Informed Democracy: Voting-based Novelty Detection for Action Recognition

10/30/2018
by   Alina Roitberg, et al.
2

Novelty detection is crucial for real-life applications. While it is common in activity recognition to assume a closed-set setting, i.e. test samples are always of training categories, this assumption is impractical in a real-world scenario. Test samples can be of various categories including those never seen before during training. Thus, being able to know what we know and what we do not know is decisive for the model to avoid what can be catastrophic consequences. We present in this work a novel approach for identifying samples of activity classes that are not previously seen by the classifier. Our model employs a voting-based scheme that leverages the estimated uncertainty of the individual classifiers in their predictions to measure the novelty of a new input sample. Furthermore, the voting is privileged to a subset of informed classifiers that can best estimate whether a sample is novel or not when it is classified to a certain known category. In a thorough evaluation on UCF-101 and HMDB-51, we show that our model consistently outperforms state-of-the-art in novelty detection. Additionally, by combining our model with off-the-shelf zero-shot learning (ZSL) approaches, our model leads to a significant improvement in action classification accuracy for the generalized ZSL setting.

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1 Introduction

Human activity recognition from video is a very active research field, with a long list of potential application domains, ranging from autonomous driving to security surveillance [Aggarwal and Ryoo(2011), Poppe(2010)]. However, the vast majority of published approaches are developed under the assumption that all categories are known a priori [Carreira and Zisserman(2017), Simonyan and Zisserman(2014), Wang et al.(2016)Wang, Xiong, Wang, Qiao, Lin, Tang, and Van Gool, Hara et al.(2018)Hara, Kataoka, and Satoh, Varol et al.(2017)Varol, Laptev, and Schmid, Ji et al.(2013)Ji, Xu, Yang, and Yu]. This closed set

constraint represents a significant bottleneck in the real world, where the system will probably encounter samples from various categories including those never seen during development. The set of possible actions is dynamic by its nature, possibly changing over time. Hence, collecting and maintaining large scale application-specific datasets of video data is especially costly and impractical. This raises a crucial need for the developed models to be able to identify cases where they are faced with samples out of their knowledge domain. In this work, we explore the field of activity recognition under

open set conditions [Scheirer et al.(2014)Scheirer, Jain, and Boult, Scheirer et al.(2013)Scheirer, de Rezende Rocha, Sapkota, and Boult, Busto and Gall(2017)], a setting which has been little-explored before especially in the action recognition domain [Moerland et al.(2016)Moerland, Chandarr, Rudinac, and Jonker].

In an open world application scenario, an action recognition model should be able to handle three different tasks: 1) the standard classification of previously seen categories; 2) knowledge transfer for generalization to new unseen classes (e.g. through zero-shot learning); 3) and knowing how to automatically discriminate between those two cases. The third component of an open set model lies in its ability to identify samples from unseen classes (novelty detection). This is closely linked to the classifier’s confidence in its own predictions, i.e. how can we build models, that know, what they do not know? A straight-forward way is to employ the Softmax

output of a neural network (NN) model as the basis for a rejection threshold 

[Richter and Roy(2017), Ramos et al.(2017)Ramos, Gehrig, Pinggera, Franke, and Rother]. Traditionally, action recognition algorithms focus on maximizing the top-1 performance on a static set of actions. Such optimization leads to Softmax scores of the winning class being strongly biased towards very high values [Nguyen et al.(2015)Nguyen, Yosinski, and Clune, Szegedy et al.(2013)Szegedy, Zaremba, Sutskever, Bruna, Erhan, Goodfellow, and Fergus, Gal and Ghahramani(2016), Kendall et al.(2017)Kendall, Gal, and Cipolla]

. While giving excellent results in closed set classification, such overly self-confident models become a burden under open set conditions. A better way to asses NN’s confidence, is to rather predict the probability distribution with Bayesian neural networks (BNN). Recently, Gal

et al [Gal and Ghahramani(2016)] introduced a way of efficiently approximating BNN modeled as a Gaussian Process [Rasmussen(2004)] and using dropout-based Monte-Carlo sampling (MC-Dropout) [Gal and Ghahramani(2016)]. We leverage the findings of [Gal and Ghahramani(2016)] and exploit the predictive uncertainty in order to identify activities of previously unseen classes.

