Robust People Detection and Tracking from an Overhead Time-of-Flight
Alvaro Fernandez-Rincon, David Fuentes-Jimenez, Cristina Losada-Gutierrez,
Marta Marron-Romera, Carlos A. Luna, Javier Macias-Guarasa and Manuel Mazo
Department of Electronics, University of Alcal
a, Ctra. Madrid-Barcelona, km. 33,600, 28805-Alcal
a de Henares, Spain
{alvaro.fernandez, david.fuentes, losada, marta, caluna, macias, mazo}
People Detection, Tracking, Time-of-Flight, ToF Camera.
In this paper we describe a system for robust detection of people in a scene, by using an overhead Time of
Flight (ToF) camera. The proposal addresses the problem of robust detection of people, by three means: a
carefully designed algorithm to select regions of interest as candidates to belong to people; the generation
of a robust feature vector that efficiently model the human upper body; and a people classification stage, to
allow robust discrimination of people and other objects in the scene. The proposal also includes a particle
filter tracker to allow people identification and tracking. Two classifiers are evaluated, based on Principal
Component Analysis (PCA), and Support Vector Machines (SVM). The evaluation is carried out on a subset
of a carefully designed dataset with a broad variety of conditions, providing results comparing the PCA and
SVM approaches, and also the performance impact of the tracker, with satisfactory results.
In the last years, automatic people detection and
tracking in a non-invasive way (without adding turn-
stiles or other contact systems for access control) has
received a lot of attention because of its different ap-
plications such as access control, video-surveillance
or behavior analysis.
In this paper, we propose a system for robust
and reliable detection and tracking of multiple people
from depth image sequences, acquired using an over-
head ToF camera. The proposal works properly even
if the number of people is high or if they are close to
each other.
There are several works in the literature that pro-
pose different approaches for people detection. The
first works (Ramanan et al., 2006; Jeong et al., 2013),
are based on the use of an RGB camera. These pro-
posals obtain suitable results under controlled con-
ditions, but they do not work properly in scenarios
with occlusions. In order to reduce the occlusions,
other approaches use a camera in an overhead po-
sition (Antic et al., 2009; Cai et al., 2014). Other
works (Dan et al., 2012; Del Pizzo et al., 2016) use
the fusion of RGB and depth information (obtained
using a Kinect
sensor (Sell and O’Connor, 2014))
in order to improve the detection.
However, using RGB images can imply an inva-
sion of users’ privacy, since there is information that
could allow knowing the identity of the people in the
scene. This can be a relevant issue in applications
where there are privacy preservation requirements,
due, among others, to legal considerations. Because
of that, in the last few years, researchers have looked
for alternatives in order to preserve the users’ privacy.
Some of them propose the use of overhead depth sen-
sors or 2.5D cameras , based on Time of Flight (ToF)
(Bevilacqua et al., 2006; Stahlschmidt et al., 2014;
Jia and Radke, 2014) or structural light (Zhang et al.,
2012; Gal
ık and Gargal
ık, 2013; Rauter, 2013; Zhu
and Wong, 2013; Del Pizzo et al., 2016)for people
detection and tracking, preserving their privacy.
The works described in (Zhang et al., 2012;
Stahlschmidt et al., 2014; Jia and Radke, 2014;
Del Pizzo et al., 2016) allow people detection preserv-
ing the users’ privacy but, since these works do not in-
clude a classification stage, they cannot discriminate
between people and other objects in the scene. Be-
cause of that, these proposals generate an important
number of false positives in realistic scenarios.
Other approaches (Gal
ık and Gargal
ık, 2013;
Rauter, 2013; Zhu and Wong, 2013) incorporate a
classification stage in order to reduce the number of
false positives. The strategies described in (Gal
and Gargal
ık, 2013), and (Zhu and Wong, 2013) ob-
tain a descriptor based on the human head and shoul-
Fernandez-Rincon A., Fuentes-Jimenez D., Losada-Gutierrez C., Marron-Romera M., Luna C., Macias-Guarasa J. and Mazo M.
Robust People Detection and Tracking from an Overhead Time-of-Flight Camera.
