Research Of H.264 Rate Control Algorithm

H.264/AVC is a new generation video coding compression standard. It was developed by JVT (Joint Video Expert Team) which was joined by ITU-T(ITU Telecommunication Standardization Sector) and MPEG(Moving Picture Experts Group) in March 2003. The goal of H.264/AVC is to ensure image quality and improve the compression efficiency. H.264/AVC has a very good compression performance. It will play an important role in the fields of digital TV broadcasting, real-time communication and network streaming meida.Firstly, the H.264/AVC coding standard was detailedly analyzed and discussed based on the introduction of the video encoding technology background and research status. The features and application of H.264/AVC and key techniques in H.264 which include intra-prediction, inter-prediction, integer transform, quantization, entropy coding and block filtering are detailedly analyzed.Secondly, the basic principle of rate control algorithm was introduced in this dissertation. The purpose of rate control is to control output bit rate, so that network bandwidth is used more efficiently and image quality will be better and more stable. We introduced several classic rate control algorithms such as RM8, TM5, VM8 and TMN8. H.264 coding control model based on the Lagrangian optimization algorithm was introduced, and the rate control algorithm of JVT-G012 used in H.264/AVC coding standard was deeply analyzed and discussed in the following. JVT-G012 rate control algorithm is composed mainly of three steps: target bit allocation; calculate the corresponding quantization parameter according to the allocated target bit; update the model parameters after the image was compressed.This dissertation focuses on the JVT-G012 rate control algorithm. There are three disadvantages of JVT-G012 algorithm have been discussed. (1). JVT-G012 rate control algorithm uses the P-frame's quantization parameters of the previous GOP to calculate the I-frame's quantization parameter in the current GOP, so rate control of the I-frame is too simple. (2). JVT-G012 algorithm is only based on the remaining number of bits of the buffer to allocate bits of P-frame, so the bit allocation of the P-frame was not sufficiently precise. (3). Because JVT-G012 algorithm does not take the problem of scene change in the video sequence into account, the image quality can decline sharply and the buffer can overflow easily when scene change occurred. To solve these problems, we dealt with them separately. (1). We calculate QP of the I-frame by weighting the QP which was calculated by using the proportion of I-frame's complexity in the previous GOP and the QP which was calculated by H.264 rate control algorithm, to control the I-frame's QP precisely. (2). We allocate bit of the P-frame based on the output buffer status, the image complexity of the P-frame and the position of the P-frame in the GOP. (3). Because JVT-G012 algorithm does not take the problem of scene change into account, we propose a rate control algorithm based on scene change in the dissertation. Firstly, we detect scene change in the sequence by using the mean absolute difference of pixel brightness component. If scene change was detected, we use an adaptive GOP structure and length of the scene change processing algorithm to process the scene change which was occurred.Finally, by using JM8.6 platform, the JVT-G012 algorithm and the proposed algorithm in the dissertation were carried out experimental verification respectively. Experimental results show that the improved algorithm can control the output bit rate more accurately, the PSNR of the image has also increased while the improved algorithm can effectively detect and deal with the scene change in the video sequences, and the image quality after the scene change has also been stabilized.