background image
ISO/IEC 10918-1 : 1993(E)
The second level of Figure B.2 specifies that a frame shall begin with a frame header and shall contain one or more scans.
A frame header may be preceded by one or more table-specification or miscellaneous marker segments as specified in
B.2.4. If a DNL segment (see B.2.5) is present, it shall immediately follow the first scan.
For sequential DCT-based and lossless processes each scan shall contain from one to four image components. If two to
four components are contained within a scan, they shall be interleaved within the scan. For progressive DCT-based
processes each image component is only partially contained within any one scan. Only the first scan(s) for the components
(which contain only DC coefficient data) may be interleaved.
The third level of Figure B.2 specifies that a scan shall begin with a scan header and shall contain one or more entropy-
coded data segments. Each scan header may be preceded by one or more table-specification or miscellaneous marker
segments. If restart is not enabled, there shall be only one entropy-coded segment (the one labeled "last"), and no restart
markers shall be present. If restart is enabled, the number of entropy-coded segments is defined by the size of the image
and the defined restart interval. In this case, a restart marker shall follow each entropy-coded segment except the last one.
The fourth level of Figure B.2 specifies that each entropy-coded segment is comprised of a sequence of entropy-
coded MCUs. If restart is enabled and the restart interval is defined to be Ri, each entropy-coded segment except the last
one shall contain Ri MCUs. The last one shall contain whatever number of MCUs completes the scan.
Figure B.2 specifies the locations where table-specification segments may be present. However, this Specification hereby
specifies that the interchange format shall contain all table-specification data necessary for decoding the compressed
image. Consequently, the required table-specification data shall be present at one or more of the allowed locations.
B.2.2
Frame header syntax
Figure B.3 specifies the frame header which shall be present at the start of a frame. This header specifies the source image
characteristics (see A.1), the components in the frame, and the sampling factors for each component, and specifies the
destinations from which the quantized tables to be used with each component are retrieved.
C
1
1
1
1
C
C
SOF
P
Lf
Y
X
Nf
H
V
Tq
2
2
V
2
H
Tq
2
Nf
Nf
Nf
Nf
H
V
Tq
n
TISO0850-93/d021
Frame header
Frame component-specification parameters
Component-specification
parameters
Figure B.3 Frame header syntax
Figure B.3 [D21], = 5.5 cm = 215.%
The markers and parameters shown in Figure B.3 are defined below. The size and allowed values of each parameter are
given in Table B.2. In Table B.2 (and similar tables which follow), value choices are separated by commas (e.g. 8, 12) and
inclusive bounds are separated by dashes (e.g. 0 - 3).
SOF
n
:
Start of frame marker Marks the beginning of the frame parameters. The subscript n identifies whether
the encoding process is baseline sequential, extended sequential, progressive, or lossless, as well as which
entropy encoding procedure is used.
SOF
0
:
Baseline DCT
SOF
1
:
Extended sequential DCT, Huffman coding
SOF
2
:
Progressive DCT, Huffman coding
CCITT Rec. T.81 (1992 E)
35
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