DOI: 10.5220/0006169905560564
In Proceedings of the 12th International Joint Conference on Computer Vision, Imaging and Computer Graphics Theory and Applications (VISIGRAPP 2017), pages 556-564
ISBN: 978-989-758-225-7
2017 by SCITEPRESS Science and Technology Publications, Lda. All rights reserved
Figure 1: General System Architecture.
ders structure. These proposals allow people detec-
tion and are able to discriminate between people and
other objects in the scene, but their detection rates
drop significantly if people are close to each other.
Regarding the tracking of multiple people, multi-
ple approaches have been developed during the last
decades (Jia et al., 2008). Among them, the main al-
ternatives can be divided into three groups: using an
estimator for each object to follow (Isard and Blake,
1998), using a single estimator based on an extended
state vector (MacCormick and Blake, 2000), and us-
ing a single multimodal estimator (Marron et al.,
2005; Marron et al., 2010). Since there can be several
people detected in any scene, it is necessary to imple-
ment an association algorithm in order to improve the
reliability of the tracking process. There are different
alternatives for this task, being the most widely used
those based on Maximum Likelihood (ML), Near-
est Neighbor (NN) and Probabilistic Data Association
(PDA) (Bar-Shalom et al., 2011).
The structure of the paper is as follows: Section 1
provides a general introduction and a review of the
literature, Section 2 describes the main modules of
the system architecture, Section 3 includes the exper-
imental setup, results and discussion, and Section 4
contains the main conclusions and future work.
Figure 1 shows the general architecture of the pro-
posed system. Its main modules will be described
next, and we will devote more attention to the Feature
Extraction strategy, and also to the Tracking Module,
as they are the most relevant due to their novelty (the
former) and the specific adaptations carried out (in the
2.1 Feature Extraction
The people detection process includes an offline stage
in charge of generating (training) the models to be
used in the people classification stage, and the online
process includes the following modules:
1. Depth Image Acquisition (height acquisition).
The ToF camera is located in an overhead position
at a h
height from the floor, and its optical
axis is perpendicular to the floor plane. To obtain
the height matrix H, we subtract h
to each
pixel height of the depth image acquired by the
2. Noise Reduction. One of the fundamental draw-
backs in ToF cameras is the high noise level that
is present in the depth image. This noise is es-
pecially significant if there are moving objects in
the scene, reaching a great number of invalid pix-
els along the objects edges. To reduce the noise
and the number of invalid pixels, we have imple-
mented a noise filtering algorithm that includes
two stages. In the first one, the invalid pixels
are corrected using the mean value of the nearest
valid pixels. We consider as invalid pixels those
which are flagged by the camera as invalid pixels,
and those with a height greater than the maximum
height for a person (220 cm). Then, a nine ele-
ment mean filter is applied to the height matrix H
to smooth the detected surfaces.
3. Regions of Interest (ROI’s) Estimation. In this
work, we use a local maxima detection algorithm
to select which regions in the height matrix H cor-
respond to people or other objects. In case there
are several people or objects in the scene, this al-
gorithm must determine which pixels belong to
each of them. The ROI’s are defined as the pixels
around each detected local maximum that belong
Robust People Detection and Tracking from an Overhead Time-of-Flight Camera
to the same object. Since the body parts of inter-
est for this solution are the head, neck and shoul-
ders, we impose the criterion that the height dif-
ference between the highest point on the head and
the shoulders, should not be greater that an inter-
est height h
. Taking into account anthropo-
metric considerations (Matzner et al., 2015), we
selected h
= 40cm. Finally, contour analy-
sis (Luna et al., 2016) is performed to assign pix-
els to each ROI. Figure 2 shows two examples of
ROI estimation in scenes with two people, some
of them wearing accessories.
(a) Scene with two people, one
of them wearing a cap.
(b) Scene with two people, one
of them wearing a hat.
Figure 2: Examples of ROI’s estimation.
4. Feature Vector Calculation. The feature vector
components will be related to the pixel associ-
ated to the person surface in different height lev-
els within the corresponding ROI. In this work,
the feature vector is composed of six compo-
nents (Luna et al., 2016). Five of these features
will be related to the visible people or objects sur-
faces at different heights, and the sixth compo-
nent will correspond to the relationship between
the higher and lower diameters of the top surface,
providing an idea on the eccentricity of the person
head. The detail of the feature vector calculation
is shown in Algorithm 1.
