On the Test-Time Zero-Shot Generalization of Vision-Language Models: Do We Really Need Prompt Learning?

The development of large vision-language models notably CLIP has catalyzed research into effective adaptation techniques with a particular focus on soft prompt tuning. Conjointly test-time augmentation which utilizes multiple augmented views of a single image to enhance zero-shot generalization is emerging as a significant area of interest. This has predominantly directed research efforts towards test-time prompt tuning. In contrast we introduce a robust MeanShift for Test-time Augmentation (MTA) which surpasses prompt-based methods without requiring this intensive training procedure. This positions MTA as an ideal solution for both standalone and API-based applications. Additionally our method does not rely on ad hoc rules (e.g. confidence threshold) used in some previous test-time augmentation techniques to filter the augmented views. Instead MTA incorporates a quality assessment variable for each view directly into its optimization process termed as the inlierness score. This score is jointly optimized with a density mode seeking process leading to an efficient training- and hyperparameter-free approach. We extensively benchmark our method on 15 datasets and demonstrate MTA's superiority and computational efficiency. Deployed easily as plug-and-play module on top of zero-shot models and state-of-the-art few-shot methods MTA shows systematic and consistent improvements.