A STEREOSCOPIC AUGMENTED REALITY SYSTEM

FOR THE VERIDICAL PERCEPTION OF THE 3D SCENE LAYOUT

M. Chessa, M. Garibotti, A. Canessa, A. Gibaldi, S. P. Sabatini and F. Solari

The Physical Structure of Perception and Computation Group, University of Genoa, Genoa, Italy

Keywords:

Stereoscopic Display, Virtual Reality, 3D Visualization, Head Tracking, 3D Position Judgment, Shape

Perception, Human and Computer Interaction.

Abstract:

The recent diffusion of the stereoscopic 3D technologies has yielded the development of affordable and of

everyday use devices for the visualization of such information. This has paved the way for powerful human

and computer interaction systems based on augmented reality environment where humans can interact with

both virtual and real tools. However, an observer freely moving in front of a 3D display could experience a

misperception of the depth and of the shape of virtual objects. Such distortions can have serious consequences

in scientiﬁc and medical ﬁelds, where a veridical perception is required, and they can cause visual fatigue

in consumer and entertainment applications. Here, we propose a novel augmented reality system capable to

correctly render 3D virtual objects, without adding signiﬁcant delay, to an observer that changes his position

in the real world and acts in the virtual scenario. The correct perception of the scene layout is assessed through

two experimental sessions with several observers.

1 INTRODUCTION

In the last decade, there has been a rapidly growing

interest in technologies for presenting stereo 3D im-

agery both for professional applications, e.g. scien-

tiﬁc visualization, medicine and rehabilitation system

(Subramanian et al., 2007; Ferre et al., 2008; Knaut

et al., 2009), and for entertainment applications, e.g.

3D cinema and videogames (Kratky, 2011).

With the diffusion of 3D stereo visualization tech-

niques, researchers have investigated the beneﬁts and

the problem associated with them. Several studies

devised some speciﬁc geometrical parameters of the

stereo acquisition setup (both actual and virtual) in

order to induce the perception of depth in a human

observer (Grinberg et al., 1994). In this way, we can

create stereo pairs that are displayed on stereoscopic

devices for human observers which do not introduce

vertical disparity, and thus causing no discomfort to

the users (Southard, 1992). Yet, other factors, re-

lated to spatial imperfections of the stereo image pair,

that yield visual discomfort have been addressed. In

(Kooi and Toet, 2004) the authors experimentally de-

termined the level of discomfort experienced by a

human observer viewing imperfect binocular image

pairs, with a wide range of possible imperfections and

distortions. Moreover, in the literature there are seve-

ral works that describe the difﬁculty of perceptually

rendering a large interval of 3D space without a visual

stress, since the eyes of the observer have to maintain

accommodation on the display screen (i.e., at a ﬁxed

distance), thus lacking the natural relationship be-

tween accommodation and vergence eye movements,

and the distance of the objects (Wann et al., 1995).

The vergence-accommodation conﬂict is out of the

scope of this paper, however for a recent review see

(Shibata et al., 2011).

Besides the previously cited causes of discom-

fort, another well-documented problem is that the 3D

shape and the scene layout are often misperceived

by a viewer freely positioned in front of stereoscopic

displays (Held and Banks, 2008). Only few works

in the literature address the problem of examining

depth judgment in augmented or virtual reality envi-

ronments in the peripersonal space (i.e. distances less

than 1.5 m). Among them, (Singh et al., 2010) in-

vestigated depth estimation via a reaching task, but in

their experiment the subjects could not freely move in

front of the display. Moreover, only correcting meth-

ods useful in speciﬁc situation, e.g. see (Lin et al.,

2008; Vesely et al., 2011), or complex and expensive

systems (Cruz-Neira et al., 1993) are proposed in the

literature. Nevertheless, to the knowledge of the au-

thors, there are no works that aim to quantitatively

Chessa M., Garibotti M., Canessa A., Gibaldi A., P. Sabatini S. and Solari F..

A STEREOSCOPIC AUGMENTED REALITY SYSTEM FOR THE VERIDICAL PERCEPTION OF THE 3D SCENE LAYOUT.

