DURING THE PASTdecades, videos have already taken the main part of our daily entertainment. When transmitting videos over thebandwidth-limited Internet, video compression should be applied to significantly save the coding bit-rate. Lossy codec technologies and standards for videos, such as the well-known HEVC,1 have achieved an impressive coding efficiency.However, due to the compromise to lower the bit-rate, the distortions are ineluctably induced in the codec. For example, there are visible discontinuities at block boundaries in the decoded images, which are known as blocking artifacts.Other artifacts may include blurring, ringing,color bias, etc. These compression artifacts may cause severe degradation in the visual quality atthe decoder side. Therefore, how to reduce the compression artifacts has attracted more and more attention in recent years.A traditional way to reduce the compression artifacts is to apply in-loop filters at the encoder.Digital Object Identifier 10.1109/MMUL.2019.2922194 Date of publication 12 June 2019; date of current version 20November 2019.