1、 INTERNATIONAL TELECOMMUNICATION UNION ITU-T T.42TELECOMMUNICATION STANDARDIZATION SECTOR OF ITU (07/2003) SERIES T: TERMINALS FOR TELEMATIC SERVICES Continuous-tone colour representation method for facsimile ITU-T Recommendation T.42 ITU-T Rec. T.42 (07/2003) i ITU-T Recommendation T.42 Continuous-
2、tone colour representation method for facsimile Summary This Recommendation defines a colour data representation method in order to make it possible to interchange continuous-tone colour image data over facsimile communication services such as Group 4 and Group 3 facsimile. CIELAB space is selected
3、as the basic colour space mainly for hard copy (printed) application. CIE Illuminant D50 and its perfectly diffuse reflecting white point (X0= 96.422; Y0= 100.000; Z0= 82.521) is selected as the basic illuminant and white point respectively. The default gamut range chosen is L* = 0, 100, a* = 85, 85
4、, b* = 75, 125. The exact expression is in terms of offset and range. The YCC space based on sYCC is selected also as the basic color space mainly for soft copy (displayed) application. CIE illuminant D65 and its perfectly diffuse reflecting white point (X0= 95.045; Y0= 100.000; Z0= 108.892) is the
5、illuminant and white point respectively. The default gamut is chosen is Y = 0, 1, Cb = 0.5, 0.5, Cr = 0.5, 0.5. The exact expression is in terms of offset and range. The main difference from the previous version is the following: ITU-YCC addition. Source ITU-T Recommendation T.42 was approved by ITU
6、-T Study Group 16 (2001-2004) under the ITU-T Recommendation A.8 procedure on 14 July 2003. ii ITU-T Rec. T.42 (07/2003) FOREWORD The International Telecommunication Union (ITU) is the United Nations specialized agency in the field of telecommunications. The ITU Telecommunication Standardization Sec
7、tor (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 worldwide basis. The World Telecommunication Standardization Assembly (WTSA), which meets ever
8、y 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 Resolution 1. In some areas of information technology which fall within ITU-Ts purview
9、, 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 administration and a recognized operating agency. Compliance with this Recommendation is volunta
10、ry. 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 are met. The words “shall“ or some other obligatory language such as “must“ and the ne
11、gative 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 possibility that the practice or implementation of this Recommendation may involve the
12、 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 the Recommendation development process. As of the date of approval of this Recommendat
13、ion, ITU had not received notice of intellectual property, protected by patents, which may be required to implement this Recommendation. However, implementors are cautioned that this may not represent the latest information and are therefore strongly urged to consult the TSB patent database. ITU 200
14、3 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. T.42 (07/2003) iii CONTENTS Page 1 Scope 1 2 Field of application. 1 3 References. 1 4 Definitions 1 5 Conventions 2 6 Colour representation model. 2
15、 6.1 Overview 2 6.2 Colour representation recommendation . 2 7 Colour reproduction information 5 Appendix I Method for colorimetric calculation from spectral measurement 6 Appendix II Calculation of CIELAB real values from CIE XYZ values . 10 Appendix III Calculation of ITU-YCC real values from CIE
16、XYZ values 10 ITU-T Rec. T.42 (07/2003) 1 ITU-T Recommendation T.42 Continuous-tone colour representation method for facsimile 1 Scope 1.1 This Recommendation defines a continuous-tone colour data representation method in order to make it possible to interchange continuous-tone colour image data ove
17、r a facsimile communication service such as Group 4 or Group 3 facsimile. Its purpose is to specify a colour space, reference white point, illuminant type, gamut range, and colour reproduction information for the interchange of colour data. 1.2 This Recommendation, together with documents such as cl
18、auses of ITU-T Recs T.4 and T.30, or T.563, T.503 and T.521, will define a colour image data format that may be used by colour facsimile service and by other telematic services. 2 Field of application 2.1 This Recommendation defines a colour data representation method which enables a receiver to rep
