2015 IEEE Winter Conference on Applications of Computer Vision

Gait-based person identiﬁcation method using shadow biometrics forrobustness to changes in the walking direction
Makoto Shinzaki1 †, Yumi Iwashita1 ‡, Ryo Kurazume
Kyushu University
744 Motooka Nishi-ku Fukuoka, Japan

Koichi Ogawara
Wakayama University
930 Sakaedani Wakayama, Japan†shinzaki@irvs.ait.kyushu-u.ac.jp, ‡yumi@ieee.org

Abstract

body area of the subject [8] [9]. The projected body area ofthe subject, i.e. the shadow of the subject, is used to extract
biometric information from images taken by overhead cameras. In [9], we focused on images taken at an oblique angle
with the ground so that both body and shadow areas of thesubject are captured enough. In these images, we can consider that body and shadow areas contain body information
captured from two different viewpoints - camera and Sun
direction. Thus the use of both information from shadows
and information from directly observed body can provide
a higher correct classiﬁcation rate than from either taken
alone. However, if the subject’s walking direction and theposition of the Sun are different from that in the database,the appearances of both body and shadow areas are different from those in the database. This causes that the correctclassiﬁcation rate decreases.
The problem of appearance variations due to changesof the subject’s walking direction compared with that in
the database is a common problem in gait recognition, andgait researchers have been working on this topic [5] [12].
Kusakunniran et al. proposed a view transformation modelusing Support Vector Regression to synthesize virtual viewpoint images from captured images [10]. Felez et al. proposed an approach which casts gait recognition as a bipartite
ranking problem [3]. The proposed method can learn features invariant to covariate condition changes from different
people and different datasets. However, if training datasetsto learn models do not include images [10] / features [3]of the same condition with the subject’s one, the performance gets worse. We proposed a visual hull-based approach which synthesizes gait images at virtual viewpoints
using sequential 3D model of the walking subject [5]. Thismethod has an assumption that 3D models of the walking
subject are reconstructed in advance. However, if the 3Dmodels are not available, the visual hull-based method cannot be used. Muramatsu et al. proposed a method, which
overcomes the drawback of [5], to generate an arbitrary
view transformation model [12]. The model is constructedusing a 3D gait database composed of non-subject multi-

Person recognition from gait images is generally not robust to changes in appearance, such as variations of thewalking direction. In general conventional methods havefocused on training a model to transform gait features or
gait images to those at a different viewpoint, but the performance gets worse in case the model is not trained at a
viewpoint of a subject. In this paper we propose a novel
gait recognition approach which differs a lot from existing
approaches in that the subject’s sequential 3D models andhis/her motion are directly reconstructed from captured images, and arbitrary viewpoint images are synthesized fromthe reconstructed 3D models for the purpose of gait recognition robust to changes in the walking direction. Moreover, we propose a gait feature, named Frame Difference
Frieze Pattern (FDFP), which is robust to high frequency
noise. The efﬁciency of the proposed method is demonstrated through experiments using a database that includes
41 subjects.

1. Introduction
Person recognition methods are useful for various applications, such as surveillance applications for security operations and service robots coexisting with humans in daily
life. Gait is one of biometrics which do not require interaction with subjects and can be performed at a distance. In
general, existing gait recognition methods assume that cameras are placed at locations where an entire body shape of aperson can be observed. However, if cameras are placed athigh locations, such as on UAV or the rooftops of tall buildings, for the purpose of wide area security operations, thecorrect classiﬁcation rate decreases. This is because the human body area, which is used to extract gait features, cannot
be captured sufﬁciently from overhead cameras.
To deal with this problem, we proposed shadow biometrics, which is a biometrics method that uses the projected
1 These authors contributed equally to the paper.

978-1-4799-6683-7/15 $31.00 © 2015 IEEE
DOI 10.1109/WACV.2015.95

670

ple people’s 3D models. However, since the subject’s 3Dmodel is not included in the training dataset to learn theview transformation model, the synthesized image may notcoincide to the real one.

