PeaCap: Patch-Level Retrieval for Lightweight Retrieval-Augmented Image Captioning

Retrieval-augmented image captioning aims to improve caption quality by grounding generation in external evidence, but most prior systems retrieve evidence using coarse whole-image similarity, which can miss small or rare objects in cluttered scenes. We propose PeaCap, a patch-based retrieval-augmented captioning framework that explicitly studies how retrieval granularity affects the quality of retrieved object evidence and downstream caption generation. PeaCap decomposes a query image into patches, performs patch-level image-to-image retrieval to obtain object tags, and fuses the retrieved tags with the whole-image embedding via a lightweight cross-attention module and an alignment loss to robustly prompt a frozen LLM. Analyses on retrieval (encoder choice, patch-vs.-whole retrieval, and patch-grid ablations) show that patch-level retrieval can improve object coverage, and experiments on COCO and out-of-domain benchmarks demonstrate competitive captioning performance under a lightweight training setup.