Chang-Ming Lee

Motion Vector Recovery for Video Error Concealment by Using Iterative Dynamic-Programming Optimization

This paper proposes an error concealment technique for video transmission, focusing on motion vector (MV) recovery for both inter- and intra-coded frames, to improve video quality at decoder when video bit stream data incur transmission errors. The proposed algorithm considers slice (i.e., a row of macroblocks (MBs)) errors and uses DP (Dynamic Programming) optimization technique to estimate the lost MVs in a global manner, differing from the traditional Boundary Matching Algorithm (BMA) and others that recover MVs independently for individual MBs in an erroneous slice. We also propose an iterative DP process based on 8 × 8 pixels blocks to resolve finer motions (for 8 × 8, 8 × 16, and 16 × 8 pixels blocks) that will aid in the enhancement of reconstruction quality. Experiment results show that our algorithm outperforms the well-known BMA by up to 7.28 dB and the DMVE and another prior work by Qian by up to 1.0 dB at a packet loss rate of 15%. Subjective evaluation shows that our algorithm is especially promising in preserving line/curve features and motion details.

Soft decoding of VLC encoded data for robust transmission of packetized video

The soft decoding of variable-length encoded texture data generated, e.g., by video coders such as H.263+ and sent over a packetized network (Internet or mixed Internet radio-mobile) is considered here. Existing soft decoding techniques usually make use of either the number of bits in a block or the number of symbols in a block. Thus, this side information has to be transmitted. This paper describes an algorithm making use only of information available in the packetized bitstream, and is thus compatible with existing standards. Simulations illustrating the efficiency of the proposed decoding procedure are provided.

Simplification of VLC tables with application to ML and MAP decoding algorithms

Many source coding standards (JPEG, H263+, H264), rely heavily on entropy coding with variable-length codes (VLC). However, bitstreams made of VLC-encoded data are particularly sensitive to transmission errors. Recent results tend to use knowledge of the VLC structure in order to perform an efficient decoding of the bitstream. These techniques use a trellis describing the structure of the VLC codebook and assume that some a priori information is available at decoder side. Significant improvements, compared with prefix decoding of bitstreams are achieved. However, the complexity of these techniques may become intractable when realistic VLC codebooks are considered. This paper presents an algorithm for compacting VLC tables. The codewords are grouped into a minimum number of classes. Decoding algorithms may then work on a reduced number of classes, instead of working on the whole set of codewords. A proof of optimality is provided for the VLC table-compaction algorithm. The algorithm is applied to the H263+ VLC codebook and merges the 204 codewords into 25 classes. The resulting compact tables are shown to be exactly equivalent to the initial ones when used with hard decoding algorithms. The properties of the associated soft decoding algorithms using these compact tables are also evaluated

Dynamic Key Block Decision with Spatio-Temporal Analysis for Wyner-Ziv Video Coding

Wyner-Ziv coding has been recognized as the most popular method up to now. For traditional WZC, side information is generated from intra-coded frames for use in the decoding of WZ frames. The unit for intra-coding is a frame and the distance between key-frames is kept constant. In this paper, the unit for intra-coding is a block, and the temporal distance between two consecutive key blocks can varying with time. A block is assigned a mode (WZ or intra-coded), depending on the result of spatio-temporal analysis, and encoded in an alternative manner. This strategy improves the overall coding efficiency, while maintaining a low encoder complexity. The performance gain can achieve up to 6 dB with respect to the traditional pixel-domain WZC.

