Design of a Low Complexity Video Encoder for High Efficiency Video Coding

다국어 초록 (Multilingual Abstract)

High Efficiency Video Coding (HEVC) is the latest video compression standard developed by the Joint Collaborative Team on Video Coding (JCT-VC) established together by the ITU-T Video Coding Experts Group (VCEG) and ISO/IEC Moving Picture Experts Group (MPEG). It achieves more than 50% bit rate savings with subjective image quality equivalent to that of the previous video coding standard H.264/AVC. HEVC employs the quad-tree structure based on the coding tree unit (CTU), in which coding unit (CU), prediction unit (PU), and transform unit (TU) are introduced to specify the basic processing unit of coding, prediction, and transform. These structures improve coding efficiency; however, the encoding complexity increases greatly because the rate-distortion (RD) optimization process should be performed for all combinations of CU, PU, and TU to obtain the optimal CTU partition. This increased complexity is a major problem especially for power constrained devices or real-time applications. Therefore, it is highly desirable to optimize the encoding process for computational complexity reduction while maintaining the coding efficiency of HEVC.
In this dissertation, an efficient CU size decision algorithm is proposed to reduce the encoder complexity of HEVC. Based on the statistical analysis, three approaches are considered with SKIP mode decision (SMD), early CU termination (ECUT), and CU skip estimation (CUSE). In SMD, it is determined that the remaining modes except for SKIP mode are preformed or not. CUSE and ECUT determine that larger CU sizes and smaller CU sizes are coded or not, respectively. Thresholds for SMD, CUSE, and ECUT are designed based on Bayes’ rule with a complexity factor. Update process is performed to estimate the statistical parameters for SMD, CUSE, and ECUT considering the characteristic of RD cost. The experimental results show that the proposed CU size decision algorithm efficiently reduces the encoder complexity of HEVC with negligible degradation of coding performance for not only homogeneous sequences but also non-homogeneous sequences. In addition, SKIP mode enforced algorithm is proposed based on motion attention model for the additional encoding time reduction. The motion attention model is introduced as considering Human Visual System (HVS) with the intensity inductor, spatial coherence inductor, and the temporal coherence inductor. No attention region is determined according to motion attention model, SKIP mode is enforced to CU with no attention region although SKIP mode is not the optimal mode to reduce the encoder complexity. The subjective test shows that the proposed SKIP mode enforced algorithm enormously reduces the encoder complexity of HEVC with similar perceptual visual quality. Finally, the combined method with two proposed algorithms performs the vast time savings by more than 86% with similar perceptual visual quality.