1、 International Telecommunication Union ITU-T H.272 TELECOMMUNICATION STANDARDIZATION SECTOR OF ITU (01/2007) SERIES H: AUDIOVISUAL AND MULTIMEDIA SYSTEMS Infrastructure of audiovisual services Coding of moving video Procedures and values for video gamma compensation in multimedia systems ITU-T Recom
2、mendation H.272 ITU-T H-SERIES RECOMMENDATIONS AUDIOVISUAL AND MULTIMEDIA SYSTEMS CHARACTERISTICS OF VISUAL TELEPHONE SYSTEMS H.100H.199 INFRASTRUCTURE OF AUDIOVISUAL SERVICES General H.200H.219 Transmission multiplexing and synchronization H.220H.229 Systems aspects H.230H.239 Communication procedu
3、res H.240H.259 Coding of moving video H.260H.279 Related systems aspects H.280H.299 Systems and terminal equipment for audiovisual services H.300H.349 Directory services architecture for audiovisual and multimedia services H.350H.359 Quality of service architecture for audiovisual and multimedia ser
4、vices H.360H.369 Supplementary services for multimedia H.450H.499 MOBILITY AND COLLABORATION PROCEDURES Overview of Mobility and Collaboration, definitions, protocols and procedures H.500H.509 Mobility for H-Series multimedia systems and services H.510H.519 Mobile multimedia collaboration applicatio
5、ns and services H.520H.529 Security for mobile multimedia systems and services H.530H.539 Security for mobile multimedia collaboration applications and services H.540H.549 Mobility interworking procedures H.550H.559 Mobile multimedia collaboration inter-working procedures H.560H.569 BROADBAND AND TR
6、IPLE-PLAY MULTIMEDIA SERVICES Broadband multimedia services over VDSL H.610H.619 For further details, please refer to the list of ITU-T Recommendations. ITU-T Rec. H.272 (01/2007) i ITU-T Recommendation H.272 Procedures and values for video gamma compensation in multimedia systems Summary This Recom
7、mendation provides guidance on default gamma correction values and gamma compensation procedures for use in multimedia systems (such as video conferencing and video telephony systems) where the treatment of video gamma is not otherwise specified. The recommended values and procedures are consistent
8、with those of ITU-R BT.709-5. Source ITU-T Recommendation H.272 was approved on 13 January 2007 by ITU-T Study Group 16 (2005-2008) under the ITU-T Recommendation A.8 procedure. ii ITU-T Rec. H.272 (01/2007) FOREWORD The International Telecommunication Union (ITU) is the United Nations specialized a
9、gency in the field of telecommunications. The ITU Telecommunication Standardization Sector (ITU-T) is a permanent organ of ITU. ITU-T is responsible for studying technical, operating and tariff questions and issuing Recommendations on them with a view to standardizing telecommunications on a worldwi
10、de basis. The World Telecommunication Standardization Assembly (WTSA), which meets every four years, establishes the topics for study by the ITU-T study groups which, in turn, produce Recommendations on these topics. The approval of ITU-T Recommendations is covered by the procedure laid down in WTSA
11、 Resolution 1. In some areas of information technology which fall within ITU-Ts purview, the necessary standards are prepared on a collaborative basis with ISO and IEC. NOTE In this Recommendation, the expression “Administration“ is used for conciseness to indicate both a telecommunication administr
12、ation and a recognized operating agency. Compliance with this Recommendation is voluntary. However, the Recommendation may contain certain mandatory provisions (to ensure e.g. interoperability or applicability) and compliance with the Recommendation is achieved when all of these mandatory provisions
13、 are met. The words “shall“ or some other obligatory language such as “must“ and the negative equivalents are used to express requirements. The use of such words does not suggest that compliance with the Recommendation is required of any party. INTELLECTUAL PROPERTY RIGHTS ITU draws attention to the
14、 possibility that the practice or implementation of this Recommendation may involve the use of a claimed Intellectual Property Right. ITU takes no position concerning the evidence, validity or applicability of claimed Intellectual Property Rights, whether asserted by ITU members or others outside of
15、 the Recommendation development process. As of the date of approval of this Recommendation, ITU had not received notice of intellectual property, protected by patents, which may be required to implement this Recommendation. However, implementers are cautioned that this may not represent the latest i
16、nformation and are therefore strongly urged to consult the TSB patent database at http:/www.itu.int/ITU-T/ipr/. ITU 2007 All rights reserved. No part of this publication may be reproduced, by any means whatsoever, without the prior written permission of ITU. ITU-T Rec. H.272 (01/2007) iii CONTENTS P
17、age 1 Scope 1 2 References. 1 3 Definitions 1 4 Abbreviations and acronyms 1 5 Conventions 1 6 Introduction 2 7 Gamma for multimedia systems . 2 7.1 Video signal composition from multiple video sources. 4 Bibliography. 5 ITU-T Rec. H.272 (01/2007) 1 ITU-T Recommendation H.272 Procedures and values f