DOI: 10.5220/0003831600150023

In Proceedings of the International Conference on Computer Vision Theory and Applications (VISAPP-2012), pages 15-23

ISBN: 978-989-8565-04-4

 2012 SCITEPRESS (Science and Technology Publications, Lda.)

analyze the 3D shape distortions, their consequences

on the perception of the scene layout, and to propose

an effective and general solution for an observer that

freely moves in front of a 3D monitor.

In entertainment applications, such distortions

might not be a serious problem, even if they can cause

visual fatigue and stress, but in medical and surgery

applications, or in cognitive rehabilitation system and

in applications for the study of the visuo-motor co-

ordination, they can have serious implications. This

is especially true in augmented reality (AR) applica-

tions, where the user perceives real and virtual stimuli

at the same time, thus it is necessary that the rendering

of the 3D information does not introduce undesired

distortions.

In this paper, we propose an AR system capable

to minimize the 3D shape misperception problem that

arises when the viewer changes his/her position with

respect to the screen, thus yielding a natural interac-

tion with the virtual environment. The performances

of the developed system are quantitatively assessed

and compared to the results obtained by using a con-

ventional system, through experimental sessions with

the aim of measuring the user’s perception of the 3D

shape and the effectiveness of his/her interaction in

the AR environment.

The paper is organized as follow: in Section 2

we describe the geometry of the standard stereoscopic

3D rendering technique, the misperception associated

with the movements of the observer, and our solution;

Section 3 describes the AR system we have devel-

oped; Section 4 presents two experimental sessions

to validate the proposed approach; the conclusion and

future work are discussed in Section 5.

2 THE GEOMETRY OF THE

STEREOSCOPIC 3D

RENDERING TECHNIQUE

2.1 The Standard Approach

To create the stereo 3D stimuli for the conventional

approach, we have adopted the stereo rendering, that

uses the method known as “parallel axis asymmetric

frustum perspective projection”, or off-axis technique

(Bourke and Morse, 2007; Grinberg et al., 1994), the

technique usually used to generate a perception of

depth for a human observer. In the off-axis technique,

the stereo images are obtained by projecting the ob-

jects in the scene onto the display plane for each cam-

era; such projection plane has the same position and

orientation for both camera projections (see Fig. 1).

Figure 1: The two skewed frustums for the off-axis tech-

nique.

We have also taken into account the geometrical

parameters necessary to correctly create stereo pairs

displayed on stereoscopic devices for human observer

(Grinberg et al., 1994). In particular:

- the image planes have to be parallel;

- the optical points should be offset relative to the

center of the image;

- the distance between the two optical centers have

to be equal to the interpupillary distance;

- the ﬁeld of view of the cameras must be equal to

the angle subtended by the display screen;

- the ratio between the focal length of the cameras

and the viewing distance of the screen should be

equal to the ratio between the width of the screen

and of the image plane.

Moreover, as in (Kooi and Toet, 2004), one should

take into account the problem of spatial imperfection

that could cause visual discomfort to the user, such as:

- crosstalk, that is a transparent overlay of the left

image over the right image and vice versa;

- blur, that is different resolutions of the stereo im-

age pair.

The previously mentioned rules are commonly

considered when designing stereoscopic virtual re-

ality systems. Nevertheless, when looking at a vir-

tual scene, in order to obtain a veridical perception of

the 3D scene layout, it is necessary that the observer

must be positioned in the same position of the virtual

stereo camera. If the observer is in the correct po-

sition, then the retinal images, originated by viewing

the 3D stereo display and the ones originated by look-

ing at the real scene, are identical (Held and Banks,

2008). If this constraint is not satisﬁed, a mispercep-

tion of the object’s shape and of the depth occurs (see

Fig. 2(a) and Fig. 3).

