  Domain Generalization (DG) endeavors to create machine learning models that
excel in unseen scenarios by learning invariant features. In DG, the prevalent
practice of constraining models to a fixed structure or uniform
parameterization to encapsulate invariant features can inadvertently blend
specific aspects. Such an approach struggles with nuanced differentiation of
inter-domain variations and may exhibit bias towards certain domains, hindering
the precise learning of domain-invariant features. Recognizing this, we
introduce a novel method designed to supplement the model with domain-level and
task-specific characteristics. This approach aims to guide the model in more
effectively separating invariant features from specific characteristics,
thereby boosting the generalization. Building on the emerging trend of visual
prompts in the DG paradigm, our work introduces the novel \textbf{H}ierarchical
\textbf{C}ontrastive \textbf{V}isual \textbf{P}rompt (HCVP) methodology. This
represents a significant advancement in the field, setting itself apart with a
unique generative approach to prompts, alongside an explicit model structure
and specialized loss functions. Differing from traditional visual prompts that
are often shared across entire datasets, HCVP utilizes a hierarchical prompt
generation network enhanced by prompt contrastive learning. These generative
prompts are instance-dependent, catering to the unique characteristics inherent
to different domains and tasks. Additionally, we devise a prompt modulation
network that serves as a bridge, effectively incorporating the generated visual
prompts into the vision transformer backbone. Experiments conducted on five DG
datasets demonstrate the effectiveness of HCVP, outperforming both established
DG algorithms and adaptation protocols.
