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# Deep autoencoder for false positive reduction in handgun detection

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## Abstract and Figures

In an object detection system, the main objective during training is to maintain the detection and false positive rates under acceptable levels when the model is run over the test set. However, this typically translates into an unacceptable rate of false alarms when the system is deployed in a real surveillance scenario. To deal with this situation, which often leads to system shutdown, we propose to add a filter step to discard part of the new false positive detections that are typical of the new scenario. This step consists of a deep autoencoder trained with the false alarm detections generated after running the detector over a period of time in the new scenario. Therefore, this step will be in charge of determining whether the detection is a typical false alarm of that scenario or whether it is something anomalous for the autoencoder and, therefore, a true detection. In order to decide whether a detection must be filtered, three different approaches have been tested. The first one uses the autoencoder reconstruction error measured with the mean squared error to make the decision. The other two use the k-NN (k-nearest neighbors) and one-class SVMs (support vector machines) classifiers trained with the autoencoder vector representation. In addition, a synthetic scenario has been generated with Unreal Engine 4 to test the proposed methods in addition to a dataset with real images. The results obtained show a reduction in the number of false positives between 22.5% and 87.2% and an increase in the system’s precision of 1.2%-47\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$-47$$\end{document}% when the autoencoder is applied.
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ORIGINAL ARTICLE
Deep autoencoder for false positive reduction in handgun detection
Noelia Vallez
1
Alberto Velasco-Mata
1
Oscar Deniz
1
Received: 24 September 2019 / Accepted: 11 September 2020 / Published online: 25 September 2020
ÓThe Author(s) 2020
Abstract
In an object detection system, the main objective during training is to maintain the detection and false positive rates under
acceptable levels when the model is run over the test set. However, this typically translates into an unacceptable rate of
false alarms when the system is deployed in a real surveillance scenario. To deal with this situation, which often leads to
system shutdown, we propose to add a ﬁlter step to discard part of the new false positive detections that are typical of the
new scenario. This step consists of a deep autoencoder trained with the false alarm detections generated after running the
detector over a period of time in the new scenario. Therefore, this step will be in charge of determining whether the
detection is a typical false alarm of that scenario or whether it is something anomalous for the autoencoder and, therefore, a
true detection. In order to decide whether a detection must be ﬁltered, three different approaches have been tested. The ﬁrst
one uses the autoencoder reconstruction error measured with the mean squared error to make the decision. The other two
use the k-NN (k-nearest neighbors) and one-class SVMs (support vector machines) classiﬁers trained with the autoencoder
vector representation. In addition, a synthetic scenario has been generated with Unreal Engine 4 to test the proposed
methods in addition to a dataset with real images. The results obtained show a reduction in the number of false positives
between 22.5% and 87.2% and an increase in the system’s precision of 1.2%47% when the autoencoder is applied.
Keywords Handgun detection False positive reduction Autoencoder One-class classiﬁcation
1 Introduction
Weapons, among other threats, need to be detected as soon
as possible to eliminate or mitigate the danger they could
cause [1]. Traditionally, the surveillance of public scenar-
ios has been accomplished by the human supervision of the
images captured by closed-circuit television (CCTV) sys-
tems. However, even an experienced guard may miss a
dangerous event due to fatigue or loss of attention [2]. To
help with this situation, the creation of automated
surveillance systems (AVSs) able to locate potentially
threatening objects (or other events) in video has been
studied during the last decades [3].
Similarly to other areas, with the introduction of the new
deep learning methods these frameworks have obtained
promising results and are closer to be used in real scenarios
[4,5]. Nevertheless, although those detectors have high
detection (D) and low false positive (FP) rates, when they
are used in a different scenario from the one used for
training, the false positive ratio increases [6]. This fact
represents a major problem since even an increase of a
0.1% of the false positive ratio may cause 90 false alarms
per hour with a video input of 25 fps. Therefore, when
running the surveillance system in a real scenario, the
outcome is usually an unsatisfactory number of false
alarms. In most cases, this may lead to the guard switching
off the system.
In this context, we propose to include an extra step that
models the false alerts that are speciﬁc of the new scenario
while approximately maintaining the capability of identi-
fying the objects it was trained for. As an speciﬁc appli-
cation, this work focuses on detecting handguns in video
surveillance. After running the detector in a new scenario,
it is possible to collect all the detector alarms. Practically,
all of these alarms are false positives since the incidence of
the true event (a handgun in the scene) is very low.