VISAPP 2012 - International Conference on Computer Vision Theory and Applications

projection plane

screen

left image

right image

virtual stereo camera

misperceived

target

observer

target

misperceived

target

observer

perceived

target

projection plane

screen

left image

right image

virtual stereo camera

target

observer

perceived

target

(a) (b)

Figure 2: A sketch of geometry of the stereoscopic augmented reality environment when using the standard stereo rendering

technique (a), and when using the proposed approach (b). (a) In the virtual environment a target is positioned in T, and a

stereo camera is placed inC

L,R

, thus generating the left and right projections t

and t

on the projection plane. A real observer

in the same position O

L,R

of the virtual camera will perceive the target correctly, whereas he will misperceive the position (

and

) of the target when looking at the screen from different positions (O

L,R

and O

L,R

). (b) In the proposed technique, the

virtual camera is moved accordingly to the different positions of the observer. This yields different projections of the target

, t

; t

, t

; t

, t

) on the projection plane, thus allowing a coherent perception of the target for the observer.

2.2 The Proposed Approach

In Figure 2(a), a virtual stereo camera positioned in

(for the sake of simplicity, we omit the superscript,

i.e. C

and C

denote the positions of the left and

right cameras, respectively) determines the left and

right projections t

and t

of the target T on the pro-

jection plane. An observer located in the same po-

sition of the virtual camera (O

= C

) will perceive

the target in a position

coincident with the true po-

sition. Otherwise, an observer located in a different

position (O

6= C

) will experience a misperception of

the location of the target (

6= T). This also causes

a deformation of the 3D shape, as it is schematically

shown in Figure 3 for a simple target represented by

a horizontal bar. When the observer is in a location

different from the position of the virtual camera, he

misperceives both the depth of the object and its spa-

tial structure.

To overcome this problem, the system we have de-

veloped, and that we present in this paper, is capable

of compensating for the movement of the observer by

computing his/her position with respect to the moni-

tor, and consequently moving the virtual stereo cam-

era (see Fig. 2(b)). For each different position of the

observer a corresponding stereo image pair is gener-

ated, and displayed on the screen. Thus, the observer

always perceives the 3D shape of the objects coher-

ently. Moreover, to maintain a consistent augmented

reality environment it is necessary to have a virtual

world that is at each moment a virtual replica of the

real world. Thus, the screen and the projection plane

should be always coincident.

3 THE PROPOSED AUGMENTED

REALITY SYSTEM

3.1 Software and Hardware

Components

Considering the availability of commercial products

with high performances and affordable costs, we de-

cided to use off-the-shelf devices to design and de-

velop our AR system. Speciﬁcally, we use the Xbox

Kinect, a motion sensing input device developed by

Microsoft for the Xbox 360 video game console.

A STEREOSCOPIC AUGMENTED REALITY SYSTEM FOR THE VERIDICAL PERCEPTION OF THE 3D SCENE

LAYOUT

projection plane

screen

left image

right image

virtual stereo camera

observer

misperceived

target

observer

target

misperceived

target

Figure 3: The deformation of the 3D shape when displaying

a horizontal bar with the standard technique. For a compar-

ison with the experimental results see Fig. 7(b).

Based on an RGB camera and on an infrared (IR)

depth sensor, this RGB-D device is capable of pro-

viding a full-body 3D motion capture. The depth

sensor consists of an IR projector combined with a

monochrome camera.

The main features of this device used for the de-

velopment of the system are:

- Frame rate: 30 Hz;

- Depth image size: VGA (640×480);

- Depth resolution: 1 cm at 2 m distance from the

sensor;

- Operation range: from 0.6 m to 3.5 m;

- Color image size: VGA (640×480);

- Horizontal Field of View: 58 deg.

All the software modules are developed in C++,

using Microsoft Visual Studio 10. To render the

stereoscopic virtual scene in quad buffer mode we use

the Coin3D graphic toolkit

, a high level 3D graphic

toolkit for developing cross-platform real time 3D vi-

sualization. To access the data provided by Microsoft

XBox Kinect, we use the open source driver (version

5.0.3.3), released by PrimeSense

, the company that

developed the 3D technology of the Kinect. The lo-

calization and the tracking of the head and the hand

rely on the framework OpenNI

(version 1.3.2.1), a

set of open source Application Programming Inter-

faces (APIs). These APIs provide support for access

www.coin3D.org

www.primesense.com

www.openni.org

to natural interaction devices, allowing the body mo-

tion tracking, hand gestures and voice recognition.