This work aims at bringing conventional activity recognition to a setting where new categories might occur at any time and has the following main contributions: 1) We present a new model for novelty detection for action recognition based on the predictive uncertainty of the classifiers. Our main idea is to estimate the novelty of a new sample based on the uncertainty of a selected group of output classifiers in a voting-like manner. The choice of the voting classifiers depends on how confident they are in relation to the currently predicted class. 2) We adapt zero-shot action recognition models, which are conventionally applied solely on samples of the unseen classes, to the generalized case (i.e.  open set scenario) where a test sample may originate from either known or novel categories. We present a generic framework for generalized zero-shot action recognition, where our novelty detection model serves as a filter to distinguish between seen and novel categories, passing the sample either to a standard classifier or a zero-shot model accordingly. 3) We extend the custom evaluation setup for action recognition to the open-set scenario and formalize the evaluation protocol for the tasks of novelty detection and zero-shot action recognition in the generalized case on two well-established datasets, UCF-101 [Soomro et al.(2012)Soomro, Zamir, and Shah] and HMDB-51 [Kuehne et al.(2013)Kuehne, Jhuang, Stiefelhagen, and Serre]. The evaluation shows, that our model consistently outperforms conventional NNs and other baseline methods in identifying novel activities and was highly successful when applied to generalized zero-shot learning.

2 Related Work

Novelty Detection

Various machine learning methods have been used for quantifying the

normality of a data sample. An overview of the existing approaches is provided by [Pimentel et al.(2014)Pimentel, Clifton, Clifton, and Tarassenko, Chandola et al.(2009)Chandola, Banerjee, and Kumar]. A lot of today’s novelty detection research is handled from the probabilistic point of view [Liu et al.(2017)Liu, Lian, Wang, and Xiao, Socher et al.(2013)Socher, Ganjoo, Manning, and Ng, Pimentel et al.(2014)Pimentel, Clifton, Clifton, and Tarassenko, Mohammadi-Ghazi et al.(2018)Mohammadi-Ghazi, Marzouk, and Büyüköztürk]

, modeling the probability density function (PDF) of the training data, with Gaussian Mixture Models (GMM) being a popular choice

[Pimentel et al.(2014)Pimentel, Clifton, Clifton, and Tarassenko]. The One-class SVM introduced by Schölkopf et al[Schölkopf et al.(2001)Schölkopf, Platt, Shawe-Taylor, Smola, and Williamson]

is another widely used unsupervised method for novelty detection, mapping the training data into the feature space and maximizing the margin of separation from the origin. Anomaly detection with NNs has been addressed several times using encoder-decoder-like architectures and the reconstruction error 

[Williams et al.(2002)Williams, Baxter, He, Hawkins, and Gu]

. A common way for anomaly detection is to threshold the output of the neuron with the highest value

[Markou and Singh(2006), Hendrycks and Gimpel(2017), Richter and Roy(2017)]. Recently, Hendrycks et al[Hendrycks and Gimpel(2017)]

presented a baseline for deep-learning based visual recognition using the top-1 Softmax scores and pointed out, that this area is under-researched in computer vision.

The research of novelty detection in videos has been very limited. A related topic of anomaly detection has been studied for very specific applications, such as surveillance [Pimentel et al.(2014)Pimentel, Clifton, Clifton, and Tarassenko, Markou and Singh(2006)] or personal robotics[Moerland et al.(2016)Moerland, Chandarr, Rudinac, and Jonker]. Surveillance however often has anomalies, such as Robbery or Vandalism, present in the training set in some form [Mohammadi et al.(2016)Mohammadi, Perina, Kiani, and Murino, Sultani et al.(2018)Sultani, Chen, and Shah] which violates our open-set assumption. The work most similar to ours is the one of Moerland et al[Moerland et al.(2016)Moerland, Chandarr, Rudinac, and Jonker]

where Hidden-Markov-Model is used to detect unseen actions from skeleton features. However,

[Moerland et al.(2016)Moerland, Chandarr, Rudinac, and Jonker] considers only a simplified evaluation setting using only a single unseen action category in testing. In contrast to [Moerland et al.(2016)Moerland, Chandarr, Rudinac, and Jonker] our model is based on a deep neural architecture for detecting novel actions which makes it applicable to a wide range of modern action recognition models. Furthermore, we consider a challenging evaluation setting on well-established datasets where novel classes are as diverse as those seen before. Additionally, we go beyond novelty detection and evaluate how well our model generalizes to classifying novel classes through zero-shot learning. Our model leverages approximation of BNN using MC-Dropout as proposed by Gal et al[Gal and Ghahramani(2016)], which has been successfully applied in semantic segmentation [Kendall et al.(2017)Kendall, Gal, and Cipolla]

and active learning 

[Gal et al.(2017)Gal, Islam, and Ghahramani]. We extend the BNN approximation to the context of open set action recognition where we incorporate the uncertainty of the output neurons in a voting scheme for novelty detection.

