Enhanced Combined Inter-Intra Prediction (CIIP) in Versatile Video Coding

Joint Video Experts Team (JVET) is developing a new video coding standard beyond High Efficiency Video Coding (HEVC) named as Versatile Video Coding (VVC). In VVC, various new prediction modes have been adopted compared to HEVC and Combined Inter-Intra Prediction (CIIP) is one of them. CIIP combines inter prediction and intra prediction with derived weights to form a final prediction. In the existing CIIP, the weights are derived from the prediction modes of the two adjacent blocks of left and above for combining the final prediction, and only planar mode is used as the intra prediction mode of CIIP. In this paper, we propose methods to enhance CIIP with more accurate weights for the combination of both predictions as well as extending intra prediction modes to be combined based on the adjacent blocks' coding modes. According to empirical observations, the below-left block and above-right block are correlated with left and above blocks in terms of prediction mode, respectively. So, the first proposed method is to derive finer weight values by using prediction modes of more adjacent blocks up to 3 blocks of left, above and above-left from the 5 adjacent blocks used for deriving regular merge candidates. The second proposed method is to use intra-coded modes of adjacent blocks of left and above blocks which are used to derive MPM candidates to be combined instead of planar mode used in the current CIIP. Experiment results show that the proposed methods slightly improve the performance of CIIP in the VVC Test Model (VTM).