Two experiments examine the abilities of ten subjects to visualize directions shown on a perspective display. Subjects indicated their perceived directions by adjusting a head-mounted cursor to correspond to the direction depicted on the display. This task is required of telerobotic operators who use map-like pictures of their workspace to determine the direction of objects seen by direct view. Results show significant open loop judgment biases that may be composed of errors arising from misinterpretation of the map geometry and overestimation of gaze direction

Ellis, Stephen R.

Hacisalihzade, Selim

Smith, Stephen

English

NASA Technical Reports Server

_N92-21483
VISUAL DIRECTION AS A METRIC OF VIRTUAL SPACE
Stephen R. Ellis,1 Stephen Smith, 2 Selim Hacisalihzade 3
NASA Ames Research Center
Moffett Field, California
ABSTRACT
Two experiments examine the abilities of I0 subjects to visualize directions shown on a perspec-
tive display. Subjects indicated their perceived directions by adjusting a head-mounted cursor to cor-
respond to the direction depicted on the display. This task is required of telerobotic operators who
use map-like pictures of their workspace to determine the direction of objects seen by direct view.
Results show significant open-loop, judgement biases that may be composed of errors arising from
misinterpretation of the map geometry and overestimation of gaze direction.
INTRODUCTION
A number of investigations and reviews of the characteristics of the virtual space perceived in
pictures have been conducted recently (Rosinski et al., 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 reported to develop a clear sense that the pictured objects were laid out in a virtual space.
Quantitative characterization of the metrics of the viewer' s perceived space will advance our under-
standing of picture perception and assist the design of displays for aircrafi and spacecraft. The
objective of the following research is to characterize patterns of errors observers make when refer-
ring a judged exocentric direction to a target presented on a perspective display to their own egocen-
tric 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 orien-
tation 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 exocentric judge-
ments for which the subjects indicated their estimates of the target position by adjusting dials to
show a target's azimuth and elevation with respect to a reference direction vector (See fig. 1).
1 NASA Ames Research Center and U.C. Berkeley School of Optometry
2Sterling Software, Palo Alto, CA
3NRC Research Associate, NASA Ames Research Center
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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 biases
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 adjusts the visual direction of head-mounted light cursor to
indicate his sense of the target's depicted azimuth and elevation with respect 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 subjects 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). Furthermore, 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 particular direction vector and then to imagine where they would have to look
to see the target.
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 unfamiliar 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 surface (light from a 3v flashlight bulb through a Wratten #25 filter). The
subject's head position was sensed by a Polhemus electromagnetic head tracker attached to an non-
metallic modified welder's helmet approximately 11 cm above the head. The head tracker was inde-
pendently calibrated against 28 theodolite-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 & Suthedand PS I graphics system. The images used were similar to earlier experiments
(McGreevy and Ellis, 1986; Grunwald and Ellis, 1986; Ellis, Smith, Grunwald, and McGreevy,
1989). The major change was the greater yaw of the view direction used to create the images. It was
set to a counterclockwise yaw of-35 °. Pitch remained -22 °. The subjects were seated at the center
of the projection in front of the computer 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 adjustable 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 fig. 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,
206
randomlypresenteddisplaysandto estimatetheazimuthandelevationdirectionof thetargetwith
respecto a referencepositionanddirection.Thentheywereto transferthisjudgementto their ego-
centricframeof reference.Theymadethejudgementby adjustingthepitchandyawof their head-
mounted,light cursorto apositionwheretheywouldexpectto seethetargetif their headwasat the
referenceposition,andinitially alignedwith thereferencedirectionin thedisplayedvirtual space.
For mostof thejudgeddirectionsthesubjectscouldnotsimultaneouslyseethedisplayandthecur-
sorposition,buthadto gazebackandforth betweenthemto accomplishthetask,generallyusing