Zero-Shot Action Recognition

Research on human activity recognition under open set conditions has been sparse so far. A related field of Zero-Shot Learning (ZSL) attempts to classify new actions without any training data by linking visual features and the high-level semantic descriptions of a class, e.g

through action labels. The description is often represented with word vectors by a skip-gram model (

e.gword2vec [Mikolov et al.(2013)Mikolov, Sutskever, Chen, Corrado, and Dean]) previously trained on a large-scale text corpus. ZSL for action recognition gained popularity over the past few years and has also been improving slowly but steadily [Xu et al.(2017)Xu, Hospedales, and Gong, Wang and Chen(2017), Xu et al.(2015)Xu, Hospedales, and Gong, Qin et al.(2017)Qin, Liu, Shao, Shen, Ni, Chen, and Wang, Zhu et al.(2018)Zhu, Long, Guan, Newsam, and Shao]. In all of these works, the categories used for training and testing are disjoint and the method is evaluated on unfamiliar actions only. This is not a realistic scenario, since it requires the knowledge of whether the activity belongs to a known or novel category a priori. Generalized zero-shot learning (GZSL) has been recently studied for image recognition and a drastic performance drop of classical ZSL approaches such as ConSE [Norouzi et al.(2013)Norouzi, Mikolov, Bengio, Singer, Shlens, Frome, Corrado, and Dean] and Devise [Frome et al.(2013)Frome, Corrado, Shlens, Bengio, Dean, Mikolov, et al.], has been reported [Xian et al.(2017)Xian, Schiele, and Akata]. As the main application of our novelty detection approach, we implement a framework for ZSL in the generalized case and integrate our novelty detection method to distinguish between known and unknown actions.

Figure 1: Distribution of predictive mean and uncertainty as a 2-D histogram of the leading classifier (highest predictive mean) for the input with known and unseen actions (HMDB-51 dataset). Red denotes common cases (high frequency), blue denotes unlikely cases.

3 Novelty Detection via Informed Voting

We present a new approach for novelty detection in action recognition. That is, given a new video sample , our goal is to find out whether is a sample of a previously known category or if it belongs to a novel action category not seen before during training.

Let be the set of all  known categories in our dataset. Then is the classifier probability of action category given sample . Conceptually, our novelty detection model is composed of two main components: 1) the leader and 2) the council. The leader refers to the classifier with the highest confidence score in predicting the class of a certain sample . For example, in classification neural networks it is common to select the leader based on the highest softmax prediction score. The leader votes for sample being of its own category and assuming that the class of is one of the known categories, i.e. . The council, on the other hand, is a subset of classifiers that will help us validating the decision of a specific leader. In other words, the council members of a leader representing the selected class are a subset of the classifiers representing the rest of the classes, i.e. . These members are elected for each leader individually, i.e. 

each category classifier in our model has its own council. A council member is selected based on its certainty variance in relation to a leader. Whenever a leader decides on the category of a sample

, its council will convene and vote on the leader decision. Then, the council members will jointly decide whether the leader made the correct decision or it was mistaken because the sample is actually from a novel category.

Next, we explain in details how we measure the uncertainty of a classifier (Section 3.1); choosing a leader and its council members (Section 3.2); and, finally, the novelty voting procedure given new sample (Section 3.3).

3.1 Measuring Classifier Uncertainty

In this section, we tackle the problem of quantifying the uncertainty of a classifier given a new sample. The estimated uncertainty is leveraged later by our model to select the council members as we will see in Section 3.2.

