Improved Side Information Generation for Distributed Video Coding by Exploiting Spatial and Temporal Correlations
© Shuiming Ye et al. 2009
Received: 22 May 2008
Accepted: 14 December 2008
Published: 1 March 2009
Distributed video coding (DVC) is a video coding paradigm allowing low complexity encoding for emerging applications such as wireless video surveillance. Side information (SI) generation is a key function in the DVC decoder, and plays a key-role in determining the performance of the codec. This paper proposes an improved SI generation for DVC, which exploits both spatial and temporal correlations in the sequences. Partially decoded Wyner-Ziv (WZ) frames, based on initial SI by motion compensated temporal interpolation, are exploited to improve the performance of the whole SI generation. More specifically, an enhanced temporal frame interpolation is proposed, including motion vector refinement and smoothing, optimal compensation mode selection, and a new matching criterion for motion estimation. The improved SI technique is also applied to a new hybrid spatial and temporal error concealment scheme to conceal errors in WZ frames. Simulation results show that the proposed scheme can achieve up to 1.0 dB improvement in rate distortion performance in WZ frames for video with high motion, when compared to state-of-the-art DVC. In addition, both the objective and perceptual qualities of the corrupted sequences are significantly improved by the proposed hybrid error concealment scheme, outperforming both spatial and temporal concealments alone.
Nowadays, the most popular digital video coding solutions are represented by the ISO/IEC MPEG and ITU-T H.26x standards , which rely on a highly complex encoder. However, in some emerging applications, such as wireless low-power video surveillance, multimedia sensor networks, wireless PC cameras, and mobile camera phone, low complexity encoding is required. Distributed video coding (DVC) , a new paradigm in coding which allows for very low complexity encoding, is well suited for these applications.
In DVC, the complex task of exploiting the source statistics, that is, the motion estimation can be moved from the encoder to the decoder. The Slepian-Wolf theorem on lossless distributed source coding states that the optimal rate of joint encoding and decoding of two statistically dependent discrete signals can be achieved by using two independent encoders and a joint decoder . Wyner-Ziv coding extends this result to lossy coding with side information (SI) in the case of Gaussian memoryless sources and mean-squared error distortion . DVC generally divides a video sequence into key frames and WZ frames. The key task to exploit source statistics is carried out in SI generation process to produce an estimation of the WZ frame being decoded. SI has a significant influence on the rate distortion (RD) performance of DVC. Indeed, more accurate SI at the decoder implies that fewer bits are requested from the encoder through a feedback channel, so that the bitrate is reduced for the same quality. In common DVC codecs, the SI is obtained by motion compensated temporal interpolation (MCTI) from the previous and next key frames and utilizes the block matching algorithm (BMA) for motion estimation. However, motion vectors from BMA are often not faithful to true object motions. Unlike classical video compression, it is more important to find true motion vectors for SI generation in DVC. Therefore, it is important to improve the SI generation in DVC in order to achieve better RD performance.
Another appealing property of DVC is its good resilience to transmission errors due to its intrinsic joint source-channel coding framework. A thorough analysis of its performance in the presence of transmission errors has been presented in , showing its good error resilience properties. This results from the fact that DVC is based on a statistical framework rather than the closed-loop prediction used in conventional video coding. Recently, the rapid growth of Internet and wireless communications has led to increased interest for robust transmission of compressed video. However, transmission errors may severely impact video quality as compressed data is very sensitive to these errors . Thus, error control techniques are necessary for efficient video transmission over error prone channels.
