The objective of research is to characterize patterns of errors observers make when relating the judged exocentric direction of a target presented on a perspective display to their egocentric sense of visual direction. This type of spatial task is commonly faced by operators of telerobotic systems when using a map-like display of their workspace to determine the visual location and orientation of objects seen by direct view. It is also essentially the same task as faced by an aircraft pilot using a cockpit perspective traffic display of his surrounding airspace to locate traffic out his windows. The results of the current study clearly show that the visual direction is a significantly biased metric of virtual space presented by flat panel perspective displays. Modeling and explanation of the causes of the observed biases will allow design of compensated perspective displays

Ellis, Stephen R.

Hacisalihzade, Selim

Smith, Stephen

English

NASA Technical Reports Server

N9o- 5553
Visualization of Three Dimensional Data
Stephen R. Ellis 1, Stephen Smith, 2 Sefm Hacisalihzade 3
NASA Ames Research Center
Moffett Field, CA 94035
Introduction
A number of investigations and reviews
of the characteristics of the virtual space
perceived in pictures have been conducted re-
cently (Rosinski et a1.,1980; Sedgwick,1986;
McGreevy and Ellis,1986; Grunwald and Ellis,
1986; Ellis, Smith and McGreevy,1987;
Barfield, Sandford, and Foley, 1989). Despite
the fact that the pictures considered were
not stereoscopic, viewers typically were re-
ported to develop a clear sense that the pic-
tured objects were laid out in a virtual space.
Quantitative characterization of the metrics
of the viewer's perceived space will advance
our understanding of picture perception and
ass;st the design of displays for aircraft and
spacecraft. The objective of the following re-
search is to characterize patterns of errors
observers make when relating the judged the
exocentric direction of a target presented on
a perspective display to their egocentric
sense of visual direction. This type of spatial
task is commonly faced by operators of teler-
obotic systems when using a map-like display
of their workspace to determine the visual
location and orientation of objects seen by
direct view. It is also essentially the same
task as faced by an aircraft pilot using a
cockpit perspective traffic display of his
surrounding airspace to locate traffic out his
windows.
Previous studies of the error pattern in
direction judgements have focused on exo-
centric judgements for which the subjects
indicated their estimates of the target posi-
1NASA Ames Research Center and U.C. Berkeley
2Sterling Software, Palo Alto, CA
3NRC Research Associate,NASA Ames ResearchCenter
tion by adjusting dials to show a target's az-
imuth and elevation with respect to a refer-
ence direction vector (See Figure 1),
This response may be described as
exocentric since the dial's frame of reference
is external to the observer and contrasts
with egocentric judgements in which target
position is indicated with respect to a body-
referenced coordinate system Accordingly, in
order to test the generality of reported bi-
ases in estimating azimuth and elevation
with exocentric judgements, it is useful to
examine the same exocentric task but request
the subjects to make egocentric judgements.
For this new response the observer ad-
justs the visual direction of head-mounted
light cursor to indicate his sense of the tar-
get's depicted azimuth and elevation with re-
spect to a reference position and reference
direction. This response will explicitly test
the generality of previously reported bias in
which exocentric directions are judged to be
away from a reference straight ahead. This
bias may be attributed to errors in the sub-
jects ability to determine the view direction
used to generate the display (McGreevy and
Ellis, 1986; Grunwald and Ellis, 1986; Ellis,
Smith, Grunwald, and McGreevy, 1989). Fur-
thermore, use of an egocentric response such
as visual direction provides a more natural
response than a dial adjustment. In a sense
we ask the subjects to imagine themselves
oriented in the virtual space along a par-
ticular direction vector and then imagine
where they would have to look to see the tar-
get.
465
https://ntrs.nasa.gov/search.jsp?R=19900016237 2020-03-19T22:39:03+00:00Z
$ 0 °
Above
Ahead@
Blhlnd
Figure 1. Schematic illustration of the di-
