limin liu, member, ieee zhen li, member, ieee edward j. delp, fellow, ieee csvt 2009
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EFFICIENT AND LOW-COMPLEXITY SURVEILLANCE VIDEO COMPRESSION USING
BACKWARD-CHANNEL AWARE WYNER-ZIV VIDEO CODING
Limin Liu, Member, IEEE
Zhen Li, Member, IEEE
Edward J. Delp, Fellow, IEEE
CSVT 2009
Outline Introduction Low-complexity Video Encoding Approaches
for SVC SVC Using Wyner-Ziv Video Coding SVC Using Backward-Channel Aware Wyner-
Ziv Video Coding Experimental Results Conclusion
Basic Diagram of Video Surveillance Systems
Introduction Video surveillance system has been widely used to
enhance public safety and privacy protection. Needs efficient transmission and storage. H.264/AVC is high computational complexity at the
encoder. This can increase the cost of a surveillance system.
Video Compression Design Considerations for Surveillance Video
General case Movie studio only needs
to encode once to put on DVD but decoded by the consumers many times.
Heavy encoder, light decoder.
Surveillance systems Encoder is implemented in a
simple and low-cost video surveillance camera.
The video is decoded and analyzed at central server.
Light encoder, heavy decoder.
Other requirements High coding efficiency Error resilience requirements
Surveillance Video Compression(SVC) Low-complexity encoder based on Wyner-Ziv
coding principles to address the tradeoff between computational complexity and coding efficiency.
Backward-Channel Aware Wyner-Ziv (BCAWZ) video coding to improve coding efficiency while maintaining low-complexity at the encoder.
Error resilience scheme for BCAWZ to keep reliable transmission in the backward-channel.
Low-complexity Video Encoding Approaches for SVC
Slepian-Wolf Theorem Construct correlation
between X and Y RX ≥ H(X|Y )
RY ≥ H(Y |X)
RX + RY ≥ H(X, Y ) Regardless of its access
to side information Y , encoder A can encode X with high fidelity as long as the decoder A has access to Y.
Lower bound
Low-complexity Video Encoding Approaches for SVC Wyner-Ziv Video Coding (WZVC)
Temporal correlation among frames is exploited at the decoder instead of encoder.
Each frame is independently encoded at the encoder.Computational intensive job of motion estimation is shifted
to decoder. WZVC is an ideal choice for SVC due to the light
encoder characteristics.
SVC Using Wyner-Ziv Video Coding
videoInitial estimateTurbo code ->parity bits
LDPC code ->syndrome bits
SVC Using Wyner-Ziv Video Coding
Video sequence is divided into two groups.Key frames
○ Encoded by H.264 INTRA encoder.
○ Serve to side information at decoder
Wyner-Ziv frames ○ Encoded by channel coding
method.○ Turbo code parity bits○ Low-density-parity-check(LDPC)
code syndrome bits
SVC Using Wyner-Ziv Video Coding
Key frames Decoded by the H.264 INTRA decoder. Supply side information to other frames
by find initial estimate. Initial estimate is derived from previously
reconstructed key frames.○ Use co-located pixel value in (n-1)th
frame.○ Take average of the co-located pixel
value at (n-1)th and (n+1)th frames.
Wyner-Ziv frames Quality is low, temporal correlation is not
fully exploited. To obtain higher quality, motion
estimation can be done at decoder.
SVC Using Wyner-Ziv Video Coding Higher quality initial estimate
method1○ Search for the motion vector MVn-1
of the co-located block in (n-1)th frame as the predictor of current block.
○ The reference block in (n-1)th frame is the initial estimate.
method2○ Obtain MVF and MVB
○ Use to find PF1
Use to find PF2
Use to find PB1
Use to find PB2
○ Initial estimate
SVC Using Wyner-Ziv Video Coding
The channel decoder uses initial estimate and the incoming parity or syndrome bits to decode the frame.
Assume decoder can communicate with the encoder to request more bits until correctly decoded. Wyner-Ziv video codec formulates the
video decoding problem as an error correction problem.
If the parity or syndrome bits are lost or corrupted during transmission, it will add complexity.
Standard video codec may not be correctly decoded when bits are lost and error can propagate to the following frame.
SVC Using Backward-Channel Aware Wyner-Ziv Video Coding WZVC with INTRA key frames coding efficiency is higher
than INTRA but much lower than INTER. The distance between two neighboring key frames too far
or too close will result in lower coding efficiency and degrading the quality of side information.
