LLM-based query paraphrasing for video search

Text-to-video retrieval answers user queries through search by concepts and embeddings. Limited by the size of the concept bank and the amount of training data, answering queries in the wild is not always effective due to the out-of-vocabulary problem. Furthermore, neither concept-based nor embedding-based search can perform reasoning to consolidate the search results for complex queries mixed with logical and spatial constraints. To address these problems, we leverage large language models (LLM) to paraphrase the query by text-to-text (T2T), text-to-image (T2I), and image-to-text (I2T) transformations. These transformations rephrase abstract concepts into simple words to address the out-of-vocabulary problem. Furthermore, the complex relationship in a query can be decoupled into simpler sub-queries, yielding better retrieval performance when fusing the search results of these sub-queries. To address the LLM hallucination problem, this paper also proposes a novel consistency-based verification strategy to filter the paraphrased queries that are factually incorrect. Extensive experiments are conducted for ad-hoc video search and known-item search on the TRECVid datasets. We provide empirical insights into how traditionally difficult-to-answer queries can be resolved by query paraphrasing.