0,3457, y = 0,3585 (D50) 6. Reference Veiling glare 1,0% 2.3 Reference Observer Conditions The reference observer is the CIE 1931 two-degree standard observer from ISO/CIE 10527. 1966 2.1  IEC: 1998 – 78 – 3 ENCODING CHARACTERISTICS 3.1 Introduction The encoding transformations between 1931 CIEXYZ values and 8 bit RGB values provide unambiguous methods to represent optimum image colorimetry when viewed on the reference display in the reference viewing conditions by the reference observer. The 1931 CIEXYZ values are scaled from 0.0 to 1.0, not 0.0 to 100.0. These non-linear sR'G'B' values represent the appearance of the image as displayed on the reference display in the reference viewing condition. The sRGB tristimulus values are linear combinations of the 1931 CIE XYZ values as measured on the faceplate of the display, which assumes the absence of any significant veiling glare. A linear portion of the transfer function of the dark end signal is integrated into the encoding specification to optimise encoding implementations. Recommended treatments for both veiling glare and viewing conditions are provided in Annexes D and E. 3.2 Transformation from RGB values to 1931 CIE XYZ values The relationship is defined as follows: 0,255 0,255 0,255 8 8 8 ÷=′ ÷=′ ÷=′ bitsRGB bitsRGB bitsRGB BB GG RR (2) If 04045,0,, ≤′′′ sRGBsRGBsRGB BGR 92,12 92,12 92,12 ÷′= ÷′= ÷′= sRGBsRGB sRGBsRGB sRGBsRGB BB GG RR (3) else 04045,0,, >′′′ sRGBsRGBsRGB BGR ( ) ( ) ( ) 4,2 4,2 4,2 055,1 055,0 055,1 055,0 055,1 055,0    +′ =    +′ =    +′ = sRGB sRGB sRGB sRGB sRGB sRGB BB GG RR (4) and                 =         sRGB sRGB sRGB B G R Z Y X 0,95050,11920,0193 0,07220,71520,2126 0,18050,35760,4124 (5). The above equations closely fit a simple power function with an exponent of 2,2. This maintains consistency with the legacy of desktop and video images. 3.3 Transformation from 1931 CIE XYZ values to RGB values The sRGB tristimulus values can be computed using the following relationship: 1966 2.1  IEC: 1998 – 88 –                 − − −− =         Z Y X B G R sRGB sRGB sRGB 0570,12040,00557,0 0415,08758,19689,0 4986,05372,12406,3 (6) In the RGB encoding process, negative sRGB tristimulus values, and sRGB tristimulus values greater than 1,00 are not retained. When encoding software cannot support this extended range, the luminance dynamic range and colour gamut of RGB is limited to the tristimulus values between 0,0 and 1,0 by simple clipping. The sRGB tristimulus values are transformed to non-linear sR'G'B' values as follows: If 0031308,0,, ≤sRGBsRGBsRGB BGR sRGBsRGB sRGBsRGB sRGBsRGB BB GG RR ×=′ ×=′ ×=′ 92,12 92,12 92,12 (7) else 0031308,0,, >sRGBsRGBsRGB BGR 055,0055,1 055,0055,1 055,0055,1 )4,2/0,1( )4,2/0,1( )4,2/0,1( −×=′ −×=′ −×=′ sRGBsRGB sRGBsRGB sRGBsRGB BB GG RR (8) The non-linear sR'G'B' values are converted to digital code values. This conversion scales the above sR'G'B' values by using the equation below where WDC represents the white digital count and KDC represents the black digital count. ( )( ) ( )( ) ( )( ) R WDC KDC R KDC G WDC KDC G KDC B WDC KDC B KDC bit sRGB bit sRGB bit sRGB 8 8 8 = − × ′ + = − × ′ + = − × ′ + (9) This standard specified a black digital count of 0 and a white digital count of 255 for 24-bit (8- bits/channel) encoding. The resulting RGB values are formed according to the following equations: ( )( ) ( )( ) ( )( ) 0,00,00,255 0,0