Reconstruction Reordered Intra Block Copy for Screen Content Coding

In the pursuit of achieving further coding gains beyond the versatile video coding (VVC) standard, the enhanced compression model (ECM) has been initiated by the Joint Video Exploration Team (JVET) with the aim of developing next generation video coding techniques. In ECM, novel coding tools are studied to improve the coding efficiency for both camera-captured content and screen content. Intra block copy (IBC) has been included as a fundamental coding tool in both VVC and ECM, yielding significant improvement in compression efficiency for screen content. This paper presents a method of reconstruction reordered IBC (RR-IBC) to further improve the compression efficiency for screen content, by taking advantage of the symmetry property inherent in screen content sequences. The reconstruction block is flipped horizontally or vertically to restore the characteristics of samples in the original block. A flip-aware adjustment is performed to regulate block vector candidates of the RR-IBC block according to the types of symmetry. Similarly, the reference template of the template-based reordering method for the RR-IBC block is adjusted accordingly to accommodate the geometry property. A motion constraint is applied to restrict the block vector of an RR-IBC coded block to a single direction displacement perpendicular to the flip axis. An RR-IBC flip mode index is signalled to specify how to flip the reconstruction block. Experimental results show that the proposed RR-IBC can provide an average Bjontegaard delta rate (BD-rate) saving of 1.61%/1.79%/1.76% and 3.90%/3.63%/3.63% on Y/Cb/Cr components for class F and class TGM sequences, respectively, with a negligible change on the runtime, compared with ECM-5.0 in all intra configurations. RR-IBC has been adopted into ECM.