Intra Block Copy Mirror Mode for Screen Content Coding in Versatile Video Coding

Intra Block Copy (IBC) cannot work well for blocks with symmetry or mirror characteristic, because it is based on the translational block matching technique. In order to solve this problem, in this paper, an adequately designed IBC-Mirror mode is proposed for Versatile Video Coding (VVC), which extends our previous work. The newly proposed IBC-Mirror mode consists of two sub-modes, including IBC-Mirror Advanced Motion Vector Prediction (AMVP) mode and IBC-Mirror Merge mode. In IBC-Mirror AMVP mode, various Motion Estimation (ME) methods are applied, including previously proposed spatial search, and newly designed methods, i.e., Hash-based, Cache-based, Predictor-based and History-based IBC-Mirror search. As for IBC-Mirror Merge mode, Motion Information, composed of Block Vector (BV) and mirror flipping information, is added to the merge list to improve coding efficiency. Compared to the traditional IBC mode in VTM-10.0, the latest test model, the proposed mode can achieve 1~2% BD-rate saving. Compared to our previous work, methods in this paper can further achieve 0.5% coding gains and reduce 4% coding time. Higher coding efficiency and lower complexity show the superiority of extensions and improvements in this paper.