Fine-grained Out-of-Distribution Detection with Mixup Outlier Exposure

Enabling out-of-distribution (OOD) detection for DNNs is critical for their safe and reliable operation in the "open world". Unfortunately, current works in both methodology and evaluation focus on rather contrived detection problems, and only consider a coarse level of granularity w.r.t.: 1) the in-distribution (ID) classes, and 2) the OOD data's "closeness" to the ID data. We posit that such settings may be poor approximations of many real-world tasks that are naturally fine-grained (e.g., bird species classification), and thus the reported detection abilities may be over-estimates. Differently, in this work we make granularity a top priority and focus on fine-grained OOD detection. We start by carefully constructing five novel fine-grained test environments in which existing methods are shown to have difficulties. We then propose a new DNN training algorithm, Mixup Outlier Exposure (MixupOE), which leverages an outlier distribution and principles from vicinal risk minimization. Finally, we perform extensive experiments and analyses in our custom test environments and demonstrate that MixupOE can consistently improve fine-grained detection performance, establishing a strong baseline in these more realistic and challenging OOD detection settings.