Title: Cross-granularity Video LLM for Long Video Understanding
Primary area: applications to computer vision, audio, language, and other modalities
Keywords: Video Large Language Model, Long Video Understanding

Abstract: Video Large Language Models (video LLMs) have demonstrated remarkable capabilities in video understanding tasks, such as video question answering and temporal localization. However, understanding long videos still remains a significant challenge. Existing video LLMs adopt uni-granularity tokens for long videos, failing to simultaneously understand both high-level semantics and low-level visual details in videos. To tackle the problem, we propose \textbf{CrossVLLM}, a video LLM framework with adaptive modules of different granularities to collaborate with each other for long video understanding, which not only retains the capabilities of high-level video semantics understanding, but also strengthens the fine-grained understanding abilities. Specifically, we propose the coarse-to-fine grounding and fine-to-coarse reflection strategies for long video understanding. In the coarse-to-fine grounding strategy, video LLM with a coarse-grained module first locates the key video segments from the long video by tackling massive frames of the long video with fewer per-frame tokens. And then video LLM adapted with the fine-grained module further analyzes the key video segments with more per-frame tokens so that it can understand fine-grained information. In case the video LLM locates the wrong key video segments, during the inference stage, our designed fine-to-coarse reflection strategy instructs the fine-grained module to reflect the effectiveness of the locating result and decide whether to return to the coarse-to-fine grounding strategy with reflection feedback. Additionally, during the training stage, the coarse-to-fine grounding strategy is optimized with our proposed cross-granularity reinforcement learning strategy to further improve grounding efficiency. Extensive experiments for long video question answering and temporal video grounding tasks demonstrate that our proposed CrossVLLM framework can significantly improve the Video Large Language Model for long video understanding.

TL;DR: A novel video LLM framework with cross-granularity inference and training method for long video understanding.