CCITT Facsmile Compression Standards

The International Telecommunications Union, ITU-T (formerly know as CCITT), is a multinational union that provides standards for telecommunication equipment and systems. Here we will explain two standards for facsimile compression, T.4 and T.6.

Recommendation T.4 defines the characteristics of Group 3 facsimile terminals which enable black and white documents and also optionally colour documents to be transmitted on the general switched telephone network, international leased circuits and the Integrated Services Digital Network (ISDN). Group 3 facsimile terminals may be operated manually or automatically and document transmission may be requested alternatively with telephone conversation.
T.6 facsimile coding schemes consist of the basic facsimile coding scheme and optional facsimile coding schemes. They are defined in Section 2, 3 and 4. Facsimile coding schemes are specified assuming that transmission errors are corrected by control procedures at a lower level. The basic T.6 facsimile coding scheme is the two-dimensional coding scheme which is in principle the same as the two-dimensional coding scheme of Group 3 facsimile specified in Recommendation T.4. Optional facsimile coding schemes are specified not only for black and white images but also for grey scale and colour images. Facsimile coding control functions are used in facsimile user information in order to change facsimile parameters or to invoke the end of facsimile block.

The CCITT T.4 and T.6 specifications outline many different kinds of requirements for the facsimile apparatus, including the resolution of scanning and printing, dimensional tolerances, timing constraints etc. The sections which tend to be used in the imaging world are those which describe the compression schemes, and these are often referred to as Group 3 and Group 4 in their own right. The compression and decompression algorithms are quite complex, as will be seen, and must be implemented either in hardware or sophisticated software in order to achieve good performance.

So how do the Group 3 and Group 4 compression mechanisms work?
Both schemes encode the source image on a horizontal scanline-by-scanline basis, corresponding to the way in which documents are scanned and printed on a facsimile machine. The difference lies in the way the two standards handle successive scanlines - in Group 3, each scanline is encoded independently, whereas in Group 4, scanlines are encoded with reference to the previous one, resulting in improved compression ratios.

An interesting side-effect of the way that Group 3 and Group 4 work is that the size of compressed images is roughly linearly proportional to the resolution at which the image is scanned, whereas that of the original is of course proportional to the square of the resolution. This is because the total compressed size is a function of the number of scanlines, with roughly the same number of codes being produced for each scanline irrespective of resolution.

Compression ratios achieved using Group 3 of course depend upon the nature of the source image. Under certain circumstances, it will be obvious that the encoded version could be larger than the original (consider the case of single alternating black and white pixels), but in general compression ratios of 10:1 can be achieved on pages of typewritten text.

Compression ratios achieved using Group 4 are several times better than Group 3, yielding upwards of 25:1. Most imaging systems now use the Group 4 compression standard, as a result of the improved compression and the development of Group 4 software compression and decompression which executes at high speed.

back to top

CCITT T.4 2-Dimensional Compression Standard

INTRODUCTION
The CCITT Group 3 two-dimensional coding uses the Modified Huffman code and the Modified Relative element address designate (MR)code.The main atvantage of the CCITT Group 3 two-dimensional coding over the CCITT Group 3 one-dimensional coding is that compression efficiency is improved.This is due to the fact that the two-dimensional coding explores correlation of pixels in two successive lines.

In Group 3, a scanline is encoded as a set of runs, each representing a number of white or black pixels, with white runs alternating with black runs. Every run is encoded using a variable number of bits, which can be uniquely identified upon decoding. This means that frequently occurring lengths of run may be encoded very efficiently, at the expense of the infrequent ones. For example, a black run of 2 or 3 pixels is encoded using just 2 bits, whereas one of 1000 pixels is encoded in 25.

In summary, for every group of K lines, the CCITT Group two-dimensional scheme encodes 1 line in one-dimensional coding and the other K-1 lines in two-dimensional coding.
In the CCITT Group 3 2-D coding there are 5 changing picture elements

At the starting the coding line, the changing element a0 is first set on imaginary white changing element located just before the first element on the coding line. During the encoding of the coding line, the position of a0 is determined by the previous coding mode.

EXAMPLE

                 b1       b2

      a0       a1         a2

MODES

In the CCITT T.4 standard, there are three coding modes: Pass Mode(P) ,Vertical Mode, and Horizontal Mode (H). Depending on the position of a changing element along the coding line, one of three coding modes can be identified.

