Optimal Bit Allocation at Frame Level for Rate Control in HEVC

Rate control plays an important role in video coding systems, which makes the output bitrate of a video encoder equal to the target bitrate while minimizing the distortion of the compressed video. However, most of the existing rate control schemes achieve accurate bitrate control at the loss of rate-distortion (R-D) performance. This paper proposes an effective frame level bit allocation method to improve the R-D performance whilst maintaining the high accuracy of the bitrate control in HEVC. First, an improved R-D model is presented at the frame level, which, by making use of the information of encoded frames more completely, achieves higher accuracy with lower bitrate mismatch rate. Second, a group of picture (GOP) level bit allocation approach is introduced with the consideration of the temporal R-D dependency among different GOPs, which can further enhance the R-D performance. Finally, to achieve optimal R-D performance globally, a formulation for optimal frame level bit allocation is developed with a GOP level Lagrange multiplier introduced, which takes into account the coding effects of a current frame on the other frames within the same GOP. A scheme of recursive Taylor expansion is employed to find the GOP level Lagrange multiplier. Experimental results demonstrate that the proposed method can achieve considerable R-D performance improvement and accurate bitrate control. Specifically, our method shows 3.9% and 3.8% BD-rate saving in average compared against the HEVC reference software HM16.7 with rate control in the low-delay B and low-delay P coding structures, respectively.