1.1. Proposed method
In this paper we propose a novel gait recognition approach which differs a lot from existing approaches in thatthe subject’s sequential 3D models and his/her motion are
directly reconstructed from captured images, and arbitrary
viewpoint images are synthesized from the reconstructed
3D models for the purpose of gait recognition robust tochanges in the walking direction. There are three challenges
we deal with in the proposed method.
1.

2.

Figure 2. An example scenario using the proposed method.

To estimate the subject’s motion, markerless motion
capture technology [13] [2] can be used. In general
the markerless motion capture system prepares a 3D
shape model with bones in advance and ﬁts the shapemodel to a reconstructed 3D model of a subject for motion estimation (Fig. 1 (a)). However, if the 3D shapemodel of the subject is not available, the motion of thesubject cannot be estimated (Fig. 1 (b)). Thus the 3D
shape model of the subject should be estimated at thesame time with the estimation of his motion.

To solve above three challenges, in the proposed method
we utilize a statistical 3D shape model to estimate the 3Dmodel of the subject at the same time with the motion estimation [14] and we introduce a statistical gait motion model
to estimate his motion robust to self-occlusion. The statistical 3D shape model consists of an average 3D shape modeland several shape parameters, and by changing the shapeparameters various types of 3D shapes can be reconstructed[14]. The statistical gait motion model consists of an average gait motion model and several gait parameters, and bychanging the gait parameters different gait motions can begenerated. In this paper, we call a statistical model which
combines both the statistical 3D shape model and the statistical gait motion model. Besides, we take advantage of the
shadow biometrics, which allow us to obtain two different
viewpoint images using a single camera.
There are three possible scenarios to use the shadow biometrics, in case that we assume that images are taken at an
oblique angle with the ground. The ﬁrst one is that shadowsare projected in images of both training and test datasets,so both shadow and body areas are available. The second
one is shadows are projected in images of test dataset, thus
both shadow and body areas are available in the test dataset
but only body area can be used in the training dataset. In
this case, after we reconstruct 3D model of a subject using
the proposed method as shown in Fig. 2, we synthesize an
image from a view point of the training dataset. The last
scenario is the other way round of the second scenario. In
this paper, we focus on the second scenario.
In the proposed method, ﬁrst we extract body andshadow areas of the subject, and then we estimate parameters of both statistical 3D shape model and statistical gait
model so that the statistical model ﬁts to the reconstructed
3D model of the subject. Next, a virtual image of the statistical model is synthesized from a viewpoint of the training
dataset. Finally we extract a newly proposed gait feature,

The accuracy of motion estimation may not be goodenough for person recognition, due to self-occlusion.
One possible way of improving the accuracy is to increase the number of cameras which captures imagesof the subject. However, if the gait recognition system is installed in public areas for security purposes,
in general an area is covered by a single camera, notmultiple cameras.

3. To obtain position information of the subject, images
taken from multiple viewpoints are necessary.
3D reconstructed
model of a subject

3D modelwith bones

3D reconstructed
model of a subject

3D modelwith bones

?

(a)

(b)

Figure 1. (a) Motion estimation with a 3D shape model with bones,(b) motion cannot be estimated correctly due to a 3D model which
does not ﬁt to the subject. For the purpose of showing models
clearly, we place objects separately.

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named Frame Difference Frieze Pattern (FDFP), from synthesized images, followed by person identiﬁcation. The advantage of the statistical 3D shape model and the statisticalgait model is that by tuning parameters whole body shapeof the subject and motion can change. Thus even thoughsome of body areas are not visible in captured images due
to occlusions, the body shape and motion can be estimatedby changing parameters of the statistical model using nonoccluded areas, which can be obtained in captured images.
The remainder of the present paper is organized as follows. Section 2 describes the statistical 3D shape modeland introduces the statistical gait motion model. Section 3 introduces the proposed method to estimate parameters of both statistical models and the newly proposed
gait features. Section 4 describes experiments performed
to demonstrate the efﬁciency of the proposed method, andSection 5 presents our conclusions.

