MPEG-4 Rate Control Research Articles

A direct non-buffer rate control algorithm for real time video compression

Rate control (RC) is crucial in controlling compression bitrates and encoding qualities for networked video applications. In this paper, we propose a direct non-buffer real-time rate control algorithm for video encoding, which has two unique features. First, unlike traditional algorithms which adopt buffers in rate control, the proposed algorithm does not use a buffer in rate regulation which can reduce the delay and improve real-time response. Second, we propose a new Proportional-Integral-Derivative (PID) bit controller to directly control encoding bitrates. In addition, we also develop a simple but effective method for real-time target bit allocation. To the best of our knowledge, this is the first work that conducts video rate control without using a buffer. Our extensive experimental results have demonstrated that the proposed algorithm outperforms the MPEG-4 rate control algorithm by achieving more accurate rate regulation and improving overall coding quality.

Region-based rate control and bit allocation for wireless video transmission

In this paper, we propose a joint source-channel region-based rate control algorithm for real-time video transmissions over wireless systems. During the video transmission, the channel throughput available to the video encoder in the wireless systems is inherently variable, due to the retransmission of the error packets using the automatic repeat request (ARQ) error control. The variable data rate of the wireless system is characterized by the packet-level Gilbert two-state Markov Model, the parameters of which are extracted from the statistical properties of the channel information obtained from the wireless channel simulator. The proposed algorithm adopts a fast but effective block-based segmentation method to extract the regions of interest. Unlike traditional bit allocation methods used in the region/content-based rate control, the algorithm exploits the most effective criteria "coding qualities" as quantitative factors to directly control bit allocation among different regions so as to achieve better visual quality in the regions of interest. The computational complexity of the algorithm is low making it suitable for real-time applications. Compared with the MPEG-4 rate control algorithm, our algorithm can effectively enhance the perceptual quality for the regions of interest and significantly reduce the number of frame skipping; thereby, improve the smoothness of the video.

On-line improvements of the rate-distortion performance in MPEG-2 rate control

Existing rate-control research can be summarized into two approaches. One attempts to optimize the rate-distortion (R-D) characteristics of the input source before any decisions about the quantizer step-size assignment are made. As such, rate-control schemes out of this approach incur iterative procedures, high computational cost and encoding delays. The other approach is highlighted by direct buffer-state feedback techniques in predicting the R-D characteristics based on either probabilistic models or on historic information. Both approaches trade suboptimality in the prediction of R-D characteristics for reduced computational cost and delay. We combine the simplicity of the predictive models with the high-complexity R-D approaches to design effective rate-control algorithms to exploit the advantages of both approaches: low encoding delay and low complexity, yet optimized estimation of the R-D statistics. Extensive experiments show that improvements over the MPEG-2 scheme with gains of 0.5-1 dB per frame are achieved by the proposed schemes for a variety of test sequences when the same target bit rates are maintained.

A study of MPEG-4 rate control scheme and its improvements

This paper discusses some practical issues in implementing the MPEG-4 Q2 rate-control scheme, and proposes a number of ways to improve it. The improved algorithm has the following main features: (1) the bits allocated to each P-frame or B-frame are in proportion to its distance from the end this GOP. i.e., more bits are allocated to the frames that are nearer to their reference I-frame; (2) the target buffer level is a function of the frame position in the GOP, so that it will be achieved gracefully at the end of a GOP; and (3) the quantization value of an I-frame is decided based on its spatial complexity. Experimental results show that the improved rate-control scheme has significantly reduced the occurrence of frame skipping, increased the average PSNR by up to 0.6 dB, and improved the perceptual quality of the reconstructed video.

Adaptive rate control using nonlinear regression

This paper presents a simple, fast, and accurate rate-control algorithm using nonlinear regression that plays a central role in estimation theory. We measure a conditional mean by estimating a joint probability density function (PDF) using Parzen's (1962) window. The training data pick up the nonlinear rate-distortion (R-D) relationship between the quantization parameter (QP) and the bits spent for each macroblock depending on the sum of absolute differences (SAD). We increase the accuracy of this joint PDF by clustering the training data depending on the QP values within admissible ranges. This localization helps understand image characteristics more accurately. Then we apply the adaptive vector quantization (AVQ) to simplify the conditional mean estimation of the rate given the SAD and QP values. This information is stored into three look-up tables. They contain the localized R-D function on macroblock basis. We use these tables to find the optimal QP values in least-mean-square sense for a given bit budget of the current frame. Simulation results show that the proposed algorithm outperforms the informative MPEG-4 rate-control algorithm in terms of reproduced image quality and coding efficiency. Our algorithm gives better image quality using much fewer bits. Most of all, it keeps the bit rate very accurately due to the accuracy of the conditional mean estimator that solves the nonlinear R-D function.

Low computational cost improvements of the rate-distortion performance in MPEG-2 rate control

Rate control in standards such as MPEG-2 or H263 plays an important role in controlling the compression rate and stabilizing the decoding and playback quality. In this paper, we combine the simplicity of the rate-distortion (R-D) predictive models with the improved R-D performance offered by the iterative approaches and we design effective rate control algorithms for low encoding delay and low complexity video compression. The proposed algorithms initially utilize the Lagrangian formulation in order to add a local R-D optimization component into the current rate control scheme of the standard. Subsequently, two online controllers are designed based on this local optimization scheme for adjusting the bit rate produced by the standard. Extensive experiments show improvements over the MPEG-2 scheme of 0.5–1 dB per frame for a variety of test sequences when the same target bit rates are maintained.

歪み最小化型ビット配分に基づくデジタル放送用ＭＰＥＧ‐２符号化制御の最適化

Distortion minimization oriented bit assignment technology is representative of approaches that have been studied to improve MPEG-2 rate control performance. This technology conducts an inclusive bit assignment for each picture while maintaining picture distortion at a minimum level. Its effectiveness has been tested and confirmed. In this paper, MPEG-2 HDTV encoding was assumed, and the distortion minimization oriented bit assignment technology was introduced as a basic scheme. We proposed additional improvement approaches with full use of the technology for improving macroblock quantization control and adaptive picture type selection. Coding experiments confirmed that an additional coding gain could be achieved with the proposed scheme, especially for low bit-rate encoding.

MPEG-4 rate control for multiple video objects

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 rate quantizer model. The algorithm is organized into two stages: a pre- and a post-encoding stage. In the pre-encoding 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).

A New MPEG-2 Rate Control Scheme Using Scene Change Detection

We propose two new rate control schemes to improve MPEG-2 rate control in view of visual quality when scene changes happen. Two proposed schemes are characterized by real-time and non real-time improvement to reduce the impact of scene changes. We also propose a new target-bit prediction method using spatial activity of pictures and present a simple and efficient scene change detection scheme using signed difference of mean absolute difference (MAD). Computer simulation results show that the proposed real-time algorithm effectively alleviates visual quality degradation after scene changes. The proposed non real-time algorithm gives maximum 2 dB improvement in peak signal-to-noise ratio (PSNR) at a scene-changed picture, compared with MPEG-2 rate control scheme and it shows better quality than the real-time one.