New R-D Optimization Criterion for Fast Mode Decision Algorithms in Video Coding and Transrating

Abstract

Mode decision has a significant effect on the quality and complexity of video coding. It is even more challenging when generating multiple bitstreams with different bitrates (BRs) in, for example, dynamic adaptive streaming for HTTP or transrating systems. Full search and simplified fast search mode decision (MD) methods either suffer from a high computational complexity or have a negative impact on quality. Furthermore, mode selection in conventional approaches strongly depends on the quantization parameter (QP). Hence, modes that have been selected for high BR compression may not be suitable for low BR when transrating a bitstream. In this paper, we propose a rate-distortion (R-D)-optimized criterion for fast MD algorithms. The proposed cost function, when adopted in different fast MD algorithms, not only improves the R-D performance by up to 6.6% in terms of Bjontegaard delta rate, but also reduces the execution time of the encoder by up to 6.8%. We also show that modes selected by the proposed criterion are less sensitive to changes in BR or QP. As a result, the same modes in an encoded bitstream may be used even after transrating using requantization, resulting in a significant R-D performance improvement of up to 33.3%.