Anne Aaron

Distributed Video Coding

Distributed coding is a new paradigm for video compression, based on Slepian and Wolfs and Wyner and Zivs information-theoretic results from the 1970s. This paper reviews the recent development of practical distributed video coding schemes. Wyner-Ziv coding, i.e., lossy compression with receiver side information, enables low-complexity video encoding where the bulk of the computation is shifted to the decoder. Since the interframe dependence of the video sequence is exploited only at the decoder, an intraframe encoder can be combined with an interframe decoder. The rate-distortion performance is superior to conventional intraframe coding, but there is still a gap relative to conventional motion-compensated interframe coding. Wyner-Ziv coding is naturally robust against transmission errors and can be used for joint source-channel coding. A Wyner-Ziv MPEG encoder that protects the video waveform rather than the compressed bit stream achieves graceful degradation under deteriorating channel conditions without a layered signal representation.

Wyner-Ziv coding of motion video

In current interframe video compression systems, the encoder performs predictive coding to exploit the similarities of successive frames. The Wyner-Ziv theorem on source coding with side information available only at the decoder suggests that an asymmetric video codec, where individual frames are encoded separately, but decoded conditionally (given temporally adjacent frames) could achieve similar efficiency. We report the first results on a Wyner-Ziv coding scheme for motion video that uses intraframe encoding, but interframe decoding.

Transform-domain Wyner-Ziv Codec for Video

In current interframe video compression systems, the encoder performs predictive coding to exploit the similarities of successive frames. The Wyner-Ziv Theorem on source coding with side information available only at the decoder suggests that an asymmetric video codec, where individual frames are encoded separately, but decoded conditionally (given temporally adjacent frames) could achieve similar efficiency. We propose a transformdomain Wyner-Ziv coding scheme for motion video that uses intraframe encoding, but interframe decoding. In this system, the transform coefficients of a Wyner-Ziv frame are encoded independently using a scalar quantizer and turbo coder. The decoder uses previously reconstructed frames to generate side information to conditionally decode the Wyner-Ziv frames. Simulation results show significant gains above DCT-based intraframe coding and improvements over the pixel-domain Wyner-Ziv video coder.

Compression with side information using turbo codes

We show that turbo codes can come close to the Slepian-Wolf bound in lossless distributed source coding. In the asymmetric scenario considered, X and Y are statistically dependent signals and X is encoded with no knowledge of Y. However, Y is known as side information at the decoder. We use a system based on turbo codes to send X at a rate close to H(X|Y). We apply our system to binary sequences and simulations show performance close to the information-theoretic limit. For distributed source coding of Gaussian sequences, our results show significant improvement over previous work. The scheme also performs well for joint source-channel coding.

Rate-adaptive codes for distributed source coding

Source coding with correlated decoder side information is considered. We impose the practical constraint that the encoder be unaware of even the statistical dependencies between source and side information. Two classes of rate-adaptive distributed source codes, both based on low-density parity-check (LDPC) codes, are developed and their design is studied. Specific realizations are shown to be better than alternatives of linear encoding and decoding complexity. The proposed rate-adaptive LDPC accumulate (LDPCA) codes and sum LDPC accumulate (SLDPCA) codes (of length 6336 bits) perform within 10% and 5% of the Slepian-Wolf bound in the moderate and high rate regimes, respectively.

Wyner-Ziv video coding with hash-based motion compensation at the receiver

In the current interframe video compression systems, the encoder performs predictive coding to exploit the similarities of successive frames. The Wyner-Ziv theorem on source coding with side information available only at the decoder suggests that an asymmetric video codec, where individual frames are encoded separately, but decoded conditionally (given temporally adjacent frames) could achieve similar efficiency. In the previous work we propose a Wyner-Ziv coding scheme for motion video that uses intraframe encoding instead of interframe decoding. In this paper we improve on our Wyner-Ziv video codec by sending hash codewords of the current frame to aid the decoder in accurately estimating the motion. This allows us to implement a low-delay system where only the previous reconstructed frame is used to generate the side information of a current frame. Simulation results show significant gains above conventional DCT-based intraframe coding. The Wyner-Ziv video codec with hash-based motion compensation at the receiver enables low-complexity encoding while achieving high compression efficiency.

Towards practical Wyner-Ziv coding of video

In current interframe video compression systems, the encoder performs predictive coding to exploit the similarities of successive frames. The Wyner-Ziv theorem on source coding with side information available only at the decoder suggests that an asymmetric video codec, where individual frames are encoded separately, but decoded conditionally (given temporally adjacent frames) achieves similar efficiency. We report results on a Wyner-Ziv coding scheme for motion video that uses intraframe encoding, but interframe decoding. In the proposed system, key frames are compressed by a conventional intraframe codec and in-between frames are encoded using a Wyner-Ziv intraframe coder. The decoder uses previously reconstructed frames to generate side information for interframe decoding of the Wyner-Ziv frames.

Rate-Adaptive Distributed Source Coding using Low-Density Parity-Check Codes

Source coding with correlated decoder side infor- mation is considered. We impose the practical constraint that the encoder be unaware of even the statistical dependencies between source and side information. Two classes of rate-adaptive distributed source codes, both based on Low-Density Parity- Check (LDPC) codes, are developed and their design is studied. Specific realizations are shown to be better than alternatives of linear encoding and decoding complexity. The proposed rate- adaptive LDPC Accumulate (LDPCA) codes and Sum LDPC Accumulate (SLDPCA) codes (of length 6336 bits) perform within 10% and 5% of the Slepian-Wolf bound in the moderate and high rate regimes, respectively.

Wyner-Ziv coding for video: applications to compression and error resilience

Two separate applications of Wyner-Ziv coding of motion video coding are considered. The Wyner-Ziv theorem on source coding with side information available only at the decoder suggests that an asymmetric video codec, where individual frames are encoded separately, but decoded conditionally could achieve efficiency comparable to current interframe video compression systems. First results on the Wyner-Ziv coding scheme for motion video are reported that use interframe encoding, but interframe decoding. Secondly, Wyner-Ziv coding for the transmission of compressed video over an error-prone channel is applied. This coding scheme is used to generate a supplementary bit stream, which contains a coarsely quantized representation of the transmitted video signal. Using the conventionally decoded, error-concealed video signal as side information to decode the Wyner-Ziv bits, the transmission errors in the decoded video waveform are corrected up to a certain residual distortion, significantly improving the visual quality of the decoded video.

Distributed compression for large camera arrays

We address the problem of compression for large camera arrays, and propose a distributed solution based on Wyner-Ziv coding. The proposed scheme allows independent encoding of each view with low-complexity cameras, and performs centralized decoding with side information from additional views. Experimental results are given for two light field data sets. The performance of the proposed scheme is compared with independently coding each view using JPEG2000 and a shape-adaptive JPEG-like coder. The Wyner-Ziv coder yields superior compression performance at low bit-rates. In addition, there is a great reduction in encoder complexity when compared to JPEG2000.