1、INTERNATIONAL TELECOMMUNICATION UNIONITU-T T.6TELECOMMUNICATIONSTANDARDIZATION SECTOROF ITUTERMINAL EQUIPMENT AND PROTOCOLS FORTELEMATIC SERVICESFACSIMILE CODING SCHEMES AND CODINGCONTROL FUNCTIONS FOR GROUP 4FACSIMILE APPARATUSITU-T Recommendation T.6(Extract from the Blue Book)NOTES1 ITU-T Recomme
2、ndation T.6 was published in Fascicle VII.3 of the Blue Book. This file is an extract from theBlue Book. While the presentation and layout of the text might be slightly different from the Blue Book version, thecontents of the file are identical to the Blue Book version and copyright conditions remai
3、n unchanged (see below).2 In this Recommendation, the expression “Administration” is used for conciseness to indicate both atelecommunication administration and a recognized operating agency. ITU 1988, 1993All rights reserved. No part of this publication may be reproduced or utilized in any form or
4、by any means, electronic ormechanical, including photocopying and microfilm, without permission in writing from the ITU.Fascicle VII.3 - Rec. T.6 1Recommendation T.6Fascicle VII.3 - Rec. T.6FACSIMILE CODING SCHEMES AND CODING CONTROL FUNCTIONSFOR GROUP 4 FACSIMILE APPARATUS(Malaga-Torremolinos, 1984
5、; amended at Melbourne, 1988)1 General1.1 Scope1.1.1 Recommendation T.6 defines the facsimile coding schemes and their control functions to be used in theGroup 4 facsimile.1.1.2 This Recommendation should be read in conjunction with the following Recommendations:T.503 A document application profile
6、for the interchange of Group 4 facsimile documentsT.521 Communication application profile for document bulk transfer based on the session service(according to the rules defined in T.62 bis)T.563 Terminal characteristics for Group 4 facsimile apparatusT.73 Document interchange protocol for the Telema
7、tic servicesT.62 Control procedures for Teletex and Group 4 facsimile servicesT.62 bis Control procedures for Teletex and Group 4 facsimile services based onRecommendations X.215/X.225T.70 Network-independent basic transport service for Telematic servicesF. 161 International Group 4 facsimile servic
8、eIn addition, in the case of Group 4 Class II/III (Teletex or mixed mode of operation), the followingRecommendations should also be read:T.60 Terminal equipment for use in the Teletex serviceT.61 Character repertoire and coded character sets for the international Teletex serviceT.72 Terminal capabil
9、ities for mixed mode of operation1.2 Fundamental principles1.2.1 Facsimile coding schemes and coding control functionsFacsimile coding schemes consist of the basic facsimile coding scheme and optional facsimile coding schemes.They are defined in 2 and 3 and 4, respectively.Facsimile coding schemes a
10、re specified assuming that transmission errors are corrected by control proceduresat a lower level.The basic facsimile coding scheme is the two-dimensional coding scheme which is in principle the same as thetwo-dimensional coding scheme of Group 3 facsimile specified in Recommendation T.4.Optional f
11、acsimile coding schemes are specified not only for black and white images but also for grey scaleimages and colour images.Facsimile coding control functions are used in facsimile user information in order to change facsimileparameters or to invoke the end of facsimile block. They are defined in 2.4.