This paper proposes a new SI generation scheme by exploiting spatio-temporal correlations at the decoder. It uses partially decoded WZ frames generated by the WZ decoder to improve SI generation. In other words, the proposed scheme is not only based on the key frames, but also on the WZ bits already decoded. Furthermore, enhanced temporal frame interpolation is applied, including motion vector refinement and smoothing to re-estimate and filter the motion vectors, and optimal compensation mode selection to select the mode with minimum matching distortion. Based on these techniques, we also propose a new hybrid spatial and temporal error concealment (EC) scheme for WZ frames in DVC. It uses the error-concealed results from spatial EC to improve the performance of the temporal EC, instead of simply switching between spatial and temporal EC. Spatial EC based on the edge-directed filter  is firstly applied to the corrupted blocks, and the results are used as partially decoded WZ frames to improve the performance of temporal EC. In other words, the temporal EC is not only based on the key frames, but also on the WZ bits already decoded. Experimental results show that the proposed scheme significantly improves the quality of the SI and RD performance of DVC, and the performance of the proposed hybrid scheme is superior to spatial EC and temporal EC alone.
This paper is organized as follows. First, the DVC architecture and other related work are introduced in Section 2. Then, the proposed SI generation scheme is presented in Section 3. Section 4 introduces a new hybrid spatio-temporal EC based on the improved SI generation technique. Simulation results are presented in Section 5. Finally, Section 6 concludes the paper.
2. Related Work
2.1. DVC Architecture
2.2. Motion-Compensated Temporal Interpolation
Motion-compensated temporal interpolation (MCTI) has been used in almost the all DVC codecs to generate SI by interpolating the current frame from key frames. The purpose of MCTI is to create an interpolation of a particular frame by using blocks from previous and next reference frames. This problem is similar to video frame rate upconversion interpolation to improve temporal resolution at the decoder [9, 10]. In contrast to the motion compensation (MC) technique used in conventional codecs, MCTI has no knowledge about the frame being decoded. To estimate frame k, bidirectional motion estimation is generally used in MCTI using a bidirectional motion estimation scheme similar to the B-frames coding mode used in current video standards. For every block in frame the most similar block in frame is found, and its motion vector is calculated. Once the motion vector is obtained, the interpolated frame can be filled by simply using bidirectional motion compensation. Due to block-matching techniques not being ideal, forward and backward searches usually do not produce the same results, and they need to be averaged. This scheme holds as long as the block has constant velocity. However, when there is large or asymmetric motion, MCTI fails to generate a good SI estimate.
Spatial motion vector smoothing was proposed to improve the performance of bidirectional MCTI in [8, 11]. It is observed that the motion vectors have sometimes low spatial coherence . Therefore, spatial smoothing filter was proposed to improve motion estimation by reducing the number of false motion vectors, that is, incorrect motion vectors when compared to the true motion field. This scheme uses weighted vector median filters, which maintains the motion field spatial coherence by looking, at each block, for candidate motion vectors at neighboring blocks. This filter is also adjusted by a set of weights controlling the filter smoothing strength depending on the prediction mean square error of the block for each candidate motion vector. However, spatial motion smoothing is only effective at removing false vectors that occur as isolated pulse spikes.
Subpixel interpolation is also proposed to improve motion estimation for SI generation . The subpixel interpolation method of H.264 is used to generate the pixel value at subpixel position. At the decoder, the side information is motion-compensated refined according to the chosen multiestimation mode from backward, forward, and bidirectional modes. This motion refinement procedure uses subpixel interpolation to improve the precision of search. The subpixel interpolation is effective at improving the generated SI, but it also fails in the presence of large or asymmetric motion. Moreover, it increases the decoder complexity.
2.3. Encoder Aided Motion Estimation for SI Generation
Encoder aided motion estimation to improve SI generation was proposed to conduct more accurate motion estimation at the decoder with the help of auxiliary information sent by the encoder, such as the cyclic redundancy check (CRC)  and hash bits . In , CRC bits for every block are calculated at the encoder and transmitted for the decoder to perform motion search and choose the candidate block that produces the same CRC. The encoder transmits a CRC check of the quantized sequence. Motion estimation is carried out at the decoder by searching over the space of candidate predictors one-by-one to decode a sequence from the set labeled by the syndrome. When the decoded sequence matches the CRC check, decoding is declared to be successful. However, the way to generate and exploit CRC is complicated, and it increases the complexity not only at the decoder, but also at the encoder. In , it is proposed to send robust hash codewords from the encoder, in addition to the Wyner-Ziv bits, to aid the decoder in estimating the motion and generating the SI. These hash bits carry the motion information to the decoder without actually estimating the motion at the encoder. The robust hash code for a block simply consists of a very coarsely subsampled and quantized version of the block. The decoder performs a motion search based on the hash to generate the best SI block from the previous frame. In this scheme, the encoder is no longer an intraframe coder because of the hash store. The hash bits do help in motion estimation, but they increase the encoder complexity and transmission payload. In addition, in , the SI is generated only based on the previous key frame, which is not as good as bidirectional motion estimation.
