Advances of MPEG Scalable Video Coding Standard

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1 Advances of MPEG Scalable Video Coding Standard Wen-Hsiao Peng, Chia-Yang Tsai, Tihao Chiang, and Hsueh-Ming Hang National Chiao-Tung University 1001 Ta-Hsueh Rd., HsinChu 30010, Taiwan Abstract. To support clients with diverse capabilities, the MPEG committee is defining a novel scalable video coding (SVC) framework that can simultaneously support multiple spatial, temporal and SNR resolutions under the constraints of low complexity and low delay. To fulfill the requirements, two major approaches have been considered as the potential technologies. One is the wavelet-based scheme and the other is the scalable extension of MPEG-4 AVC/H.264. This paper aims to give a brief overview for the latest advances of these technologies. 1 Introduction Scalable video coding attracts wide attention with the rapid growth of multimedia applications over Internet and wireless channels. In such applications, the video may be transmitted under a heterogeneous environment. To support clients with diverse capabilities in complexity, bandwidth, power and display resolution, the MPEG committee is defining a scalable video coding (SVC) framework that can simultaneously support multiple spatial, temporal and SNR resolutions under the constraints of low complexity and low delay. To fulfill the requirements of SVC, more than 20+ proposals were submitted during the stage of call-for-proposal [1] in February According to the spatial transform, these proposals can be roughly classified into two categories, which are the wavelet-based scheme and the AVC/H.264-based approach [2]. In addition, depending on the transform order in the spatio-temporal domain, the wavelet-based scheme can further have two variations, which are the 2D+t and t+2d structures [2]. To distinguish the differences, Fig. 1 shows a comparison at architectural level. As shown, in order to achieve the temporal scalability, both the waveletbased scheme and the AVC-based approach adopt the technique of motion compensated temporal filtering (MCTF). In addition, to achieve the SNR scalability with fine granularity, the AVC-based scheme uses a context-adaptive bit-plane coding [3][4]. On the other hand, the wavelet-based scheme employs a zero-tree coding [5][6][7] for the same purpose. As for the spatial scalability, the waveletbased scheme takes the advantages of multi-resolution property of wavelet transform, while the AVC-based scheme exploits the layered coding concept used in MPEG-2, H.263, and MPEG-4. R. Khosla et al. (Eds.): KES 2005, LNAI 3684, pp , c Springer-Verlag Berlin Heidelberg 2005

2 890 Wen-Hsiao Peng et al. (a) An AVC/H.264-based approach (also DCT-based) (b) A wavelet-based approach with t+2d structure (c) A wavelet-based approach with 2D+t structure Fig. 1. An architectural level comparison for various SVC algorithms [2] Subjective test has been conducted for comparing these technologies [8]. Having better subjective quality in different scenarios, the AVC-based scheme is adopted as the working draft of SVC. On the other hand, MPEG committee also establishes an ad-hoc group to further study the wavelet-based technologies

3 Advances of MPEG Scalable Video Coding Standard 891 for the further video coding applications. In this paper, we give a brief overview for the latest advances of these technologies. The rest of this paper is organized as follows: Section 2 elaborates the detail for each dimension of scalability in AVC-based approach and Section 3 describes the corresponding algorithms in the wavelet-based scheme. Lastly, Section 4 summarizes the latest activities in SVC. 2 Scalable Extension of AVC/H.264 To simultaneously support spatial, temporal and SNR scalability, a scalable extension of AVC/H.264 was proposed [3]. Fig. 1 (a) shows the encoder structure of the AVC-based scheme. To facilitate the spatial scalability, the input video is decimated into various spatial resolutions and the sequence in each spatial resolution is coded in a separated layer using AVC/H.264. Within each spatial layer, the motion compensated temporal filtering (MCTF) is employed in every group of pictures (GOPs) to provide the temporal scalability. In addition, to remove the redundancy among different spatial layers, a large degree of inter-layer prediction is incorporated. The residual frames after the inter-layer prediction are then transformed and successively quantized for the SNR scalability. In the following subsections, we elaborate the details for each dimension of scalability. 2.1 Temporal Scalability In each spatial layer, the temporal scalability is achieved by the motion compensated temporal filtering (MCTF) technique, which performs the wavelet decomposition/reconstruction along the motion trajectory. Particularly, the MCTF is mostly restricted to the short-length (5, 3) wavelet, which can be implemented by a lifting scheme with only one prediction/update step. In this special case, the prediction and update can be realized using bidirectional prediction as shown in Fig. 2, where{l n } stands for the low-pass frames of level n and {H n } denotes the associated high-pass frames. Inside the MCTF, an odd-indexed frame is predicted from the adjacent and even-indexed frames to produce the high-pass frame. Accordingly, an even-index frame is updated using the combination of adjacent high-pass frames to generate the lowpass frame. To remove temporal redundancy, motion compensation is conducted before the prediction and update. By using n decomposition stages, up to n levels of temporal scalability can be provided. Specifically, the video of lower frame rate can be obtained from the low-pass frames at higher level. 2.2 Spatial Scalability and Inter-layer Prediction For the spatial scalability, sequences of different spatial resolutions are coded in separated layers. To remove the redundancy among different spatial layers, the residues and motion vectors of an enhancement layer are predicted from the ones of the subordinate layer.