&Noelia Vallez
Noelia.Vallez@uclm.es
1
VISILAB, University of Castilla La-Mancha, ETSI
Industriales, Av. Camilo Jose Cela SN, 13071 Ciudad Real,
Spain
123
Neural Computing and Applications (2021) 33:5885–5895
https://doi.org/10.1007/s00521-020-05365-w(0123456789().,-volV)(0123456789().,-volV)
Therefore, all of these detections can be stored and used to
model the new scenario.
The new step will act as a ﬁlter able to recognize typical
FPs of the detector in the particular scenario. Therefore,
this problem can be seen as an anomaly detection problem
where the anomalies are those detections that are not
similar to the FPs modeled by the ﬁlter [7,8]. In fact, the
anomalies detected on this step will be the real alarms.
To detect abnormal and extreme data, one-class classi-
ﬁers have been widely used in the literature [9]. More
concretely, autoencoders have proven to be the most suit-
able of the techniques, obtaining good results even where
other methods fail [10]. In order to use the autoencoder as a
ﬁlter and decide whether a detection is an anomaly or not,
we have tested different approaches: using the autoencoder
reconstruction error as a threshold and using the central
vector representation to train a nearest neighbor (NN) and a
one-class support vector machine (SVM) classiﬁers.
Although autoencoders have been applied in anomaly
detection problems, to the best of our knowledge, this is the
ﬁrst time they have been applied to reduce false positive
detections when the detector runs in a new scenario from
which it is not possible to obtain labeled data.
For the purpose of testing our idea, we have generated
an entirely synthetic dataset from the frames captured from
a realistic 3D scenario. The synthetic scenario resembles a
school hallway from the point of view of a surveillance
camera. This allow us to generate as much data as needed
with and without handguns to train and test the
autoencoder.
The rest of the paper is organized as follows. Section 2
performs an overview of the advances in handgun detec-
tion. Section 3shows the handgun detector used as base
detector of the proposed false positive reduction method.
Section 4describes the datasets used including the syn-
thetic dataset that has been generated. Section 5provides
detailed information about the proposed autoencoder-based
ﬁltering step. Finally, Sect. 6shows the results and Sect. 7
summarizes the main conclusions.
2 Related work
In addition to automatic CCTV video surveillance, several
approaches have been proposed to deal with concealed
handguns in X-ray or millimetric wave images. These types
of image are commonly used in airports, train stations or
the entrance of some public buildings. In 2008, Nercessian
et al. presented a system for handgun detection using X-ray
luggage scan images [11]. The approach was based on the
Gaussian mixture expectation maximization (EM) method
to perform image segmentation prior to the obtention of the
edge-based feature vectors. Gesick et al. compared three
different approaches for the detection of handguns inside
luggage [12]. The ﬁrst method employs edge detection
combined with pattern matching with reliable results.
However, both the computational time and the number of
false positives were high. The second method uses Dau-
bechies wavelet transforms with inconclusive results as the
authors commented. The third algorithm proposed in that
work was based on the scale-invariant feature transform
(SIFT). Later, in 2010, Harmer et al. used a completely
different approach based on the modeling of the complex
natural resonances of handguns and compared them with
those of other objects [13]. In addition, the work of Flitton
et al. concluded that using simpler 3D feature, descriptors
outperform even complex RIFT/SIFT solutions with an
accuracy of more than 95% [14]. In [15], Xiao et al.
employed an extension of the Haar-like features with an
passive millimeter wave (PMMW) images. Following a
similar approach, the study of Kundegorski et al. combines
bag of visual words (BoVW) based on feature point
descriptors and support vector machines (SVMs) and ran-
dom forest classiﬁers [16].
While there are numerous methods and devices that can
detect concealed weapons, unfortunately, the incidence of
mass shootings requires the use of RGB surveillance
images. Tiwari and Verma proposed a framework that
applies color-based segmentation and k-means clustering to
remove irrelevant objects and then uses Harris interest
point detector and Fast Retina Keypoint (FREAK) to locate
the handguns [17]. This resulted in high robustness when
detecting the desired object at different scales and rota-
tions. In addition, Halima and Hosam worked on a detector
that combined SIFT features, k-means clustering, a word
vocabulary histogram, and SVM [18].