The processing of the images acquired by Kinect

RGB camera is performed through the OpenCV 2.3

library.

Both the development and the testing phases have

been conducted on a PC equipped with an Intel Core

i7 processor, 12 GB of RAM, a Nvidia Quadro 2000

video card enabled to 3D Vision Pro with 1 GB of

RAM, and a Acer HN274H 3D monitor 27-inch.

3.2 System Description

Figure 4 shows the setup scheme of our system. The

XBox Kinect is located on the top of the monitor, cen-

tered on the X axis, and slightly rotated around the

same axis. This conﬁguration was chosen because

it allows the Kinect to have good visibility on the

user, without having the Kinect interposed between

the user and the monitor. To align the two coordinate

systems, we performed a calibration step by taking

into consideration a set of world points, whose co-

ordinates are known with respect to the monitor co-

ordinate system, and their positions acquired by the

Kinect. In this way, it is possible to obtain the angle

between the Y axis of the Kinect coordinate sys-

tem and the monitor one. In the setup conﬁguration

considered for the experiments presented in Section 4

the value of α

is about 8 deg.

The proposed system, during the startup phase, is

responsible to recognize and track the position of the

skeleton of the user. After the startup phase, each time

new data is available from the Kinect, the system per-

forms a series of steps to re-compute the representa-

tion of the 3D virtual world, and to achieve the inter-

action with the user. These steps can be summarized

as follows:

- Positioning of the stereo camera of the virtual

world in the “tracked” position of the user’s head,

obtained from the Kinect. The stereo camera has

a ﬁxed angle of view, far wider than the one that is

created between the real user and the monitor, that

is his/her “window” on the virtual world. For this

reason, once repositioned the camera, our system

changes the viewport to display on the monitor

only the portion of the virtual world that the cam-

era sees through the area that represents the mon-

itor in the virtual world. Just as if we are looking

through a window.

- Detection of the position of the index ﬁnger of the

user, through a light marker, in the image plane

of the Kinect RGB camera. This can be achieved

opencv.willowgarage.com

VISAPP 2012 - International Conference on Computer Vision Theory and Applications

Target

Position: 0, 0, 800 mm

movement area

of the observer

Monitor Width: 600 mm

Minimum Distance: 600 mm

Side View

Monitor

Coordinate System

Kinect

Coordinate System

ZY plane

angle αy = 8°

Monitor Height: 340 mm

movement area

of the observer

Target

Position: 0, 0, 800 mm

Minimum Distance: 600 mm

Top View

ZX plane

Figure 4: The developed setup for the augmented reality system. The reported measures refer to the speciﬁc setup considered

for the experimental results. The position of the target is the same of the one of the experiment shown in Figure 5.

ﬁrst by obtaining the position of the hand from the

skeleton, produced by OpenNI libraries, and then

performing a segmentation and a tracking of the

light source in the sub-image, centered in the de-

tected position of the hand. This image processing

step is based on the color information, and is per-

formed by using OpenCV libraries.

- Computation of the position of the ﬁnger in the

real world by combining its position in the im-

age plane and the corresponding depth (from the

Kinect D camera). The spatial displacement be-

tween the RGB and D cameras has been taken into

account.

The computed 3D position both of the head and of

the ﬁnger have been stabilized and smoothed in time

through a small averaging window over 5 frames.

In order to evaluate the precision of the measure-

ments of the system we performed a test session, from

which we derived an uncertainty of about 1 cm (at 1 m

of distance from the sensor) on the three axes in real

world coordinates, both for the head and the ﬁnger

localization.

