Title: Closing the Modality Gap for Mixed Modality Search
Primary area: unsupervised, self-supervised, semi-supervised, and supervised representation learning
Keywords: mixed modality search, modality gap, CLIP, VLM

Abstract: Mixed modality search, retrieving information across a heterogeneous corpus composed of images, texts, and multimodal documents, is an important yet underexplored real-world application. In this work, we investigate how contrastive vision-language models, such as CLIP, perform on the mixed modality search task. Our analysis reveals a critical limitation: these models exhibit a pronounced modality gap in the embedding space, where image and text embeddings form distinct clusters, leading to intra-modal ranking bias and inter-modal fusion failure. To address this issue, we propose GR-CLIP, a lightweight post-hoc calibration method that removes the modality gap in CLIP’s embedding space. Evaluated on MixBench, the first benchmark specifically designed for mixed modality search, GR-CLIP improves NDCG@10 by up to 26% over CLIP, surpasses recent vision-language generative embedding models by 4%, while using 75x less compute.