N OVELTY DETECTION VIA BLURRING
A BSTRACT
Conventional out-of-distribution (OOD) detection schemes based on variational autoencoder or Random Network Distillation (RND) have been observed to assign lower uncertainty to the OOD than the target distribution.
In this work, we discover that such conventional novelty detection schemes are also vulnerable to the blurred images.
Based on the observation, we construct a novel RND-based OOD detector,
SVD-RND, that utilizes blurred images during training.
Our detector is simple, efﬁcient at test time and outperforms baseline OOD detectors in various domains.
Further results show that SVD-RND learns better target distribution representation than the baseline RND algorithm.
Finally, SVD-RND combined with geometric transform achieves near-perfect detection accuracy on the CelebA dataset
I NTRODUCTION
Out-of distribution (OOD), or novelty detection aims to distinguish samples in unseen distribution from the training distribution.
A majority of novelty detection methods focus on noise ﬁltering or representation learning.
For example, we train an autoencoder to learn a mapping from the data to the bottleneck layer and use the bottleneck representation or reconstruction error to detect an OOD ( ?? ).
Recently, deep generative models ( ????? ) are widely used for novelty detection due to their ability to model high dimensional data.
However, OOD detection performance of deep generative models has been called into question since they have been observed to assign a higher likelihood to the OOD data than the training data ( ?? ).
On the other hand, adversarial examples are widely employed to fool the classiﬁer, and training classiﬁers against adversarial attacks has shown effectiveness in detecting unknown adversarial attacks ( ? ).
In this work, we propose blurred data as the adversarial example.
When we test novelty detection models on the blurred data generated by Singular Value Decomposition (SVD), we found that the novelty detection models assign higher conﬁdence to the blurred data than the original data.
Motivated by this observation, we employ blurring to prevent the OOD detector from overﬁtting to low resolution.
We propose a new OOD detection model, SVD-RND, which is trained using the idea of Random Network Distillation (RND) ( ? ) to discriminate the training data from the blurred image.
SVD-RND is evaluated in the hard target to OOD domain where vanilla generative models show nearly 50% detection accuracy, such as CIFAR-10 to SVHN and ImageNet to CIFAR-10 ( ? ).
Compared to conventional baselines, SVD-RND shows a signiﬁcant performance gain from 50% to over 90% in these domains.
Moreover, SVD-RND shows improvements over baselines on domains where conventional OOD detection schemes show moderate results, such as CIFAR-10 to LSUN.
P ROBLEM F ORMULATION AND R ELATED W ORK
The goal of OOD detection is to determine whether the data is sampled from the target distribution
D .
Therefore, based on the training data D train ⊂ D , we train a scalar function that expresses the conﬁdence, or uncertainty of the data.
The performance of the OOD detector is tested on the D test ⊂
D against the OOD dataset D OOD .
We denote a target and OOD pair as target : OOD in this paper, e.g., CIFAR-10 : SVHN.
In this section, we mention only closely related ﬁelds to our research.
For a broader survey on deep
OOD detection, we recommend the paper from ? .
OOD Detection: A majority of OOD detection methods rely on a reconstruction error and representation learning. ?
trained a deep neural network to map data into a minimum volume hypersphere.
Generative probabilistic novelty detection (GPND) ( ? ) employed the distance to the latent data manifold as the conﬁdence measure and trained the adversarial autoencoder (AAE) to model the manifold.
Deep generative models are widely employed for latent space modeling in OOD detection ( ?? ).
However, a recent paper by ? discovered that popular deep generative models, such as variational autoencoder (VAE) ( ? ) or GLOW ( ? ), fail to detect simple OOD from the training distribution.
While adversarially trained generative models, such as generative adversarial networks (GAN) ( ? ) or AAE, are not discussed in ? , our experiments in GPND show that such models can also struggle to detect such simple OODs.
OOD Detection with Additional Data: Some methods try to solve OOD detection by appending additional data or labels for training. ?
trained the detector to additional outlier data independent of the test OOD data. ?
employed semi-supervised learning for anomaly detection in the scenario where we have ground truth information on few training data. ?
designed geometrically transformed data and trained the classiﬁer to distinguish geometric transforms, such as translation, ﬂipping, and rotation. ?