First, features related to the pixels associated to
the head, neck and shoulders surfaces are calculated.
To do this, we divide the h
in 20 slices with a
slice height h (in this work, h = 2cm), counting the
number of pixels found in each slice s
, and building
a vector s =
The components of the s vector are very sensitive
to the appearance changes of a person (hair style, hair
length, neck height, etc.), the person height, and, ad-
ditionally, the effects of noise on the distance mea-
sures. To minimize the noise measurement errors, the
first three components of s (spanning 6cm) are inte-
grated in component ϕ
of the feature vector ϕ. If the
maximum value of the components s
is s
, we as-
sume that s
is corrupted by noise, and it is not taken
into account. In this case, ϕ
will integrate s
The feature vector components ϕ
(corresponding to
the head region too), and ϕ
(corresponding to the
shoulders) integrate three s
As the number of pixels associated to each compo-
nent ϕ
depends of the person height, it is necessary
to normalize them. To carry out the normalization,
the relationship between the maximum height hmax
and ϕ
was calculated. As an initial approximation, a
quadratic relationship has been defined:
ϕ = a
+ a
hmax + a
where a
, a
and a
are the coefficients to estimate.
The Levenberg-Marquardt algorithm was used for
the determination of those coefficients, using a sam-
ple set of people with heights between 140 cm and
213 cm. The final estimated values are a
= 0.138,
= 36.94, and a
= 2997.
The normalized components (ϕ
) of the fea-
ture vector provide information on the top view sur-
faces of people and objects, but initial experiments
on people detection showed the need to also include
more information related to the overhead geometry of
the head. So, a sixth component ϕ
has been added
to the feature vector. This component is calculated
as the relationship between the major and minor axes
of the region located 6cm below the maximum height
). In Algorithm 1 the function that calculates ϕ
is referred to as rba
Figures 3, 4 and 6 show several examples of real
depth frames for different situations, including the
profile of the feature vectors obtained for selected el-
2.2 People Class Selection
Prior to defining the required classes that will be used
to classify the detected ROIs as corresponding to a
person or not, we designed a dataset that was meant
to consider people with different heights, hair styles
and colors, complexions, and wearing or not acces-
sories that could heavily affect the feature vector com-
ponents (wearing hats, caps, etc.).
From the study of the acquired data, we initially
decided to define two classes, corresponding to peo-
VISAPP 2017 - International Conference on Computer Vision Theory and Applications
Input : height matrix H, number of ROI
s K, ROI
for k=1..K // Find feature vector for each ROI
for n=1..N // N is number of pixels belonging to ROI
i = 1 + round
// (x
) are the coordinates of pixel n belonging to ROI
and h = 2cm
= s
+ 1 // s
is the number of pixels in slice i, where i = 1,...,20
u = argmax
// Find the maximum value of s
where i = 1,...,3
for j=1..3 do
if u < 3 then
k+3( j1)
// Calculate ϕ
taking s
into account
k+3( j1)
// Calculate ϕ
without taking s
into account
u = argmax
// Find the maximum value of s
where i = 10,...,16
for j=1..2 do
k+3( j1)
// Calculate ϕ
= rba
// Calculate ϕ
Output: feature vector ϕ
Algorithm 1: Algorithm for Feature Vector Calculation.
(a) 3D point cloud measures. (b) 2D depth map. (c) Sample feature vector values.
Figure 3: Example of a scene with eight people. In Subfigure (c), top graphic corresponds to a person 165cm tall and long
hair, and the bottom graphic to a person 202cm tall and short hair.
(a) 3D point cloud measures. (b) 2D depth map. (c) Sample feature vector values.
Figure 4: Example of a frame with two people, one of them wearing a hat. In Subfigure (c), top graphic corresponds to the
person without hat, and the bottom graphic to a person wearing a hat.
ple with or without accessories (classes 1 and 2, re-
spectively). Some examples of training ROIs for peo-
ple without accessories are shown in Figure 7, while
Figure 8 shows some examples of ROIs for people
with accessories (hats and caps in this case).
