Ñæàòèå âèäåî - BitRateControl
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| Seungyup Paek and Shih-Fu Chang | Video-Server Retrieval Scheduling and Resource Reservation for Variable Bit Rate Scalable Video |
Abstract—State-of-the-art digital video compression produces bursty, variable bit rate video. The bursty nature of compressed video raises challenges in the design of video servers. In this paper, we first present a method for the efficient retrieval of bursty video data from the disk system to the memory of a digital video server. For a single video stream, the proposed retrieval schedule minimizes the buffer requirement for continuous retrieval, given that a fixed disk bandwidth is reserved for the entire duration of retrieval. Secondly, we present an optimal resource-reservation algorithm for multiple video streams based on the proposed retrieval schedule. The resource-reservation algorithm maximizes the number of bursty video streams that can be supported by a video server, given any disk bandwidth and memory resource. Thirdly, we present a progressive display scheme for scalable video that is based on the retrieval schedule and resource-reservation algorithm. Performance evaluations based on simulations using MPEG-2 trace data are presented. For a personal computer with four disks and a memory resource of 120 MB, our approach can support 50%–275% more video streams than previously proposed approaches, depending on the pre-fetch delay that users are willing to tolerate in interactive viewing of videos. RAR 228 êáàéò |
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| Hung-Ju Lee, Tihao Chiang, and Ya-Qin Zhang, Fellow | Scalable Rate Control for MPEG-4 Video |
Abstract—This paper presents a scalable rate control (SRC) scheme based on a more accurate second-order rate-distortion model. A sliding-window method for data selection is used to mitigate the impact of a scene change. The data points for updating a model are adaptively selected such that the statistical behavior is improved. For video object (VO) shape coding, we use an adaptive threshold method to remove shape-coding artifacts for MPEG-4 applications. A dynamic bit allocation among VOs is implemented according to the coding complexities for each VO. SRC achieves more accurate bit allocation with low latency and limited buffer size. In a single framework, SRC offers multiple layers of controls for objects, frames, and macroblocks (MBs). At MB level, SRC provides finer bit rate and buffer control. At multipleVOlevel, SRC offers superiorVOpresentation for multimedia applications. The proposed SRC scheme has been adopted as part of the International Standard of the emerging ISO MPEG-4 standard RAR 568 êáàéò |
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| Lilla Böröczky, Agnes Y. Ngai, and Edward F. Westermann | Joint Rate Control with Look-Ahead for Multi-Program Video Coding |
Abstract—In this paper, we present a new joint rate control algorithm for a multi-program video compression system using MPEG-2 compatible video encoders. The proposed joint rate control is based on both the feedback and look-ahead approaches. It dynamically distributes the channel bandwidth among the program encoders according to the relative complexities of the programs using picture and coding statistics. As opposed to previous works in this area, our algorithm does not restrict the encoders to operate with identical group of pictures (GOP) structures, i.e. the GOP boundaries need not be synchronized among the different encoders. The proposed algorithm allows adaptive distribution of the channel bandwidth among the programs, even at the start of encoding. Furthermore, it assures quick reaction to scene changes, where a feedback approach requires an unavoidable delay. Experimental results show that the proposed joint rate control with look-ahead results in improved picture quality in comparison with a pure feedback approach. RAR 120 êáàéò |
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| Jordi Ribas-Corbera, and Shawmin Lei | JRate Control in DCT Video Coding for Low-Delay Communications |
Abstract—An important motivation for the development of the emerging H.263+ and MPEG4 coding standards is to enhance the quality of highly compressed video for two-way, real-time communications. In these applications, the delay produced by bits accumulated in the encoder buffer must be very small, typically below 100 ms, and the rate control strategy is responsible for encoding the video with high quality and maintaining a low buffer delay. In this work, we present a simple rate control technique that achieves these two objectives by smartly selecting the values of the quantization parameters in typical discrete cosine transform video coders. To do this, we derive models for bit rate and distortion in this type of coders, in terms of the quantization parameters. Using Lagrange optimization, we minimize distortion subject to the target bit constraint, and obtain formulas that indicate how to choose the quantization parameters. We implement our technique in H.263 and MPEG4 coders, and compare its performance to TMN7 and VM7 rate control when the encoder buffer is small, for a variety of video sequences and bit rates. This new method has recently been adopted as a rate control tool in the test model TMN8 of H.263+ and (with some modifications) in the verification model VM8 of MPEG4. RAR 492 êáàéò |
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| Jos´e I. Ronda, Martina Eckert, Fernando Jaureguizar, and Narciso Garc´ýa | Rate Control and Bit Allocation for MPEG-4 |
Abstract—In recent years, an interest has developed in the coded representations of video signals allowing independent manipulation of semantically independent elements (objects). Along these lines, the ISO standard MPEG-4 enhances the traditional concept of video sequence to convert it into a synchronized set of visual objects organized in a flexible way. The real-time generation of a bitstream according to this new paradigm, and suitable for its transmission through either fixed- or variablerate channels, results in a challenging new bit-allocation and rate-control problem, which has to satisfy complex application requirements. This paper formalizes this new issue by focusing on the design of rate-control systems for real-time applications. The proposed approach relies on the modelization of the source and the optimization of a cost criterion based on signal quality parameters. Different cost criteria are provided, corresponding to a set of relevant definitions of the object priority concept. Algorithms are introduced to minimize the average distortion of the objects, to guarantee desired qualities to the most relevant ones, and to keep constant ratios among the object qualities. The techniques have been applied to a coder implementing the MPEG-4 video verification model, showing good properties in terms of achievement of the control objectives. RAR 722 êáàéò |
