Immersion, Presence, and Performance in Virtual Environments
Figure 1
Tri-Dimensional Chess
3. Experiment
3.1 Background: Tri-Dimensional Chess
Tri-Dimensional Chess (TDC) is a board game which has many characteristics
in common with conventional chess . More specifically, it is a type of chess
played on a number of boards suspended at a different heights. The pieces
used in this game are the same as conventional chess and capable of the
same movements, but also may be moved from one board to another. Moreover,
the layout of the different boards is irregular, and the initial positions
of the sixteen pieces of each side are different than in conventional chess.
Finally, TDC has a set of four movable attack boards, which are also considered
to be pieces, and can be moved according to certain rules (Figure 1).
TDC was chosen because it provides a complex geometrical structure and it
is this complexity of the layout of the boards and the pieces, which make
it a suitable vehicle for the study. The actual rules of the game were of
no importance for this experiment. Twenty four subjects were chosen for
the experiment according to the factorial design of Table 1.
Table 1: Number of Subjects Per Cell in the
Factorial Design | Immersion | Exocentric: 7 moves |
Exocentric: 9 moves | Egocentric: 7 moves | Egocentric:
9 moves | | Plain | 3 | 3 | 3 | 3 |
| Garden | 3 | 3 | 3 | 3 |
3.2 Factorial Design
(a) Immersion: Exocentric/Egocentric
This factor relates to the surrounding aspect of immersion discussed above.
Half of the subjects were immersed with an egocentric view into a virtual
environment. This was achieved using a DIVISION ProVision100, with a Virtual
Research Flight Helmet and a DIVISION 3D Mouse. Polhemus Fastrak sensors
were used for position tracking of the head and the mouse. The generated
image has a resolution of 704x480 which is relayed to two colour LCDs each
with a 360¥240 resolution. The HMD provides a horizontal field of view
of about 75 degrees, and about 40 degrees vertically. Forward movement in
the VE is accomplished by pressing a left thumb button on the 3D mouse,
and backward movement with a right thumb button. A virtual hand was slaved
to the 3D mouse - there was no virtual body representation other than this.
Objects could be touched by the hand and grabbed by using the trigger finger
button on the 3D mouse.
The other 12 subjects experienced the VE from an exocentric view. In order
to keep all conditions as similar as possible apart from egocentric or exocentric,
the exocentric subjects used exactly the same system, except that they viewed
the images on a TV screen. They controlled movement by the 3D mouse. This
time the HMD was placed on the left shoulder of the subject so that viewpoint
could be controlled with the left hand.
One condition could not be controlled - the resolution of the different
displays (HMD and TV screen). The image generated from the same source as
the HMD had a resolution of 704¥480 which was fed to an NTSC 3.58 28
inch TV. Hence the exocentric group observed a higher resolution display.
(b) Environment: Plain/Garden
The environment factor is related to vividness. Half of the subjects ("garden")
participated in an environment where the TDC system was located in a realistic
setting. This consisted of an open field, populated by a table, a chair,
a tree and small plant. The TDC board was located on the table. This model
had a large horizontal plane forming the ground, and a spherical cone representing
the sky. This was called the "garden" environment. All surfaces
in the VE garden were appropriately texture mapped. The remaining subjects
("plain") saw the TDC game suspended in a void. Examples of these
environments are shown in the colour plates.
(c) Number of Moves
Each subject had to witness the first few moves of a computer versus computer
game. The number of moves was either 7 or 9, to give tasks of slightly differing
degrees of complexity. The subject was responsible for initiating the sequence
of moves, as well as "instructing" each consecutive move. To be
more precise, the subject had to initiate this game by pressing a red button
situated next to the base of the virtual TDC. As soon as the button was
pressed, one of the pieces on the board would change its colour to bright
red, indicating the first computer move. The move was not performed by the
computer until the subject decided to "instruct" the computer
to do so. To give this instruction, the subject had to touch the red piece
with the virtual hand. Doing this caused the piece to leave its current
position and move to a new position on the board. As soon as this piece
moved to its new position, another piece on the board changed its colour
to bright red. The subject had then to touch this piece in order to make
it move to its new position on the board, following a predetermined path.
Another piece would then in turn change to bright red, and so on. This process
carried on for a certain number of moves - 7 or 9. When the subject could
not find any other bright red piece on the board the sequence had finished.
The subject could repeat the identical complete sequence from the beginning
by again pressing the red button.
The task of the subject was to remember which pieces were moved and where
they were moved to. They then had to reproduce the final state of the board
on the real life TDC board from which the virtual TDC had been modeled.