When we introduced the use of the SVM classi-
fier (more on this below), a new class was added to
be able to model a general “non-people” class, com-
prising partial people ROIs, out of ROIs areas, chairs,
floor areas, fists from people in the sequences, etc.
Robust People Detection and Tracking from an Overhead Time-of-Flight Camera
(a) 3D point cloud
(b) 2D depth map. (c) Sample feature vector values.
Figure 5: Example of a frame with one person moving his fists up and down. In Subfigure (c), top graphic corresponds to the
person, and the bottom graphic to the detected fists.
(a) 3D point cloud measures. (b) 2D depth map. (c) Sample feature vector values.
Figure 6: Example of a frame with one person pushing a chair. In Subfigure (c), top graphic corresponds to the person, and
the bottom graphic to the detected chair.
Figure 7: Image samples for the people without accessories
class (class 1).
Figure 8: Image samples for the people with accessories
class (class 2).
Figure 9 provides some examples of training material
for the non-people class.
Figure 9: Image samples for the non-people class (class 3).
2.3 Classifier
Two approaches were selected in order to classify
each feature vector as corresponding or not to a per-
son: Principal Component Analysis (PCA), and Sup-
port Vector Machines (SVM), which will be briefly
described next.
2.3.1 PCA based Classifier
Our first approach was using a classifier based on
Principal Component Analysis (PCA) (Shlens, 2014;
enez et al., 2005), due to its simplicity and robust-
ness. This strategy required an offline estimation of
the models for each class, prior to the online classifi-
cation process.
In the offline process, the two transformation ma-
trices required in the PCA strategy are calculated. To
do so, a number of training vectors were used, associ-
ated to different people representative of each of the
two people classes.
The transformation matrices for each class are
formed by the eigenvectors associated to the high-
est eigenvalues of the corresponding scatter matri-
ces (Shlens, 2014; Jim
enez et al., 2005). In our
case, three eigenvectors have been chosen, follow-
ing the criterion that the average normalized residual
quadratic error (RMSE) is higher than 90%.
In the classification process (online process), the
feature vector of each ROI is calculated, and for each
class, the difference between this vector and the aver-
age vector class is projected in the transformed space.
VISAPP 2017 - International Conference on Computer Vision Theory and Applications
The projected vector will then be recovered in the
original space. The Euclidean distance between the
projected and recovered vectors is computed, and re-
ferred to as the reconstruction error. This process is
applied for each of the two classes.
Finally, a feature vector is classified as corre-
sponding to a person if its reconstruction error is
lower than a given threshold for any of both transfor-
mations (classes). The threshold for each class was
determined experimentally for each class, calculated
from the average value of the reconstruction error and
its its standard deviation (evaluated on the training
2.3.2 SVM based Classifier
As an alternative to the PCA classifier described
above, we also addressed the use of a SVM as the
final people classifier(Burges, 1998), also requiring a
supervised training stage.
We initially planned to use a binary SVM (to dis-
tinguish between people and non people), but the rel-
atively bad results we obtained in preliminary exper-
iments, lead us to use a multiclass SVM (Burges,
1998; Hsu and Lin, 2002), in which we included
classes for people with and without accessories, in ad-
dition to the non-people class.
The SVM models were trained from manually se-
lected areas covering a broad range of conditions in
what respect to people and non-people characteristics,
and their distribution along the recording area.
Preliminary experiments were run in order to de-
cide the SVM kernel type, and the optimal values
for the C and gamma coefficients. The final config-
uration used was a radial kernel with C = 0.5 and
gamma = 0.00015.
2.4 Tracking System
As shown in Figure 1, the global system includes a fi-
nal tracking stage, that is executed from the results of
the people detector final classifier. This tracking pro-
cess allows to obtain each detected person trajectory
along the video sequence, i.e., its position and speed
at each time t.
