Paper:
An important property of image classification systems in the real world is that they both accurately classify objects from target classes ("knowns") and safely reject unknown objects ("unknowns") that belong to classes not present in the training data. Unfortunately, although the strong generalization ability of existing CNNs ensures their accuracy when classifying known objects, it also causes them to often assign an unknown to a target class with high confidence. As a result, simply using low-confidence detections as a way to detect unknowns does not work well. In this work, we propose an Unknownaware Deep Neural Network (UDN for short) to solve this challenging problem. The key idea of UDN is to enhance existing CNNs to support a product operation that models the product relationship among the features produced by convolutional layers. This way, missing a single key feature of a target class will greatly reduce the probability of assigning an object to this class. UDN uses a learned ensemble of these product operations, which allows it to balance the contradictory requirements of accurately classifying known objects and correctly rejecting unknowns. To further improve the performance of UDN at detecting unknowns, we propose an information-theoretic regularization strategy that incorporates the objective of rejecting unknowns into the learning process of UDN. We experiment on benchmark image datasets including MNIST, CIFAR-10, CIFAR-100, and SVHN, adding unknowns by injecting one dataset into another. Our results demonstrate that UDN consistently outperforms state-of-the-art methods at rejecting unknowns -up to 20 point gains in accuracy, while still preserving the classification accuracy.

Review:
This paper is about a novel method to detect unknown samples which are of a different class than the trained ones. The idea is to use an output subnet which use a fully connected layer and a binary tree which encode the product relationship instead of the sum currently used in state of the art method (particularly the softmax with low confidence). The binary tree is made of split nodes which are responsible to produce a probability distribution from the root to each leaf. The max path i.e. the path with the largest probabilities determines the class of the input and can be used to measure how confident the classifier is about the classification decision. Combine multiple subnets and the tool obtained is able to do complex predictions and maintain a good generalization performance. The method also uses an information theory based regularization which decrease the probability of having subnets with uniform probability distribution i.e. a large entropy. Experiments on CIFAR-10 and MNIST against CIFAR-100, SVHN show that the method has an improved rejection accuracy while maintaining a good classification accuracy on the test set.
The topic of the paper is interesting and the approach seems to be solid, however the experiments are not so convincing. They are limited on two very easy datasets and do not show if the method is able to scale when more difficult and more realistic amount of classes are considered (like in CIFAR-100, SVHN or maybe ImageNet). Given that a simple dataset like these require 9 hour of training, it is also not clear how much the method is able to scale computationally and if it is applicable realistically. Moreover the presentation could be improved as figure 1 and its section 2 are complex and not easy to follow at several points. Hence, I’m leaning towards rejecting this paper.
In particular:
- It would be interesting to see experiments where the number of classes is higher than the ones in MNIST and CIFAR-10. CIFAR-100 and SVHN would be a good testbed for such case.
- How is the complexity of the method and how does it scale with the number of training classes?
- It is stated that the method bring a 25% percentage points in the accuracy of unknown rejection detection, however table 1 shows a large improvement only in the case of SVHN. Hence the claims seems a bit off compared to the measured data. Moreover, using only CIFAR-10 is insufficient to back up the claim.
- Why the related work section is at the end of the paper? It confuses the reader and would be more useful to be put after the introduction section.