SL-Ground: Sentence-Level Grounding for Fine-Grained Open-Vocabulary Object Detection

Yang-Syu Li, Cheng-Huan Yang, Chia-Yung Jui, Peggy Joy Lu

Grounding-based open-vocabulary detectors provide a widely used framework for text-conditioned localization, with a phrase-grounding formulation that aligns object queries to individual text tokens and recovers class scores through token-to-class aggregation. We observe that this token-level scoring design becomes a bottleneck for fine-grained open-vocabulary object detection, where the target category is defined by a complete compositional description rather than by isolated tokens. Based on this observation, we propose SL-Ground, a sentence-level grounding reformulation for grounding-based open-vocabulary detection. Instead of scoring queries against fragmented token evidence, SL-Ground encodes each category description as a complete sentence-level representation and directly scores object queries against these representations. To evaluate fine-grained recognition across label granularity, we construct a multi-granularity Stanford Cars benchmark spanning vehicle type, brand, and car model. At the finest car-model granularity, SL-Ground raises AP over the token-level variant from 14.5 to 70.6 under matched training candidate sampling and to 86.0 with full-vocabulary training,…