The recent advances in deep learning have also been
applied to the handgun detection problem using CCTV
images. The ﬁrst contribution in this area came in the work
of Olmos et al. where two different approaches were used
[4]. The ﬁrst one uses a classiﬁcation CNN to detect
handguns with the sliding window method, whereas the
second one is based on the Faster R-CNN detection
architecture. The latter obtained the best results when tes-
ted in a dataset composed of several YouTube videos. On
the other hand, Gelana et al. followed a more traditional
approach using edge detection and a classiﬁcation CNN
with the sliding window method [19]. In addition, Romero
and Salamea trained a YOLO object detection and local-
ization system to detect ﬁrearms with the particularity of
running the detector only in areas where there are people
[20]. Another study of Olmos et al. proposed using a
symmetric dual camera system to increase the performance
of the detection model in low quality surveillance videos
improving both the false positive and the detection rates.
5886 Neural Computing and Applications (2021) 33:5885–5895
123
To model outliers, discordant objects or simply data that
has a different behavior or pattern, anomaly detection
techniques have been used [7]. Anomaly detection has a
wide range of applications. For example, it can be used to
detect anomalies in stock prices and time series [21,22],
abnormal medical images of ﬁndings [2325], abnormal
events in video [5,26,27], intrusion detection [28], or
disaster areas from radar images [29].
A simple method to model anomalies is to use neighbor-
based methods such as the k-nearest neighbor [3032]
where anomalies are identiﬁed as those points in the data
space that differ from the surrounding data points. The
advantage of these methods is the independence of the data
distribution. However, their performance relies on the
values of the parameters selected such as the number of
neighbors.
An alternative to use neighbor-based methods is to
detect anomalies taken into account that they are grouped
in a zone of the data space. Thus, the anomaly detection
problem is solved as a subspace learning problem [3336].
Although this method work well in some cases, ﬁnding the
number of subspaces in which the anomalies are distributed
is not trivial.
As in classiﬁcation and detection tasks, CNNs have
demonstrated to improve the performance in anomaly
detection problems [26]. More concretely, convolutional
autoencoders have been used to model input data and
reduce data space dimensionality [37]. Their use has
reduced the need of reprocessing input data and compute
handcrafted features from it [10]. Following this approach,
Mabu et al. proposed to use a convolutional autoencoder
followed by a one-class SVM to model normal areas in
satellite images and detect abnormal areas caused by nat-
ural disasters in Japan [29]. Lu and Xu demonstrated the
potential of using variational autoencoders to detect
anomalies in skin disease images [23]. The authors rec-
sarial networks) due to their training stability and
interpretable results. Sugimoto et al. use an autoencoder
followed by a k-NN classiﬁer to detect myocardial
infarction.
Another approach is the one followed by Gutoski et al.
in which autoencoders and stacked denoising autoencoders
are used for clustering [38]. With the clustering, repre-
sentation in possible to deﬁne whether a new sample is an
anomaly or not according to its distance to the clusters.
Gutoski et al. also followed this approach for one-class
classiﬁcation [38].
In some cases, there are more than one group of
abnormalities as in the work carried out by Mirsky et al. in
[28]. The authors proposed to use an ensemble of autoen-
coders instead of one to detect online network intrusions.
The decision of what is an anomaly or not is based on the
RMSE (root-mean-squared errors) score output by the
autoencoders.
For video input, Singh and Mohan use deep stacked
autoencoders to obtain a deep representation of spa-
tiotemporal video volumes to detect road accidents [37].
The anomaly score is obtained with a one-class SVM as in
other works.
Finally, non-symmetric autoencoders have also been
used to learn space representations. An example of this is
the work carried out by Tran and Hogg where the
autoencoder representation is used for detecting anomalies
in video [39]. In addition, recurrent autoencoders with
LSTM (long short-term memory) layers have also been
applied for anomaly detection in video in the work carried
out by Yan et al. in [40].
3 Handgun detector
Before addressing the false positive rate reduction through
the use of the autoencoder, we needed to train and test a
handgun detector. As shown in Sect. 2, there are several
approaches that can be selected, from the use of classiﬁ-
cation CNNs with the sliding window approach to the most
modern CNN detection architectures. While the former
examines every subregion of the image, the latter uses
region proposal algorithms to reduce the number of
examined windows or process the full image in one pass
[41]. The advantage of the new architectures is the ability
to detect objects in different locations of the image without
being restricted to a certain aspect ratio. Moreover, the
number of regions to be examined is drastically reduced in
comparison with other methods. The most representative
architectures for object detection that follow a region
proposals approach are R-CNN, Fast R-CNN, and Faster
R-CNN [42].
