HEVC ALF decode complexity analysis and reduction

This paper analyzes the decoder implementation complexity of a new tool called Adaptive Loop Filtering (ALF) being considered for the ITU-T/ISO/IEC High Efficiency Video Coding (HEVC) standard, and proposes new luma filters (Nx7 and Nx5) for ALF that reduce memory bandwidth, memory size requirements, and number of computations. The luma filters in ALF of the initial version HEVC Test Model (HM-1.0) have a maximum vertical size of 9. The vertical size of the ALF filters determines the memory size (line buffers) and memory bandwidth requirements. Accordingly, this paper proposes reducing the vertical size of ALF filters to 7 and 5, which are referred to as Nx7 and Nx5 filter sets respectively. These filters reduce memory bandwidth and size requirements by 25% and 50% respectively with minimal impact on coding efficiency. In addition, the worst case computational complexity is reduced by -10% and -20% respectively. Reduced vertical size luma ALF filters are under consideration for inclusion in HEVC standard with Nx7 being been adopted into HM- 2.0 and Nx5 being under consideration for HM-4.0.