There was no limit to the amount of times a subject could repeat the sequence
of moves. This was done so that different rates of learning between the
subjects be eliminated as a source of experimental variation. The importance
of feeling confident in being able to accurately reproduce the moves in
real life was clearly explained to each subject, and the main experiment
did not commence until the subject confirmed a high degree of confidence.
3.3 Virtual Model and Performance
The boards and pieces were modeled in AutoCAD. There were on the average
290 vertices and 230 triangles in each chess piece. The entire board, including
the board base, the bottom, middle, and top boards, the attack-boards, and
the poles suspending the attack-boards consisted of 438 vertices and 344
triangles, all texture mapped. The activation button consisted of 56 vertices
and 42 triangles. The garden, including a table, a chair, a plant, a tree,
a ground, and a sky-dome, consisted of 2543 vertices and 1456 triangles,
all texture mapped. Altogether there were a total of 7732 triangles in the
garden environment and 6276 in the plain environment (consisting only of
the TDC system). Further description of the process of object construction
can be found in in (Linakis, 1995).
The frame rate offered on the ProVision system is not guaranteed at any
particular level of performance. It varied between 15 and 20Hz depending
on the complexity of the data in view at any particular time. Clearly subjects
in the plain environment would have generally experienced a faster frame
rate than those in the garden environment. This does confound the experiment
to some extent since on the one hand the more realistic environment is,
in the terminology of this paper, a more "immersive" one, yet
its lower frame rate makes it less immersive. However, an experiment of
Barfield and Hendrix (1995) found
that frame rates of between 15 and 20Hz resulted in the same degree of reported
"presence".
3.4 Procedures
(a) Selection of Subjects
The range of subjects was chosen to be as broad as possible in terms of
their background knowledge of chess and computer literacy. The subjects
varied from computer science students with previous computer and chess knowledge,
to people with no previous knowledge of chess and almost no computer experience.
Allocation to the cells was carried out randomly except that in cases where
the subjects had previously experienced Virtual Reality they were evenly
distributed in the design, so as to maintain an average of VR expertise
amongst the subjects of each cell. There were 16 males and 8 females.
(b) The Pre-Questionnaire
A pre-questionnaire was given to subjects at the time of their agreement
to participate. This gathered basic demographical and other information
such as prior experience with chess, TDC, computers and VR.
(c) The Spatial Awareness Test
The Spatial Awareness Test (SAT) is one of four General Ability Tests which
aim to measure how well a person can identify similarities and relationships
in words, shapes, or numbers. The specific purpose of the SAT is to test
the ability of a person to create, retain and manipulate mental images by
mentally "folding" flat patterns into 3D objects (Smith
and Whetton, 1988).
Each subject had to do this test, and a standard score was derived from
their answers. The higher the score, the greater the ability of the subject
to mentally derive 3D structure when from 2D visual input, according to
the principles of these tests. The purpose of administering the test was
to attempt to take into account differing background abilities in mental
imagery.
(d) Introducing Subjects to the Tri-Dimensional Chess
Most of the subjects were not expected to have seen the Tri-Dimensional
chess before this introductory session so that a training session should
be given to each subject.
Each subject was given the same introductory talk on the TDC. They were
told that the TDC has three main boards and four attack boards. It was made
very clear to them that those attack boards were considered to be pieces,
and that they could be part of a legal move. Finally, it was decided that
the subjects should not be told that the TDC pieces are capable of the same
movements as conventional chess pieces, as it would be very easy to deduce
the correct position of a moved piece based on elimination of impossible
moves. Subjects were therefore told that pieces can move in any one of six
directions i.e. forward, backwards, left, right, up, and down (from one
level to another).
Finally, the experimenter performed a number of moves on the real TDC using
the pieces of one side (the Gold side), and then asked the subjects to copy
the moves with the pieces of the opposite side (Silver). If a subject made
an error in copying the move the experimenter would immediately report this
to the subject and the corrected version of the move was performed by the
experimenter and explained to the subject.
(e) The Virtual Kitchen Task
As with the TDC, the subjects were not expected to have any existing knowledge
or experience in Virtual Reality. It was therefore important to familiarise
them with the VR equipment before the main experimental task was to be carried
out.
A virtual kitchen demonstration was chosen for this purpose since it involved
an environment with which people are naturally familiar. Moreover, this
environment is well designed (i.e. precise in size, and detailed in geometrical
description), and is highly interactive since most of the objects can be
picked up or moved.
