DURING THE PASTdecades, videos have

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