background image
ISO/IEC 10918-1 : 1993(E)
4.8.2
Minimum coded unit
Related to the concepts of multiple-component interleave is the minimum coded unit (MCU). If the compressed image
data is non-interleaved, the MCU is defined to be one data unit. For example, in Figure 12 the MCU for the non-
interleaved case is a single data unit. If the compressed data is interleaved, the MCU contains one or more data units from
each component. For the interleaved case in Figure 12, the (first) MCU consists of the three interleaved data units A
1
, B
1
,
C
1
. In the example of Figure 13, the (first) MCU consists of the four data units A
1
, A
2
, B
1
, C
1
.
4.9
Structure of compressed data
Figures 1, 2, and 3 all illustrate slightly different views of compressed image data. Figure 1 shows this data as the output
of an encoding process, Figure 2 shows it as the input to a decoding process, and Figure 3 shows compressed image data
in the interchange format, at the interface between applications.
Compressed image data are described by a uniform structure and set of parameters for both classes of encoding processes
(lossy or lossless), and for all modes of operation (sequential, progressive, lossless, and hierarchical). The various parts of
the compressed image data are identified by special two-byte codes called markers. Some markers are followed by
particular sequences of parameters, as in the case of table specifications, frame header, or scan header. Others are used
without parameters for functions such as marking the start-of-image and end-of-image. When a marker is associated with a
particular sequence of parameters, the marker and its parameters comprise a marker segment.
The data created by the entropy encoder are also segmented, and one particular marker the restart marker is used to
isolate entropy-coded data segments. The encoder outputs the restart markers, intermixed with the entropy-coded data, at
regular restart intervals of the source image data. Restart markers can be identified without having to decode the
compressed data to find them. Because they can be independently decoded, they have application-specific uses, such as
parallel encoding or decoding, isolation of data corruptions, and semi-random access of entropy-coded segments.
There are three compressed data formats:
a)
the interchange format;
b)
the abbreviated format for compressed image data;
c)
the abbreviated format for table-specification data.
4.9.1
Interchange format
In addition to certain required marker segments and the entropy-coded segments, the interchange format shall include the
marker segments for all quantization and entropy-coding table specifications needed by the decoding process. This
guarantees that a compressed image can cross the boundary between application environments, regardless of how each
environment internally associates tables with compressed image data.
4.9.2
Abbreviated format for compressed image data
The abbreviated format for compressed image data is identical to the interchange format, except that it does not include all
tables required for decoding. (It may include some of them.) This format is intended for use within applications where
alternative mechanisms are available for supplying some or all of the table-specification data needed for decoding.
4.9.3
Abbreviated format for table-specification data
This format contains only table-specification data. It is a means by which the application may install in the decoder the
tables required to subsequently reconstruct one or more images.
4.10
Image, frame, and scan
Compressed image data consists of only one image. An image contains only one frame in the cases of sequential and
progressive coding processes; an image contains multiple frames for the hierarchical mode.
A frame contains one or more scans. For sequential processes, a scan contains a complete encoding of one or more image
components. In Figures 12 and 13, the frame consists of three scans when non-interleaved, and one scan if all three
components are interleaved together. The frame could also consist of two scans: one with a non-interleaved component,
the other with two components interleaved.
CCITT Rec. T.81 (1992 E)
21
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