Two well-known architectures are YOLO (You Only
Look Once) and SSD (single-shot detector). YOLO
addresses object detection as a regression problem with
spatially separated bounding boxes and their corresponding
class probabilities [43]. SSD is able to predict, with only
one pass over the entire image, the bounding boxes and the
class probabilities for them [44].
In addition to all the above, there is a recently developed
CNN-based detector called RetinaNet [45]. RetinaNet was
designed to solve the problem of having extreme fore-
ground–background class imbalanced problems and has
been also applied to X-ray images [46].
For the particular problem of weapon (handgun and
knife) detection, [47] reviews recent work and shows that
Faster R-CNN has been the prevalent method. For that
reason, we have selected the Faster R-CNN architecture to
train a handgun detector with a dataset provided by the
Neural Computing and Applications (2021) 33:5885–5895 5887
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University of Seville [1]. The dataset is composed of 871
images that contain 177 annotated handguns. Those images
were extracted from the video captured by 2 CCTV cam-
eras located in two different college hallways.
4 Datasets
The collection and labeling of the data necessary to train
deep learning models are tasks that require signiﬁcant time
and effort. This is even more complicated in detection or
segmentation problems in which someone has to select the
area of the image in which the object is located, or the
exact contour of the object, in addition to the category. A
possible solution to this problem is the use of public
datasets, but, depending on the problem, it is not always
possible to have one available. The use of synthetic images
facilitates the work required to obtain large datasets. For
this work, a completely synthetic dataset has been gener-
ated with Unreal Engine 4 [48], rendering a scenario that
represents a high-school hallway where people are walk-
ing. There are other popular alternatives such as Unity [49]
and Lumberyard/CryEngine [50] that can also be used for
the same purpose. While some of the people on the sce-
nario carry everyday objects in their hands, such as mobile
phones, others carry guns or nothing (see Fig. 1).
Another advantage of having the data generation fully
controlled by the researcher is that it is also possible to
automatically generate a mask image with the desired
objects for each frame. In this case, each generated image
contains the people in white, the background covered in
black, and each handgun ﬁlled with a different color to help
are obtained, the coordinates of the bounding boxes that
contain the weapons are extracted storing the annotations
in XML ﬁles with the format deﬁned by the Pascal VOC
2012 Challenge [51].
A total of 4000 images were generated with this method
with a resolution of 1280720. From these, 3000 frames
were used to train and adjust the proposed autoencoder
ﬁlter, containing 5437 annotated handguns. The remaining
1000 frames were used to evaluate and compare the
detector and detector ?autoencoder systems.
In addition to the synthetic dataset, the Gun Movies
Database [52] has also been used to ensure the differences
are caused by the proposed method and not by changes in
the texture by the origin of the data. This dataset contains
Fig. 1 Synthetic scenario with a zoom on the elements of interest (in
this case, a mobile phone and a handgun)
Fig. 2 Sample frame from the Gun Movies dataset
Fig. 3 Proposed system
Fig. 4 Autoencoder training phase
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images of size 640480 pixels from 7 laboratory-shot
movies with a total of 817 frames and 686 annotated
handguns (Fig. 2).
For this second dataset, a total of 817 images were used.
From these, 571 frames were used to train and adjust the
proposed autoencoder ﬁlter and the remaining 246 frames
were used to evaluate and compare the detector and
detector ?autoencoder systems.
5 Proposed method
As introduced above, when a detector runs in a new sce-
nario, the false positive rate increases due to its particu-
larities that were not seen in the training data. To deal with
this problem, we propose to add a ﬁltering step after the
detector inference (see Fig. 3). The ﬁlter is used to discard
the FP detections of the object detector produced by the
particularities of the new scenario.
The ﬁlter can be considered as a one-class classiﬁer that
learns how to identify a certain type of samples. Thus, the
rest of the samples can be considered as anomalies. This
problem has been addressed in the literature through the
use of the one-class versions of the SVM, k-nearest
neighbor (k-NN), random forests classiﬁers, and more
recently with deep autoencoders [9,38,53]. In our case, an
autoencoder is trained to model the class of the typical FP
detections.