19、roduce colour image data as specified by the sender. The basic values are for hard copy (printed) colour image data and for soft copy (displayed) colour image data. 2.2 It is assumed that when a service is performed using this Recommendation, all non-basic features are subject to negotiation. 3 Refe
20、rences 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; use
21、rs of 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 Recommen
22、dation does not give it, as a stand-alone document, the status of a Recommendation. CIE Publication No. 15.2 (1986), Colorimetry, Second edition. ISO 5-1 to 5-4 (1984-2001), Photography Density measurements. ISO 13655:1996, Graphic technology Spectral measurement and colorimetric computation for gra
23、phic arts images. IEC 61966-2-1 (1999) Amd.1 Ed. 1.0 (2003), Multimedia systems and equipment Colour measurement and management Part 2-1: Colour management Default RGB colour space sRGB. 4 Definitions The definitions in ITU-T Rec. T.411 apply to this Recommendation. The definitions in CIE Publicatio
24、n No. 15.2 apply to this Recommendation. The definitions in IEC 61966-2-1 Amd.1 Ed. 1.0 Annex F apply to this Recommendation. 2 ITU-T Rec. T.42 (07/2003) This Recommendation defines the following terms: 4.1 CIELAB: This refers to 1976 CIE L*a*b* colour space. 4.2 ITU-YCC: This refers to sYCC in IEC
25、61966-2-1 Amd.1 Ed. 1.0, Annex F. 5 Conventions The conventions in CIE Publication No. 15.2 apply to this Recommendation. The conventions in the part of IEC 61966-2-1 Amd.1 Ed. 1.0, Annex F, apply to this Recommendation The conventions in ISO 13655 apply to this Recommendation. 6 Colour representati
26、on model 6.1 Overview In order to represent continuous-tone colour data accurately and uniquely, a device-independent interchange colour space is needed. This colour image space should encode the range of hard copy image data the range of soft copy image data. The following represents an example for
27、 the use of this model: a Sender scans an original colour image using a specific device-dependent colour space which may depend on the illuminant and/or filters of a particular scanner system. The Sender converts the device-dependent colour data to the interchange colour representation. The Sender t
28、hen encodes the data using a coding algorithm such as ITU-T Rec. T.81 (JPEG). The Receiver receives the encoded data. The data is decoded and converted to the colour space which is device dependent. In order to define the colour representation, it is necessary to specify the white point, illuminant
29、and gamut range used in the interchange data representation. Certain additional information for better and/or more desirable colour reproduction may optionally be specified. 6.2 Colour representation recommendation 6.2.1 CIELAB space 6.2.1.1 Colour space specification In this Recommendation, CIELAB
30、space is the basic value mainly for hard copy (printed) colour image. Conversion from spectral measurement data to CIE XYZ is defined in ISO 13655. (See Appendix I.) Conversion from CIE XYZ colour space to CIELAB real values is the same as that defined in CIE Publication No. 15.2. (See Appendix II.)
31、 White point and illuminant data, and gamut range are specified in 6.2.1.2 and 6.2.1.3. 6.2.1.2 White point and illuminant data CIE Illuminant D50 and its perfectly diffuse reflecting white point (X0= 96.422; Y0= 100.000; Z0= 82.521) are the basic values. Other illuminants and/or white points are op
32、tional and must be negotiated before use. Optional values are for further study. ITU-T Rec. T.42 (07/2003) 3 6.2.1.3 Gamut range The basic gamut range is chosen to span the union of available hard copy device gamuts as observed under D50 illumination. This range is as follows, with the exact definit
33、ion expressed below: L* = 0, 100 a* = 85, 85 b* = 75, 125 The gamut range is expressed as an OFFSET in the communication space and a RANGE, with the exception of the minimum and maximum values, which are explicitly defined in the definitions above. The calculations from real values L*a*b*to nLnanbbi
34、t integers, which are expressed by NLNaNb, are made as follows: NL = (2*nL 1)/RANGEL L* + OFFSETLNa = (2*na 1)/RANGEa a* + OFFSETaNb = (2*nb 1)/RANGEb b* + OFFSETbThe RANGE, OFFSET pairs for basic range L*, a* and b* for the case where NLNaNbare nLnanbbit integers are: Variable Range Offset L*100.00
35、 0 a*170.00 2*(na 1) b*200.00 2*(nb 2) + 2*(nb 3) In the case of eight-bit values, the calculations are as follows for the basic range: NL = round (255./100.) L* Na = round (255./170.) a* + 128. Nb = round (255./200.) b* + 96. In the case of twelve-bit values, the calculations are as follows for the
