Modern applications from Autonomous Vehicles to Video Surveillance generate massive amounts of image data. In this work we propose a novel image outlier detection approach (IOD for short) that leverages the cutting-edge image classifier to discover outliers without using any labeled outlier. We observe that although intuitively the confidence that a convolutional neural network (CNN) has that an image belongs to a particular class could serve as outlierness measure to each image, directly applying this confidence to detect outlier does not work well. This is because CNN often has high confidence on an outlier image that does not belong to any target class due to its generalization ability that ensures the high accuracy in classification. To solve this issue, we propose a Deep Neural Forest-based approach that harmonizes the contradictory requirements of accurately classifying images and correctly detecting the outlier images. Our experiments using several benchmark image datasets including MNIST, CIFAR-10, CIFAR-100, and SVHN demonstrate the effectiveness of our IOD approach for outlier detection, capturing more than 90% of outliers generated by injecting one image dataset into another, while still preserving the classification accuracy of the multi-class classification problem.
Pros
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[Originality/Clarity]
The manuscript presents a novel technique for outlier detection in a supervised learning setting where something is considered an outlier if it is not a member of any of the "known" classes in the supervised learning problem at hand. The proposed solution builds upon an existing technique (deep neural forests). The authors clearly explain the enhancements proposed and the manuscript is quite easy to follow.
[Clarity/Significance]
The enhancements proposed are empirically evaluated in a manner that clearly shows the impact of the proposed schemes over the existing technique. For the data sets considered, the proposed schemes have demonstrated significant improvements for this scoped version of outlier detection.
[Significance]
The proposed scheme for improving the performance of the ensemble of the neural decision trees could be of independent interest in the supervised learning setting.
Limitations
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[Significance]
Based on my familiarity with the traditional literature on outlier detection in an unsupervised setting, it would be helpful for me to have some motivation for this problem of outlier detection in a supervised setting. For example, the authors mention that this outlier detection problem might allow us to identify images which are incorrectly labelled as one of the "known" classes even though the image is not a true member of any of the known classes, and might subsequently require (manual) inspection. However, if this technique would actually be used in such a scenario, the parameters of the empirical evaluation, such as a threshold for outliers that considers 5000 images as outliers, seem unreasonable. Usually number of outliers (intended for manual inspection) are fairly low. Empirical evaluations with a smaller number of outliers is more meaningful and representative of a real application in my opinion.
[Significance]
Another somewhat related question I have is the applicability of this proposed outlier detection scheme in the unsupervised scheme where there are no labels and no classification task in the first place. Is the proposed scheme narrowly scoped to the supervised setting?
[Comments on empirical evaluations]
- While the proposed schemes of novel inlier-ness score (weighted sum vs. max route), novel regularization scheme and ensemble of less correlated neural decision trees are extremely interesting and do show great improvements over the considered existing schemes, it is not clear to me why the use of something like Isolation Forest (or other more traditional unsupervised outlier detection schemes such as nearest/farthest neighbour based) on the learned representations just before the softmax is not sufficient. This way, the classification performance of the network remains the same and the outlier detection is performed on the learned features (since the learned features are assumed to be a better representation of the images than the raw image features). The current results do not completely convince me that the proposed involved scheme is absolutely necessary for the considered task of outlier detection in a supervised setting.
- [minor] Along these lines, considering existing simple baselines such as auto-encoder based outlier detection should be considered to demonstrate the true utility of the proposed scheme. Reconstruction error is a fairly useful notion of outlier-ness. I acknowledge that I have considered the authors' argument that auto-encoders were formulated for dimensionality reduction.
[Minor questions]
- In Equation 10, it is not clear to me why (x,y) \in \mathcal{T}. I thought \mathcal{T} is the set of trees and (x,y) was the sample-label pair. 
- It would be good understand if this proposed scheme is limited to the multiclass classification problem or is it also applicable to the multilabel classification problem (where each sample can have multiple labels).