Figure 3. Statistical 3D shape model consisting of an average 3D
shape model and shape parameters

2. Statistical models
In this section, we ﬁrst brieﬂy describe the statistical 3D
shape model and then introduce the statistical gait motion
model. Both models are generated from training datasets ofmultiple people.
Figure 4. An example sequence of 3D models in the 4D gait
database.

2.1. Statistical 3D shape model
The statistical 3D shape model [14] was constructed byusing an AIST/HQL dataset [1], which consists of 3D models of 97 people (49 male and 48 female). The model consists of an average 3D shape model and 11 shape parameters, and by changing the shape parameters various typesof 3D shapes can be reconstructed. Assume that the average
3D shape x̄, which is deﬁned as x̄ = N1 ΣN
i=1 xi (N is the total number of 3D models). Here, a 3D model of a person i isdeﬁned as xi = (xi1 , yi1 , zi1 , · · · , xik , yik , zik )T (k is thenumber of vertex points of a 3D model). The difference between the average 3D shape x̄ and a 3D model xi is deﬁned
as dxi = xi − x̄. By applying eigenvalue decomposition to(dx1 , dx2 , · · · , dxN )T , eigenvectors and eigenvalues are
obtained. Finally, a statistical 3D shape model is deﬁned asx = x̄ + Eb.

of one gait cycle. The database comprises 41 subjects withfour sequences for each subject (Fig. 4).
First, we estimate motion of each walking person in the
database using a markerless motion capture system [13]. As
we mentioned in Section 1.1, a 3D model with bones foreach person is necessary, so we estimate the 3D model withbones using the statistical 3D shape model ﬁrst. In concrete,
we estimate parameters of the statistical 3D shape modelso that the statistical 3D shape model can ﬁt to the reconstructed 3D model of each person. Moreover, we embedded
a bone model, which has 30 degree of freedom in total, tothe statistical 3D shape model, so that positions of all joints
can move with the change of parameters of the statistical
3D model.
After motions of all gait sequences are estimated, we
generate the statistical gait motion model as follows. First,
since the number of frames in one gait cycle differs amongwalking sequences, we normalize the number of frames in
one gait cycle into a certain number F (we set F = 22
in experiments). We basically use the same methodology
to generate the statistical gait model with the method forthe statistical 3D shape model, but the difference is that weapply a frequency analysis to motions before we apply the
eigenvalue decomposition. The reason why we apply a fre-(1)Here, E = (e1 , e2 , · · · , et ) is eigenvectors and b =(b1 , b2 , · · · , bt )T is shape parameters. t = 11 with 95 %contributing rate. By changing the shape parameters, various types of 3D shapes can be reconstructed as shown in
Fig. 3.

2.2. Statistical gait motion model
The 3D models used in section 2.1 are the ones of people
standing. To generate a statistical gait model we use a 4D
gait database [7] consisting of 3D models for the duration
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quency analysis is as follows. Gait is a repetitive motion,
thus once parameters are tuned properly, gait motion for theduration of one gait cycle can be reconstructed even thoughgait motion for a part of one gait cycle is not obtained.
Let us deﬁne an average angle vector Ujs =s(u(1,j) , us(2,j) , · · · , us(F,j) )T , and us(i,j) is deﬁned as anangle of a joint j (1 ≤ j ≤ J, J is the number of joints) and a gait sequence s (1 ≤ s ≤ S,
S is the total number of gait sequences). We apply
a discrete cosine transform to Ujs and obtain a power
spectrum vector dsj = (ds(1,j) , ds(2,j) , · · · , ds(F,j) )T . A
power spectrum vector of all joints is deﬁned as D s =(ds(1,1) , ds(2,1) , · · · , ds(F −1,J) , ds(F,J) ), and ﬁnally we deﬁnea vector K = (D 1 , D 2 , · · · , D S ).
The eigenvalue decomposition is applied to K, and thestatistical gait motion model is deﬁned as
K = K̄ + N w.

