MM-Flow: Multi-modal Flow Network for Point Cloud Completion

Point cloud is often noisy and incomplete. Existing completion methods usually generate the complete shapes for missing regions of 3D objects based on the deterministic learning frameworks, which only predict a single reconstruction output. However, these methods ignore the ill-posed nature of the completion problem and do not fully account for multiple possible completion predictions corresponding to one incomplete input. To address this problem, we propose a flow-based network together with a multi-modal mapping strategy for 3D point cloud completion. Specially, an encoder is first introduced to encode the input point cloud data into a rich latent representation suitable for conditioning in all flow-layers. Then we design a conditional normalizing flow architecture to learn the exact distribution of the plausible completion shapes over the multi-modal latent space. Finally, in order to fully utilize additional shape information, we propose a tree-structured decoder to perform the inverse mapping for complete shape generation with high fidelity. The proposed flow network is trained using a single loss named the negative log-likelihood to capture the distribution variations between input and output, without complex reconstruction loss and adversarial loss. Extensive experiments on ShapeNet dataset, KITTI dataset and measured data demonstrate that our method outperforms the state-of-the-art point cloud completion methods through qualitative and quantitative analysis.