ﬁne-tuned the image classiﬁer to predict word embedding.
However, the intuition behind these methods is to beneﬁt from potential side information, while our self-generated blurred image focuses on compensating the deep model’s vulnerability to OOD data with a lower effective rank.
Adversarial Examples and OOD Detection on Labeled Data: Some methods combine OOD detection with classiﬁcation, resulting in OOD detection on labeled data.
Adversarial examples can be viewed as generated OOD data that attacks the conﬁdence of a pretrained classiﬁer.
Therefore, two ﬁelds share similar methodologies.
For example, ? set the conﬁdence as the maximum value of the probability output, which is vulnerable against the adversarial examples generated by the Fast Sign Gradient Method (FSGM) ( ? ).
On the other hand, ? employed FSGM counterintuitively to shift the OOD data from the target further, therefore improving OOD detection. ?
employed Mahalanobis distance to measure uncertainty in the hidden features of the network, which also proved efﬁcient in adversarial defense.
Bayesian Uncertainty Calibration: Bayesian formulation is widely applied for better calibration of the model uncertainty.
Recent works employed bayesian neural networks ( ? ) or interpreted a neural network’s architecture in the bayesian formulation, such as dropout ( ? ), and Adam optimizer ( ? ).
Our baseline, RND ( ? ), can be viewed as the bayesian uncertainty of the model weight under randomly initialized prior ( ? ).
M ETHODOLOGY
In this section, we motivate our use of blurred data as adversarial examples to conventional deep OOD detection methods.
Motivatied by the observation, we present our proposed algorithm, SVD-RND, and provide intuitions why SVD-RND help OOD detection.
3. G ENERATING B LURRED D ATA
In this work, blurred images function as adversarial examples.
We directly employ the SVD on the data matrix of the single image in the training data and force the bottom non-zero singular values to zero to construct a blurred image.
Suppose that data image d ∈ D consists of i channels, where the j -th channel has N j nonzero singular values σ j 1 ≥ σ j 2 ≥ . . .
σ jN j > 0 .
Then, the j -th channel can be represented as the weighted sum of orthonormal vectors.
[Equation]
We prune the bottom K non-zero singular values of each channel to reconstruct the blurred image.
We test conventional novelty detection methods on blurred images.
We ﬁrst train the VQ-VAE ?
in the CIFAR-10 ( ? ) dataset. Figure ??
shows the loss of VQ-VAE on the test data and blurred test data ( K = 28 ).
We follow the settings of the original paper.
VQ-VAE assigns higher likelihood to the blurred data than the original data.
[Figure]
[Figure]
We note that this phenomenon is not constrained to the generative models.
We trained the RND on the CIFAR-10 dataset and plot the l 2 loss in the test data and blurred test data in Figure ?? . We refer
Appendix ?? for detailed explanation and employed architecture for the RND in the experiment.
We plot the l 2 loss on SVHN ( ? ) data for relevance.
Multiple skip connections in residual blocks don’t resolve the information leakage on their own.
Furthermore, we plot the average loss on the blurred test data and original test data during the training procedure.
Throughout the training phase, the model assigns lower uncertainty to the blurred data.
This trend is similar to the CIFAR-10 : SVHN phenomenon observed by ? , where the generative model assigns more conﬁdence to the OOD data from the beginning.
While we employ SVD for our main blurring technique, conventional techniques in image processing can be applied for blurring, such as Discrete Cosine Transform (DCT) or Gaussian Blurring.
However,
DCT squares the size of the hyperparameter search space, therefore much harder to optimize than
SVD.
We further compare the performance between SVD and other blurring techniques in Section ?? .
3. OOD D ETECTION V IA SVD-RND
We now present our proposed algorithm, SVD-RND.
SVD-RND trains the predictor network f to discriminate between original and blurred datasets.
We ﬁrst generate blurred datasets D K i from
D train by zeroing the bottom K i non-zero singular values of each data channel ( i = 1 , . . .
, b train , where b train is the number of generated blurred datasets used for training).