3. Person identiﬁcation robust to viewpointchangesIn this section, we explain details of the proposed method
and a new gait feature, named Frame Difference Frieze Pattern (FDFP).

3.1. Shape and motion estimation using shadow
biometrics
The advantage of the use of both body and shadow areasis that we can use two viewpoint images using a single camera. This is useful to estimate the position information of
the subject, which is necessary for motion estimation withhigh accuracy. The position information of the subject canbe estimated using both shadow and body areas with the assumption that the pose of the Sun is known with respect tothe camera. In this paper, we assume that the position information of the subject for the duration of one gait cycle is
known, but instead we focus on showing the effectivenessof the proposed concept. The position estimation will be
our future work.
As we mentioned in section 1.1, the scenario we focus
on is that both shadow and body areas are available in thetest dataset but only body area can be used in the training
dataset. Thus, after we estimate a 3D shape and gait motionof a subject, we synthesize an image from a viewpoint of
the training dataset as shown in Fig. 2. To estimate the 3D
shape and gait motion of the subject, we estimate shape parameters and gait parameters of the statistical model using
the following procedure.

(2)Here, N = (n1 , n2 , · · · , nt ) is eigenvectors and w =(w1 , w2 , · · · , wt )T is gait parameters. t = 20 with 95 %contributing rate. To reconstruct joint angles, we apply
an inverse discrete cosine transform to K. By changing
the gait parameters, various types of motions can be reconstructed as shown in Fig. 5.

1. First, we set an initial position of the statistical modelto the position the subject. Here, since we have the information of positions of the subject for the duration of
one gait cycle, his walking direction with respect to thecamera can be calculated. Next, we virtually project a
shadow of the statistical 3D model to the ground using
the position information of the Sun, and synthesize an
image from a virtual viewpoint as shown in Fig. 6.

(a) Average gait model for the duration of one gait cycle2.

After we obtain a virtual image, we extract a silhouette area, followed by a contour extraction of body andshadow areas (Fig. 7 (a)). We apply the same process
to an image of the subject to extract a contour of thesubject as shown in Fig. 7 (b).

3. At each point pi on the contour of the image of the subject the nearest neighbor point qj is obtained, and theevaluation value d is calculated by d = Σi (pi − qj )2 .
The shape and gait motion parameters are estimatedusing the steepest descent method to minimize theevaluation value.

(b) Motion variations with the change of gait parameters

Figure 5. Statistical gait motion model consisting of an average
gait motion and gait parameters. (a) average gait motion and (b)motion variations with the change of gait parameters.

673this may result in higher frequency noise. So in this section, we propose a new gait feature, named Frame Difference Frieze Pattern (FDFP), which is designed to be robust
to higher frequency noise. As the name suggests, the FDFP
is inspired by the Frieze Pattern [11].
First, we calculate a frame difference image between aframe f and a frame f − Δf as follows.

255 (I(f, x, y) = I(f − Δf, x, y))S(x, y) =(3)0(I(f, x, y) = I(f − Δf, x, y))Here, f is a frame ID in the duration of one gait cycle (1≤ f ≤ F and F =22), and Δf is set from 1 to 15. Example
images of Δf = 1 and Δf = 2 are shown in Figs. 8 (a) and(b). The FDFP is calculated using frame difference images
which are generated from silhouette images for the durationof one gait cycle. The FDFP has two feature vectors, one is
F DF Prow generated along the vertical axis and the otherone is F DF Pcol generated along the horizontal axis.

Figure 6. Flow of the proposed method to estimate shape and motion parameters of the statistical model.

