Rate control algorithm based on quality factor optimization for Dirac video codec

摘要

Rate control plays an essential role in video coding and transmission to provide the best video quality at the receiver end given the constraint of certain network conditions. This paper proposes a rate control algorithm for the wavelet-based open-source Dirac video codec. The existing Dirac architecture has a constant-quality control mechanism based on rate–distortion optimization (RDO), giving variable bitrate. The proposed algorithm exploits the existing constant-quality control, which is governed by a parameter called quality factor (QF) to give a constant bitrate. A mathematical model called the rate–quality factor (R–QF) is derived to generate optimum QF for the current coding frame using the bitrate resulting from the encoding of the previous frame in order to meet the target bitrate. The proposed algorithm is a complete one-pass process and does not require complex mathematical computation. The process of calculating the QF is simple and, further, calculation is not required for each coded frame. It also provides the rate control solution for both intra-frame-only and inter-frame coding modes. The experimental results show that the proposed algorithm can control the bitrate precisely (i.e. within 1% of target bitrate in average for inter-frame coding mode and near-perfect flat response in generated bits vs. frame number curve in intra-frame-only coding mode). Moreover, the variation of bitrate over each group of pictures (GOP) in inter-frame coding mode is lower than that of H.264 using JM11. This is an advantage in preventing the buffer overflow and underflow for real-time multimedia data streaming. More importantly, there is no PSNR performance loss because of application of the proposed rate control algorithm. It gives superior quality over relatively static motion sequences and fast motion sequences with average quality as shown in the analysis and evaluations presented in this paper.