36、 basic range: NL = round (4095./100.) L* Na = round (4095./170.) a* + 2048. Nb = round (4095./200.) b* + 1536. Other gamut range values are optional and must be negotiated before use. For example, the following optional range: L* = 0, 100 a* = 128, 127 b* = 128, 127 would be expressed using range an
37、d offset in the case of eight-bit values by: NL= round (255./100.) L* Na= round (255./255.) a* + 128. Nb= round (255./255.) b* + 128. 4 ITU-T Rec. T.42 (07/2003) Note that values of L* greater than 100 are not disallowed, but are not in general reproducible, as they correspond to colours which may b
38、e produced by fluorescence or specular reflection in the hard copy case. In the soft copy case, tristimulus values X, Y, Z of image data shall be scaled under the condition that the maximum Y should not be over 100. Usually, the maximum of a monitor or an image may be used for the scaling. Values of
39、 L* less than 0 have no physical meaning. In addition, values of a* outside the range 500, 500 and values of b* outside the range 200, 200 are not representable from X, Y, Z tristimulus values, and have no meaning. 6.2.2 ITU-YCC space 6.2.2.1 Colour space specification In this Recommendation, ITU-YC
40、C space is the basic value mainly for soft copy (displayed) color image data. Conversion from spectral measurement data to CIE XYZ is defined in ISO 13655. (See Appendix I.) Conversion from CIE XYZ colour space to ITU-YCC real values is the same as that defined in IEC 61966-2-1 Amd.1 Ed. 1.0, Annex
41、F. (See Appendix III.) White point and illuminant data, and gamut range are specified in 6.2.2.2 and 6.2.2.3. 6.2.2.2 White point and illuminant data CIE Illuminant D65 and its perfectly diffuse reflecting white point (X0= 95.045; Y0= 100.000; Z0= 108.892) are the basic values. Other illuminants and
42、/or white points are not permitted for ITU-YCC. 6.2.2.3 Gamut range The basic gamut range is as follows, with the exact definition expressed below: Y = 0, 1 Cb = 0.5, 0.5 Cr = 0.5, 0.5 The gamut range is expressed as an OFFSET in the communication space and a RANGE, with the exception of the minimum
43、 and maximum values, which are explicitly defined in the definitions above. The calculations from real values YCbCr to nYnCbnCrbit integers, which are expressed by NYNCbNCr, are made as follows: NY= (2*nL 1)/RANGEY Y + OFFSETYNCb= (2*na 1)/RANGECb Cb + OFFSETCbNCr= (2*nb 1)/RANGECr Cr + OFFSETCrThe
44、RANGE, OFFSET pairs for basic range Y, Cb and Cr for the case where NYNCbNCrare nYnCbnCrbit integers are: Variable Range Offset Y 1.00 0 Cb 1.00 2*(nCb 1) Cr 1.00 2*(nCr 1) ITU-T Rec. T.42 (07/2003) 5 In the case of eight-bit values, the calculations are as follows for the basic range: NY= round(255
45、./1.) Y NCb= round(255./1.) Cb + 128. NCr= round(255./1.) Cr + 128. Other gamut range values are optional and must be negotiated before use. For example, the following optional range: Y = 0, 1.0 Cb = 1.0, 1.0 Cr = 1.0, 1.0 would be expressed using range and offset in the case of ten-bit values by: N
46、Y= round(1023./1.) Y NCb= round(1023./2.) Cb + 512. NCr= round(1023./2.) Cr + 512. 7 Colour reproduction information Colour reproduction information may be optionally provided, in addition to absolute values (CIELAB values or ITU-YCC values) for colour image data. This information may be used for be
47、tter and/or more desirable colour reproduction. This information is described in Table 1. Colour reproduction information is a non-basic feature. Table 1/T.42 Colour reproduction information list No. From Receiver to Sender From Sender to Receiver Type of data 1 Device White (Paper white for hard co
48、py, display white for soft copy) Original (Note) White CIELAB or ITU-YCC 2 Device Black (Cyan + magenta + yellow or + black colorant for hard copy, display black for soft copy) Original (Note) Black CIELAB or ITU-YCC 3 Device Cyan (Cyan colorant for hard copy, green + blue phosphor for soft copy) Or
49、iginal (Note) Cyan CIELAB or ITU-YCC 4 Device Magenta (Magenta colorant for hard copy, blue + red phosphor for soft copy) Original (Note) Magenta CIELAB or ITU-YCC 5 Device Yellow (Yellow colorant for hard copy, red + green phosphor for soft copy) Original (Note) Yellow CIELAB or ITU-YCC 6 Device Red (Magenta + yellow colorant for hard copy, red phosphor for soft copy) Original (Note) Red CIELAB or ITU-
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