4 892 Wen-Hsiao Peng et al. Fig. 2. MCTF structure for the (5, 3) wavelet In the prediction process, the residues and motion vectors of the subordinate layer are firstly interpolated if the subordinate layer has a lower resolution. In addition, the partition of an inter MB can be derived from the relevant sub-blocks at the subordinate layer and the motion vectors can be obtained by refining and scaling the ones from the corresponding sub-blocks. On the other hand, for an intra MB, the inter-layer prediction is allowed only if the corresponding 8x8 block of the subordinate layer is within an intra-coded MB. 2.3 SNR Scalability For the SNR scalability, the residues after the inter-layer prediction are transformed with the 4x4 integer transform. Then the transform coefficients are successively quantized into multiple quality layers. The coefficients in a quality layer is coded by a hybrid approach of bit-plane and (Run, Level) coding. For the scalability with fine granularity, the bit-planes in a quality layer are coded using a cyclical block coding [4]. The coding step is partitioned into the significant and refinement passes. The significant pass first encodes the insignificant coefficients that have zero values in the subordinate layers. Then, the refinement pass refines the remaining significant coefficients ranging from -1 to +1. During the significance pass, the transform blocks are coded in a cyclical and block-interleaved manner. On the other hand, the coding of refinement pass is conducted subband-by-subband. To further reduce bit rate, a context-adaptive binary arithmetic coder is employed.

5 Advances of MPEG Scalable Video Coding Standard Scalable Approach Using Inter-frame Wavelet Different from the aforementioned AVC-based scheme that exploits the hybrid coding structure, a wavelet-based scheme using (t+2d) structure was proposed in [5][9]. Like the AVC-based approach, the wavelet-based scheme can produce a fully embedded bit-stream that simultaneously supports spatial, temporal, and SNR scalability.however, a major difference between the AVC-based approach and the wavelet-based scheme is that all the predictions in the latter case are conducted in an open-loop manner. The open-loop prediction provides more flexibility on bit-stream extraction and is more robust to transmission errors. Fig. 2 (b) shows the framework of wavelet-based scheme using (t+2d) structure. Within each GOP, MCTF is used for temporal decomposition. Particularly, the temporal low-pass frames come from the motion compensation of the original images. The low-pass frames are used for the temporal scalability. For the spatial scalability, 2-D wavelet decomposition is conducted for each frame after the MCTF. The spatial scalability is achieved by the multi-resolution property. Furthermore, after the spatial decomposition, the wavelet coefficients are coded by a zero-tree entropy coder (or other arithmetic coder) to generate an embedded bit-stream for the SNR scalability. By means of proper extraction, a single bit-stream with scalable spatial, temporal and SNR parameters can be produced. Similar wavelet video coding schemes but with specific features have been latter suggested by several other researchers [10][11]. In the following subsections, we will give a brief overview for each dimension of scalability in the wavelet-based scheme. 3.1 Temporal Scalability To provide temporal scalability, the wavelet-based scheme also adopts the MCTF asdescribedinsection2.1. In fact, the MCTF concept was first proposed for wavelet coding [9]. Typically, Haar or (5, 3) wavelet are used. To improve the accuracy of motion field so as to achieve a better coding performance, several techniques for better motion estimation have been proposed [12][13]. Particularly, in [13], a novel structure, known as Barbell-Lifting, is presented. The basic idea is to use a barbell function for generating the prediction/update values for the lifting scheme. Specifically, for each pixel in the high-pass frame, the prediction value is obtained by using a set of pixels as the input to the barbell function. It has been proved that the prediction using the barbell function offers a superior performance as compared to the one using a single-pixel. Moreover, it often reduces the mismatch of motion in the prediction and update steps. However, due to the coupling of different spatial subbands, this scheme may result in an inaccurate prediction/update when extracting the video of lower resolution. To solve this problem, an in-band MCTF can be used instead [10].

6 894 Wen-Hsiao Peng et al. 3.2 Spatial and SNR Scalability To achieve the spatial scalability, a separable 2-D wavelet transform is applied to both low-pass and high-pass frames. Similar to JPEG2000, the lower subbands are used to reconstruct the lower resolution images. To provide the SNR scalability with fine granularity, an embedded zerotree coding as in [6] is used after the spatial decomposition. In addition to [6], other methods like embedded zero-block coding (EZBC) [5] and embedded block coding with optimized truncation (EBCOT) [7] are also commonly used for coding the wavelet transform coefficients. Particularly, to reduce the temporal redundancy among successive frames, these coding techniques can be further extended to have 3-D structure such as 3-D EBCOT in [11]. 3.3 Motion Scalability The compressed bit-stream includes both the texture and the motion information. The bit-stream for the texture part can be arbitrarily truncated. However, the one for the motion information can not be easily partitioned. Since the motion information may be a considerable portion, it can consume a large percentage of the transmitted data in the low bit rate applications. This leads to few available bits for transmitting texture data and thus results in poor subjective quality. To solve this problem, motion information should be represented in a scalable manner. In [14], a scalable representation of motion information is proposed for the MC-EZBC. Further, in [15], the motion information is partitioned into multiple motion layers. Each layer records the motion vectors with a specified accuracy. The lowest layer denotes a rough representation of the motion vectors and the higher layers are used to refine the accuracy. Different layers are coded independently so that the motion information can be truncated at the layer boundary. Due to the mismatch between the truncated motion information and the residual data, the schemes with scalable motion information may have drifting error. As a result, a linear model is proposed in [12] toprovideabettertrade-off between the scalable representation and the rate-distortion performance. 4 Conclusion In this paper, we have reviewed the fundamentals of SVC and its latest development in MPEG standard. Both the AVC-based approach and the wavelet-based scheme are capable of offering a fully scalable bit-stream. Although the AVCbased approach has been selected as the working draft in MPEG, the waveletbased scheme has the potential for future video coding applications/standard. Research activities on the wavelet-based technology have been growing rapidly in the past a few years and the MPEG committee continuously keeps an ad-hoc group working on this subject. At the moment, a number of core experiments are set up for the AVC-based SVC in MPEG. The target date of completing the MPEG SVC standard is 2006.

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