Detecting Out-of-Distribution Through the Lens of Neural Collapse

Out-of-distribution (OOD) detection is essential for the safe deployment of AI. Particularly, OOD detectors should generalize effectively across diverse scenarios. To improve upon the generalizability of existing OOD detectors, we introduce a highly versatile OOD detector, called Neural Collapse inspired OOD detector (NC-OOD). We extend the prevalent observation that in-distribution (ID) features tend to form clusters, whereas OOD features are far away. Particularly, based on the recent observation, Neural Collapse, we further demonstrate that ID features tend to cluster in proximity to weight vectors. From our extended observation, we propose to detect OOD based on feature proximity to weight vectors. To further rule out OOD samples, we leverage the observation that OOD features tend to reside closer to the origin than ID features. Extensive experiments show that our approach enhances the generalizability of existing work and can consistently achieve state-of-the-art OOD detection performance across a wide range of OOD Benchmarks over different classification tasks, training losses, and model architectures. Machine learning models deployed in practice will inevitably encounter samples that deviate from the training distribution. As a classifier cannot make meaningful predictions on test samples that belong to unseen classes during training, it is important to actively detect and handle Out-of-Distribution (OOD) samples. Considering the diverse application scenarios, an effective OOD detector should generalize across classification tasks of different input resolutions, number of classes, classification accuracy, as well as classifiers under different training schemes and architectures. Since Nguyen et al. (2015) reveals that neural networks tend to be over-confident on OOD samples, an extensive body of research has been focused on developing effective OOD detection algorithms.