In the context of deep learning, it is common to consider the single point estimates for each category, represented by the output of the softmax layer, as a confidence measure 

[Markou and Singh(2006), Hendrycks and Gimpel(2017), Augusteijn and Folkert(2002), Richter and Roy(2017)]. However, this practice has been highlighted in literature to be inaccurate since a model can be highly uncertain even when producing high prediction scores [Nguyen et al.(2015)Nguyen, Yosinski, and Clune, Szegedy et al.(2013)Szegedy, Zaremba, Sutskever, Bruna, Erhan, Goodfellow, and Fergus]. Bayesian neural networks (BNNs) offer us an alternative to the point estimate models and are known to provide a well calibrated estimation of the network uncertainty in its output. Given the network parameters and a training set , the predictive probability of the BNN is obtained by integrating over the parameter space. The prediction

is therefore the mean over all possible parameter combinations weighted by their posterior probability:

(1)

However, BNNs are known to have a difficult inference scheme and high computation cost [Gal and Ghahramani(2016)]. Therefore, we leverage the robust model proposed by [Gal and Ghahramani(2016)] to approximate the predictive mean and uncertainty of the BNN posterior distribution with network parameters modeled as a Gaussian Process (GP). This method is based on dropout regularization [Srivastava et al.(2014)Srivastava, Hinton, Krizhevsky, Sutskever, and Salakhutdinov]

, a widely used technique which has proven to be very effective against overfitting. That is, it leverages the dropout at each layer in the network to draw the weights from a Bernoulli distribution with probability

. At test time, the dropout is iteratively applied for forward passes for each individual sample. Then, the statistics of the neural network output represents a Monte-Carlo (MC) approximation of the neuron’s posterior distribution This approach is referred to as MC-Dropout [Gal and Ghahramani(2016)].

Specifically, let

be a representation generated by a convolutional neural network (CNN) for an input sample

. We add a feedforward network on top of the CNN with two fully-connected layers with weight matrices and . Instead of using a deterministic Softmax estimate in a single forward pass as it is common with CNNs, we now compute the mean over stochastic iterations as our prediction score:

(2)

where

is the rectified linear unit (ReLU) activation function,

is the bias vector of the firs layer. Additionally,

and are diagonal matrices where the diagonal elements contain binary values, such that they are set to with probability and otherwise to .

We further empirically compute the model’s predictive uncertainty as the distribution variance:

(3)

Fig. 1 shows how predictive mean and uncertainty are distributed for samples of known and novel classes. The plot depicts clearly different patterns for the resulting probability distributions in these two cases which illustrates the potential of Bayesian uncertainty for novelty detection.

3.2 Selecting the Leader and its Council

Figure 2: Council members and uncertainty statistics for three different leaders (HMDB-51). The classifier’s average uncertainty and its variance (area surrounding the point) illustrate how it changes its belief in the leader for different data inputs. Blue points are in the council of the current leader, while red points are classifiers that did not pass the credibility threshold.

Now that we can estimate the confidence and uncertainty of each category classifier in our model, we describe in this section how to choose the leader and select it council members.

The Leader.

Rather than selecting the leader using a point estimate based on the softmax scores of the output layer, we leverage here the more stable dropout-based estimation of the prediction mean. Hence, the leader is selected as the classifier with highest expected prediction score over sampling iterations:

(4)

where is estimated according to Eq. 2.

The Council.

The leader by itself can sometimes produce highly confident predictions for samples of unseen categories [Szegedy et al.(2013)Szegedy, Zaremba, Sutskever, Bruna, Erhan, Goodfellow, and Fergus]. Hence, we can not rely solely on the leader confidence to estimate whether a sample is of a novel category or not. Here, the rest of the classifiers can help in checking the validity of the leader’s decision. We notice that these classifier exhibit unique patterns in regard to a certain leader. They can be grouped into two main groups: the first shows high uncertainty when the leader is correctly classifying a sample; while the second shows a very low uncertainty and are in agreement with the leader.

Guided by this observation, we select the members of the Council for a certain leader based on their uncertainty variance in regards to samples of the leader’s category, i.e. . In other words, those classifiers that exhibit very low uncertainty when the leader is classifying samples of its own category are elected to join its council. During the training phase, we can select the council members for each classifier in our model. Here, we randomly split the initial set into a training set which is used for model optimization and parameter estimation, and a holdout set which is used for choosing the council member for all the classifiers iteratively. Specifically, we use a 9/1 split for the training and the holdout splits. We first estimate the parameters of our deep model using . Then, we evaluate our model over all samples from . For each category classifier in our model, we construct a set of true positive samples . For each sample , we estimate the uncertainty of the rest of the classifiers using the MC-Dropout approach. Then, the variance of these classifiers’ uncertainty is estimated as:

(5)

where and is the expectation of the uncertainty of the classifier over samples . Finally, classifiers with a variance lower than a fixed credibility threshold are then elected as members of council.