Then, we assign a different randomly initialized target network g i to each D K i .
Finally, we assign g 0 as the target network for the original dataset.
Predictor network f is trained to minimize the l 2 loss against the corresponding target network on each dataset.
We do not update the target network during the training procedure.
[Equation]
[Table] When the new sample x is given, SVD-RND outputs (cid:107) f ( x ) − g 0 ( x ) (cid:107) 22 as the uncertainty to the sample.
Figure ?? shows the training process of SVD-RND.
While the original RND paper employs a single target network to train the predictor network, SVD-RND employs multiple target networks to discriminate the original data from the blurred images.
No other regularization techniques or explicit metrics are employed in SVD-RND.
While SVD-RND directly regularizes only on the blurred images, we expect such regularization generally improve OOD detection for the following two reasons.
First, while RND fails on OODs generated by blurring, it performs moderately on OODs generated by the orthogonal direction to the dataset.
For the evidence, we show in Appendix ?? on CIFAR-10 dataset that RND is able to detect
OODs generated by adding noise orthogonal to the data.
RND outputs higher uncertainty to every
OOD dataset generated from 20 independent runs.
Second, Equation ?? forces the predictor network f to output g 0 ( x ) for the original data x ∈ D train , and g i ( x ) for the blurred data x ∈ D K i .
Therefore, f naturally learns to discriminate between the data and its low-rank projection.
From such regularization, we expect f to learn the target distributionspeciﬁc information from the projection vector, which is previously neglected in
conventional deep
OOD detection methods.
We will verify our reasoning in further experiments.
E XPERIMENTAL R ESULTS
In this section, we examine the performance of SVD-RND on the target : OOD domain, where deep generative models have been observed to assign a higher likelihood to the OOD data.
4. E XPERIMENT S ETTING
SVD [Equation] RND is examined in the cases in Table ?? .
CIFAR-10 :
SVHN, CelebA ( ? ) : SVHN, and
TinyImageNet ( ? ) : (SVHN, CIFAR-10, CIFAR-100) are the cases reported by ? .
We expect SVDRND outperform conventional OOD detection methods by a large margin.
We also studied CIFAR- :
(LSUN ( ? ), TinyImageNet), LSUN : (SVHN, CIFAR-10, CIFAR-100) and CelebA: (CIFAR-10, CIFAR-100) target : OOD pairs to examine potential tradeoffs of our method.
We implement the baselines and SVD-RND in the PyTorch framework.
1 For a uniﬁed treatment, we resized all images in all datasets to 32 × 32.
We refer to a detailed setting in Appendix ?? .
For SVD-RND, we optimize the number of blurred non-zero singular values over different datasets.
We choose the detector with the best performance across the validation data.
We refer to all the parameter settings in Appendix ?? . We also examine the case where the image is blurred by DCT and Gaussian blurring.
For DCT, we apply the DCT to the image, discard low magnitude signals, and generate the blurred image by inverse DCT.
In DCT-RND, we optimize the number of unpruned signals in the frequency domain.
For gaussian blurring, we optimize the shape of the Gaussian kernel.
We denote this method as GB-RND.
We compare the performance of SVD-RND, DCT-RND, and GB-RND to the following baselines.
Generative Probabilistic Novelty Detector : GPND ( ? ) is the conventional generative model based novelty detection method that models uncertainty as a deviation of data to the latent representation, which is modeled by the adversarial autoencoder.
We trained GPND with further parameter optimization.
[Table]
[Figure] Discarded Nonzero Singular Values [Figure] Geometric Transforms : We compare the effectiveness of the blurred image against geometric transforms ( ? ).
The authors use four types of geometric transforms: ﬂip, rotation, vertical translation, and horizontal translation.
We compute the independent effects of each transformation by setting them as OOD proxies in the RND framework.
Moreover, we also investigate the effect of pixel inversion, contrast reduction, and shearing.
We refer ?
for detailed explanation of the augmentation strategies.
RND :
We employ RND ( ? ) to show the effectiveness of our regularizer directly.
Typicality Test : ? set the OOD metric of the generative model as the distance between the mean log likelihood of the model on the training data and the log likelihood of the model on the incoming data.