Initially, the VR equipment was shown to the subjects. They were told how
they could navigate through the virtual environment, how they could pick
objects up, and release them. They were given written instructions on what
they had to do in the virtual kitchen. The same instructions were repeated
verbally by the experimenter. Each subject had to navigate around the kitchen,
find a particular object in the kitchen, pick it up and drop it, after having
taken it to another part of the environment. The particular object turned
its colour to bright red when touched, hence indicating that it could be
picked up. This was done so that consistency would be maintained with the
change of colour of pieces in the main experimental task. The subjects were
told that objects in Virtual Environments are not solid and are hence penetrable
by the virtual hand. They were also made aware of the fact that the virtual
environment does not simulate gravity. Finally, the experimenter guided
each one of the subjects through this familiarisation process by talking
to them and by making himself clearly present in the real world. This was
a practice that was avoided during the main experimental task, since talking
to subjects during their VR experience is likely to adversely affect the
sense of presence. During the main experiment, assistance and guidance was
given only in cases of emergency (e.g. when a subject was in danger of colliding
with an object in the real world), or in cases where the subject had specifically
requested some help.
As a result of this training session, the subjects were expected to be familiar
enough with the VR controls so as to be able to operate efficiently in the
main experimental Virtual Environment. Finally, it should be noted that
subjects who were in the non-immersed subject group for the main experiment,
were also non-immersed during this VR experience.
(f) The Virtual Tri-Dimensional Chess Task and the Reproduction Session
In these sessions the subjects had to carry out the tasks as described in
section 3.2(c). All relevant instructions were given in written form to
the subjects and were also verbally repeated by the administrator. For example,
"Your task is to remember the new positions of the pieces on the board.
You may take as long as you want to look at the board, until you feel confident
that you remember the new positions. If you feel unsure, you can repeat
the process, by pressing the red button, as many times as you wish."
It was made clear to them that they would have as much time as possible
at their disposal, and that it was important that they felt confident that
they would be able to reproduce the moves at the end of the VR session.
They were also reminded of the fact that their administrator would not interact
with them unless specifically asked to do so.
Finally, during the task reproduction session the administrator took clear
and precise notes of the moves performed by each subject. These notes were
the main source of experimental data. Also, the times between button presses
were internally recorded in relevant files.
(g) The Post-Questionnaire
The post-questionnaire was given to subjects at the end of all the sessions.
This included questions on their confidence about their performance, nausea
caused by the VR, and three questions relating to presence. These were the
same three questions that we have used in a majority of our previous studies,
and recorded on a 1 to 7 scale.
- the sense of "being there" in the environment depicted by
the display;
- the extent to which there were times that the virtual world seemed
more the presenting reality than the real world, and
- the sense of having visited a place rather than seeing images.
3.5 Variables Measured
(a) Response (Dependent) Variables
The major response variable was the number of correct moves (out of 7 or
9) that the subject made using the real TDC. We denote this variable by
C.
Presence was a dependent variable in one analysis, and an explanatory variable
in another. We used the same measures of reported presence as in our previous
studies. The presence variable (p) was taken as the number of 6 or 7 answers
to the three questions as stated in 3.4(g), and hence was a count out of
3. As before (Slater, Usoh and Steed,
1995) an alternative score was constructed by combining the three presence
question scores into a single scale using principal components analysis
(Kendall, 1975). The first principal
component is the linear combination of the original variables that maximises
the total variance. The second is orthogonal to the first and maximises
the total residual variance. The first two principal components accounted
for 83% of the total variation in the original three variables. The single
presence score was taken as the norm of the vector given by the first two
principal components.
(b) Independent Variables
These were given by the experimental design as Immersion: Egocentric or
Exocentric, Environment: Plain or Garden, and Number of Moves: 7 or 9, as
discussed in Section 3.2.
(c) Explanatory Variables
These were recorded from the questionnaires and during the experiment. The
major ones are recorded in Table 2. The most important ones are given earlier
in the table. The "practice" variable was recorded in order to
allow for different natural learning times amongst the subjects. "Remember"
and "capable" although not used directly in the analysis were
useful as cross checks (correlating with "practice") to check
whether subjects saw sufficient practice sessions according to the needs
of their level of confidence and memory.
Table 2
The Main Explanatory Variables
- Practise (P)
The number of practice sequences initiated by the subjects.
- Gender
Male (1), Female (2).
- Spatial (SAT)
Results of spatial awareness test. Higher score means "better"
spatial ability.
- Chess
Whether the subject knows how to play chess: Yes (1), No(2).
- Computer literacy
Whether or not the subject is a regular computer user: Yes(1), No(2).
- Time
The overall viewing time of the moves in the VE.
- Sick
Level of sickness as a result of the VE experience (1-7 scale).
- Remember
The confidence with which the moves were remembered (1-7 scale).
- Capable
The confidence that they had correctly reproduced the moves (1-7 scale).
- Age
Age of subject in years