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ISO/IEC 10918-1 : 1993(E)
Td
j
:
DC entropy coding table destination selector Specifies one of four possible DC entropy coding table
destinations from which the entropy table needed for decoding of the DC coefficients of component Cs
j
is
retrieved. The DC entropy table shall have been installed in this destination (see B.2.4.2 and B.2.4.3) by the
time the decoder is ready to decode the current scan. This parameter specifies the entropy coding table
destination for the lossless processes.
Ta
j
:
AC entropy coding table destination selector Specifies one of four possible AC entropy coding table
destinations from which the entropy table needed for decoding of the AC coefficients of component Cs
j
is
retrieved. The AC entropy table selected shall have been installed in this destination (see B.2.4.2 and B.2.4.3)
by the time the decoder is ready to decode the current scan. This parameter is zero for the lossless processes.
Ss:
Start of spectral or predictor selection In the DCT modes of operation, this parameter specifies the first
DCT coefficient in each block in zig-zag order which shall be coded in the scan. This parameter shall be set to
zero for the sequential DCT processes. In the lossless mode of operations this parameter is used to select the
predictor.
Se:
End of spectral selection Specifies the last DCT coefficient in each block in zig-zag order which shall be
coded in the scan. This parameter shall be set to 63 for the sequential DCT processes. In the lossless mode of
operations this parameter has no meaning. It shall be set to zero.
Ah:
Successive approximation bit position high This parameter specifies the point transform used in the
preceding scan (i.e. successive approximation bit position low in the preceding scan) for the band of coefficients
specified by Ss and Se. This parameter shall be set to zero for the first scan of each band of coefficients. In the
lossless mode of operations this parameter has no meaning. It shall be set to zero.
Al:
Successive approximation bit position low or point transform In the DCT modes of operation this
parameter specifies the point transform, i.e. bit position low, used before coding the band of coefficients
specified by Ss and Se. This parameter shall be set to zero for the sequential DCT processes. In the lossless
mode of operations, this parameter specifies the point transform, Pt.
The entropy coding table destination selectors, Td
j
and Ta
j
, specify either Huffman tables (in frames using Huffman
coding) or arithmetic coding tables (in frames using arithmetic coding). In the latter case the entropy coding table
destination selector specifies both an arithmetic coding conditioning table destination and an associated statistics area.
Table B.3 Scan header parameter size and values
Values
Parameter
Size (bits)
Sequential DCT
Progressive DCT
Lossless
Baseline
Extended
Ls
16
6
+
2
Ns
Ns
1
8
1-4
Csj
1
8
0-255
a)
Tdj
1
4
0-1
0-3
0-3
0-3
Taj
1
4
0-1
0-3
0-3
0
Ss
1
8
0
-1
0
-1
0-63
1-7
b)
Se
1
8
63
-
63
-
Ss-63
c)
0
Ah
1
4
0
-1
0
-1
0-13
0
Al
1
4
0
-1
0
-1
0-13
0-15
a)
Cs
j
shall be a member of the set of C
i
specified in the frame header.
b)
0
for lossless differential frames in the hierarchical mode (see B.3).
c)
0
if Ss equals zero.
38
CCITT Rec. T.81 (1992 E)
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