We experiment typicality test on the RND framework by employing the test error of RND as the role of negative log likelihood in the generative models.
Five metrics on binary hypothesis testing are used to evaluate the OOD detectors: area under the
Receiver Operating Characteristic curve (AUROC), area of the region under the Precision-Recall curve (AUPR), detection accuracy, and TNR (True negative rate) at 95 % TPR (True positive rate).
All criterions are bounded between 0 and 1 , and the result close to 1 implies better OOD detection.
4. OOD D ETECTION R ESULTS
We summarize our results on the TNR in 95 % TPR in Table ?? .
We refer to full results in appendix ?? .
In all target : OOD domains except the CelebA : (CIFAR-10, CIFAR-100) domain, SVD-RND outperforms all other baselines in every metric.
Furthermore, all the proposed techniques outperform
[Figure] GPND and RND on all target : OOD domains, especially in CIFAR-10 : (LSUN, TinyImageNet) domains and CelebA: (CIFAR-10, CIFAR-100) domains where even GPND and RND show moderate results.
For better understanding, we visualize the CIFAR-10 data before and after blurring in
Appendix ?? . We plot the performance of SVD-RND in 50 epochs over different K 1 in Figure ?? .
We increase the number of seeds to 4 to check the stability of our result.
In the best performing parameter for each OOD data, SVD-RND shows narrow conﬁdence intervals.
Furthermore, we extend the Figure ??
to small K 1 for comparison.
We refer the results in Appendix ?? .
Furthermore, we plot the output of SVD-RND to target CIFAR-10 data and OOD SVHN data in
K 1 = 28 .
SVD-RND further separates SVHN data compared to Figure ?? .
Also, we compare the test uncertainty of SVD-RND against the test uncertainty of the RND on each (CIFAR-10, SVHN, LSUN,
TinyImageNet) data.
For SVD-RND, test-loss of each (CIFAR-10, SVHN, LSUN, TinyImageNet) data increases (150 , 38400 , 1516 ,
2102)% over its test loss of RND.
Therefore, SVD-RND further discriminates OOD from the target distribution.
GPND and RND fail to discriminate OOD from the targets in CIFAR-10 :
SVHN, LSUN : (SVHN,
CIFAR-10, CIFAR-100), TinyImageNet : (SVHN, CIFAR-10, CIFAR-100), and CelebA : SVHN domains.
Moreover, GPND performs the SVD of the jacobian matrix in test time, which makes
GPND slower than SVD-RND.
Furthermore, we visualize the uncertainty prediction of RND and
SVD-RND.
Figure ?? shows the top-9 examples on CIFAR-10 test data, where SVD-RND and RND assign the highest uncertainty.
We observe that SVD-RND tends to assign higher uncertainty to blurry or hardly recognizable image compared to RND.
On the other hand, OOD detection schemes based on geometric transformations ( ? ) show generally improved results against GPND and RND on detecting OOD data compared to RND and GPND.
Especially in CelebA : (SVHN, CIFAR-10, CIFAR-100) domain, rotation and translation based methods show prominent performance.
However, in the CIFAR-10 target domain, OOD detection schemes based on geometric transformations show degraded performance against RND or GPND on
LSUN and TinyImageNet OOD data.
Furthermore, typicality test shows mixed results compared to the baseline algorithms.
Finally, we also investigate the case where limited training data is available.
We examined the performance of SVD-RND and RND in CIFAR-10 : (LSUN, TinyImageNet) domains.
Figure ?? shows the TNR at 95% TPR metric of each method when the number of training data is reduced.
For each OOD data, we denote result on SVD-RND as OOD SVD, and denote result on RND as
OOD RND.
Compared to RND, SVD-RND shows consistent performance when only 20% of training data is available.
Furthermore, SVD-RND outperforms RND until 16% of training data is available.
[Table]
F URTHER A NALYSIS
In this section
[Table]
we further analyze and apply SVD-RND for speciﬁc scenarios.
In Section ??
[Table]
we examine whether SVD-RND learns better representation compared to the baseline.
