Cross-Domain and Cross-Modal Knowledge Distillation in Domain Adaptation for 3D Semantic Segmentation

With the emergence of multi-modal datasets where LiDAR and camera are synchronized and calibrated, cross-modal Unsupervised Domain Adaptation (UDA) has attracted increasing attention because it reduces the laborious annotation of target domain samples. To alleviate the distribution gap between source and target domains, existing methods conduct feature alignment by using adversarial learning. However, it is well-known to be highly sensitive to hyperparameters and difficult to train. In this paper, we propose a novel model (Dual-Cross) that integrates Cross-Domain Knowledge Distillation (CDKD) and Cross-Modal Knowledge Distillation (CMKD) to mitigate domain shift. Specifically, we design the multi-modal style transfer to convert source image and point cloud to target style. With these synthetic samples as input, we introduce a target-aware teacher network to learn knowledge of the target domain. Then we present dual-cross knowledge distillation when the student is learning on source domain. CDKD constrains teacher and student predictions under same modality to be consistent. It can transfer target-aware knowledge from the teacher to the student, making the student more adaptive to the target domain. CMKD generates hybrid-modal prediction from the teacher predictions and constrains it to be consistent with both 2D and 3D student predictions. It promotes the information interaction between two modalities to make them complement each other. From the evaluation results on various domain adaptation settings, Dual-Cross significantly outperforms both uni-modal and cross-modal state-of-the-art methods.