4 EXPERIMENTS AND RESULTS

To quantitatively assess the proposed augmented re-

ality system, and to verify if it effectively allows a

veridical perception of the 3D shape and a better inter-

action with the user, we have performed two types of

experiments. In the ﬁrst one, the observer was asked

to reach a virtual target (i.e. the nearest right bottom

vertex of a frontal cube whose width is 2.5 cm), in

the second one, the task was to trace the proﬁle of

a virtual horizontal bar. In both cases, the scene has

been observed by different positions, and we have ac-

quired the position of the head and of the ﬁnger dur-

ing the execution of the tasks. The experiments have

been performed both by using a standard rendering

technique (in the following denoted as tracking OFF)

and the proposed approach that actively modiﬁes the

rendered images with respect to the position of the

observer’s head (in the following denoted as tracking

ON). The virtual scenes have been designed in order

to minimize the effects of the depth cues other than

the stereoscopic cue, such as shadows and perspec-

tive. It is worth noting that since we use a real ﬁnger

to estimate the perceived depth of a virtual object, we

obtain a veridical judgment of the depth. If the task is

performed by using a virtual ﬁnger (a virtual replica

of the ﬁnger), we obtain a relative estimation of the

depth, only.

For the experiments six subjects were chosen,

with ages ranging from 28 to 44. All participants

(males and females) had normal or corrected-to-

normal vision. Each subject performed the two tasks

looking at the scene from 5 different positions both in

A STEREOSCOPIC AUGMENTED REALITY SYSTEM FOR THE VERIDICAL PERCEPTION OF THE 3D SCENE

LAYOUT

-300 -200 -100 0 100 200 300

-200

-150

-100

-50

100

-2000200

600

800

1000

1200

1400

1600

1800

X [mm]

Z [mm]

Y [mm]

X [mm]

Target

Head position with tracking ON

Head position with tracking OFF

Perceived position of the target with tracking OFF

Perceived position of the target with tracking ON

Figure 5: Perceived positions of the target point, and the related head positions for the reaching task. The perceived positions

of the target with our tracking system enabled (green circles) are less scattered than the ones using a conventional system (red

circles).

tracking ON and in tracking OFF mode.

4.1 First Experiment: 3D Position

Judgment

Figure 5 shows the results of the ﬁrst experiment.

When the tracking is ON, the positions of the

observer’s head (represented by yellow circles) are

spread in a smaller range than the positions acquired

when the tracking is OFF (represented by cyan cir-

cles). This happens since, as a consequence of the ac-

tive modiﬁcation of the point of view, the target could

not be visible from lateral views. This behaviour is

consistent with real world situations, in which objects

that are seen through a window disappear when the

observer moves laterally. Nevertheless, the perceived

target points when the tracking is ON (green circles)

are less scattered than the perceived positions of the

target point when the tracking is OFF (red circles),

also taking into account the more limited movements

of the observer in the ﬁrst situation.

Table 1 shows the mean errors and their standard

deviations of the perceived points. The tracking of the

head position of the observer and the re-computation

of the stereo pairs projected onto the screen, per-

formed by our system, yield a consistent reduction in

the error of the perceived position of the target and in

its standard deviation.

A graphic representation of the scattering areas of

the perceived points with respect to the movements of

Table 1: The mean error and the standard deviation values

(expressed in mm) of the perceived position of the target

for the ﬁrst experiment. The actual position of the target is

(12.5, −12.5, 812.5).

X Y Z

tracking OFF 81±68 22±16 146±119

tracking ON 14±10 6±5 74±43

the observer in the two situations is shown in Figure 6.

It is worth noting that the area where the target

is perceived, when our tracking system is enabled,

is very small, thus indicating a veridical perception

of the virtual object. In particular, when the track-

ing is OFF, the mean and the standard deviation val-

ues of the perceived 3D points, expressed in mm, are

(34 ± 104, −34 ± 17, 898 ± 168), whereas we obtain

(21 ± 15, −17 ± 6, 774± 76) with the tracking ON.

Since the positions of the observers are uniformly dis-

tributed in the work space, the perceived positions of

the target are uniformly distributed around the actual

target position (see Fig. 2), thus yielding mean values

comparable between the two systems. The misper-

ception is represented by the spread of the perceived

positions of the target, that it can be quantiﬁed by the

standard deviation values.

VISAPP 2012 - International Conference on Computer Vision Theory and Applications

Z [mm]

X [mm]

1400

1200

1600

1000

800

600

400

200

0-200 200 400

Target

Head position with tracking ON

Head position with tracking OFF

Perceived position of the target

with tracking OFF

Perceived position of the target

with tracking ON

Figure 6: Graphic representation of the results of the ﬁrst

experiment. The boxes are centered in the mean values of

the head positions and of the perceived positions of the tar-

get. The size of the boxes is represented by the standard

deviation values. The smaller size of the green box repre-

sents a veridical perception (i.e. without misperception) of

the position of the target.