The resulting data from the people detector inform
about the number of persons P
detected in the corre-
sponding image I
, as well as their position (x
with p
= 1..P
. These data are used as in a proba-
bilistic filtering and tracking process based on a single
particle filter that is thus used for multimodal model-
ing of the dynamics hypotheses of the people in the
A constant speed model is used to perform the
probabilistic filtering and tracking, whose state (2)
and output (3) equations show that the state vec-
tor includes information about both the person posi-
tion and speed hypothesis x
= (x
while the output vector just includes the position hy-
pothesis, as it is generated by the people detector,
= (x
p,t +1
p,t +1
1 0 1 0
0 1 0 1
0 0 1 0
0 0 0 1
+ w
1 0 0 0
0 1 0 0
+ v
More specifically, the tracking system used here
is based on the eXtended Particle Filter with Cluster-
ing Process (XPFCP (Marron et al., 2005)), with a set
of n = 1..N
particles (people hypotheses), from
which N
are renewed at each iteration of the filter,
and N
are kept in order to ensure the estimation
multimodality and skip the impoverish problem that
this proposal may suffer from (A. Doucet, 2001), thus
= N
+ N
The filter is therefore conformed by five stages, as
shown in Figure 10, whose functionality is described
Figure 10: XPFCP functional diagram.
Prediction. Using the state model in equation 2,
a propagation from time t 1 to time t of all
n = 1..N
hypotheses (particles) state vector
= (x
)) is performed.
Association+Correction. The reliability of each
n = 1..N
particle (person position and speed
hypothesis, represented by the state vector) in the
tracking is obtained through the particle weight
. This is computed with the Mahalanobis dis-
tance in the image of the position represented
Robust People Detection and Tracking from an Overhead Time-of-Flight Camera
by the particle output vector (from the predicted
state vector x
= (x
) with the
output model 3) and the nearest person detec-
tion y
= (x
): the smallest distance will
give the biggest weight through a Gaussian model
of the noise v
in equation 3, and thus, the
biggest reliability of the hypothesis represented
by the corresponding particle x
, using a Near-
est Neighbor association strategy (Ekman, 2008).
Selection. Using their normalized weights w
(with n = 1..N
), the most reliable particles are
selected with a residual resampling algorithm (Liu
and Chen, 1998), giving a final set of N
cles, and taking out the least N
reliable ones
from the set, that will be substituted by new ones
at the next re-initialization filter step.
Clustering. A K-means clustering is then per-
formed over the final N
set of particles, thus
obtaining the result of the global people detection
and tracking system: a list of the filtered trajec-
tory estimations for all the persons in the input
sequence, represented by the clusters’ centroids
of the particles’ set, i.e., a state vector contain-
ing the position and speed of their represented per-
sons in the image x
= (x
). An-
other NN association process is finally carried out
between this global output in I
, and its previous
result in I
, allowing the identification of each
person track, with a certainty value.
Re-initialization. Before finishing, the filter pre-
pares the set of particles for its next iteration,
recruiting the needed N
hypotheses to com-
plete the N
set. These particles are generated
from the people detection output y
= (x
with p
= 1..P
(a set of points next of each of
its classification output), increasing its robustness
and avoiding the impoverish problem of the multi-
modal particle filter thanks to the re-initialization
strategy in the XPFCP that reinforces the weakest
modes in the probabilistic people location density
function that the global set represents.
3.1 Experimental Setup
In order to provide data for training and evaluating
the proposal, we used a preliminary subset of a depth
database that is being recorded with a Kinect
v2 de-
vice located at a height of 3.4m
The recordings tried
The GOTPD1 database ( (Macias-Guarasa et al., 2016),
that is available to the academic community for research
to cover a broad variety of conditions, with scenarios
Single and multiple people
Single and multiple non-people (such as chairs)
People with and without accessories (hats, caps)
People with different complexity, height, hair
color, and hair configuration
People actively moving and performing additional
actions (such as using their mobile phones, mov-
ing their fists up and down, etc.).
The data used was split in two subsets, one for
training and the other for testing. The subsets are fully
independent, so that no person present in the training
database was present in the testing subset.
Table 1 and Table 2 show the details of the train-
ing and testing subsets, respectively. #Samples refers
to the number of all the heads over all the frames in
the recorded sequences (in our recordings we used 39
different people). The database contains sequences in
which the users were instructed on how to move un-
der the camera (to allow for proper coverage of the
recording area), and sequences where people moved
freely (to allow for a more natural behavior).
