Page 167 - ITU Journal, ICT Discoveries, Volume 3, No. 1, June 2020 Special issue: The future of video and immersive media

ITU Journal: ICT Discoveries, Vol. 3(1), June 2020

Other applications like colour facsimile group 3 (4.1.1) have even today some limited use, but in importance
far beyond, for example, a JPEG image attached to an email. Computer conferences (4.1.4) with ITU-T T.120
[13] were popular in the 1990s and in the early 2000s, but it is difficult to estimate what market share remains
today. ISDN videophone (ITU-T H.320 [9] and ITU-T H.261 [10]) saw some modest use in the past, but no
longer.

Note that all of these applications had their own communication protocols, file formats, colour models, etc.
This was the reason, from the CCITT side, why these components were not required and so not included in the
JPEG-1 standard.
4.1.1 Colour facsimile group 3: Annex E of ITU-T T.30 [14]; Annex E of ITU-T T.4 [15]

Facsimile group 3 was especially popular during the 1980s, but in the 1990s the Internet and the worldwide
web gradually replaced this service. It has still some advantages; however, when colour facsimile group 3 was
implemented in products, interest in this application was already in decline. Annex E of ITU-T T.30 [14] opens,
“This annex describes the additions to ITU-T Rec. T.30 to enable the transmission of continuous-tone
(multilevel) colour and gray-scale images for Group 3 facsimile mode of operation.
The objective is to enable the efficient transmission of high quality, full colour and gray-scale images over
the general switched telephone network and other networks. The images are normally obtained by
scanning the original sources with scanners of 200 pels/25.4 mm or higher, and bit depths of eight bits per
picture element per colour component or higher. The original sources are typically colour or gray-scale
photographs or hard copies from high-quality printing systems.
The method specified here performs well on full-colour images, but for transmission of multi-colour images
such as business graphics, other methods may be more efficient. Two such methods would be the
transmission of images using ITU-T Recs T.434 (Binary File Transfer) and T.82 (JBIG encoding). This annex
does not address the encoding of multi-colour images. This topic is left for further study.
The encoding methodology for continuous-tone (multilevel) images is based on the JPEG (ITU-T Rec. T.81 |
ISO/IEC 10918-1) image encoding standard. The JPEG image coding method includes both a lossy mode
and a lossless mode of encoding. This annex adopts the lossy mode of encoding which is based on the
Discrete Cosine Transform.
The technical features of encoding and decoding the continuous-tone colour and gray-scale image data are
described in Annex E/T.4. It describes two modes of image encoding (lossy gray-scale and lossy colour)
which are defined using ITU-T Rec. T.81.”
ITU-T T.30 [14] is actually in use because facsimile group 3 terminals that were popular before the web era
are still around, along with colour printers with facsimile sending and receiving capability. Nevertheless, its
importance is substantially less than that of, for example, web pages with photographic content.

4.1.2 Colour group 4 facsimile: ITU-T T.563 [16]; ITU-T T.42 [17]
Clause 2.5 of ITU-T T.563 [16] reads,

“For the continuous tone colour image, the continuous tone colour representation method for G4 facsimile
is defined in Recommendation T.42.”
Clause 6.1 of ITU-T T.42 [17] reads,

“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.

© International Telecommunication Union, 2020 145