4.2 Second Experiment: Shape

Perception

Figure 7 shows the perceived start and end points of

the bar traced in the second experiment.

The considerations on the positions of the head

done for the previous experiment are still valid. When

our system is enabled, the perceived positions of the

bar are less scattered than in the conventional system.

This is conﬁrmed by the mean error and the standard

deviation values of the perceived mid point of the bar,

shown in Table 2.

Table 2: The mean error and the standard deviation values

(expressed in mm) of the perceived mid point of the hori-

zontal bar for the second experiment. The actual position of

the mid point of the target bar is (0, 0, 800).

X Y Z

tracking OFF 47±47 26±29 149±88

tracking ON 9±6 13±8 76±73

Moreover, when the tracking is OFF, the observer

misperceives the orientation of the horizontal bar, as

we have previously discussed (see Fig. 3). Table 3

conﬁrms that with the proposed system the mean er-

ror and the standard deviation of the perceived orien-

tation of the bar are reduced. The same applies for the

perceived length of the bar.

Table 3: The mean error and the standard deviation values

of the perceived length and orientation (angle with respect

to the horizontal bar) for the second experiment. The actual

length of the target bar is 100 mm, and the orientation is

0 deg.

length (mm) angle (deg)

tracking OFF 36±38 11±13

tracking ON 26±22 7±5

5 CONCLUSIONS AND FUTURE

WORK

A novel stereoscopic augmented reality system has

been developed and presented in this paper. Such sys-

tem allows a coherent perception of both virtual and

real objects to an user acting in a virtual environment,

by minimizing the misperception of the depth and of

the 3D layout of the scene. This is achieved through

a continuous tracking of the position of the observer

and a consequent re-computing of the left and right

image pair displayed on the screen.

In the conventional systems, when the user freely

moves in front of the screen, distortions of the shape

and of the distance of virtual objects occur. This is-

sue is relevant when an accurate interaction of a real

observer in a virtual world is required, especially in

scientiﬁc visualization, rehabilitation systems, or in

psychophysical experiments.

The proposed system relies on off-the-shelf tech-

nologies (i.e., Microsoft XBox Kinect for the track-

ing, and a 3D monitor with shutter glasses for the ren-

dering) and it allows a natural interaction between the

A STEREOSCOPIC AUGMENTED REALITY SYSTEM FOR THE VERIDICAL PERCEPTION OF THE 3D SCENE

LAYOUT

-2000200

600

800

1000

1200

1400

1600

1800

Z [mm]

X [mm]

-2000200

600

800

1000

1200

1400

1600

1800

Z [mm]

X [mm]

(a) (b)

Figure 7: Perceived positions of the target horizontal bar, and the related head positions (denoted by the yellow and cyan

circles for tracking ON and tracking OFF, respectively) for the tracing task. The perceived positions of the end-points of the

target bar with our tracking system enabled (green circles) are closer to the actual values, than the ones obtained by using a

conventional system (red circles).

user and the virtual environment, without adding sig-

niﬁcant delay in the rendering process.

The performances of the developed augmented re-

ality system has been assessed by a quantitative anal-

ysis in reaching and tracing tasks. The results have

been compared with the ones obtained by using a con-

ventional system that does not track the position of the

head. The results conﬁrmed a better perception of the

depth and of the 3D scene layout with the proposed

system.

As a future work, we plan to improve the tech-

niques used to render the stereoscopic environment

and to track the user in the scene. Moreover, in order

to further validated the proposed system, an extensive

experimental phase is foreseen, with a larger number

of participants and with different tasks.

ACKNOWLEDGEMENTS

This work has been partially supported by “Progetto

di Ricerca di Ateneo 2010”, and by National Project

“PRIN 2008”.

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A STEREOSCOPIC AUGMENTED REALITY SYSTEM FOR THE VERIDICAL PERCEPTION OF THE 3D SCENE

LAYOUT