Point cloud videos effectively capture real-world spatial geometries and temporal dynamics, which are essential for enabling intelligent agents to understand the dynamically changing 3D world we live in. Although static 3D point cloud processing has witnessed significant advancements, designing an effective 4D point cloud video backbone remains challenging, mainly due to the irregular and unordered distribution of points and temporal inconsistencies across frames. Moreover, recent state-of-the-art 4D backbones predominantly rely on transformer-based architectures, which commonly suffer from large computational costs due to their quadratic complexity, particularly when processing long video sequences. To address these challenges, we propose a novel 4D point cloud video understanding backbone based on the recently advanced State Space Models (SSMs). Specifically, our backbone begins by disentangling space and time in raw 4D sequences, and then establishing spatio-temporal correlations using our newly developed Intra-frame Spatial Mamba and Inter-frame Temporal Mamba blocks. The Intra-frame Spatial Mamba module is designed to encode locally similar or related geometric structures within a certain temporal searching stride, which can effectively capture short-term dynamics. Subsequently, these locally correlated tokens are delivered to the Inter-frame Temporal Mamba module, which globally integrates point features across the entire video with linear complexity, further establishing long-range motion dependencies. Experimental results on human action recognition and 4D semantic segmentation tasks demonstrate the superiority of our proposed method. Especially, for long video sequences, our proposed Mamba-based method has an 87.5% GPU memory reduction, 5.36 times speed-up, and much higher accuracy (up to +10.4%) compared with transformer-based counterparts on MSR-Action3D dataset.

基于最近提出的状态空间模型（SSMs）的新型4D点云视频理解骨干网络，通过解开原始4D序列中的空间和时间，使用新开发的Intra-frame Spatial Mamba和Inter-frame Temporal Mamba模块建立了点云视频内部和帧间的时空关联，并在人体动作识别和4D语义分割任务上证明了该方法的优越性。

MAMBA4D: 高效的长序列点云视频理解及解耦空间-时间状态空间模型