Abstract
In current standardized hybrid video encoders, the Lagrange multiplier determination model is a key component in rate-distortion optimization. This originated some 20 years ago based on an entropy-constrained high-rate approximation and experimental results obtained using an H.263 reference encoder on limited test material. In this paper, we present a comprehensive analysis of the results of a Lagrange multiplier selection experiment conducted on various video content using H.264/AVC and HEVC reference encoders. These results show that the original Lagrange multiplier selection methods, employed in both video encoders, are able to achieve optimum rate-distortion performance for I and P frames, but fail to perform well for B frames. The relationship is identified between the optimum Lagrange multipliers for B frames and distortion information obtained from the experimental results, leading to a novel Lagrange multiplier determination approach. The proposed method adaptively predicts the optimum Lagrange multiplier for B frames based on the distortion statistics of recent reconstructed frames. After integration into both H.264/AVC and HEVC reference encoders, this approach was evaluated on 36 test sequences with various resolutions and differing content types. The results show consistent bitrate savings for various hierarchical B frame configurations with minimal additional complexity. BD savings average approximately 3% when constant quantization parameter (QP) values are used for all frames, and 0.5% when non-zero QP offset values are employed for different B frame hierarchical levels.
Highlights
V IDEO compression, has been a key enabler for video storage, conferencing, broadcasting and streaming since the early 1980s, when the first widely adopted international coding standard, H.261 [1], was established
After integration into the H.264/AVC and High Efficiency Video Coding (HEVC) reference software, the proposed adaptive Lagrange multiplier determination method was tested on a video dataset with various content at different resolutions
Experimental results demonstrate the optimality of existing λ determination methods in H.264/AVC and HEVC reference encoders for encoding I and P/Bp frames, but highlight the shortcomings of these models for Bb frames
Summary
V IDEO compression, has been a key enabler for video storage, conferencing, broadcasting and streaming since the early 1980s, when the first widely adopted international coding standard, H.261 [1], was established. With recent advances in video and communication technologies, the demand for video content is ever increasing, with 73% of all Internet bandwidth consumed by video in 2016 This figure is predicted to increase to 82% in 2021 [2]. The first introduces the rate distortion optimization (RDO) problem in the context of hybrid video encoders. Hybrid video encoders select optimum coding parameters popt by minimizing a Lagrange cost function of rate R and distortion D [10]: min{J }, where J = D(p) + λ · R(p), (1). Where p is the vector of coding parameters including prediction modes, block partitions, etc., and λ represents the Lagrange multiplier. This optimization process is iterated during the compression process at various block levels for different types of frame
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