ISO/IEC 10918-1 : 1 1993(E)
ISO/IEC 10918-1 : 1 1993(E)
CCITT Rec. T.81 (1992 E)
Annex B
Compressed data formats
(This annex forms an integral part of this Recommendation | International Standard)
ISO/IEC 10918-1 : 1993(E)
CCITT Rec. T.81 (1992 E)
This annex specifies three compressed data formats:
a)
the interchange format, specified in B.2 and B.3;
b)
the abbreviated format for compressed image data, specified in B.4;
c)
the abbreviated format for table-specification data, specified in B.5.
B.1 describes the constituent parts of these formats. B.1.3 and B.1.4 give the conventions for symbols and figures used in
the format specifications.
B.1
General aspects of the compressed data format specifications
Structurally, the compressed data formats consist of an ordered collection of parameters, markers, and entropy-coded data
segments. Parameters and markers in turn are often organized into marker segments. Because all of these constituent parts
are represented with byte-aligned codes, each compressed data format consists of an ordered sequence of 8-bit bytes. For
each byte, a most significant bit (MSB) and a least significant bit (LSB) are defined.
B.1.1
Constituent parts
This subclause gives a general description of each of the constituent parts of the compressed data format.
B.1.1.1
Parameters
Parameters are integers, with values specific to the encoding process, source image characteristics, and other features
selectable by the application. Parameters are assigned either 4-bit, 1-byte, or 2-byte codes. Except for certain optional
groups of parameters, parameters encode critical information without which the decoding process cannot properly
reconstruct the image.
The code assignment for a parameter shall be an unsigned integer of the specified length in bits with the particular value
of the parameter.
For parameters which are 2 bytes (16 bits) in length, the most significant byte shall come first in the compressed data's
ordered sequence of bytes. Parameters which are 4 bits in length always come in pairs, and the pair shall always be
encoded in a single byte. The first 4-bit parameter of the pair shall occupy the most significant 4 bits of the byte. Within
any 16-, 8-, or 4-bit parameter, the MSB shall come first and LSB shall come last.
B.1.1.2
Markers
Markers serve to identify the various structural parts of the compressed data formats. Most markers start marker segments
containing a related group of parameters; some markers stand alone. All markers are assigned two-byte codes: an X'FF'
byte followed by a byte which is not equal to 0 or X'FF' (see Table B.1). Any marker may optionally be preceded by any
number of fill bytes, which are bytes assigned code X'FF'.
NOTE Because of this special code-assignment structure, markers make it possible for a decoder to parse the compressed
data and locate its various parts without having to decode other segments of image data.
B.1.1.3
Marker assignments
All markers shall be assigned two-byte codes: a X'FF' byte followed by a second byte which is not equal to 0 or X'FF'.
The second byte is specified in Table B.1 for each defined marker. An asterisk (*) indicates a marker which stands alone,
that is, which is not the start of a marker segment.
CCITT Rec. T.81 (1992 E)
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