Fig. 2 shows three leaders and their elected councils according to our approach. We see, for example, that eight classifiers did not pass the credibility threshold for the leader drink and were excluded from its council. The variance of the uncertainty is especially high for sit and eat in this case. This is expected since those actions often occur in a similar context.

3.3 Voting for Novelty

Given the trained deep model and the sets of all council members from the previous step, we can now generate a novelty score for a new sample as follows. First, we calculate the prediction mean and uncertainty of all the action classifiers using stochastic forward passes and MC-Dropout. Then, the classifier with the maximum predicted mean is chosen as the leader. Finally, the council members of the chosen leader vote for the novelty of sample based on their estimated uncertainty (see Algorithm 3.3).

Examples of such voting outcome for three different leaders are illustrated in Fig. 3. In case of category cartwheel, we can see that when the leader is voting indeed for the correct category, all council members show low uncertainty values therefore resulting in a low novelty score, as uninformed classifiers (marked in red) are excluded. However, we observe very different measurements for an example from an unseen category clap which is also predicted as cartwheel. Here, multiple classifiers which are in the council (marked in blue) show unexpected high uncertainty values (e.g. eat, laugh), therefore discrediting the leader decision and voting for a high novelty score.

1:Input: Input sample x, Classification Model , sets of Council members for each Leader:
2:Output: Novelty score
3:Inference using MC-Dropout Preform stochastic forward passes: ;
4:for all  do
5:   Calculate the prediction mean and uncertainty: and
6:end for
7: Find the Leader: 
8: Select the Council: 
9:Compute the novelty score :
Algorithm Novelty Detection by Voting of the Council Neurons

Model variants.

We refer to our previous model as the Informed Democracy model since voting is restricted to the council members which are chosen in an informed manner to check the decision of the leader. In addition to the previous model, we consider two other variants of our model:

  1. The Uninformed Democracy model: Here, there is no council and all classifiers have the right to vote for any leader. Hence, step in Algorithm 3.3 is replaced with .

  2. The Dictator model: unlike the previous model, this one leverages only the leader’s uncertainty in its own decision to predict the novelty of the sample, i.e. .

Open set and zero-shot learning

Once our model generated the novelty score , we can decide whether is a sample from a novel category or not using a sensitivity threshold

. This threshold can be estimated from a validation set using the equal error rate of the receiver operating characteristic curve (ROC). Then, if

the Council votes in favor of the Leader and its category is taken as our final classification result. Otherwise, an unknown activity class has been identified. In this case, the input could be passed further to a module in charge of handling unfamiliar data, such as a zero-shot learning model or a user to give the sample a new label in the context of active learning.

Figure 3: Examples of selective voting for the novelty score of different activities. The first row depicts the case where the samples are of known classes and second row for those of novel classes. Red points highlight classifiers, which were excluded from to the council of the current leader. Their uncertainty is, therefore, ignored when inferring the novelty score.

4 Evaluation

Evaluation setup

Since there is no established evaluation procedure available for action recognition in open-set conditions, we adapt existing evaluation protocols for two well-established datasets, HMDB-51 [Kuehne et al.(2013)Kuehne, Jhuang, Stiefelhagen, and Serre] and UCF-101 [Soomro et al.(2012)Soomro, Zamir, and Shah], for our task222Dataset splits used for novelty detection and generalized zero-shot action recognition are provided at https://cvhci.anthropomatik.kit.edu/aroitberg/novelty_detection_action_recognition. We evenly split each dataset into seen/unseen categories (26/25 for HMDB-51 and 51/50 for UCF-101). Samples of unseen classes will not be available during training, while samples of the remaining set of seen classes is further split into training (70%) and testing (30%) sets, thereby adapting the evaluation framework of [Wang and Chen(2017)] for the generalized

ZS learning scenario. For each dataset, we randomly generate 10 splits and report the average and standard deviation of the recognition accuracy. Using a separate validation split, we optimize the credibility threshold

and compute the threshold for rejection for each category as the Equal Error Rate of the ROC.