Use Backward-Channel Aware Motion estimation
(BCAME). The basic idea BCAME is to perform motion estimation at
decoder and send the motion information back to encoder through backward-channel.
May increase the latency and cause problem for some SVC.
SVC Using Backward-Channel Aware Wyner-Ziv Video Coding
video
Even frames
Odd frames(excluding 1, 3)
1, 3 frames H.264 INTRA encoder H.264 INTRA decoder
Mode selection
SVC Using Backward-Channel Aware Wyner-Ziv Video Coding
Encode first and third frame as INTRA frames.
All the other odd frames are encoded with BCAME called BP frames.
All the even frames are encoded as a Wyner-Ziv frame.
SVC Using Backward-Channel Aware Wyner-Ziv Video Coding BP frames
Mode1 Mode2
Motion vectors are sent back to encoder. Encoder do mode selection by MSE or
SAD. Encoder use received motion vector with
previous reconstructed BP frames to generate MC reference for current BP frame.
Residual between current BP and reference is than transformed and entropy-coded.
Error Resilience In Backward-Channel Aware Wyner-Ziv Video Coding Problem
If there is error or delay at backward-channel. Motion vector is not updated and encoder continues to use it.
Encoder and decoder use different motion vectors in motion compensation for the same frame.
Error Resilience In Backward-Channel Aware Wyner-Ziv Video Coding
Two-stage error-resilient procedure
Insert the index of the frame as header info before sending
Report error when receive index not match the frame number at the encoder
Encodes the key frames as INTRA and send frame type to decoder
Coding key frames as BP
frames
decoder encoder
De-synchronization
OK
Synchronization
channel
Experimental Results H.264/AVC JM8.0 WZVC with INTRA, INTER
or BP key frames Odd-frames -> key frame Even-frame ->Wyner-Ziv frame
Block-size -> 8*8 WZVC with INTER key
frames First-frame -> INTRA Other key frames -> P frame
H.264/AVC INTER coding First-frame -> INTRA Other frames -> IBPBP GOP
structure Quarter-pel motion search Search range -> 32 Number of reference frames -> 3
Experimental Results Comparison
WZVC with INTRA key frames○ 3-4.5 dB gain for low motion
sequences○ 1.5-2 dB gain for high motion
sequences H.264 INTRA coding
Discussions Low motion sequence is more
continuous. Hence motion vectors derived from previous reconstructed frame is good.
Many surveillance video are low motion.
WZVC encoder complexity is comparable to H.264 INTRA coding.
WZVC can provide significant rate reduction compared to H.264 INTRA coding.
Experimental Results Comparison
WZVC with INTRA key frames○ 7-8 dB loss○ 10-13 dB loss
H.264 INTER coding
Discussions INTER coding is much more
efficient than INTRA coding especially in low motion sequences.
Experimental Results Comparison
BCAWZ○ 5-7 dB gain
WZVC with INTRA key frames
Comparison BCAWZ
○ 1.5-2 dB loss H.264 INTER coding
Discussions This gain is achieved with marginal
complexity increase at the encoder. BCAWZ
○ Two motion vector send from decoder H.264 INTER coding
○ 4*2M*2M candidate motion vector for a search range of M. (in this paper is 32)
Experimental Results Comparison
BCAWZ○ 2-3 dB loss
WZVC with INTER key frames
Discussions Use INTER key frames
significantly increase the encoder complexity.
Experimental Results Comparison
WZVC with INTRA key frames BCAWZ (BP key frame)
Discussions The visual quality of BP frame is
significantly higher than WZVC with INTRA key frames.
Experimental Results Backward-channel /forward-
channel bandwidth Lower rate : 10-15% Higher rate : 5% This usage can be satisfied in
SVC.
Error resilience performance for BCAWZ Without error resilience
○ Sharply drops With error resilience
○ Effectively recover from backward-channel delay or erasure.
Conclusion Present a low-complexity video encoding framework for surveillance video
compression and transmission to address the tradeoff between computational complexity and coding efficiency.
BCAWZ can achieve significantly higher coding efficiency than H.264/AVC INTRA coding as well as existing Wyner–Ziv video coding methods and is close to H.264/AVC INTER coding, while maintaining similar coding complexity with INTRA coding.
Error resilience scheme for BCAWZ to address the concern of reliable transmission in the backward-channel, which is essential to the quality of video data for real-time and reliable activity and event analysis.