Frequency-Based Comprehensive Prompt Learning for Vision-Language Models

This paper targets to learn multiple comprehensive text prompts that can describe the visual concepts from coarse to fine, thereby endowing pre-trained VLMs with better transfer ability to various downstream tasks. We focus on exploring this idea on transformer-based VLMs since this kind of architecture achieves more compelling performances than CNN-based ones. Unfortunately, unlike CNNs, the transformer-based visual encoder of pre-trained VLMs cannot naturally provide discriminative and representative local visual information. To solve this problem, we propose Frequency-based Comprehensive Prompt Learning (FCPrompt) to excavate representative local visual information from the redundant output features of the visual encoder. FCPrompt transforms these features into frequency domain via Discrete Cosine Transform (DCT). Taking the advantages of energy concentration and information orthogonality of DCT, we can obtain compact, informative and disentangled local visual information by leveraging specific frequency components of the transformed frequency features. To better fit with transformer architectures, FCPrompt further adopts and optimizes different text prompts to respectively align with the global and frequency-based local visual information via a dual-branch framework. Finally, the learned text prompts can thus describe the entire visual concepts from coarse to fine comprehensively. Extensive experiments indicate that FCPrompt achieves the state-of-the-art performances on various benchmarks.