12、2 Facsimile coding schemes and coding control functions for black and white images2.1 GeneralThis section specifies the facsimile coding schemes, and associated control functions for black and whiteimages.Facsimile coding schemes consist of the basic facsimile coding scheme and optional coding schem
13、es.2 Fascicle VII.3 - Rec. T.6The use of the optional facsimile coding schemes is subject to mutual agreement between terminals and shall beinitiated by the appropriate procedural steps.2.2 Basic facsimile coding scheme2.2.1 Principle of the coding schemeThe coding scheme uses a two-dimensional line
14、-by-line coding method in which the position of each changingpicture element on the current coding line is coded with respect to the position of a corresponding reference elementsituated on either the coding line or the reference line which is immediately above the coding line. After the coding line
15、has been coded, it becomes the reference line for the next coding line. The reference line for the first coding line in apage is an imaginary white line.2.2.2 Definition of changing picture elements (see Figure 1 /T.6)A changing element is defined as an element whose “colour” (i.e. black or white) i
16、s different from that of theprevious element along the same scan line.2.2.3 Coding modesOne of the three coding modes are chosen according to the coding procedure described in 2.2.4 to code theposition of each changing element along the coding line. Examples of the three coding modes are given inFig
17、ure 2/T.6, 3/T.6 and 4/T.6.2.2.3.1 Pass modeThis mode is identified when the position of b2lies to the left of a1. (See Figure 2/T.6.)However, the state where b2occurs just above a1as shown in Figure 3/T.6 is not considered as a pass mode.Fascicle VII.3 - Rec. T.6 32.2.3.2 Vertical modeWhen this mod
18、e is identified, the position of a1is coded relative to the position of b1. The relative distance a1b1can take on one of seven values V(0), VR(1), VR(2), VR(3), VL(1), VL(2) and VL(3), each of which is represented by aseparate code word. The subscripts R and L indicate that a1is to the right or left
19、 respectively of b1, and the number inbrackets indicates the value of the distance a1b1(see Figure 4/T.6).2.2.3.3 Horizontal modeWhen this mode is identified, both the run-lengths a0a1and a1a2are coded using the code words H + M(a0a1) +M(a1a2). H is the flag code word 001 taken from the two-dimensio
20、nal code table (Table 1/T.6). M(a0a1) and M(a1a2)are code words which represent the length and “colour” of the runs a0a1and a1a2respectively and are taken from theappropriate white or black run-length code tables (Tables 2/T.6 and 3/T.6).4 Fascicle VII.3 - Rec. T.6TABLE 1/T.6Code tableMode Elements
21、to be coded Notation Code wordPass b1, b2P 0001Horizontal a0a1, a1a2H 001 + M(a0a1) + M(a1a2)(see Note)Vertical a1just under b1a1b1= 0 V(0) 1a1to the right of b1a1b1= 1 VR(1) 011a1b1= 2 VR(2) 000011a1b1= 3 VR(3) 0000011a1to the left of b1a1b1= 1 VL(1) 010a1b1= 2 VL(2) 000010a1b1= 3 VL(3) 0000010Exte
22、nsion 0000001xxxNote Code M( ) of the horizontal mode represents the code words in Tables 2/T.6 and 3/T.6.Fascicle VII.3 - Rec. T.6 5TABLE 2/T.6Terminating codesWhite run length Code word Black run length Code word01234567891011121314151617181920212223242526272829303132333435363738394041424344454647
23、484950515253545556575859606162630011010100011101111000101111001110111110011101000011101000001000000011110100110101101010101011010011100011000001000001011100000110000100010100001010110010011010010000110000000001000000011000110100001101100010010000100110001010000010101000101100001011100101000001010010
24、010101000101011001011000010110100000100000001010000101000001011010100100101001101010100010101010010010000100101010110000101100101011010010110110100101001001011001100100011001100110100012345678910111213141516171819202122232425262728293031323334353637383940414243444546474849505152535455565758596061626
25、300001101110101110011001100100001100010100010000001000000101000011100000100000001110000110000000010111000001100000000010000000110011100001101000000011011000000011011100000101000000000101110000001100000001100101000001100101100001100110000001100110100000110100000000110100100000110101000000110101100001
26、101001000001101001100001101010000001101010100001101011000001101011100000110110000000110110100001101101000001101101100000101010000000101010100000101011000000101011100000110010000000110010100000101001000000101001100000010010000000011011100000011100000000010011100000010100000000101100000000101100100000
27、01010110000001011000000010110100000011001100000011001116 Fascicle VII.3 - Rec. T.6TABLE 3/T.6Make-up codes between 64 and 1728White run length Code word Black run length Code word6412819225632038444851257664070476883289696010241088115212161280134414081472153616001664172811011100100101110110111001101
28、100011011101100100011001010110100001100111011001100011001101011010010011010011011010100011010101011010110011010111011011000011011001011011010011011011010011000010011001010011010011000010011011641281922563203844485125766407047688328969601024108811521216128013441408147215361600166417280000001111000011
29、001000000011001001000001011011000000110011000000110100000000110101000000110110000000011011010000001001010000000100101100000010011000000001001101000000111001000000011100110000001110100000000111010100000011101100000001110111000000101001000000010100110000001010100000000101010100000010110100000001011011
30、00000011001000000001100101Make-up codes between 1792 and 2560Run length(black and white)Make-up codes1792185619201984204821122176224023042368243224962560000000010000000000110000000001101000000010010000000010011000000010100000000010101000000010110000000010111000000011100000000011101000000011110000000