F DF Prow (y, f ) = Σx S(x, y, f )(4)

F DF Pcol (x, f ) = Σy S(x, y, f )(5)

Examples of F DF Prow and F DF Pcol are show in Fig. 8(c).
The concept of the FDFP is similar to the Shape
Variance-Based (SVB) Frieze Pattern [11], but the difference is as follows. In the SVB Frieze Pattern, the authors
set a key frame, which is a double-support position, and calculated a frame difference as the difference between a frame
f and the key frame. On the other hand, our FDFP does not
require to detect the key frame and a frame difference iscalculated as the difference between a frame f and a frame
f − Δf . Moreover, by changing the value Δf , the FDFP
can represent much more information.

(a) Virtual image(b) Test dataset

Figure 7. Silhouette and contour extraction of a virtual image (a)and an image in the test dataset (b).

3.3. Person identiﬁcation
4.

We identify the subject based on the combination of thenearest neighbor approach and the voting-based method [7].
During the training phase, we extract gait features using the
FDFP from each image in the training sequences and thenbuild a database. Then, during the identiﬁcation phase, gaitfeatures are extracted from each synthesized image of a subject’s sequence in the test dataset. At each frame of the subject’s sequence, a person who has the nearest gait feature in
the training dataset is voted, and this process is repeated forthe duration of one gait cycle. Finally, a person who getsthe most votes is chosen as the subject.

Step 1 to 3 are repeated for each image of one gait
cycle.

After we reconstruct the statistical model with estimatedshape and gait parameters, we synthesize a virtual image
which are taken with the same viewpoint of the training
dataset.

3.2. Frame Difference Frieze Pattern (FDFP)In the statistical shape model and the statistical gait motion model, we use eigenvectors up to 95 % contributing
rate, thus we omit eigenvectors with higher frequency. This
causes a small difference between the estimated statisticalmodel and the 3D reconstructed model of the subject, and

4. Experiments
This section presents the results of person identiﬁcation
robust to changes in the walking direction. To prove the674

(a) An example of images taken
at a virtual camera θ1 , which areused as test datasets.

(b) An example of images taken
at a virtual camera θ2 , which
are used as training datasets.

(a) Frame difference (Δf = 1)

Figure 9. Example images of test and training datasets.

four sequences for each subject, so totally there are 164 gait
sequences. In this experiment, we synthesized images byusing a virtual camera which is placed at a viewpoint θ1with respect to 3D models in the database as shown in Fig.
9(a), and we used these images as test images. Regarding
the training datasets, we placed a virtual camera at a viewpoint θ2 , and synthesized images as shown in Fig. 9(b).

4.1. Evaluation of the estimated statistical modelFirst we evaluated the statistical model with estimatedparameters by the proposed method qualitatively and quantitatively. In a top row in Fig. 10 (a) we show examplesof silhouette images at a view point θ1 in the test dataset.
The second row shows example silhouette images synthesized at a view point θ1 from the statistical models whoseparameters were estimated by the proposed method. Since
parameters of the statistical models are estimated using images at view point θ1 , it is easily expected that synthesizedimages from the estimated statistical models are similar tosilhouette images in the test dataset. As we mentioned in
section 1.1, the advantage of the use of the statistical modelis that it can estimate motion and 3D model in areas which
are not visible from the camera due to occlusions. To see
the effectiveness of the statistical model, let us see the results from a view point θ2 as shown in Fig. 10 (b). Fromthese results, we can verify the proposed method can reconstruct 3D model and motion in occluded areas.
We evaluated the accuracy of the proposed method quantitatively. Since in experiments test images are synthesizedfrom 3D reconstructed models of walking subjects, we cancompare the accuracy of the statistical model with estimatedparameters from reconstructed 3D models. We calculated
a sum of minimum distances between the statistical modeland the reconstructed 3D model at each frame, and we obtain average distance and standard deviation using models
for the duration of one gait cycle. Figure 11 (a) shows results of all 41 people in case that no parameters of the statistical model are estimated, and Fig. 11 (b) shows results in(b) Frame difference (Δf = 2)(c) Examples of Frame Difference Frieze Pattern (Δf = 1)

Figure 8. Frame Difference Frieze Patterneffectiveness of the statistical shape model and the statistical gait motion model, we carried out experiments using the
4D gait database. The database comprises 41 subjects with675

case that parameters are estimated by the proposed method.
From these results, it is clear the proposed method worked
well even though we use just two viewpoint images (i.e.
shadow and body areas).