               -  V  (0):a1 just under b1,
               -  Vr (1):a1 one pixel to the right of b1,
               -  Vr (2):a1 two pixels to the right of b1,
               -  Vr (3):a1 three pixels to the right of b1,
               -  Vl (1):a1 one pixel to the left b1,
               -  Vl (2):a1 two pixels to the left b1,
               -  Vl (3):a1 three pixels to the left b1.

The code words for three coding modes (pass, horizontal and vertical coding modes) are shown in Table 4. with the optional extension. In Table 4. M(ai,aj) represents the code words of 1-D compression standard for the run ai aj.

back to top

CCITT T.6 2-Dimensional Compression Standard

INTRODUCTION
In 1984, the CCITT developed a specification defining the facsimile coding schemes to be used in the Group 4 black and white facismile. The standard, known as CCITT Recommendation T.6, is designed for error-free digital facsimile transmission mainly on public data networks. This specification has been widely used in document imaging systems for compressing black and white images. It is sometimes referred to as the CCITT Group 4 compression standard.

The CCITT T.6 facsimile coding standard consists of this basic facsimile coding scheme and future optional facsimile coding schemes. It is assumed that transmission errors are corrected by control procedures at a lower level. Therefore, CCITT T.6 is a noiseless coding scheme.
Since the CCITT T.6 coding scheme is designed for noise free transmission, a single bit error can cause corruption of the image. Compared to the CCITT T.4 coding scheme, this scheme is very sensitive to error, and is less robust.

The one-dimensional coding scheme of Group 3 facsimile makes use of only horizontal dependencies between picture elements in the same scan line. The advantage of the CCITT T.6 coding scheme over the one-dimensional coding scheme of Group 3 facsimile is that it exploits both horizontal and vertical dependencies, resulting in the greater compression efficiency. Since the CCITT T.4 two-dimensional coding scheme requires that every Kth line of image data must be encoded by the one-dimensional coding scheme, it is less efficient compared to the CCITT T.6 coding scheme.

The essence of the scheme is as follows. Consider the pattern shown in the figure below, which represents a few scanlines from an image. Because each changing pixel is underneath, or at least very close to, that on the line above, its position is encoded using a 'Vertical' code. This encoding is used where the relative positions are within three pixels of each other, and is the most commonly occurring type of code. In particular, the case where the positions of the changing pixels are identical, known as V(0) (the others being VL(3) to VR(3)), is encoded using a single bit. It can be seen immediately that a vertical line, of any thickness, can be encoded very efficiently.
When the bottom of image is detected (because the position of the changing pixel is very different to that on the line above), this is encoded as a 'Pass' code, and represents skipping over the two changing pixels, to black and back to white, on the line above. The positions of changing pixels which are not in close proximity to those above are encoded in pairs, using the Group 3 encoding mechanism.


For 200 DPI (dots per inch) A4 documents, the typical compression ratio of the CCITT T.6 coding scheme is from about 15:1 to 20:1. For 400 DPI document images (e.g., eight CCITT test documents), it ranges from 30:1 to 40:1.

back to top

Books

• Data and Image Compression : Tools and Techniques, 4E"
  by Gilbert Held and Thomas R. Marshall
• FAX : Facsimile Technology and Systems (Arctech House Telecommunications Library)"
  by Dennis Bodson, Stephen Urban and Kenneth R. McConnell
• The Fax Modem Sourcebook/Book and Disk
  by Andrew Margolis

back to top

References

• Specs for Group 3 and 4 image coding (group 3 is very similar to group 4) are in CCITT (1988) volume VII fascicle VII.3.
• The following paper covers T.4, T.6 :
  "Review of standards for electronic imaging for facsimile systems" in Journal of Electronic Imaging, Vol. 1, No. 1, pp. 5-21, January 1992.

back to top

Related Links

• There is also an updated spec contained in 1992 recommendations T.1 to T.6. CCITT (now ITU-T) specs are available by anonymous ftp. The T.4 and T.6 specs are in : ftp://sunsite.doc.ic.ac.uk/computing/ccitt/ccitt-standards/1988/7_3_01.ps.gz ftp://sunsite.doc.ic.ac.uk/computing/ccitt/ccitt-standards/1988/7_3_02.ps.gz
• Here you can find original T.4 recommendation document
• Software for reading and writing CCITT Group 3 and 4 images is also available in directory : ftp://merry.cs.monash.edu.au/pub/alanf/TIFF_FAX

back to top