Unsupervised out-of-distribution detection using kernel density estimation

Deep neural networks achieve significant advancement to the state-of-the-art in many computer vision tasks. However, accuracy of the networks may drop drastically when test data come from a different distribution than training data. Therefore, detecting out-of-distribution (OOD) examples in neural networks arises as a crucial problem. Although, majority of the existing methods focuses on OOD detection in classification networks, the problem exist for any type of networks. In this paper, we propose an unsupervised OOD detection method that can work with both classification and non-classification networks by using kernel density estimation (KDE). The proposed method estimates probability density functions (pdfs) of activations at various levels of the network by performing KDE on the in-distribution dataset. At test time, the pdfs are evaluated on the test data to obtain a confidence score for each layer which are expected to be higher for in-distribution and lower for OOD. The scores are combined into a final score using logistic regression. We perform experiments on 2 different classification networks trained on CIFAR-10 and CIFAR-100, and on a segmentation network trained on Pascal VOC datasets. In CIFAR-10, our method achieves better results than the other methods in 4 of 6 OOD datasets while being the second best in the remaining ones. In CIFAR-100, we obtain the best results in 2 and the second best in 3 OOD datasets. In the segmentation network, we achieve the highest scores according to most of the evaluation metrics among all other OOD detection methods. The results demonstrate that the proposed method achieves competitive results to the state-of-the-art in classification networks and leads to improvement on segmentation network.