The testing subset only included sequences with
two or more people (up to eight), and it was further
divided in two subsets (C1 and C2), to evaluate the
developed systems.
3.2 Results and Discussion
Our baseline system was the one based on the PCA
classifier and with no tracking stage. In the tables be-
low, FP and FN are the number of false positives and
false negatives respectively, and %ERR is the system
error rate (ERR = 100 · [(FP + FN)/#Samples]). The
tables also include confidence intervals calculated on
the ERR metric, for a confidence level of 95%.
Table 3 shows the results of our first experiment
comparing the performance of the PCA classifier (row
PCA) to that of the SVM one (row SVM), using test-
ing subset C1. From the table, it can be clearly seen
that the SVM classifier is much better at accurately
modeling people: The simple linear approach by the
PCA strategy is not able to cope with the variability
of people characteristics and varying positions along
the recorded space.
Table 4 shows the effect of using the tracker (row
PCA+XPF) as compared to only using the PCA clas-
sifier (row PCA), using testing subset C2. In this case,
it’s also clear that the tracker provides an improve-
ment as compared to the baseline system, although
purposes .
VISAPP 2017 - International Conference on Computer Vision Theory and Applications
Table 1: Details of the training subset.
Sequence ID #Samples Description Class
Single person
Class 1:
Person without
S0041S046 1682
S0047S0048 373
Multiple people with
accessories (hats, caps)
Class 2: Person with
accessories (hats, caps)
Table 2: Details of the testing subsets.
Testing subset #Samples #Class1 #C lass2
C1: Sequences with two or more people 8592 7013 1579
C2: Sequences with two or more people 9510 7762 1748
Table 3: Comparison betwen the PCA and SVM classifiers
(using testing subset C1).
Classifier FN FP %ERR
PCA 756 25 9.09 ± 0.61
SV M 335 27 4.21 ± 0.42
Table 4: Comparison betwen the use of not of the tracker
(using testing subset C2).
Tracking use FN FP %ERR
PCA without tracking 355 4 3.77 ± 0.38
PCA plus tracking 335 9 3.60 ± 0.37
the differences are not statistically significant. This
result is consistent with the idea that the people de-
tection process is very accurate, so that the tracking
stage can only achieve minor improvements, specially
in the reduction of false negatives, at the expense of a
very slight increase in the number of false positives.
Both effects are due to the ability of the tracker to
provide additional hypothesis that the people detector
could not generate (due to either misclassifications or
occlusions in the depth image).
In this work, we proposed a system for the robust
detection of people in depth images, captured by an
overhead ToF camera. The proposal comprises sev-
eral stages, and it allows achieving the detection of
multiple people in the scene in a robust way.
First, the isolated maximums in the scenes are de-
tected. Then, a Region of Interest (ROI) is precisely
defined around each maximum, and from the pixels
included in the ROI, a 6-component feature vector is
extracted, with their component values related to the
number of pixels in given areas of the ROI. The se-
lected feature vector has proved its efficacy for prop-
erly characterizing the people upper body geometry.
For the feature vector classification, we have pre-
sented two alternatives, based on PCA and SVM re-
spectively. The obtained results show that the SVM
classifier exhibits a higher performance, as the PCA
based strategy is not able to cope with the high vari-
ability of people and scene characteristics.
The proposal also includes a particle filter tracker
to allow people identification and tracking. The per-
formance impact of the tracker have been analyzed by
comparing the results with and without this stage. The
results demonstrate that the incorporation of the track-
ing stage not only allows to have information about
the trajectory and velocity of each person, but also
improve the detection results, reducing the error rate.
Future work will include a more exhaustive exper-
imental work, exploiting more sophisticated classifi-
cation strategies, and applying the system to actual
people counting solutions in realistic scenarios.
This work has been supported by the Spanish
Ministry of Economy and Competitiveness under
projects SPACES-UAH (TIN2013-47630-C2-1-R)
and HEIMDAL (TIN2016-75982-C2-1-R), and by
the University of Alcal
a under projects SCALA
(CCG2016/EXP-010), DETECTOR (CCG2015/
EXP-019) and ARMIS (CCG2015/EXP-054).
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