18、or video gamma compensation in multimedia systems 1 Scope This Recommendation addresses procedures and values for video gamma compensation in multimedia systems (such as video conferencing and video telephony systems) in cases where a gamma correction function is not otherwise specified. 2 Reference
19、s The following ITU-T Recommendations and other references contain provisions which, through reference in this text, constitute provisions of this Recommendation. At the time of publication, the editions indicated were valid. All Recommendations and other references are subject to revision; users of
20、 this Recommendation are therefore encouraged to investigate the possibility of applying the most recent edition of the Recommendations and other references listed below. A list of the currently valid ITU-T Recommendations is regularly published. The reference to a document within this Recommendatio
21、n does not give it, as a stand-alone document, the status of a Recommendation. ITU-R BT.709-5 ITU-R Recommendation BT.709-5 (2002), Parameter values for the HDTV standards for production and international programme exchange. 3 Definitions This Recommendation defines the following term: 3.1 gamma: Th
22、e exponent used in an opto-electronic input-output relationship transfer characteristics equation between a video signal level used to stimulate a visual display element and the intensity of the resulting light, ordinarily based on red, green and blue display element components. Alternatively, gamma
23、 can be used to describe the input-output relationship between the intensity of a source colour component and the corresponding video signal level generated from a camera sensor. The relationship is generally approximately exponential, with the brightness of a display element within a scaled nominal
24、 range from 0 to 1 being approximately proportional to the signal level input to the display within a scaled nominal range from 0 to 1, raised to an exponent gamma. The actual ideal opto-electronic transfer characteristics that are informally referred to by the exponent gamma may differ somewhat fro
25、m an exponential input-output relationship, especially in the range near zero. However, such a functional relationship is often still referred to as “gamma“. 4 Abbreviations and acronyms This Recommendation uses the following abbreviations and acronyms: CRT Cathode Ray Tube MCU Multipoint Control Un
26、it NTSC National Television Standards Committee PAL Phase Alternating Line 5 Conventions The word “shall“ is used in this Recommendation to specify a mandatory requirement. The word “should“ is used in this Recommendation to specify a suggested, but not required, course of action. 2 ITU-T Rec. H.272
27、 (01/2007) The word “may“ is used in this Recommendation to specify an optional course of action, without expressing a preference. 6 Introduction The proper interpretation and display of coded video material requires knowledge of the colour space characteristics of the decoded video. A complete colo
28、ur space definition for digital video representation may include specification of the following aspects: The chromaticity coordinates of the source colour primaries (ordinarily referred to as red, green and blue, or R, G and B) and reference white point. The opto-electronic transfer characteristics
29、(i.e., gamma) of the source components (e.g., definition of E R , E Gand E Bas a function of R, G and B). Matrix coefficients for transformation of the RGB components into luma and chroma components (e.g., definition of components E Y , E PBand E PRas a function of E R , E Gand E B ). Definition of
30、scaling, offsets, and quantization for digital representation. A gamut boundary definition specifying the range of values over which effective representations of colours can be achieved. 7 Gamma for multimedia systems A key aspect of the colour representation system is the opto-electronic transfer c
31、haracteristics, which are informally referred to as gamma. Gamma can be used to describe the input-output relationship between the video signal level used to stimulate a display element and the intensity of the resulting light. Alternatively, gamma can be used to describe the input-output relationsh
32、ip between the intensity of a source colour component and the corresponding video signal level generated from a camera sensor. The relationship is generally non-linear, with the brightness of a display element within a scaled nominal range from 0 to 1 being approximately proportional to the signal l
33、evel input to the display within a scaled nominal range from 0 to 1, raised to an exponent gamma. Typical displays, for example those intended to reproduce the ITU-R BT.709-5 signal format recommended by the ITU-R, use a gamma exponent of approximately 1/0.45 (approximately 2.2). To compensate for t
