Study On Rate Control Techniques For H.264/AVC Video Coding

In recent years, with the development of information society,the demands for multimedia increase largely. Corresponding digital video products are popular in broad application areas such as business, amusement and surveillance, etc. This drives the digital video techniques of encoding, processing and transmission to advance rapidly. Being the newest international video coding standard, H.264/AVC achieves great improvements in coding efficiency, quality and network adaptability. As a result, H.264 gradually becomes dominant in video applications.Meanwhile, rate control plays an important role in the application of video coding standards. For a video encoder, rate control is employed to maximize the coding quality at certain channel bandwidth and buffer constraints. However, the interdependence of rate control and RDO in H.264 makes it more difficult to implement than that in prior standards. In this thesis, we focus on improving the perceptual quality of real time video applications by optimization of H.264 rate control scheme.Firstly, the advanced coding features of H.264 are introduced, especially for the Intra/Inter prediction methods and novel quantization scheme combined with interger DCT-like transform, which are closely related with rate control. Then we give an analysis of rate distortion theory in video encoder and rate control optimization. The classical rate distortion models for rate control are reviewed. Subsequently, we sum up some key issues in rate control. Based on the JVT adopted rate control schemes, we give a discussion on the shortage of existing H.264 rate control algorithms and the probabilities of improvements. To relieve the video quality degradation caused by high motions and scene changes, an improved frame-layer bit allocation scheme based on pre-encoding is proposed. First, the frame is pre-encoded in 16×16 modes with a fixed quantization parameter (QP). The coding complexity of current frame is then measured by rate-distortion ratio according to the coding results of pre-encoding stage. The frame bit budget is finally decided by the frame coding complexity and inter-frame PSNR fluctuation, combined with the buffer status. Simulation results show that, in comparison with the H.264 adopted rate control scheme, our method is more efficient to suppress the sharp PSNR drops due to high motions and scene changes. The pre-encoding stage introduces no more than 30% extra computational complexities and need no any following frame, which makes it applicable to real time video coding.To futher improve the visual quality of real time video applications by means of smoothing the frame quality fluctuations, we develop a two-stage rate control scheme. It integrates theρ-domain R-D model into a two-stage H.264 coding framework. In the first stage, a coarse QP is used for motion estimation and RDO. Based on the coding results of this stage, the relatedρ-domain R-D model is constructed. Then a low-pass filter is used to estimate the target frame distortion. The frame bit allocation is determined by target distortion, remaining coding bits and buffer status. If the bit rate or distortion of the first stage exceeds related thresholds, the second coding stage is performed, which adopts theρ-domain rate control scheme to get the refined QP of each MB for residual signal recoding. Experimental results show the proposed scheme can efficiently smooth out the vidual quality fluctuation between successive frames, on the basis of meeting bit rate constraints accurately.Finally, the development of video quality measurement is reviewed, based on which we give a discussion on the necessities of video encoder optimization in terms of HVS characteristics. And the relationship between mode decision and bit allocation within a frame is analyzed. Then we focus on the benefits and feasibilities of perceptual optimization for H.264 mode decision. According to above analysis, a perceptual importance analysis model is developed for video segmentation from the view of video encoding. Base on this model, we propose a perceptual mode decision scheme for H.264, which adaptively adjusts the Lagrangian multiplier in RDO process in terms of the perceptual importance of each MB. Therefore, in a frame, more bits can be allocated to the regions with high perceptual importance for visual quality improvement and few bits to the regions with low perceptual importance for bit rate saving. To meet bit rate constraints, we proposed a MB-layer rate control algorithm on the basis of adaptive mode decision scheme. The simulation results show the efficiency of the proposed scheme.