It was also proposed to split the Wyner-Ziv frame into two subsets at the encoder based on a checkerboard pattern, in order to exploit spatial correlations between these subsets at the decoder . Each subset is encoded independently. At the decoder, the first subset is decoded using the SI obtained by MCTI, thus exploiting only temporal correlation. Then, the second subset is decoded by MCTI based on key frames, or by interpolating the first decoded subset. When the estimated temporal correlation is high, the temporal SI is used. Otherwise, the spatial SI is used. However, this approach can only achieve a modest improvement, and the encoder must be modified accordingly.
2.4. Iterative Decoding and Motion Estimation for SI Generation
The idea of iterative decoding and motion estimation has also been proposed to improve SI, such as motion vector refinement via bitplane refinement  and iterative MCTI techniques [17, 18], but with a high cost of several iterations of motion estimation and decoding. In , the reconstructed image and the adjacent key frames are used in order to refine the motion vectors and, thus, obtain a new and improved version of the decoded and SI frames, including matching criteria function to perform motion estimation and three decoding interpolation modes to select the best reference frame. This scheme is based on bitplane refinement for pixel-domain DVC, and only minor improvements have been achieved. In , the first outcome of the distributed decoder is called partially decoded picture. A second motion-compensated interpolation is applied, which uses the partially decoded picture as well as the previous and next key frames. For each aligned block in the partially decoded picture, the most similar block is searched in the previous frame, the next key frames, the motion-compensated average of the previous and the next frames, and the result from the MCTI previously performed. However, only minor improvement has been achieved. In this paper, we use the same idea of using partially decoded picture, but it is further augmented with suspicious vector detection, new matching criteria for motion estimation, and motion vector filtering, resulting in much better improvements. An iterative approach based on multiple SI with motion refinement has also been proposed in . Multiple SI streams are used at the decoder: the first SI stream is predicted by motion extrapolating the previous two closest key frames and the second SI stream is predicted using the immediate key frame and the closest Wyner-Ziv frame. Based on the error probability, the turbo decoder decides which SI stream is used for decoding a given block.
2.5. Error Concealment
EC consists in estimating or interpolating corrupted data at the decoder from the correctly received information. It can improve the quality of decoded video corrupted by transmission errors, without any additional payload on the encoder or channel. EC can be classified into three categories: spatial concealment [19–21], temporal concealment [22–24], and hybrid spatial and temporal concealments [25–27].
Spatial EC is used to interpolate the lost block from its spatially neighboring available blocks or coefficients in the current frame. It relies on the inherent spatial smoothness of the data. For example, the technique proposed in  exploits the smoothness property of image signals and recovers the damaged blocks by a smoothness measure based on second-order derivatives. However, this smoothness measure would lead to blurred edges in the recovered frame due to the simple second-order derivative-based measure to represent the edges. In , through benchmarking results on the existing error concealment approaches, it was observed that none of the existing approaches is an all time champion. A classification-based concealment approach was proposed which can combine the better performance of different spatial approaches .
Temporal EC techniques use the temporally neighboring frames to estimate the lost blocks in the current frame, based on the assumption that the video content is smooth and continuous in the temporal domain. A very simple scheme for temporal error concealment is used to just copy the block at the same spatial location in the previous frame to conceal the lost block. A bidirectional temporal error concealment algorithm that can recover loss of a whole frame was proposed in . However, the accuracy of the motion estimation may affect the results significantly.