34、his non-linearity, typical video cameras apply a gamma pre-correction on their output signal, using a gamma of approximately 0.45. The ideal opto-electronic transfer characteristics that are informally referred to by the exponent gamma differ somewhat from this exponential input-output relationship,
35、 especially in the range near zero. The exact opto-electronic transfer characteristics equation specified for such use is typically that specified in clause 1.2 of ITU-R BT.709-5. In order to ensure that the colour intensities are consistently signalled to the decoder, encoded video transmitted by a
36、 terminal should thus use a gamma of 0.45. A different value may be used if explicitly specified, given as a default value, negotiated, or, in the case that the decoding system has signalled the ability to display an indicated gamma value, indicated. If the video input source (camera, playback devic
37、e, etc.) provides a video signal with a different gamma, a gamma correction functional module should be inserted between the video source and video encoder to correct the gamma of the output from the video source to a value of 0.45 before encoding, as illustrated in Figure 1 below. A different value
38、 may be used if explicitly specified, given as a default value, negotiated, or, in the case that the decoding system has signalled the ability to display an indicated gamma value, indicated. ITU-T Rec. H.272 (01/2007) 3 Decoded video should be assumed to have a gamma of 0.45 unless a different value
39、 is explicitly specified, given as a default value, negotiated, or indicated. Herein, the phrase “gamma of 0.45“ is intended to refer to the opto-electronic transfer characteristics equation specified in clause 1.2 of ITU-R BT.709-5. If the terminals video display has a gamma other than 1/0.45 (appr
40、oximately 2.2), a gamma correction functional module may be inserted between the video decoder and the display to correct the video signal gamma to a value of 1/(display gamma) before display, as illustrated in Figure 1. NOTE 1 In some cases, for example to compensate for the ambient illumination le
41、vel at the display location, a different display gamma may be desirable. In the case of display of locally captured video, the gamma of the camera output and display input should also be corrected as described above. NOTE 2 The method used for gamma pre-correction by the gamma correction function is
42、 left to implementers, but in many cases a look-up table with 256 or 1024 values for each of the R, G and B channels of video will be sufficient. NOTE 3 If both terminals in Figure 1 correct gamma as described in this Recommendation, the brightness of the display elements will be proportional to the
43、 brightness of the video source. In practice, some terminals will not fully correct gamma. If only one terminal corrects gamma, only partial gamma correction will be achieved. NOTE 4 Video signals generated according to NTSC specifications (see Part A of b-ITU-R BT.1700) are specified as using a gam
44、ma value of 0.45 already, and therefore do not require the application of an additional gamma correction function. NOTE 5 Video signals generated according to PAL specifications (see Part B of b-ITU-R BT.1700) are currently specified as using a gamma value of 1/2.8. However, professional television
45、broadcast cameras used for SDTV and HDTV typically use a gamma value of 0.45 with slight variations; the older 1/2.8 specified value no longer being in current use. NOTE 6 b-ITU-T H.261 and b-ITU-T H.263 refer only to b-ITU-R BT.601 for definition of the colour space used for video representation. b
46、-ITU-R BT.601 did not actually specify a gamma correction function until its 2007 version. The gamma value of 0.45 is specified in the 2007 version of b-ITU-R BT.601, and it represents typical current industry practice for source acquisition using that Recommendation. Thus, if source acquisition is
47、conducted using some other gamma value, a gamma correction function should be applied prior to the encoding process for b-ITU-T H.261 or b-ITU-T H.263 use in the absence of some means for encoder-decoder negotiation of the use of a different gamma function. NOTE 7 b-ITU-T H.262 and b-ITU-T H.264 pro
48、vide a transfer_characteristics syntax element by which means an encoder may indicate the gamma pre-correction applied to the source video. However, these Recommendations do not require the transfer_characteristics syntax element to be present in all video bitstreams and specify that the default int
49、erpretation to be inferred in the absence of the use of the transfer_characteristics syntax element for this purpose is to be determined by the application. Thus, in the absence of the use of the transfer_characteristics syntax element and in the absence of other system-specified default values, negotiations or indicators, the inferred value for use in multimedia systems (such as video conferencing and video telephony systems) should be 0.45 for b-ITU-T H.262 and b-ITU-T H.264 use. The gamma exponent
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