31、011111Fascicle VII.3 - Rec. T.6 72.2.4 Coding procedureThe coding procedure identifies the coding mode that is to be used to code each changing element along thecoding line. When one of the three coding modes has been identified according to Step 1 or Step 2 mentioned below,an appropriate code word
32、is selected from the code table given in Table 1/T.6. The coding procedure is as shown in theflow diagram of Figure 5/T.6.8 Fascicle VII.3 - Rec. T.6Step 1i) If a pass mode is identified, this is coded using the word 0001 (Table 1/T.6). After this processing, pictureelement a0just under b2is regarde
33、d as the new starting picture element a0for the next coding (seeFigure 2/T.6).ii) If a pass mode is not detected, then proceed to Step 2.Note It does not affect compatibility to restrict the use of pass mode in the encoder to a single pass mode. Variations of the algorithm which do not affect compat
34、ibility should be the subject of further study.Step 2i) Determine the absolute value of the relative distance a1b1.ii) If | a1b1| 3, as shown in Table 1/T.6, a1b1is coded by the vertical mode, after which position a1isregarded as the new starting picture element a0for the next coding.iii) If | a1b1|
35、 3, as shown in Table 1/T.6, following horizontal mode code 001, a0a1and a1a2are respectivelycoded by one-dimensional run length coding.Run lengths in the range of 0 to 63 pels are encoded with their appropriate terminating code word ofTable 2/T.6. Note that there is a different list of code words f
36、or black and white run lengths. Run lengths inthe range of 64 to 2623 pels are encoded first by the make-up code word representing the run length whichis nearest, not longer, to that required. This is then followed by the terminating code word representing thedifference between the required run leng
37、th and the run length represented by the make-up code. Runlengths in the range of lengths longer than or equal to 2624 pels are coded first by the make-up code of2560. If the remaining part of the run (after the first make-up code of 2560) is 2560 pels or greater,additional make-up code(s) of 2560 a
38、re issued until the remaining part of the run becomes less than 2560pels. Then the remaining part of the run is encoded by terminating code or by make-up code plusTerminating code according to the range as mentioned above.After this processing, position a2is regarded as the new starting picture elem
39、ent a0for the next coding.Note Coding examples are given in Recommendation T.4, 4.2.5.2.2.5 Processing the first and last picture element in a line2.2.5.1 Processing the first picture elementThe first starting picture element a0on each coding line is imaginarily set at a position just before the fir
40、stpicture element, and is regarded as a white picture element (see 2.2.2).The first run length on a line a0a1is replaced by a0a1 1. Therefore, if the first actual run is black and isdeemed to be coded by horizontal mode coding, then the first code word M(a0a1) corresponds to an imaginary whiterun of
41、 zero length (see Figure 10/T.4).2.2.5.2 Processing the last picture elementThe coding of the coding line continues until the position of the imaginary changing element situated just afterthe last actual element has been coded. This may be coded as a1or a2. Also, if b1and/or b2are not detected at an
42、y timeduring the coding of the line, they are positioned on the imaginary changing element situated just after the last actualpicture element on the reference line.2.3 Optional facsimile coding schemes for black and white images2.3.1 Uncompressed modeUncompressed mode is an optional coding scheme as
43、sociated to the basic facsimile coding scheme and is usedto transmit the image information without data compression techniques as shown in Table 4/T.6.The extension code in 2.2.4 with the xxx bits set to 111 is used as an entrance code from the basic codingscheme in 2.2 to the uncompressed mode.Whil
44、e using the uncompressed mode, the last picture elements of the end of the line and the first pictureelements of the beginning of the following line are concatenated to one pattern.Fascicle VII.3 - Rec. T.6 9TABLE 4/T.6Uncompressed mode code wordsEntrance code touncompressed modeBasic coding scheme:
45、 0000001111Image pattern Code wordUncompressedmode code10100100010000100000101001000100001000001Exit fromuncompressedmode code00000000000000001T00000001T000000001T0000000001T00000000001TT denotes a tag bit which tells the colour of the next run (black = 1, white = 0).2.4 Facsimile coding control fun
46、ctions2.4.1 Control functions for basic facsimile coding scheme2.4.1.1 End-of-facsimile blockThe end-of-facsimile block (EOFB) code is added to the end of every coded facsimile block. The format ofEOFB is as follows:Format: 00000000000100000000000124 bits2.4.1.2 Pad bitsPad bits may be used after th
47、e end-of-facsimile block code if it is necessary to align on octet boundaries or to afixed block size. The format used is as follows.Format: Variable length string of 0s.2.4.1.3 ExtensionExtension code is used to indicate the change from the current mode to another mode, e.g., another codingscheme.F
48、ormat: 0000001xxx,where xxx = 111 indicates uncompressed mode which is specified in 2.3.1.Further study is needed to define other unspecified xxx bit assignments and their use for any further extensions.3 Optional grey scale facsimile coding schemes and their coding control functionsFor further study.4 Optional colour facsimile coding schemes and their coding control functionsFor further study.
