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41

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Annual Meeting of Human Factors and Ergonomics Society, Albuquerque, New Mexico. Sept. 1997.

PERCEPTUAL EFFECTS IN ALIGNING VIRTUAL AND REAL OBJECTS

IN AUGMENTED REALITY DISPLAYS

Paul Milgram and David Drascic

Department of Mechanical and Industrial Engineering

University of Toronto

Toronto, Ontario, Canada

The concept of Augmented Reality (AR) displays is defined, in relation to the amount of real (unmodelled) and

virtual (modelled) data presented in an image, as those displays in which real images, such as video, are

enhanced with computer generated graphics. For the important class of stereoscopic AR displays, several

factors may cause potential perceptual ambiguities, however, which manifest themselves in terms of decreased

accuracy and precision whenever virtual objects must be aligned with real ones. A review is given of research

conducted to assess both the magnitude of these perceptual effects and the effectiveness of a computer assisted

Virtual Tape Measure (VTM), which has been developed for performing quantitative 3D measurements on real-

world stereo images.

BACKGROUND

This paper deals with visual perceptual factors

which influence performance when using

Augmented Reality (AR) displays as a remote

measurement or control tool in application domains

such as telerobotics and medicine. AR displays are

defined here as a subset of the class of "Mixed

Reality" (MR) displays, which in turn are defined

within the larger context of the Reality-Virtuality

(RV) continuum (Milgram & Kishino, 1994). As

depicted in Fig. 1, the RV continuum is presented

as a framework for describing the spectrum of

cases that define whether the primary world being

experienced by an observer is real or virtual. One

way to display real world objects is by scanning,

transmitting and reproducing image data, as is the

case with ordinary video displays

1

-- without the

need for the display system to "know" anything

about the objects. Another way is by viewing real-

world scenes either directly or via some optical lens

system. Virtual images, on the other hand, can be

produced only if the computer display system

1

Note that, although we are limiting our discussion here to

visual displays, similar classfications may be made with

respect to other display modalities. For example, real sound

sources may be directly transduced or replayed, whereas a

virtual sound source could be produced through computer

modelling and synthesis.

generating the images has a model of the objects

being portrayed.

Fig. 1 shows that MR refers to the class of

all displays in which there is some kind of

combination of real and virtual environments.

Within this context, the meaning of the term

“Augmented Reality", depicted on the left side of

the continuum, becomes quite clear: AR displays

are those in which the primary image is of a real

environment, which is enhanced, or augmented,

with computer-generated imagery. As shown in

the figure, in other words, the difference between

the purely real environment on the left, depicting a

video image of a person next to a robot, and the

AR example to the right is the addition of the

graphical robot on the table. In general, Augmented

Reality enables one to make virtual images appear

before the viewer in well specified locations in the

real world image. Such images can display task

related data, or can serve as interactive tools for

measuring or controlling the environment, using

either direct viewing (DV) or head-mounted video

"see-through" displays or ordinary display

monitors.

In contrast to AR, “Augmented Virtuality"

(AV) displays are those in which a primarily virtual

environment is enhanced, or augmented, through

some addition of real world images or sensations.

Such additions can take the form of directly viewed

(DV) objects, where users might see their own

Reality-Virtuality (RV) Continuum

Reality

e.g. Direct View,

(Stereo) Video (SV)

Augmented

Reality (AR)

e.g. DV or SV

with SG

Augmented

Virtuality (AV)

e.g. SG with

DV or SV

Virtual

Environment (VE)

e.g. Stereo

Graphics (SG)

Mixed Reality (MR)

Figure 1: Simplified representation of the Reality-Virtuality (RV) Continuum, showing how real and virtual

worlds can be combined in various proportions, according to the demands of different tasks.

limb instead of a computer-generated simulation, as

is common with surround type virtual

environments (VE's) where one might reach into

the scene to grasp an object with one's own hand.

Another AV mode is when video images are added

to otherwise completely simulated displays. This

concept is shown in Fig. 1 by the completely

virtual (modelled) image at the extreme right side of

the RV continuum, which is augmented by adding

an (unmodelled) video background in the AV

example to the left.

In this paper we deal with (visual)

Augmented Reality displays only, and we further

limit ourselves to the special, but very significant,

case in which all viewing systems are stereoscopic.

Our particular interest lies in situations in which the

available 3D cues do not completely support each

other, and may even be in conflict, thereby leading

to distorted perceptions of depth, distance or shape.

(Drascic & Milgram, 1996).

One class of tasks which is particularly

influenced by such distortions is that of aligning

virtual objects with real ones (RV alignment). In

AR environments one may require this capaibility

for visualising how, as shown in the AR example

of Figure 1, a virtual 3D graphic object would

appear against the real 3D video (SV) background

into which the model has been constructed to fit. In

a conceptually similar application, we have super-

imposed simulated human operator mannequins

onto real SV workplaces, for the purposes of ergo-

nomic workplace analysis. In such cases the

important perceptual issues involve having the

virtual mannequin appear to fit in properly with the

background and having its limbs appear to make

contact realistically with the floor, chairs, tools and

other instruments.

In other cases, it may be necessary to make

reliable 3D measurements of the dimensions or

locations of various objects within the SV image,

as well as distances between those objects. This

latter capability comprises the essence of our AR

Virtual Tape Measure (VTM) (Milgram et al,

1997), one of the fundamental capabilities of our

ARGOS (Augmented Reality through Graphic

Overlays on Stereo-video) display system (Drascic

et al, 1993). One important application of the

VTM, presented elsewhere in this proceedings

REAL-VIRTUAL ALIGNMENT ERRORS

IN AUGMENTED REALITY