In order to collect the training samples for the autoen-
coder, the detector is run in the particular scenario for a
certain period of time, storing all the FP detections (Fig. 4).
Initially, all detections can be considered as FPs in a real
scenario since the incidence of handguns is very low.
Deep autoencoders learn the input data distribution
using an intermediate representation. They are able to
compress the data into a small vector and then reconstruct
the input from it with accurate results. If new input data
come from a different distribution, the reconstruction error
will be higher.
Finally, according to the autoencoder structure, we
deﬁne and compare 3 different methods to check whether
the output of the detector is a typical false positive. The
Fig. 5 Autoencoder architecture
used
Table 1 Detailed description of the autoencoder architecture used.
The output of the conv2d_7 layer (in bold) is used as the FP inter-
mediate representation
Layer (type) Output shape Param #
input_1 (InputLayer) (None, 64, 64, 3) 0
conv2d_1 (Conv2D) (None, 64, 64, 4) 112
max_pooling2d_1 (MaxPooling2) (None, 32, 32, 4) 0
conv2d_2 (Conv2D) (None, 32, 32, 28) 1036
max_pooling2d_2 (MaxPooling2) (None, 16, 16, 28) 0
conv2d_3 (Conv2D) (None, 16, 16, 52) 13156
max_pooling2d_3 (MaxPooling2) (None, 8, 8, 52) 0
conv2d_4 (Conv2D) (None, 8, 8, 76) 35644
max_pooling2d_4 (MaxPooling2) (None, 4, 4, 76) 0
conv2d_5 (Conv2D) (None, 4, 4, 100) 68500
max_pooling2d_5 (MaxPooling2) (None, 2, 2, 100) 0
conv2d_6 (Conv2D) (None, 2, 2, 124) 111724
max_pooling2d_6 (MaxPooling2) (None, 1, 1, 124) 0
conv2d_7 (Conv2D) (None, 1, 1, 148) 165316
conv2d_8 (Conv2D) (None, 1, 1, 148) 197284
up_sampling2d_1 (UpSampling2) (None, 2, 2, 148) 0
conv2d_9 (Conv2D) (None, 2, 2, 124) 165292
up_sampling2d_2 (UpSampling2) (None, 4, 4, 124) 0
conv2d_10 (Conv2D) (None, 4, 4, 100) 111700
up_sampling2d_3 (UpSampling2) (None, 8, 8, 100) 0
conv2d_11 (Conv2D) (None, 8, 8, 76) 68476
up_sampling2d_4 (UpSampling2) (None, 16, 16, 76) 0
conv2d_12 (Conv2D) (None, 16, 16, 52) 35620
up_sampling2d_5 (UpSampling2) (None, 32, 32, 52) 0
conv2d_13 (Conv2D) (None, 32, 32, 28) 13132
up_sampling2d_6 (UpSampling2) (None, 64, 64, 28) 0
conv2d_14 (Conv2D) (None, 64, 64, 3) 759
Fig. 6 Subsets of the synthetic dataset used. The Gun Movies dataset
is similarly split
Neural Computing and Applications (2021) 33:5885–5895 5889
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simplest one is to establish a threshold for the reconstruc-
tion error. Therefore, detections with low reconstruction
error will be discarded as typical FPs of the scenario. The
other two methods are based on the use of the central
vector as a compact representation of the images and then
train a one-class classiﬁer with it. For that, SVM and k-NN
with k¼1 were used, and the thresholds were selected
according to the scores and the distance to the closest
neighbor, respectively.
5.1 Autoencoder architecture
The structure of an autoencoder consists of an encoder path
that ignores the noise and reduces the dimensionality and a
decoder path that makes the reconstruction. The compres-
sive path of the autoencoder used consists of a set of 6
convolutional and max-pooling layers (Fig. 5). Similarly,
the reconstruction path has also 6 convolutional and up-
sampling layers. The input is a 3-channel image of size
6464, and the central vector has 148 elements (conv2d_7
layer). A more detailed description of the architecture can
be seen in Table 1.
