Enhanced MPEG G-PCC: Addressing Challenges in the OBUF Entropy Coding Framework

The Moving Picture Experts Group (MPEG) has published the geometry-based point cloud compression (G-PCC) standard. It converts the compression of irregular point coordinates to the coding of structured binary octree node occupancy, where the Context-based Adaptive Binary Arithmetic Coding (CABAC) can be applied. The context model, constructed by intra and inter-octree layer information, drives the probability update of the arithmetic coder with a so-called Optimal Binarization with Update On-the-Fly (OBUF) scheme. The original OBUF design, while effective, lacks a probability range limitation for each binary coder, leading to issues in probability estimation accuracy and convergence speed. Moreover, when coding dynamic point clouds, the inter-frame information is not efficiently considered in OBUF, leading to excessive memory consumption for storing and tracking context states. To address these challenges, we propose an initialization strategy for both fine-grained context states (Fine-CtxS) and coarse-grained context states (Coarse-CtxS) in OBUF, alongside an adaptive probability bound determination method for each Coarse-CtxS to confine probability estimation. Furthermore, the paper delves into improvements for inter-frame geometry coding, including the construction of Fine-CtxS, and reducing memory consumption of Fine-CtxS in OBUF. The proposed methods have been adopted in recent G-PCC Edition 2 standardization activities, demonstrating enhanced performance.