Fast Adaptive Loop Filter Algorithm Based on the Optimization of Class Merging

Adaptive loop filter (ALF) is one of the new tools adopted in the next generation video coding standard Versatile Video Coding (VVC). ALF leads to a performance improvement of 2%∼6% at the expense of high computational complexity and long processing time. Especially when encoding, ALF accounts 5%∼20% for total encoding runtime. To solve this problem, this paper proposes a fast algorithm for ALF by optimizing the process of class merging, which is an important step of rate-distortion optimization (RDO) in ALF. Experimental results indicate that compared to the VVC Test Model (VTM-22.0), this method can reduce the ALF encoding runtime on average 48.16%, 60.65%, 56.15% under all-intra, low-delay P and random access configurations, with only 0.15%, 0.31%, 0.19% increase in luma Bjontegaard Delta-Bit Rate (BD-BR).