RefZVC: Refinable Zero-Shot Video Captioning by Test-Time Reinforcement Polishing

Recently, the zero-shot image captioning (zero-shot IC) method based on pre-trained visual language models (VLMs) and large language models (LLMs) has made significant progress. However, how to adapt it to the zero-shot video captioning (zero-shot VC) scenario (without video-text paired supervision) has not been well explored. Inspired by various recent test-time strategies (sacrificing additional test time to improve performance), we try to introduce a new paradigm of Test-time Reinforcement Polishing in zero-shot VC scenario. We take temporal dependency modeling as the starting point and propose a novel framework for Refinable Zero-shot VC, called RefZVC. RefZVC can greatly cover the long-term context of the video and continuously polish and refine the generated captions in a reward-feedback manner. We first design an Adaptive Frame Skipping module (AdaSkip) to skip redundant frames and select diverse keyframe sequences. Subsequently, we propose a Multi-granularity Reinforcement Polishing (MRP) mechanism, which iteratively polishes captions by leveraging Gaussian Kernel Cache (GKC) to capture temporal dynamics, store and reuse relevant historical context. In addition, MRP calculates rewards for generated captions at both the sentence-level and entity-level to achieve test-time polishing. With the MRP mechanism, RefZVC achieves superior zero-shot generalization performance, outperforming previous zero-shot VC methods on benchmarks such as MSVD, MSR-VTT, and VATEX.