Leveraging Perturbation Robustness to Enhance Out-of-Distribution Detection

Out-of-distribution (OOD) detection is the task of identifying inputs that deviate from the training data distribution. This capability is essential for the safe deployment of deep computer vision models in open-world environments. In this work, we propose a post-hoc method, Perturbation-Rectified OOD detection (PRO), based on the insight that prediction confidence for OOD inputs is more susceptible to reduction under perturbation than IND inputs. From this observation, we proposed a meta-score function that searches for local minimum scores near original inputs by applying gradient descent. This procedure enhances the separability between in-distribution (IND) and OOD samples. Importantly, the approach improves OOD detection performance without complex modifications to the underlying model architectures or training protocol. To validate our approach, we conduct extensive experiments using the OpenOOD benchmark. Our approach further pushes the limit of softmax-based OOD detection and is the leading post-hoc method for small-scale models. On a CIFAR-10 model with adversarial training, PRO effectively detects near-OOD inputs, achieving a reduction of more than 10% on FPR@95 compared to state-of-the-art methods.