Cross-modal Unsupervised Domain Adaptation for 3D Semantic Segmentation via Bidirectional Fusion-then-Distillation

Cross-modal Unsupervised Domain Adaptation (UDA) becomes a research hotspot because it reduces the laborious annotation of target domain samples. Existing methods only mutually mimic the outputs of cross-modality in each domain, which enforces the class probability distribution agreeable in different domains. However, these methods ignore the complementarity brought by the modality fusion representation in cross-modal learning. In this paper, we propose a cross-modal UDA method for 3D semantic segmentation via Bidirectional Fusion-then-Distillation, named BFtD-xMUDA, which explores cross-modal fusion in UDA and realizes distribution consistency between outputs of two domains not only for 2D image and 3D point cloud but also for 2D/3D and fusion. Our method contains three significant components: Model-agnostic Feature Fusion Module (MFFM), Bidirectional Distillation (B-Distill), and Cross-modal Debiased Pseudo-Labeling (xDPL). MFFM is employed to generate cross-modal fusion features for establishing a latent space, which enforces maximum correlation and complementarity between two heterogeneous modalities. B-Distill is introduced to exploit bidirectional knowledge distillation which includes cross-modality and cross-domain fusion distillation, and well-achieving domain-modality alignment. xDPL is designed to model the uncertainty of pseudo-labels by self-training scheme. Extensive experimental results demonstrate that our method outperforms state-of-the-art competitors in several adaptation scenarios.