6 Results
The Faster R-CNN model trained with the dataset from the
University of Seville obtained an mAP of 0.7933. Training
took 2 days and executed 62 epochs. An Ubuntu 14.04 LTS
(a) (b) (c) (d)
Fig. 7 Typical false positives of
the handgun detector in the
synthetic scenario (enlarged)
(a) (b) (c)
(d) (e) (f)
Fig. 9 Autoencoder reconstruction of: aTP and d) FP of the detector
from the synthetic dataset. band eare the reconstructed images, and
cand fare the absolute difference between the reconstructions and
their corresponding original images
(a) (b) (c)
Fig. 8 Typical false positives of the handgun detector in the Gun
Movies dataset (enlarged)
Table 3 Increase in the detector precision when the autoencoder is
applied by method and dataset
MSE kNN SVM
Synthetic 1.46% 1.77% 1.2%
th =0.0057 th =0.34 th =14600
Gun Movies 47% 20% 8%
th =0.047 th =1.92 th =175
Table 2 Percentage of FPs that are ﬁltered by method and dataset
MSE kNN SVM
Synthetic 26.4% 30% 22.5%
Gun Movies 87.2% 74.1% 49%
5890 Neural Computing and Applications (2021) 33:5885–5895
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machine with 2 nVIDIA Quadro M4000 cards, Keras with
TensorFlow backend and CUDA 8.0 were used to perform
the training.
After obtaining this base detector, both datasets were
divided into 4 parts to (1) train and (2) validate the
autoencoder, (3) ﬁt the k-NN and SVM classiﬁers, and (4)
test the system variants (Fig. 6). The detector was then run
on each of the subsets, and the FP and TP patches were
stored. Although only the FP detections are used to train
and validate the autoencoder and ﬁt the classiﬁers, the
correct detections were also generated and stored for the
test subset to check that the detection rate is minimally
affected.
Overall, for the autoencoder training and validation, two
sets with 4913 and 3607 FPs, respectively, were obtained
for the synthetic dataset and 586 and 405 FPs for the Gun
Movies dataset. Another set composed of 3712 FP regions
for the synthetic dataset and 95 FP regions for the Gun
Movies dataset was used to ﬁt the classiﬁers used to per-
form the decision. Finally, a set of 4832 regions (4632 FPs
and 200 TPs) for the synthetic dataset and 499 regions (359
FPs and 40 TPs) for the Gun Movies dataset was reserved
for testing. Figure 7shows some examples of the typical FP
detections obtained in the synthetic scenario.
The autoencoders were then trained and validated with
the stored FP detections of the training and validation
subsets. This process took only about an hour for each
dataset to complete 500 epochs in a Windows 10 PC with
an nVIDIA GTX 1060 MaxQ card using Keras with Ten-
sorFlow backend and CUDA 9.0. At this point, if the
autoencoder is used with some test images from TP and FP
detections, the ability to effectively reconstruct FPs is
evidenced (see Fig. 9).
The stored FP detections from the ﬁt subset of each
dataset were used to feed each of the autoencoders and get
intermediate vectors to train the SVM and k-NN one-class
classiﬁers. The SVM selected uses a linear kernel. On the
other hand, k¼1 was selected for the k-NN algorithm.
To illustrate the performance of both the detector and
detector ?autoencoder approaches on the two datasets,
they were tested with the 1000 images from the fourth
subset of the synthetic scenario and the 246 frames of the
Gun Movies dataset. The histograms of the reconstruction
error for the MSE thresholding-based method, the proba-
bility score for the SVM one-class classiﬁer, and the
Fig. 10 Synthetic dataset.
Histograms of the MSE
reconstruction error, SVM
score, and K-NN distance (y-
axis uses logarithmic scale)
Neural Computing and Applications (2021) 33:5885–5895 5891
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distance to the nearest neighbor for the k-NN were obtained
(Figs. 10 and 11). Although TPs and FPs are overlapped in
all cases, the ﬁrst part of the MSE and k-NN histograms
and the last part of the SVM histogram do not contain TPs.
Therefore, the FPs that lie on those parts of the histograms
can be potentially ﬁltered selecting the value of the ﬁrst bin
(or the last for the SVM) in the histogram that contains TPs
as threshold. For the synthetic dataset, this shows that,
without affecting the detection rate, up to 26.4% of all the
FPs can be ﬁltered using the MSE reconstruction error,
22.5% using the one-class SVM, and 30% with the distance
to the nearest neighbor of the k-NN (2). On the other hand,
in the histograms obtained from the Gun Movies dataset
TPs and FPs are less overlapped making it possible to
remove up to 87.2% of all the FPs using the MSE recon-
struction error, 49% using the one-class SVM, and 74.1%
with the distance to the nearest neighbor of the k-NN
without affecting the detection rate.