rection judgement task. The subject adjusted
the angles ,r and e shown on the dials at the
right until they appeared equal to the az-
imuth angle I' and the elevation angle e of the
target cube relative to reference at the cen-
ter. Dotted lines, labels and arrows did not
appear on the map display.
Methods
Two groups of 5 subjects participated
as independent groups in two experiments.
The subjects were male laboratory personnel
ranging in age from 20 to 43 who were unfa-
miliar with the purpose of the experiment.
The experiments were conducted inside
a 1.5 m planetarium dome that served as a
projection surface for a head mounted, light
pointer which projected a red filament image
shaped as a 1.5 ° chevron onto the dome's sur-
face (light from a 3v flashlight bulb through a
Wratten #25 filter). The subject's head posi-
tion was sensed by a Polhemus electromag-
netic head tracker attached to an nonmetalic
modified welder's helmet approximately 11
cm above the head. The head tracker was
independently calibrated against 28 theodo-
lite-positioned, reference markers which
were visible during Calibration but not during
testing.
The subjects were presented with an
exocentric judgement task generated by a
PDP 11/40 - Evans & Sutherland PSI graphics
system. The images used were similar to
earlier experiments (McGreevy and Ellis,
t986; Grunwald and Ellis, 1986; Ellis, Smith,
Grunwald, and McGreevy, 1989). The major
change was the greater yaw of the view di-
rection used to create the images. It was set
to a counterclockwise yaw of -35 °. Pitch re-
mained -22 °. The subjects were seated at the
center of the projection in front of the com-
puter calligraphic monitor about 80 cm from
the display surface and looked downward into
it with a -22 deg. pitch angle matching that
of the view vector. The viewport was 17 cm
square.
Subjects were first positioned in an ad-
justable chair so that their head-mounted
light cursor pointed to a subjective straight-
ahead, eye level that corresponded to the
calibration point at 0° pitch and 0° yaw. (See
Figure 2) While in this position, a reference
reading was taken from the head sensor for
all future measurements. The subjects then
were instructed to examine a series of
automatically, randomly presented displays
and to estimate the azimuth and elevation
direction of the target with respect to a ref-
erence position and direction. Then they were
to transfer this judgement to their egocen-
tric frame of reference. They made the
judgement by adjusting the pitch and yaw of
their head-mounted, light cursor to a posi-
tion where they would expect to see the tar-
get if their head was at the reference posi-
tion, and initially aligned with the reference
direction_n_he disp[ay_ -virtual_-spa_e. _-_ro/
most of the judged directions the Subjects
could not simultaneously see the display and
the cursor position, but had to gaze back and
forth between them to accomplish the task,
generally using head movements for excen-
tricites greater than 15°. After adjusting the
cursor, they held their position and moved a
toggle switch that signaled the computer to
take the data. The data for a 1 sec. period
prior to the switch signal were averaged to
give a single measurement. Three replications
of each position were taken from each sub-
ject in a randomized sequence of 64
measurements that took about 2. hours to
complete.
The interpretation of the head-direction
data is complicated by the different centers
of rotation associated with pitch, yaw and
roll of the head. Pure yaws did not displace
the center of rotation very much and the mea-
sured head yaw to the calibrated positions
were within 2° of the calibrated angles
within +60 ° of the straight ahead, the great-
466
8O
Mean Pointin_ Error
=,o60-
q.)
'I=
40-
20-
_kJ
i
0
-20
-100
._xrA
.... m .... i .... i,---
-50 0 50 100
Azimuth (deg.)
Figure 2. Schematic illustration of the experimental arrangement by
which the subject indicated the visual direction at which he would ex-
pect to see the target presented on the CRT perspective display if he
were positioned at the reference point and aligned with the reference
direction. The data in the right portion of the figure represent the av-
erage error arcs in a rectangular projection of the forward sphere
when both experimental conditions are combined• Each arrow repre-
sented the average pitch and yaw error in visual direction to a point at
the tail of the arrow.
est deviations being at the most extreme an-