Architecture details

We augment the RGB-stream of the I3D architecture [Carreira and Zisserman(2017)] with MC-Dropout. The model is pre-trained on the Kinetics dataset, as described in [Carreira and Zisserman(2017)]

. The last average pooling is connected to two fully connected layers: a hidden layer of size 256 and the final softmax-classifier layer. These are optimized using SGD with momentum of 0.9, learning rate of 0.005 and dropout probability of 0.7 for 100 epochs. We sample the output scores for

stochastic forward passes applied on the two layers preceding the classifier, while the credibility threshold is set to 0.001.

Baselines

We compare our model to three popular methods for novelty and outlier detection: 1) a One Class SVM

[Schölkopf et al.(2000)Schölkopf, Williamson, Smola, Shawe-Taylor, and Platt, Schölkopf et al.(2001)Schölkopf, Platt, Shawe-Taylor, Smola, and Williamson] with RBF kernel (upper bound on the fraction of training errors set to 0.1); 2) a GMM [Zorriassatine et al.(2005)Zorriassatine, Al-Habaibeh, Parkin, Jackson, and Coy, Pimentel et al.(2014)Pimentel, Clifton, Clifton, and Tarassenko] with 8 components; 3) and Softmax probabilities [Richter and Roy(2017), Hendrycks and Gimpel(2017)] as the value for thresholding. Both SVM and GMM were trained on normalized features obtained from last average pooling layer of I3D pre-trained on the Kinetics dataset [Carreira and Zisserman(2017)].

Novelty Detection

We evaluate the novelty detection accuracy in terms of a binary classification problem, using the area under curve (AUC) values of the receiver operating characteristic (ROC) and the precision-recall (PR) curves.

We show the robustness of our approach in comparison to the baseline methods in Table 1. All variants of our model clearly outperform the conventional approaches and achieve an ROC-AUC gain of over 7% on both datasets. Along our model variants, Informed Democracy has proven to be the most effective strategy for novelty score voting, outperforming the Dictator by 5.5% and 1.4%, while Uninformed Democracy achieved second-best results. We believe that smaller differences in performance gain on the UCF-101 data are due to the much higher supervised classification accuracy on this dataset. Since the categories of UCF-101 are easier to distinguish visually and the confusion is low, there is more agreement between the neurons in terms of their confidence.

Generalized Zero-Shot Learning (GZSL)

Next, we evaluate our approach in the context of GZSL, where our novelty detection model serves as a filter to distinguish whether the observed example should be classified with the I3D model in the standard classification setup, or mapped to one of the unknown classes via a ZSL model. We compare two prominent ZSL methods: ConSE [Norouzi et al.(2013)Norouzi, Mikolov, Bengio, Singer, Shlens, Frome, Corrado, and Dean] and DeViSE [Frome et al.(2013)Frome, Corrado, Shlens, Bengio, Dean, Mikolov, et al.]. The ConSE model starts by predicting probabilities of the seen classes, and then takes the convex combination of word embeddings of the top K most possible seen classes and select its nearest neighbor from the novel classes in the word2vec space. For DeViSE, we train a separate model to regress word2vec representations from the visual features. We use the publicly available word2vec model that is trained on Google News articles [Mikolov et al.(2013)Mikolov, Sutskever, Chen, Corrado, and Dean].

For consistency, we first report the results for the standard ZS case (i.e. UU) and further extend to the generalized case (i.e. UU+S and U+SU+S) as shown in Table 2. In the more realistic GZSL setup, our model is not restricted to any group of target labels and is evaluated on actions of seen and unseen category using the harmonic mean of accuracies for seen and unseen classes as proposed by [Xian et al.(2017)Xian, Schiele, and Akata]. Table 2 shows a clear advantage of employing novelty detection as part of a GZSL framework. While failure of the original ConSE and DeViSE models might be surprising at first glance, such performance drops have been discussed in previous work on ZSL for image recognition [Xian et al.(2017)Xian, Schiele, and Akata] and is due to the fact that both models are biased towards labels that were used during training. Our Informed Democracy model yields the best recognition rates in every setting and can therefore be indeed successfully applied for multi-label action classification in case of new activities.

