Local to Global: A Sparse Transformer-Based Small Object Detector for Remote Sensing Images

The object detection plays a crucial role in remote sensing due to the urgent demands of various applications, such as urban planning and environmental monitoring. Despite the notable progress, current methods still struggle with detecting challenging small objects. At the object level, the limited pixel representation, blurred details, and background interference of small objects impose greater demands on feature extractors. At the network level, resource bias fails to provide adequate learning signals for these objects. In this article, we propose a sparse Transformer-based detector (STDet) to tackle these challenges. Specifically, we design a local-to-global Transformer network (LGFormer) to explore essential feature representations. The local Transformer block (LTB) establishes correlations between tokens and their surrounding data, while the global Transformer block (GTB) captures long-distance dependencies related to the objects. Meanwhile, we introduce a scale-balanced label assignment (SBLA) strategy that considers more samples to small objects. SBLA dynamically shifts the learning focus to easily overlooked objects and alleviates the issue of sample imbalance. Extensive experiments on three large-scale remote sensing datasets demonstrate the effectiveness of STDet and its superiority in small object detection.