Threshold-Adaptive Unsupervised Focal Loss for Domain Adaptation of Semantic Segmentation

Semantic segmentation is an important task for intelligent vehicles to understand the environment. Current deep learning based methods require large amounts of labeled data for training. Manual annotation is expensive, while simulators can provide accurate annotations. However, the performance of the semantic segmentation model trained with synthetic datasets will significantly degenerate in the actual scenes. Unsupervised domain adaptation (UDA) for semantic segmentation is used to reduce the domain gap and improve the performance on the target domain. Existing adversarial-based and self-training methods usually involve complex training procedures, while entropy-based methods have recently received attention for their simplicity and effectiveness. However, entropy-based UDA methods have problems that they barely optimize hard samples and lack an explicit semantic connection between the source and target domains. In this paper, we propose a novel two-stage entropy-based UDA method for semantic segmentation. In stage one, we design a threshold-adaptative unsupervised focal loss to regularize the prediction in the target domain. It first introduces unsupervised focal loss into UDA for semantic segmentation, helping to optimize hard samples and avoiding generating unreliable pseudo-labels in the target domain. In stage two, we employ cross-domain image mixing (CIM) to bridge the semantic knowledge between two domains and incorporate long-tail class pasting to alleviate the class imbalance problem. Extensive experiments on synthetic-to-real and cross-city benchmarks demonstrate the effectiveness of our method. It achieves state-of-the-art performance using DeepLabV2, as well as competitive performance using the lightweight BiSeNet with great advantages in training and inference time.