A Rate Control Scheme For H.264 Under Low Delay Constraint

H.264/AVC video coding standard is established by ITU-T and ISO/IEC. The framework of H.264 is similar to its previous standards such as H.26x and MPEG. However, a lot of new techniques are utilized in H.264. These new techniques include intra-prediction, multi-block mode inter-prediction, 1/4 pixel motion search and 4x4 integer transform. The application of these new techniques greatly improves the coding efficiency of H.264. Compared with previous standards, H.264 can save about 50% bit rates while having the same visual quality as other standards. H.264 is composed with network abstraction layer (NAL) and video coding layer (VCL). NAL enhances its network function, and makes it possible to integrate the coded bits to transportation layer easily. That is why H.264 is widely applied in real-time video communications such as video telephony and mobile video.It is very important to minimize the end-to-end delay for transmitting video data in real-time, two way video communications. In this paper, we introduce a rate control scheme under low time delay constraint for H.264 standards. Since encoding the I-frame by different quantization parameter will generate different rate and distortion for the whole GOP, we propose a Lagrange optimization scheme which is based on prediction to choose the QP for the I-frames. We use the quadratic R-D model to predict the rate of I-frame and the whole GOP. And then perform RDO for the whole prediction bits of the GOP to find the tradeoff between the rate and distortion for the GOP. We also introduce a scheme to charge the encode buffer, allow some less important frames to be skipped even the buffer fullness is not reach the threshold, thus these important frames could have enough space to be encoded, and this method is also effective to stop the consecutive frame skipping . The experiments show that compare to the JVT-H017 scheme, our proposal gained higher PSNR with less frame skipping. Firstly, the basic theory, methods and international standards of image coding are introduced.Secondly, H.264/AVC is then introduced in detail including its architecture and new features such as intra-prediction, multi-block mode. The rate control and RDO theory are concentrated. We also provide the most popular rate control method for the main video coding standards, and the JVT-H017 rate control proposal for H.264 is high lighted.Thirdly, we do experiments on H.264 proposal JVT-H017 under low time delay constraint. We analyze the poor results and fix out how to improve the rate control under low time delay constraint.In the fourth part, we introduce our rate control scheme for H.264 under low delay constraint. We divide our scheme in to GOP layer rate control and frame layer rate control. In GOP layer, we propose a Lagrange optimization scheme which is based on prediction to choose the QP for the I-frames. We perform RDO for the whole prediction bits of the GOP to find the tradeoff between the rate and distortion for the GOP. In frame layer, we introduce an active frame skipping scheme, which allows less important frames to be skipped even the buffer is not full, to maintain other important frames be encoded, also it is effective to stop the consecutive frame skipping.In the fifth part, we do experiments to compare our scheme with H.264 proposal JVT-H017. The results show that our algorithm could gain a better PSNR and reduce the frame skipping obviously.In the last part, we sum up the tasks we have done for this scheme, state the further research for H264 low time delay rate control.