Furthermore, we propose a novel heuristic for training SVD-RND in Section ??
[Table]
where no validation OOD data is available.
Finally, we show that we can further improve the performance of SVD-RND by incorporating geometric transformations.
5. R EPRESENTATION LEARNING FROM SVD-RND
While SVD-RND outperforms RND on every target : OOD domains in Section ??
[Table]
we provide further evidence that SVD-RND learns superior target distribution representation compared to RND.
For the evidence, we ﬁne-tune a classiﬁer over the ﬁxed activation map of SVD-RND and RND.
We set the activation map as the output of the ﬁrst 15 or 27 layers of RND and SVD-RND predictor network trained in CIFAR-10 datasets.
For the ﬁne-tuning, we either appended three residual blocks and a linear output layer with softmax activation (denoted as 7-layer in Table ?? ) or a linear layer (denoted as linear in Table ?? ).
Then, we ﬁne-tune the appended network for the CIFAR-10 classiﬁcation task.
The SGD optimizer with learning rate 0.1 is used for ﬁne-tuning, and the learning rate is annealed to 0.01 and 0.001 after 30 and 60 epochs over 100 epochs of training, respectively.
We average the result across three ﬁxed random seeds.
We show our results in Table ?? .
SVD-RND consistently outperforms RND and the randomly initialized network on the ﬁne-tuning task.
Therefore, the result supports that SVD-RND learns better target distribution-speciﬁc knowledge.
Surprisingly, when we ﬁne-tune over 7-layer neural network,
RND consistently underperforms over randomly initialized weights.
5. L OG E FFECTIVE R ANK C RITERION IN SVD-RND IN ZERO OOD VALIDATION DATA
In our main experiments in Section ?? , we used the OOD validation data for tuning the novelty detection methods.
However, in realistic scenarios, OOD data are generally unknown to the detector.
We propose an effective rank ( ? ) based design of SVD-RND that does not use the OOD validation dataset and compare its performance against the results in Section ?? .
Log effective rank of the single image matrix D is deﬁned as the entropy of the normalized singular values ( σ 1 , . .
, σ N ) of the image matrix.
[Equation]
In equation ?? , H is deﬁned as the entropy function.
Then, effective rank is set to two to the power of log effective rank.
We set the effective rank of image data as the averaged effective rank of each channel.
In SVD-RND, selecting each K 1 , . . .
, K b train corresponds to regularization against OOD with similar effective rank.
We propose selecting each K i such that average of log effective rank on each blurred dataset is equally spaced to each other.
Speciﬁcally, suppose the log effective rank of the data averaged in training dataset D train is LER D train .
Then, we set the target log effective rank
LER 1 , LER 2 , . .
. , LER b train as follows.
[Equation]
[Table] [Table]
Then, we select K i such that the average of the log effective rank in the blurred dataset with K i discarded singular values is closest to LER i .
We test our criterion in CIFAR-10 and TinyImageNet data with different b train .
We train SVD-RND for 25 epochs for b train = 3 , and 20 epochs for b train = 4 .
We show the performance of SVD-RND based on uniform spacing of log effective rank in equation ?? in Table ?? , which is denoted as SVD-RND (uniform).
We also show results of SVD-RND optimized with the validation OOD data from Table ?? and denote them as SVDRND (optimized) in Table ?? .
Uniform SVD-RND already outperforms the second-best methods in Table ?? .
Furthermore, as b train increases, uniform SVD-RND approaches the performance of the optimized SVD-RND.
5. F URTHER IMPROVEMENT OF SVD-RND
While SVD-RND achieves reasonable OOD detection performance, combining SVD-RND with other baseline algorithms may further enhance the performance.
For example, as shown in Table ?? , training against rotated data beneﬁts OOD detection in CelebA dataset.
Therefore, we unify SVD-RND and geometric transform-based method to further improve SVD-RND.
We treat both blurred data and geometrically transformed data as OOD and train the target network to discriminate the original data from the OOD.
We combine rotation and vertical translation with SVD-RND and denote them as SVD-ROT-RND and SVD-VER-RND, respectively.
