TIFF 6.0 Specification
Final—June 3, 1992
Section 11: CCITT Bilevel Encodings
The following fields are used when storing binary pixel arrays using one of the
encodings adopted for raster-graphic interchange in numerous CCITT and ISO
(International Organization for Standards) recommendations and standards. These
encodings are often spoken of as “Group III compression” and “Group IV com-
pression” because their application in facsimile transmission is the most widely
known.
For the specialized use of these encodings in storing facsimile-transmission images,
further guidelines can be obtained from the TIFF Class F document, available on-line
in the same locations as this specification. This document is administered by another
organization; paper copies are not available from Adobe.
Compression
Tag
= 259 (103.H)
Type = SHORT
N
=1
3 = T4-encoding: CCITT T.4 bi-level encoding as specified in section 4, Coding, of
CCITT Recommendation T.4: “Standardization of Group 3 Facsimile apparatus
for document transmission.” International Telephone and Telegraph Consultative
Committee (CCITT, Geneva: 1988).
See the T4Options field for T4-encoding options such as 1D vs 2D coding.
4 = T6-encoding: CCITT T.6 bi-level encoding as specified in section 2 of CCITT
Recommendation T.6: “Facsimile coding schemes and coding control functions
for Group 4 facsimile apparatus.” International Telephone and Telegraph Consul-
tative Committee (CCITT, Geneva: 1988).
See the T6Options field for T6-encoding options such as escape into
uncompressed mode to avoid negative-compression cases.
Application in Image Interchange
CCITT Recommendations T.4 and T.6 are specified in terms of the serial bit-by-
bit creation and processing of a variable-length binary string that encodes bi-level
(black and white) pixels of a rectangular image array. Generally, the encoding
schemes are described in terms of bit-serial communication procedures and the
end-to-end coordination that is required to gain reliable delivery over inherently
unreliable data links. The Group 4 procedures, with their T6-encoding, represent a
significant simplification because it is assumed that a reliable communication
medium is employed, whether ISDN or X.25 or some other trustworthy transport
vehicle. Because image-storage systems and computers achieve data integrity and
communication reliability in other ways, the T6-encoding tends to be prefered for
imaging applications. When computer storage and retrieval and interchange of
facsimile material are of interest, the T4-encodings provide a better match to the
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