ISO/IEC 10918-1 : 1993(E)
Table F.4 ­ Statistical model for DC coefficient coding
Context-index
Value
Coding decision
S0
DC_Context(Da)
V
=
0
SS
S0
+
1
Sign of V
SP
S0
+
2
Sz < 1 if V > 0
SN
S0
+
3
Sz < 1 if V < 0
X1
20
Sz < 2
X2
X1
+
1
Sz < 4
X3
X1
+
2
Sz < 8
.
.
.
.
.
.
X15
X1
+
14
Sz < 2
15
M2
X2
+
14
Magnitude bits if Sz < 4
M3
X3
+
14
Magnitude bits if Sz < 8
.
.
.
.
.
.
M15
X15
+
14
Magnitude bits if Sz < 2
15
F.1.4.4.1.4
Default conditioning for DC statistical model
The bounds, L and U, for determining the conditioning category have the default values L
=
0 and U
=
1. Other bounds
may be set using the DAC (Define Arithmetic coding Conditioning) marker segment, as described in Annex B.
F.1.4.4.1.5
Initial conditions for DC statistical model
At the start of a scan and at the beginning of each restart interval, the difference for the previous DC value is defined to be
zero in determining the conditioning state.
F.1.4.4.2
Statistical model for coding the AC coefficients
As shown in Table F.5, each statistics area for AC coding consists of a contiguous set of 245 statistics bins. Three bins are
used for each value of the zig-zag index K, and two sets of 28 additional bins X2,...,X15,M2,...,M15 are used for coding
the magnitude category and magnitude bits.
The value of SE (and also S0, SP and SN) is determined by the zig-zag index K. Since K is in the range 1 to 63, the
lowest value for SE is 0 and the largest value for SP is 188. SS is not assigned a value in AC coefficient coding, as the
signs of the coefficients are coded with a fixed probability value of approximately 0.5 (Qe
=
X'5A1D', MPS
=
0).
The value of X2 is given by AC_Context(K). This gives X2
=
189 when K
Kx and X2
=
217 when K > Kx, where Kx is
defined using the DAC marker segment (see B.2.4.3).
Note that a X1 statistics bin is not used in this sequence. Instead, the 63
×
1 array of statistics bins for the magnitude
category is used for two decisions. Once the magnitude bound has been determined ­ at statistics bin Xn, for example ­ a
single statistics bin, Mn, is used to code the magnitude bit sequence for that bound.
F.1.4.4.2.1
Default conditioning for AC coefficient coding
The default value of Kx is 5. This may be modified using the DAC marker segment, as described in Annex B.
F.1.4.4.2.2
Initial conditions for AC statistical model
At the start of a scan and at each restart, all statistics bins are re-initialized to the standard default value described in
Annex D.
CCITT Rec. T.81 (1992 E)
101
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