We compare the performance of SVD-ROT-RND and SVD-VER-RND against rotation and vertical translation in CelebA : (SVHN, CIFAR-10, CIFAR-100) domain.
We refer readers to the results in
Table ?? .
We observe that SVD-ROT-RND and SVD-VER-RND outperform their counterparts and
SVD-RND.
Especially, SVD-ROT-RND and SVD-VER-RND show signiﬁcant performance gain in
CelebA : (CIFAR-10, CIFAR-100) domains.
C ONCLUSION
In this work, a blurred image is introduced as an adversarial example to the deep OOD detection method.
SVD-RND is employed for adversarial defense against blurred images.
SVD-RND achieves signiﬁcant performance gain in all target : anomaly domains.
Even without the validation OOD data, we can design SVD-RND to outperform conventional OOD detection models.
We stress that such performance gain is achieved without external data or additional regularization techniques.
Furthermore, experiments on SVD-RND and RND show that the neural network can potentially learn to perform OOD detection, however overﬁts to blurred data.
Understanding this phenomenon will be beneﬁcial to performance of the image-based models.
A CKNOWLEDGEMENT
This work was supported by Samsung Electronics and the ICT R&D program of MSIP/IITP.
[20160-00563, Research on Adaptive Machine Learning Technology Development for Intelligent Autonomous Digital Companion]
A F ULL OOD DETECTION R ESULTS
[Table]
B R ANDOM N ETWORK D ISTILLATION
We use RND ( ? ) as the base model of our OOD detector.
RND consists of the trainable predictor network f , and randomly initialized target network g .
The predictor network is trained to minimize the l 2 distance against the target network on training data.
We do not update the target network g throughout the training phase.
[Equation]
Then, for the newly encountered data x , RND outputs (cid:107) f
( x ) − g ( x ) (cid:107) 22 as an uncertainty to the data.
The main intuition of the RND is to reduce the distance between f and g only on the target distribution, hence naturally threshold between the target and the OOD distribution.
We employ RND for our base OOD detector due to its simplicity over generative models.
Also, RND has already shown to be effective in novelty detection on MNIST dataset ( ? ).
In RND, f is generated by appending two fully connected layers in the network of g , where g consists of 3 convolution layers and a fully connected layer.
In our experiments, we set g as the ﬁrst 33 layers of ResNet34 without ReLU activation in the end.
f is constructed by appending two sequential residual blocks.
The output size of each residual block is 1024 and 512.
We also discard ReLU activation in the second residual block to match the form of g .
C D ATA PREPROCESSING , NETWORK SETTINGS , PARAMETER SETTINGS FOR MAIN EXPERIMENT
To make the OOD detection task harder, we reduce CelebA, TinyImageNet, and LSUN data into 50000 training data (for test dataset, we reduce the CelebA test data to 26032 examples).
For
TinyImageNet data, we discard half of the images in each class, resulting in 250 training samples for each 200 class.
Reduction in LSUN dataset results in 5000 data for each 10 class.
Also, the ﬁrst 1000 images of the test OOD data are used for validation.
For SVD-RND, and all other RND based detectors, we use the same structure for f and g deﬁned in Appendix ?? .
The number of parameter updates is ﬁxed across the experiments.
The Adam optimizer, with a learning rate of 10 − 4 , is used for RND based OOD detection methods.
The learning rate is annealed to 10 − 5 in half of the training process.
For our main experiment, we average the result across two ﬁxed random seeds.
In SVD-RND, DCT-RND, and GB-RND, we used one blurred data for CIFAR-10 and CelebA dataset, and two blurred data for TinyImageNet and LSUN dataset.
For SVD-RND, We optimize across
K 1 ∈ { 18 , 20 , 22 , 24 , 25 , 26 , 27 , 28 } in the CIFAR-10 and CelebA datasets.
For TinyImageNet and LSUN datasets, we optimize over K 1 ∈ { 8 , 10 , 12 , 14 } and K 2 ∈ { 22 , 24 , 26 , 28 } .
In DCTRND, we deﬁne K i as the number of unpruned signals in the frequency domain.