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| Anthony Vetro, Huifang Sun, and Yao Wang | MPEG-4 Rate Control for Multiple Video Objects |
Abstract—This paper describes an algorithm which can achieve a constant bit rate when coding multiple video objects. The implementation is a nontrivial extension of the MPEG-4 rate control algorithm for single video objects which employs a quadratic ratequantizer model. The algorithm is organized into two stages: a pre- and a postencoding stage. In the preencoding stage, an initial target estimate is made for each object. Based on the buffer fullness, the total target is adjusted and then distributed proportional to the relative size, motion, and variance of each object. Based on the new individual targets and rate-quantizer relation for texture, appropriate quantization parameters are calculated. After each object is encoded, the model parameters for each object are updated, and if necessary, frames are skipped to ensure that the buffer does not overflow. A preframeskip control is exercised to avoid buffer overflow when the motion and shape information occupies a significant portion of the bit budget. The rate control algorithm switches between two operation modes so that the coder can reduce the spatial coding accuracy for an improved temporal resolution. A shape-coding control mechanism is also proposed, which provides a tradeoff between texture and shape coding accuracy. Overall, the algorithm is able to successfully achieve the target bit rate, effectively code arbitrarily shaped objects, and maintain a stable buffer level. These techniques have been adopted by the MPEG committee in July 1997 as part of the video Verification Model (VM8). RAR 484 êáàéò |
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| Liang-Jin Lin, and Antonio Ortega | Bit-Rate Control Using Piecewise Approximated Rate–Distortion Characteristics |
Abstract—Digital video’s increased popularity has been driven to a large extent by a flurry of recently proposed international standards (MPEG-1, MPEG-2, H.263, etc.). In most standards, the rate control scheme, which plays an important role in improving and stabilizing the decoding and playback quality, is not defined, and thus different strategies can be implemented in each encoder design. Several rate–distortion (R–D)-based techniques have been proposed to aim at the best possible quality for a given channel rate and buffer size. These approaches are complex because they require the R–D characteristics of the input data to be measured before making quantization assignment decisions. In this paper, we show how the complexity of computing the R–D data can be reduced without significantly reducing the performance of the optimization procedure. We propose two methods which provide successive reductions in complexity by: 1) using models to interpolate the rate and distortion characteristics, and 2) using past frames instead of current ones to determine the models. Our first method is applicable to situations (e.g., broadcast video) where a long encoding delay is possible, while our second approach is more useful for computation-constrained interactive video applications. The first method can also be used to benchmark other approaches. Both methods can achieve over 1 dB peak signal-to-noise rate (PSNR) gain over simple methods like the MPEG Test Model 5 (TM5) rate control, with even greater gains during scene change transitions. In addition, both methods make few a priori assumptions and provide robustness in their performance over a range of video sources and encoding rates. In terms of complexity, our first algorithm roughly doubles the encoding time as compared to simpler techniques (such as TM5). However, complexity is greatly reduced as compared to methods which exactly measure the R–D data. Our second algorithm has a complexity marginally higher than TM5 and a PSNR performance slightly lower than that of the first approach. RAR 429 êáàéò |
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| Huifang Sun, Wilson Kwok, Max Chien, and C. H. John Ju | MPEG Coding Performance Improvement by Jointly Optimizing Coding Mode Decisions and Rate Control |
Abstract—This paper presents a new algorithm for determining the optimal MPEG [1] coding strategy in terms of the selection of macroblock coding modes and quantizer scales. In the algorithm proposed in the Test Model [2] the rate control operates independently from the coding mode selection for each macroblock. The coding mode is decided based only upon the energy of predictive residues. Actually, the two processes of coding mode decision and rate control are intimately related to each other and should be determined jointly in order to achieve optimal coding performance. We formulate the constrained optimization problem and present solutions based upon rate-distortion characteristics, or R(D) curves, for all the macroblocks that compose the picture being coded. Distortion for the entire picture is assumed to be decomposable and expressible as a function of individual macroblock distortions, with this being the objective function to minimize. The determination of the optimal solution is complicated by the MPEG differential encoding of motion vectors and dc coefficients, which introduce dependencies that carry over from macroblock to macroblock for a duration equal to the slice length. As an approximation, a near-optimum greedy algorithm is proposed. Once the upper bound in performance is calculated, it can be used to assess how well practical suboptimum methods perform. Finally, such a practical suboptimum algorithm is proposed and evaluated. RAR 291 êáàéò |
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| Tihao Chiang and Ya-Qin Zhang | A New Rate Control Scheme Using Quadratic Rate Distortion Model |
Abstract—A new rate control scheme is used to calculate the target bit rate for each frame based on a quadratic formulation of the rate distortion function. The distortion measure is assumed to be the average quantization scale of a frame. The rate distortion function is modeled as a second-order function of the inverse of the distortion measure. We presented a closed form solution for the target bit allocation which includes the MPEG-2 TM5 rate control scheme as a special case. Model parameters are estimated using statistical linear regression analysis. Since the estimation uses the past encoded frames of the same picture prediction type (I; P; B pictures), the proposed approach is a single pass rate control technique. Because of the improved accuracy of the rate distortion function, the fluctuations of the bit counts are significantly reduced by 20–65% in standard deviation of the bit count while the picture quality remains the same. Thus, the buffer requirement is reduced at a small increase in complexity. This technique has bee adopted by the MPEG committee as part of VM5.0 in November 1996. RAR 157 êáàéò |
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