Asymmetric image retrieval (AIR), which typically employs a compact model for the query side and a large model for the database server, has garnered significant attention in resource-constrained environments. While deep hashing methods have shown great potential in large-scale image retrieval, current attempts for the asymmetric image retrieval overlook the differences in quantization capabilities between query and gallery networks. In AIR, the conventional quantization scheme forces the outputs of small query models to approximate the discrete outputs of large models, imposing overly rigid and stringent constraints that severely limit the optimization of small query models. Furthermore, existing deep hashing methods for AIR necessitate labeled datasets from large models, which also limits their practical applicability. To this end, we reconsider the necessity of strict discretization in AIR and propose a novel asymmetric hashing method, named Deep Correlation Alignment Hashing (DCAH). Rather than explicitly quantizing continuous query features to match discrete gallery representations, we distill the correlation across both models and introduce a Correlation Alignment based Quantization (CAQ) scheme, thereby implicitly accomplishing quantization. To preserve the similarity consistency between the query and gallery models, we further employ a correlation alignment-based knowledge distillation strategy which is intrinsically compatible with the CAQ. Notably, the proposed quantization scheme can function as a plug-and-play module that seamlessly integrates with existing AIR methods. Comprehensive evaluations on three real-world benchmark datasets demonstrate the effectiveness of the proposed quantization scheme CAQ, and also show that DCAH achieves state-of-the-art performance in asymmetric image retrieval scenarios.

Graph prompt learning (GPL) is designed to bridge the gap between graph pretraining models and downstream graph tasks, providing advantages in terms of graph knowledge transfer. However, GPL is vulnerable to poisoned graph attacks that induce abnormal training via adversarial malicious perturbations. We observe that the prevalent meta-gradient attacks, which heavily rely on the training of surrogate graph neural networks (GNNs), fail to account for the impact of perturbations on GPL where the pretrained GNN remains frozen and graph prompt tokens are tuned. Moreover, their gradient-assigned strategies tend to corrupt the topological semantics on a few influential labeled graphs, which in turn diminishes the trustworthiness of the surrogate training. To address this issue, we propose a dynamic equilibrium gradient-assigned attack against GPL, named MetaGpro. To guarantee the transferability of MetaGpro, the surrogate GPL is utilized in our simulation across various downstream tasks. To dynamically equilibrate the relationships between the reliability of surrogate models and instable structures, the over-robust contrastive learning is integrated into the surrogate training. In this way, the gradient bias caused by excessive perturbations of labeled nodes can be effectively mitigated. Subsequently, the topology perturbation generation is exploited to assign more gradient weights to nodes that are closer to the misclassification area. The experimental results reveal that the surrogate GPL outperforms the surrogate GNN in 96% of downstream evaluations, and our MetaGpro reduces the accuracy of GPL by 2%∼20% compared to the state-of-the-art (SOTA) works mostly. The code for our MetaGpro is available here.