Controllable Image Captioning with Prompt-Conditioned Scene Rewards

Jongyeop Hyun, Taeyoung Kim, Hyounghun Kim


Abstract
Large Vision-Language Models produce fluent image descriptions but offer limited semantic control: users cannot reliably specify whether captions should emphasize attributes, relations, or particular image regions. We present Fine-grained Captioning Control Using Scene-rewards (FoCUS), a controllable image captioning method that lets users steer captions toward specific semantic emphases through natural-language control prompts. The core idea is a prompt-conditioned control objective based on scene-graph-aligned component scores. Generated captions are parsed and aligned to scene-graph components—objects, attributes, and relations—and these components are differentially weighted, including negative weights, according to the requested emphasis. We optimize this objective with GRPO and further improve its reliability through a stricter object validity threshold and reasoning-based verification for attribute and relation scoring. To evaluate controllability, we introduce Semantic Control and Precision Evaluation (SCoPE), a benchmark with contrastive Include/Avoid constraints for measuring both target content coverage and out-of-scope suppression. Experiments on two VLM backbones show that FoCUS consistently improves controllability and fine-grained caption quality without degrading general caption performance. The FoCUS code and SCoPE benchmark will be publicly released.
Anthology ID:
2026.emnlp-main.2622
Volume:
Proceedings of the 2026 Conference on Empirical Methods in Natural Language Processing
Month:
October
Year:
2026
Address:
Budapest, Hungary
Editors:
Hung-Yi Lee, Sunipa Dev, Jessy Li, Desmond Elliott
Venue:
EMNLP
SIG:
SIGDAT
Publisher:
Association for Computational Linguistics
Note:
Pages:
59590–59615
Language:
URL:
https://aclanthology.org/2026.emnlp-main.2622/
DOI:
Bibkey:
Cite (ACL):
Jongyeop Hyun, Taeyoung Kim, and Hyounghun Kim. 2026. Controllable Image Captioning with Prompt-Conditioned Scene Rewards. In Proceedings of the 2026 Conference on Empirical Methods in Natural Language Processing, pages 59590–59615, Budapest, Hungary. Association for Computational Linguistics.
Cite (Informal):
Controllable Image Captioning with Prompt-Conditioned Scene Rewards (Hyun et al., EMNLP 2026)
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PDF:
https://aclanthology.org/2026.emnlp-main.2622.pdf