Temporal EC usually leads to better results when compared to spatial concealment, given the typically high temporal correlation in video. However, for video with scene changes or with very large or irregular motion, spatial EC is preferred. Some attempts have been made to combine both spatial and temporal ECs to improve performance [26, 27]. These schemes use some mode selection methods to decide whether to use spatial or temporal EC. For example, temporal activity (measured as the prediction error in the surrounding blocks) and spatial activity (measured as the variance of the same surrounding blocks) are used to decide which concealment mode to use . In general, however, these methods have achieved very limited success, mostly due to the simple mode selection mechanisms at the decoder to merge the results from both spatial and temporal ECs.
In , a forward error correcting coding scheme is proposed for traditional video coding, where an auxiliary redundant bitstream generated at the encoder using Wyner-Ziv coding is sent to the decoder for error concealment. However, in the literature, few error concealment schemes for DVC can be found.
3. Proposed SI Generation Scheme
Common MCTI techniques use only the previous and next key frames to generate the SI. In comparison, the proposed SI generation scheme appears to perform much better than common MCTI, since it has additional information (from WZ bits) about the frame it is trying to estimate. Moreover, the spatio-temporal correlations are exploited based on the PDWZ frame using the SI refinement procedure. The decoded frame obtained here could then be used again as PDWZ frame for a subsequent iteration. However, our experiments show that extra iterations do not provide a significant performance improvement. In other words, the additional information carried by parity bits is fully exploited in a single run of our SI generation scheme. Therefore, only one iteration is used in the proposed scheme, avoiding additional complexity at the decoder.
3.1. Matching Criterion
where is the coordinate of the top left point of the original block in the current frame is the reference frame, is the candidate motion vector MV, and are the dimensions of the block. However, when there are changes in pixel intensity and noises, minimizing MAD often leads to false motion vectors.
Unfortunately, BAD is not efficient at picking out bad motion vectors when local variation is large .
where α is a weighting factor, and MV is the candidate motion vector. MAD is utilized to measure how well the candidate MV can keep temporal continuity. The smaller MAD is, the better the candidate MV keeps temporal continuity. On the other hand, BAD is used to measure how well the candidate MV can keep spatial continuity. The smaller BAD is, the better the candidate MV keeps spatial continuity.
This matching criterion is exploited in suspicious vector detection, motion vector refinement and smoothing, and optimal motion compensation mode selection in the proposed SI generation pipeline.
3.2. Suspicious Vector Detection
3.3. Motion Vector Refinement and Smoothing
The spatio-temporal correlations between the PDWZ frame and the reference key frames are exploited to refine and smooth the estimated motion vectors. More specifically, the motion vectors are re-estimated by bidirectional motion estimation using the matching criterion defined in (3) and the PDWZ frame. They are then filtered using a spatial smoothing filter. This process generates a new estimation of the motion vector for the block to be interpolated.
3.4. Optimal Motion Compensation Mode Selection
The objective of this step is to generate an optimal motion-compensated estimate. In most DVC schemes, while bidirectional prediction is shown to be effective, it is limited to motion-compensated average of the previous and the next key frames.
Backward mode: the block in the SI is interpolated using only one block from the previous key frame.
Forward mode: the block in the SI is interpolated using only one block from the next key frame.
Bidirectional mode: the block in the SI is interpolated using the average of one block in the next key frame and another block in the previous key frame, at arbitrary positions.
Among these modes, the decision is performed according to the matching criterion defined in (3), and the one with the minimum matching error is retained.
Based on the refined motion vectors and the selected interpolation mode, motion compensation is applied to generate the final SI. Based on this SI, the final decoded frame is got after running the WZ decoder again.