In addition, the precision–recall curves corresponding to
the detector and the autoencoder with the three proposed
decision methods were obtained (see Figs. 12 and 13). The
experimental results show a reduction in the number of
false positives while roughly maintaining the detection
capabilities [54]. Since the precision–recall curve is cal-
culated by varying the detector output threshold and the
autoencoder has another threshold that can be varied too,
each curve was obtained under a speciﬁc value for the
autoencoder and varying the threshold of the detector (3).
For the synthetic dataset, comparing all the curves for a
speciﬁc autoencoder thresholding method, they show a
maximum increase in the precision of 1.46% at the same
recall values when the autoencoder and the MSE are used
(threshold =0.0057), of 1.2% using the autoencoder and
the SVM classiﬁer (threshold =14600), and of 1.77% in
case of the autoencoder and K-NN (threshold =0.34). For
the Gun Movies dataset, results show a maximum increase
in the precision of 47% at the same recall values when the
autoencoder and the MSE are used (threshold =0.047), of
8% using the autoencoder and the SVM classiﬁer (thresh-
old =175), and of 20% in case of the autoencoder and K-
NN (threshold =1.92).
Overall, the results show that the autoencoder is able to
ﬁlter part of the new FPs in all cases without affecting the
detection rate of the original system. All thresholding
Fig. 11 Gun movies dataset.
Histograms of the MSE
reconstruction error, SVM
score, and K-NN distance (y-
axis uses logarithmic scale)
5892 Neural Computing and Applications (2021) 33:5885–5895
123
methods are able to reduce the number of FPS to some
degree.
7 Conclusions
In this work, a step to ﬁlter the false positive detections that
appear when a pre-trained handgun detector is deployed in
the ﬁnal surveillance scenario has been proposed. This step
consists of training a deep autoencoder with the false
positive regions obtained from the particular scenario.
Once the autoencoder is trained, it can be used to decide
whether a detection is similar to the already known typical
false alarms and can be ﬁltered, or otherwise if an alert
should be triggered.
The ability of the autoencoder to reduce the number of
FPs has been demonstrated with a potential reduction by up
to 30% for the synthetic scenario when it is combined with
Fig. 12 Precision–recall curves for the synthetic dataset. Best viewed
in color (color ﬁgure online)
Fig. 13 Precision–recall curves for the Gun Movies dataset. Best
viewed in color (color ﬁgure online)
Neural Computing and Applications (2021) 33:5885–5895 5893
123
ak-NN classiﬁer trained with the vector representation of
the detector FPs regions and up to 78% of the FPs for the
Gun Movies dataset when the autoencoder is combined
with the MSE error metric. Furthermore, the handgun
detection capability of the system is not compromised by
the added ﬁltering step under a wide range of threshold
levels.
Although the proposed approach has been only applied
to two particular scenarios, it can be extended to more than
one since having different perspectives, lighting conditions
or background objects will generate different false posi-
tives. Thus, during the system’s deployment only a generic
detector (in this case a handgun detector) is required and
one autoencoder will be trained for each camera feed.
Acknowledgements We thank Professor Dr. J.A. Alvarez for the
surveillance images provided for training the handgun detector and J.
J. Corroto for generating the synthetic dataset. This work was par-
tially funded by projects TIN2017-82113-C2-2-R by the Spanish
Ministry of Economy and Business and SBPLY/17/180501/000543
by the Autonomous Government of Castilla-La Mancha and the
ERDF.
Compliance with Ethical Standards
Conflict of interest The authors declare that they have no conflict of
interest.
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... The first studies in this context improved the detection in videos by building new datasets [17,10,27,12,11]. The most recent studies applied preor post-processing techniques to further reducing the errors [16,26,4,19]. While other studies used data fusion to improve the overall performance [25,2,18,1,23]. ...
... Several works addressed reducing the number of FP and FN in handgun detection in realistic video surveillance environments using pre-processing [16,4] and post-processing approaches [26,19]. For example, the authors in [4] presented a pre-processing technique that help improving the quality of the videos in changing illumination environments. ...
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