For CIFARand CelebA datasets, we optimize K 1 across { 4 , 8 , 12 , 14 , 16 , 20 , 24 , 28 } .
For TinyImageNet and
LSUN datasets, we optimize over K 1 ∈ { 20 , 24 , 28 , 32 } and K 2 ∈ { 40 , 44 , 48 , 52 } .
For gaussian blurring, we optimize over the shape ( x i , y i ) of the Gaussian kernel.
We optimized the parameter over x i ∈ { 1 , 3 , 5 } , y i ∈ { 1 , 3 , 5 } for each blurred data.
To ﬁx the number of updates, we train
SVD-RND, DCT-RND, and GB-RND for 50 epochs in the CIFAR-10 and CelebA datasets, andepochs for the rest.
For GPND, the settings for the original paper are followed.
Furthermore, we optimize the reconstruction loss λ 1 and adversarial loss λ 2 for discriminator D z across λ 1 ∈ { 8 , 9 , 10 , 11 , 12 } and λ 2 ∈ { 1 , 2 , 3 } .
We choose the parameters with the best validation performance in 100 epochs,
For RND, we trained over 100 epochs.
For geometric transforms, we optimize the magnitude of the shift of shear, horizontal translation and vertical translation methods.
We optimize the magnitude of translation across { 4 , 8 , 12 , 16 } and choose the parameter with the best validation performance.
Detector is trained for (cid:100) 100 | T | +1 (cid:101) epochs, where | T | is the number of transformations.
The number of transformations is 1 in ﬂipping and invert, 2 for horizontal translation, vertical translation, and shear, and 3 for rotation and contrast.
Finally, for typicality test, we estimated the average test loss of the RND for 50000 training data as the mean.
Then, when every time a sample is given, we set the distance of the test loss of the sample
[Table]
to the mean as the OOD metric.
For fair comparison, we assumed that one sample is given in a test time.
D RND ON GENERATED OOD S BY INCREMENTING ORTHOGONAL VECTOR
In Section ?? , we proposed that data in the blurred direction is the main weakness of the conventional novelty detection methods.
For the evidence, we present the results on OODs generated by adding vectors orthogonal to the data.
Precisely, we sample a Gaussian vector z and compute the component of the random vector z orth,x that is orthogonal to the data x.
[Equation]
We scaled the l 2 norm of the orthogonal vector z orth,x on each data to be α % of the l 2 norm of the signal.
We plot the average uncertainty of RND on the original data, blurred data, and the perturbed data in Table ?? .
From the 20 independent runs on the perturbed data, we report the case with smallest test uncertainty in Table ?? .
We varied α from 5 to 20.
While blurring reduces average test uncertainty of RND, adding orthogonal vector to the data incerases the test uncertainty of RND.
[Figure]
Discarded Nonzero Singular Values
[Figure]
E V ISUALIZATION OF DIFFERENT BLURRING TECHNIQUES
For the visualization, we plot the CIFAR-10 image and blurred version processed by SVD-RND,
DCT-RND, and GB-RND in Figure ?? .
Images in the same column are processed with the same technique.
Furthermore, column (b), (d), (e) is the best performing parameter of SVD-RND, DCTRND, and GB-RND on SVHN OOD data.
Likewise, (c), (e), (f) is the best performing parameter of
SVD-RND, DCT-RND, and GB-RND on TinyImageNet
OOD data.
F E XTENDED VERSION OF F IGURE ??
We further extend Figure ?? to analyze the behavior of SVD-RND when small number of singular values are discarded.
Therefore, we experiment SVD-RND where K 1 = 0 , 5 , 10 , 15 and plot the result in Figure ?? .
When K 1 = 0 , this corresponds to training a target network to follow between g 0 and g 1 .
Therefore, we use the test metric for sample x as min( (cid:107) f
( x ) − g 0 ( x ) (cid:107) 22 , (cid:107) f
( x ) − g 1 ( x ) (cid:107) 22 ) .
We also set the test metric as (cid:13)(cid:13)(cid:13) f
( x ) − g 0 ( x )+ g 1 ( x )(cid:13)(cid:13)(cid:13)2 and this shows TNR(at 95% TPR) of
[Table]