4. Application of Improved SI Generation Technique to EC
4.1. Spatial Concealment Based on Edge Directed Filter
4.2. Enhanced Temporal Error Concealment
The techniques used in the improved SI generation scheme, as described in Section 3, are exploited to improve the performance of the temporal EC. The approach is based on MCTI and motion vector filtering as proposed in . One of the key novelties is that the partially error-concealed frame is used to improve the temporal EC, unlike , where MCTI is based on the previous and next key frames. Indeed, the reconstructed frame by spatial concealment contains additional information about the current frame carried by the correctly received WZ bits. Therefore, by using the partially error-concealed frame resulting from spatial EC, the spatio-temporal correlations between this frame and the reference key frames can be better exploited. Hence, the performance of the temporal EC is improved.
The matching criterion in (3) is used to evaluate the error in motion estimation based on the partially error-concealed frame and the reference key frames. The spatio-temporal correlations between the partially error-concealed frames and the key frames are then exploited to refine and smooth the estimated motion vectors using the technique presented in Section 3.3. The block most similar to the corrupted block is selected from a number of sources: the previous frame, the next frame, the bidirectional motion-compensated average of the previous and the next frames, as presented in Section 3.4. Based on the estimated motion vectors and the interpolation modes, motion compensation is applied to generate the reconstructed blocks as the result of temporal concealment.
5. Results and Discussions
The TDWZ DVC codec proposed in  is used in our experiments, and only luminance data is coded. The video sequences Foreman, Soccer, Coastguard, and Hallmonitor are used in QCIF format and at 15 fps. The DVC codec is run for the first 149 frames. Eight RD points are computed per sequence. The results are compared to the TDWZ codec . The quantization parameters of the key frames in our experiments were selected in such a way that the qualities of the output key frames are kept similar with those of the WZ frames. The weight α in (3) and the threshold T in (4) are empirically set to 0.3 and 10, respectively.
5.1. Performance Improvement by the Proposed SI Generation Method
α = 0.0
α = 0.3
α = 0.7
α = 1.0
5.2. Error Concealment Simulations
In simulations on error concealment for DVC, a communication channel, characterized by the error pattern files provided in  with different Packet Loss Ratios (PLRs), is used. The test sequences are corrupted with packet loss rates of 3%, 5%, 10%, and 20%. For the various test conditions in terms of PLR and Quantization Parameters (QPs), the average PSNR is measured. Results are obtained by averaging over ten runs using different error patterns. In video streaming, due to the tight delay constraint, a feedback channel is not preferred since it will cause additional delay, and the received retransmitted bits beyond time constraint will be useless. In this case, no feedback channel is used when errors occurred during the transmission. However, the rate control of the DVC architecture (Figure 1) used in this paper uses a feedback channel driven rate control. Therefore, in this section, we suppose that the encoder performs ideal rate control, that is, the number of requested bits for each bitplane for the error-free case is determined a priori and used for decoding the corresponding corrupted bitstream. Furthermore, as used in , if the bit error probability of the decoded bitplane in WZ decoder is higher than the decoder uses the corresponding bitplane of the SI. The header of the WZ bitstream, which contains critical information such as frame size, quantization parameters, and intraperiod, is assumed as correctly received.
A new side information generation scheme is proposed to improve the performance of DVC. The use of the partially decoded WZ frame, which is exploited in motion vector refinement, smoothing, and optimal compensation mode selection improves the performance of SI generation. Simulation results show that a large improvement of performance has been achieved by the proposed approach, compared to the-state-of-the-art DVC. The proposed approach achieves better RD performances than TDWZ for all test sequences. For sequences with large motion, such as Soccer, the proposed method improves the objective quality (up to 1.0 dB), compared with TDWZ. These results were obtained with no additional complexity or modification to the DVC encoder.
The new SI generation technique is also very helpful in the proposed hybrid EC scheme for WZ frames. Simulation results on the proposed error concealment method also show that the proposed method can bring significant improvements in terms of both objective and perceptual qualities for corrupted sequences. The proposed hybrid EC scheme is only applied at the decoder, without any modification to the DVC encoder or transmission channels.
This work was partially supported by the European Network of Excellence VISNET II (http://www.visnet-noe.org) IST Contract 1-038398, funded under the European Commission IST 6th Framework Program.
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