gles. The reason for the residual error was
the difficulty of exactly positioning the sub-
ject to the calibration reference point. Pitch
in contrast tends to be around a moving cen-
ter of rotation somewhat behind the neck and
consequently tends to translate the head up-
wards and backwards from the initial refer-
ence point which was used to provide a
straight ahead, level reference for all subse-
quent measures. Consequently, when the sub-
jects pointed their head-mounted cursors to
the extreme pitches, the sensor reading un-
dershot the calibrated value by from 5 to 8
degrees! We have calculated geometrical
corrections for the effects of this displace-
ment from the reference point since we could
measure it, but generally found that they
were small (2-4 °) and for reasons discussed
below may not in principle be proper to use.
After calibration of the head tracker in
the light, the two experiments were con-
ducted in the dark with the CRT display
turned down so that only the frame of the
monitor was faintly visible to provide an
egocentric direction reference. In one ex-
periment the head cursor was kept on. In the
other the cursor was turned off and the sub-
jects had to rely principally on vestibular
and proprioceptive cues to "look" to the direc-
tion they would expect to see the target.
Results and Discussion
The results from both experiments were
similar and are analyzed together in thls
summary. Multivariate analysis of variance
conducted with BMDP 4V on the elevation and
azimuth errors showed that for both judge-
ments the target elevation, target azimuth
had statistically reliable effects on both the
pitch and the yaw of tl_e errors in the head
pointing error. Pitch direction errors: Target
Elevation: F=I6.14 df=4,5 p <.,009; Target
Azimuth: F=7.08, df=4,5 p<.027;
errors: Target Elevation: ns; Target Azimuth:
F=29.5 df=4,32 p<.001. Standard errors for
the mean error ranged between 1 and 10 de-
grees. The main effect of the presence of the
light cursor was not significant and did not
interact with other independent variables
(See Figures 3 and 4 )
467
Since we did not anticipate errors as
large as actually measured, we did not use
spherical statistics to correct the problems
of mapping spherical data into a linear analy-
sis. Since the analysis was conducted on the
error data corrected for wrap-around of the
scale and most of the errors were less than
15 degrees, use of spherical statistics is not
likely to substantially change the major re-
sults.
The proper method to use to correct for
movement of the subject from his calibrated
reference position while he positions the
cursor depends upon his interpretation of the
of the meaning of the cursor position. If he
considers its image on the inner surface of
the sphere to represent the location of a tar-
get cube at that distance, about 1.5m, he
would have to introduce parallax corrections
to his body-referenced, head direction as he
90
?0
Mean Egocentric DirectionError
CursonOn
v _10 _ _ •
10'
-30' N-5
-50 -- D - . • . , - _, • , •
-_ -50 -30 -10 10 30 50
Azimuth (de_
Figure 3. The data in the figure represent
the average error arcs in a rectangular pro-
jection of the forward sphere for the condi-
tion in which the head driven cursor was
translated with respect to the original refer- turned on. Each arrow represents the average
ence point so as to keep the cursor on the pitch and yaw error in visual direction to a
same place on the sphere as he moved. Alter- point at the tail of the arrow.
natively, if he considered, as he in fact was
instructed, the cursor image to represent a
body-referenced direction toward the target,
head displacement in itself would not require
adjustment of head direction to keep to cur-
sor properly pointed. This condition is partic-
ularly true since he was instructed that the
target was at a relatively great distance
from the reference cube. For the layouts used,
the distance between target and reference
was 6m and the viewing distance was mod-
eled at 28m. At this distance the parallax
correction for a 5 cm lateral movement would
have to be only about 0.5 °, comparable to the
biological noise associated with head direc-
tion. Thus, since the head-angle was mea-
sured with respect to a body-referenced
straight ahead, correction for head displace-
ment need not be made.
The observed mean body-referenced
errors for both experiments are plotted in
Figures 3 and 4 as error arcs on a rectangular
