EURASIP Journal on Image and Video Processing 3
Key
WZ
BP 0
Buffer
Reconstruction
Side information
Delay
Quantizer
Extract
bit-planes
Turbo
encoder
Intra
encoding
CA-VLC
encoder
CA-VLC
decoder
Intra
decoding
SA-MCE SI
generation
Turbo
decoder
Auxiliary
information
generation
BP
DCT
DCT
DCT
Request bits
Decoder
Encoder
.
.
.
−
2
M
quantizer
n
Figure 1: Framework of spatial-aided low-delay WZ codec.
information. As a result, the resolution of the spatial auxiliary
information is a quarter of the original frame. To reduce
the temporal redundancy, DPCM is performed between the
adjacent LL subbands to encode the LL subband. For DPCM
coding, the difference between the current LL subband and
its previously reconstructed reference frame is calculated.
Then the residues are DCT transformed and quantized by
a quantizer. Finally, the quantized coefficients are encoded
by a CA-VLC entropy encoder used in H.264/AVC. If the
reference frame is a key frame, the LL subband of full-
resolution reconstructed intra-frame needs to be yielded by
DWT to form the reference frame for DPCM coding.
2.3. Wyner-Ziv Frame Coding. At the encoder, the whole WZ
frame is encoded by DCT transform domain WZ coding [3].
First, a block-wise DCT is applied to the whole WZ frame
and the statistical dependencies within a frame are exploited.
The transform coefficients are grouped together to form
the coefficient bands. Then for each band, different M-level
uniform scalar quantizers are applied. Next, the bit-planes
are extracted and each bit-plane is organized to fixed length
binary codewords. Each codeword is sent to the Slepian-Wolf
(SW) encoder as input and the output is the parity bits. The
SW coder is implemented using a rate-compatible punctured
turbo code (RCPT). Then, these parity bits are punctured
into different blocks and stored in a buffer. The blocks of
parity bits, which are also called as WZ bits, are successively
transmitted to decoder upon request.
At the decoder, the spatial auxiliary information of
current WZ frame is decoded first. Then, the SI of whole
WZ frame is generated with the help of the auxiliary
information by an SA-MCE method which is presented in
Section 2.5. Subsequently, DCT is applied on the generated
full-resolution SI and the coefficients in each DCT block
are extracted into different subbands corresponding to the
DCT bands partition patterns. The DCT coefficient Y
i
of
SI at the ith position in current subband is used for the
bit-plane probabilities evaluation. This means that for every
original coefficient X
i
the value of Y
i
is used to evaluate
the probability of every bit of X
i
being 1 or 0. The detailed
description about the probability evaluation and correlation
model being used is introduced in the next subsection.
2.4. Correlation Model. As the turbo decoder obtains the side
information, a priori probability of current decoding bit-
planes should be calculated first. According to simulation
results, the probability distribution of the difference between
the source and its SI conforms to a Laplacian model and
thus, the Laplacian model is taken as the probability density
function for calculating the a priori probability. To estimate
the values of the jth bit of X
i
being 0 or 1, the probability can
be calculated as
p
b
j
i
| y
i
, s
i
, b
0
i
, ..., b
j−1
i
=
α
2
e
−α|d|
(1)
with
d
= a ·
Z
i
+ offset
− y
i
= a
b
0
i
· 2
m
+ ···+ b
j−1
i
· 2
m− j+1
+ b
j
i
· 2
m− j
+2
m− j−1
− y
i
.
(2)
Let b
j
i
denote the jth bit-plane at the position i in current
subband and its estimation is
b
j
i
.However,{b
0
i
, ..., b
j−1
i
} are
those previously decoded bits and b
0
i
is the most significant
bit. In (1), S
i
is the sign bit. If the coefficient X
i
is positive,
S
i
equals 0; otherwise S
i
equals 1. For each coefficient
band, different standard deviation of Laplacian model 1/α is
adopted. The value of 1/α is determined by offline training.
In (2), Z
i
represents the integer number that has
the jth bit
b
j
i
and those previously more significant bits
{b
0
i
, ..., b
j−1
i
}. Offset is an estimated value used to compen-
sate the lower part of Z
i
.IfX
i
is partitioned into m bins,
offset equals 2
m− j−1
. a is used to adjust the sign of the value
(Z
i
+ offset), which is defined as
a
=
1 s
i
= 0,
−1 s
i
= 1.
(3)