Error (distance)
120
100
80
60
40
20
0

5

10

15

20

25

30

35

40

25

30

35

40

Person ID(a)Error (distance)
120
100
80
60
40
20

(a) View point θ1

0

5

10

15

20

Person ID(b)

Figure 11. Average and standard deviation of distances between
reconstructed 3D models and statistical models with estimated parameters.Results for statistical models without parameter estimation and (b) results for statistical models with parameters estimatedby the proposed method.

First, we used image sequences at the viewpoint θ1 fortest and training datasets, and Table 1 shows results using
GEI, AIE, AMI and the proposed FDFP. Here, in the AMIbased method there is a parameter which is used to divide
a silhouette area into multiple areas. Thus we changed thisparameter in experiments. From these results, it is clear the
proposed gait feature FDFP showed the best performance.
Next, we used images sequences at the viewpoint θ1 fortest dataset and images sequences at the viewpoint θ2 fortraining dataset, and Table 2 shows results. The results ofexisting methods, GEI, AIE, and AMI are worse than thoseof the viewpoint θ2 . On the other hand, the proposed FDFP
showed the best performance thanks to the robustness to thehigh frequency noise.

(b) View point θ2

Figure 10. 3D shape and motion estimation. Synthesized images
from a viewpoint θ1 (a) and a viewpoint θ2 (b).

4.2. Person identiﬁcation using the proposed
methodIn next experiments, we identiﬁed people using the proposed method and compared the performance of the proposed gait feature (FDFP) with other existing gait features,such as Gait Energy Image (GEI) [4], Active Energy Image (AEI) [15], and Afﬁne Moment Invariants (AMI) [6],which showed good performance in other gait databases.
The evaluation was performed using a rule similar to leaveone-out cross validation. In other words, when we selected
one gait image sequence from test datasets, we eliminatedan image sequence corresponding to this selected image sequence from the database.

5. Conclusions
We herein proposed a method by which to identify people robust in changes in the walking direction. The proposed method differs signiﬁcantly from existing approaches
in that the subject’s sequential 3D models and his/her motion were directly reconstructed from captured images, byintroducing the statistical shape model and the statisticalgait motion model. Then arbitrary viewpoint images aresynthesized from the reconstructed 3D models for the purpose of gait recognition robust to changes in the walking676

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GEI [4]
AEI [15]
AMI [6]

Proposed
FDFP

78.66
78.05
The number of divided areas
1
5
9
13
15
54.27 89.02 90.24
89.02
89.02
Frame interval : Δf
1
5
9
13
15
73.17 98.17 99.39 99.39 99.39

Table 1. Person identiﬁcation using image sequences at the viewpoint θ1 for test and training datasets [%].

GEI [4]
AEI [15]
AMI [6]

Proposed
FDFP

34.15
46.95
The number of divided areas
1
5
9
13
15
9.76 23.78 24.39 28.66 28.66
Frame interval : Δf
1
5
9
13
15
39.02 74.39 84.15 85.98 83.54

Table 2. Person identiﬁcation using image sequences at the viewpoint θ2 for test and training datasets [%].

direction. We also proposed the new gait feature, named
Frame Difference Frieze Pattern (FDFP). We carried out experiments using the 4D database, which included data for 41
people, and demonstrated that the proposed method outperformed conventional methods.
Our future work includes to identify people using imagesby a camera placed outside.

Acknowledgment
This work is supported by Kyushu University Interdisciplinary Programs in Education and Projects in Research
Development (FS-type, No. 26338) and JGS-S ScholarshipFoundation.

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