projection of the response sphere. The pat-
tern shows a tendency to err towards the
subject's egocentric straight ahead, but with
a significant asymmetry. The results may be
interpreted as a composition of errors: 1) the
asymmetrical pattern previously reported
for exocentric dial responses which is gener-
ally away from the straight ahead and 2) a
larger but symmetric tendency to overesti-
MeanEsocentrlcDirectionError
Cursoroff
gO
70-
50-
10-
®'°i
-50' !
-70 -SO -30 -10 10 30 50 70
Azimuth (deg)
Figure 4. The data in the figure represent
the average error arcs in a rectangular pro-
jection of the forward sphere for the condi-
tion in which the head driven cursor was
turned off. Each arrow represents the average
pitch and yaw error in visual direction to a
point at the tail of the arrow.
mate the extent of the gaze direction in-
dicated by the head mounted cursor. Over es-
timates like this have been reported by
Biguer et al. (1984) for hand pointing to vi-
sual target and for head pointing to brief
auditory targets (Perrott, Ambarsoom, and
Tucker, 1987 ). In the case of hand pointing
without visual feedback of pointing error
such overestimates result in overshoot er-
rors. In the case of head pointing without
pointing error feedback, the overestimates
result in undershoot errors similar to those
observed.
The observation that the errors were
not effected by turning off the light cursor
supports the idea that one source of error
arises from the proprioceptive and vestibular
estimate of head rotation. But whether the
phenomena is truly one of gaze remains to be
determined by future experiments examining
gaze angles produced by different combina-
tions of eye and head angles. The results of
the current study clearly show however, the
visual direction is a significantly biased
metric of virtual space presented by flat
panel perspective displays. Modeling and ex-
planation of the causes of the observed bi-
ases will allow design of compensated per-
spective displays.
References
Barfield W., Sandford, J. and Foley, J. (1989) The men-
tal rotation and perceived realism of computer
generated three dimensional images. International
Journal of Man-machine studies (in press)."
Biguer, B., Prablanc, C., Jeannerod, M. (1984) The
contribution of eye and head movements in hand
pointing accuracy. Experimental Bra/n Research,
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Grunwald, Arthur, & Ellis, Stephen R. (1986) Spatial
orientalion by familiarity cues. Proceedings of
the 6th EuropeanAnnual Conference on Manual
Control, June, 1986, University of Keele, Great
Britain.
Ellis, Stephen R., Smith, Stephen, McGreevy, Michael
W. (1987) Distortions of perceived visual direc-
tions out of pictures. Perception and Psy-
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Mardia, K.V. (1972) The statistics of directional data Aca-
demic Press, New York.
McGreevy, Michael W. and Ellis, Stephen R. (4.986) The
effect of perspective geometry on judged direc-
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Perrotl, David (1987) Changes In head positionas a mea-
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Rosinski, R.R., Mulholland,T, Degelman, D., and Faber, J.
(1980) Picture perception: an analysis of com-
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Boff, L. Kaufman, J.P. Thomas eds, New York: Wi-
ley, 21:19 - 21.
Figure 5. Circular plots for perspective dis-
plays in which subjects indicated target az-
imuth for targets at 0 degrees elevation by
adjusting an angle on a dial. The errors are
plotted as directed arcs with the tail of each
arrow at the correct position of the target.
The length of each arrow represents the aver-
age error from 8 subjects. Though the viewing
azimuth was -22 ° compared to the -35 ° used
in the current experiments, the conditions
are otherwise comparable. The error arcs
clearly show a bias away from the straight
ahead rather than towards it and also show an
asymmetry with greater errors in the right
quadrant than in the left. Thus, if this bias
were to cancel a larger one, perhaps due to
overestimation of gaze direction, that was
toward the straight ahead, the resulting bias
would be smaller in the right quadrant than in
the left. This expected pattern in found the
the data for zero degree target orientation in
Figures 3 and 4.
469



Visualization of three dimensional data

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