ConPCAC: Conditional Lossless Point Cloud Attribute Compression via Spatial Decomposition

A conditional lossless point cloud attribute compression method, dubbed ConPCAC, is proposed. The previous work typically codes point attributes in a point cloud in an autoregressive way, incurring unbearable coding time. By contrast, ConPCAC proposes a group-wise conditional entropy model for fast coding while preserving coding performance. Specifically, ConPCAC adopts a “Group Decomposition - Attribute Initialization - Latent Distribution Prediction” framework. First, it flexibly decomposes the original point cloud into multiple groups according to the geometry coordinate distribution. Then, the first group is coded using a base coder, e.g., the standardized G-PCC, and the following groups are progressively coded using a neural coder conditioned on their preceding groups. Two key units, Attribute Initialization (Init) and Latent Distribution Prediction (LDP), are devised in the neural coder. The Init unit employs the nearest neighbor to initialize the attributes of a group, and the LDP unit further predicts the attribute probability distribution for the group. In this way, ConPCAC enables full correlation exploration across groups and parallel processing among points in a group. Finally, the predicted probabilities are fed into the arithmetic engine to code the true attribute values of each group. Extensive experiments demonstrate the performance of ConPCAC. It achieves 14.59%, 10.32%, and 12.26% improvements over the latest G-PCC on the widely used 8iVFB, Owlii, and MVUB datasets, respectively, significantly outperforming state-of-the-art lossless PCAC methods. Moreover, its computational complexity is comparable to G-PCC and much lower than existing learning-based methods. Associated code and models will be released on the website https://github.com/3dpcc/ConPCAC.