Extension of Matrix-Based Intra Prediction to 4:4:4 Chroma Formats

Abstract

The final version of the Versatile Video Coding (VVC) standard incorporates the Matrix-Based Intra Prediction (MIP) tool. It consists of additional intra prediction modes which were derived from a data-driven training. These modes, in general, are applied to the luma component only. This paper describes how to apply the MIP modes to the chroma components in certain cases. In the generic case, if a chroma block uses the intra direct mode (DM), its intra mode is derived from the co-located luma block. If this luma block uses MIP, the chroma block uses the planar mode, because a MIP mode is not applicable for all block shapes. However, if the chroma format is 4:4:4 and single-tree coding is used, luma and chroma blocks share the same partitioning. The paper investigates the impact of using the MIP mode of the luma component when the chroma mode is the DM in these cases. This extension also harmonizes MIP with the Adaptive Color-Space Transform (ACT) for RGB-content, because the chroma mode of blocks using the ACT is inferred to be the DM. For the extended MIP tool in the Versatile Video Coding Test Model 11.2 (VTM-11.2) that implements the final VVC standard, bit-rate savings of 1.51%/0.60%/0.62% in terms of the Bj⊘ntegaard-Delta bit rate (BD-rate) are reported compared to the VTM-11.2 with MIP disabled for the All-Intra configuration and natural content in the RGB 4:4:4 format.