Robust Video Transmission Over Mixed IP - Wireless Channels using Motion-Compensated Oversampled Filterbanks

Robust video coding has attracted increasing attention during the past few years. This paper proposes a joint source channel coding scheme able to resist transmission errors over mixed Internet-wireless channels. It involves motion-compensated oversampled filterbanks (OFBs). The redundancy introduced by the overcomplete representation in signals at the output of OFBs is employed for error correction of the motion compensated frames. The errors may be due to the wireless part of the channel (random noise), but also to the Internet part (packet losses). The performance of the proposed approach is illustrated for compressed streams transmitted through a packet erasure channel with an averaged packet loss of 6.25% followed by a binary symmetric channel with a crossover probability of 10-2

Robust reconstruction of motion vectors using frame expansion

Transmitting video streams on channels impaired with transmission errors is a very demanding task, mainly when images are predicted from previous ones. In this case, errors on motion vectors can be very harmful. In order to overcome this problem, this paper presents a modified H263+ scheme without motion vector transmission. This is obtained by reestimating these motion vectors at the receiver, based on properties of frame expansions. This procedure is obtained at the cost of an increased bit rate, but shows that robust (and efficient) transmission can indeed be obtained in conjunction with image prediction.

Block-based distributed video coding with variable block modes

In this paper, a new block-based pixel domain distributed video coding scheme featured with variable block modes is proposed. In addition to intra mode and Wyner-Ziv mode employed in conventional block-based distributed video coding scheme, two supplementary block modes “SKIP mode” and “zero motion mode” are introduced in the proposed scheme to improve the overall coding efficiency, as well as to reduce the decoding complexity. Moreover, the channel coding is performed on macroblcok level to reduce the coding loss due to inserted information in the parity bits. The simulation results show that the proposed scheme outperforms both the conventional frame-based transform-domain and the block-based pixel-domain distributed coding schemes.

Distributed video coding with block mode decision to reduce temporal flickering

The common distributed video coding (DVC) systems treat input video frames group by group. In each group of pictures (GOP), usually the first frame, called key frame, is intra-coded and the others are Wyner-Ziv (WZ)-coded. This GOP coding structure presents a fixed and inefficient coding mode switch (key or WZ) at group boundaries, thus preventing a DVC system from adapting the rate-distortion (R-D) performance to varying video content. In this work, a structure of temporal group of blocks (TGOB) with dynamic coding mode (key/WZ) decision is presented. This dynamic TGOB coding architecture determines each image block to be a key block or a WZ block based on spatiotemporal image analysis, resulting in a mode switch of fine granularity in both the spatial and temporal domains. As a consequence, not only the overall coding efficiency is improved, but also the temporal flickering artifacts for the reconstructed video are reduced substantially. Experimental results show that our proposed DVC scheme with block mode decision achieves up to 2.85 dB of quality gain and also up to 51% of temporal flickering reduction, compared to the well-known DISCOVER system.

Practical estimation of adaptive correlation noise model for Distributed Video Coding

In contrast with the traditional video compression system, Distributed Video Coding (DVC) architecture dramatically shifts the complexity from the encoder to the decoder. This low-cost encoding concept can be exploited in the emerging applications, e.g. wireless sensor networks. In order to increase the compression efficiency, improvement of side information generation and refinements of Correlation Noise Model (CNM) are main streams to improve DVC. However, most of these schemes are theoretical and expensive for the decoder. In order to retain low-cost and efficient system, a side information refinement with a practical CNM estimation is proposed. While maintaining the video quality, our proposed mechanism totally improves the system compression efficiency about 18% for the bit-rate with a low complexity decoder.

Enhanced Side Information Generator with Accurate Evaluations in Block-Based Wyner-Ziv Video Coding

Wyner-Ziv coding (WZC) has received a lot of attention lately. Based on the block unit for WZ-/intra-coding and the temporal distance between two consecutive key blocks, two techniques are proposed to improve the performance of block-based distributed video coding. Depending on the spatio-temporal analysis, the first method adjusts the mode assignment to a more precise generation of side information. In addition, the second method carries out the correlation calculation in the encoder side to ensure a better statistical estimation between the side information frame and the original frame. Thus, the more accurate side information combined with the more reliable statistical parameters results in an improved coding efficiency. The simulation results show that the compound of these two proposed methods has an improvement up to 2.7 dB with respect to the previous work in block-based WZC.