C-GAP: Class-Aware and Online Prompting Improves Vision-Language Models on Imbalanced Classes
C-GAP is a novel framework that improves detection of rare object classes in vision-language models by iteratively refining language prompts using a large language model (LLM), without retraining or additional annotations. It operates in two phases: first, establishing a composite caption baseline combining scene descriptions and class-quantity context; second, an LLM iteratively refines each image's caption based on minority-class average precision (AP) thresholds. Experiments show up to 53% improvement in minority-class AP, and ~81% relative improvement on COCO.
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[Submitted on 10 Jul 2026]
Title:C-GAP: Class-Aware and Online Prompting Improves Vision-Language Models on Imbalanced Classes
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Abstract:Safety-critical perception systems must reliably detect rare object classes within small label spaces, a setting that long-tailed detection methods, designed for hundreds of classes with dense annotation, fundamentally do not address. Open-vocabulary detectors offer a promising alternative, as they use natural language queries at inference time, making prompt quality a first-class lever for detection performance. We exploit this property to address class imbalance: rather than retraining models or collecting additional annotations, we ask whether iteratively refining the language prompts, fed to frozen detectors, can improve minority class detection. We introduce C-GAP Caption-Guided Augmentation and Prompting), a detector-agnostic, annotation-free framework that operates in two phases. First, we establish a composite caption baseline combining per-image scene descriptions with class-quantity context, which we show outperforms scene-description only or class-quantity-only prompts across multiple open-vocabulary architectures and benchmarks. Second, an LLM iteratively refines each image's caption individually, with trials triaged into accept, tentative, or regenerate buckets based on minority-class [email protected] against a dynamic threshold derived from the composite baseline. Refinement terminates early once sufficient [email protected] gain is achieved. No detector weights are updated at any stage. Our experiments shows that C-GAP improves minority-class average precision up to 53% over the baselines. On COCO, C-GAP improves minority-class [email protected] by ~81% relative over the composite baseline (17.69 -> 32.09). Experiments confirm that composite captions provide the critical foundation for effective refinement: using scene-description-only or class-quantity-only prompts as the refinement starting point yields diminishing returns, supporting both stages of C-GAP as necessary contributions.
Subjects:
Computer Vision and Pattern Recognition (cs.CV)
Cite as: arXiv:2607.09008 [cs.CV]
(or arXiv:2607.09008v1 [cs.CV] for this version)
https://doi.org/10.48550/arXiv.2607.09008
arXiv-issued DOI via DataCite (pending registration)
Submission history
From: Francis Fernandez [view email] [v1] Fri, 10 Jul 2026 00:24:41 UTC (5,692 KB)
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