Novelty Detection Model HMDB-51 UCF-101
ROC AUC % PR AUC % ROC AUC % PR AUC %
Baseline Models
One-class SVM 54.09 (3.0) 77.86 (4.0) 53.55 (2.0) 78.57 (2.4)
Gaussian Mixture Model 56.83 (4.2) 78.40 (3.6) 59.21 (4.2) 79.50 (2.2)
Conventional NN Confidence 67.58 (3.3) 84.21 (3.0) 84.28 (1.9) 93.92 (0.7)
Our Proposed Model based on Bayesian Uncertainty
Dictator 71.78 (1.8) 86.81 (2.5) 91.43 (2.3) 96.72 (1.0)
Uninformed Democracy 73.81 (1.7) 87.83 (2.3) 92.13 (1.8) 97.15 (0.7)
Informed Democracy 75.33 (2.7) 88.66 (2.3) 92.94 (1.7) 97.52 (0.6)
Table 1: Novelty detection results evaluated as area under the ROC and PR-curves for identifying previously unseen categories (mean and standard deviation over ten dataset splits).
Zero-Shot Approach HMDB-51 UCF-101
UU UU+S U+SU+S UU UU+S U+SU+S
Standard ConSe Model 21.03 (2.07) 0 (0) 0 (0) 17.85 (1.95) 0.07 (0.10) 0.13 (0.20)
Standard Devise Model 17.27 (2.01) 0.26 (0.37) 0.52 (0.73) 14.48 (1.13) 0.81 (0.36) 1.61 (0.71)
ConSe + Novelty Detection
One-class SVM 21.03 (2.07) 10.99 (1.83) 17.40 (2.41) 17.85 (1.95) 10.37 (1.59) 16.55 (1.91)
Gaussian Mixture Model 21.03 (2.07) 13.30 (2.58) 19.91 (3.32) 17.85 (1.95) 9.31 (1.30) 15.98 (1.99)
Conventional NN Confidence 21.03 (2.07) 10.96 (0.87) 18.56 (1.22) 17.85 (1.95) 12.19 (1.72) 20.91 (2.59)
Informed Democracy (ours) 21.03 (2.07) 13.67 (1.31) 22.27 (1.79) 17.85 (1.95) 13.62 (1.94) 23.42 (2.97)
Devise + Novelty Detection
One-class SVM 17.27 (2.01) 8.92 (1.89) 14.67 (2.74) 14.48 (1.13) 8.65 (1.59) 14.25 (2.00)
Gaussian Mixture Model 17.27 (2.01) 10.61 (2.22) 16.72 (3.1) 14.48 (1.13) 7.26 (0.84) 12.88 (1.40)
Conventional NN Confidence 17.27 (2.01) 8.68 (1) 15.17 (1.56) 14.48 (1.13) 10.08 (1.59) 17.69 (2.33)
Informed Democracy (ours) 17.27 (2.01) 10.73 (1.47) 18.18 (2.21) 14.48 (1.13) 11.03 (1.42) 19.48 (2.21)
Table 2: Accuracy for GZS action recognition with the proposed novelty detection model. UU: test set consists of unseen actions, the prediction labels are restricted to the unseen labels (standard). UU+S: test set consists of unseen actions, both unseen and seen labels are possible for prediction. U+SU+S: generalized ZSL case, both unseen and seen categories are among the test examples and in the set of possible prediction labels (harmonic mean of the seen and unseen accuracies reported.)

5 Conclusion

We introduce a new approach for novelty detection in action recognition. Our model leverages the estimated uncertainty of the category classifiers to detect samples from novel categories not encountered during training. This is achieved by selecting a council of classifiers for each leader (i.e. the most confident classifier). The council will validate the decision made by the leader through voting. Hence, either confirming the classification decision for a sample of a known category or revoking the leader decision and deeming the sample to be novel. We show in a thorough evaluation on two challenging benchmark, that our model outperforms the state-of-the-art in novelty detection. Furthermore, we demonstrate that our model can be easily integrated in a generalized zero-shot learning framework. Combining our model with off-the-shelf zero-shot approaches leads to significant improvements in classification accuracy.

Acknowledgements

This work has been partially funded by the German Federal Ministry of Education and Research (BMBF) within the PAKoS project.

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