headmovementsfor excentricitesgreaterthan15°. After adjusting the cursor, they held their posi-
tion 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 subject 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 measured head yaw to the calibrated positions were within 2 ° of the calibrated angles
within -I-60 ° of the straight ahead, the greatest deviations being at the most extreme angles. The rea-
son for the residual error was the difficulty of exactly positioning the subject to the calibration refer-
ence point. Pitch in contrast tends to be around a moving center of rotation somewhat behind the
neck and consequently tends to translate the head upwards and backwards from the initial reference
point which was used to provide a straight ahead, level reference for all subsequent measures. Con-
sequently, when the subjects pointed their head-mounted cursors to the extreme pitches, the sensor
reading undershot the calibrated value by from 5 to 8 degrees! We have calculated geometrical cor-
rections for the effects of this displacement 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 conducted in the dark
with the CRT display turned down so that only the frame of the monitor was faintly visible to pro-
vide an egocentric direction reference. In one experiment the head cursor was kept on. In the other
the cursor was turned off and the subjects had to rely principally on vestibular and proprioceptive
cues to "look" to the direction they would expect to see the target.
RESULTS AND DISCUSSION
The results from both experiments were similar and are analyzed together in this summary.
Multivariate analysis of variance conducted with BMDP 4V on the elevation and azimuth errors
showed that for both judgements the target elevation, target azimuth had statistically reliable effects
on both the pitch and the yaw of the errors in the head pointing error. Pitch direction errors: Target
Elevation: F=16.14 df--.4,5 p <.009; Target Azimuth: F=7.08, df---4,5 p<.027; yaw direction 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 degrees. The main effect of the presence of the light cursor was not signifi-
cant and did not interact with other independent variables (See figs. 3 and 4 )
207
Sincewedid not anticipateerrorsaslargeasactuallymeasured,wedid notusesphericalstatis-
tics to correcttheproblemsof mappingsphericaldatainto a linearanalysis.Becausetheanalysis
wasconductedon theerrordatacorrected 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 results.
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 target cube at that distance, about 1.5m, he would have to introduce parallax corrections to his
body-referenced, head direction as he translated with respect to the original reference point so as to
keep the cursor on the same place on the sphere as he moved. Alternatively, 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 cursor properly
pointed. This condition is particularly 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 refer-
ence was 6m and the viewing distance was modeled 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 direction. Thus, since the head-angle was measured with respect to a body-
referenced straight ahead, correction for head displacement 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 pattern 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 generally away from the straight ahead and 2) a larger but symmetric ten-
dency to overestimate the extent of the gaze direction indicated by the head mounted cursor. Over
estimates like this have been reported by Biguer et al. (1984) for hand pointing to visual 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 errors.
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 examin-
ing gaze angles produced by different combinations of eye and head angles. The results of the cur-
rent study clearly show however, 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.
208
REFERENCES
Barfield,W.; Sandford,J.; and Foley, J.: The mental rotation and perceived realism of computer
generated three dimensional images. International Journal of Man-machine studies, 1989,
(in press).
Biguer, B.; Prablanc, C.; Jeannerod, M.: The contribution of eye and head movements in hand
pointing accuracy. Experimental Brain Research, vol. 55, 1984, pp. 462-469.
Grunwald, Arthur; and Ellis, Stephen R.: Spatial orientation by familiarity cues. Proceedings of the
6th European Annual Conference on Manual Control, June 1986, University of Keele,
Great Britain.
Ellis, Stephen R.; Smith, Stephen; McGreevy, Michael W.: Distortions of perceived visual directions
out of pictures. Perception and Psychophysics, vol. 42, 1987, pp. 535-544.
Mardia, K. V.: The statistics of directional data Academic Press, New York, 1972.
McGreevy, Michael W.; and Ellis, Stephen R.: The effect of perspective geometry on judged direc-
tion. Human Factors, vol. 28, 1986, pp. 439-456.
Perrott, David: Changes in head position as a measure of auditory localization performance: auditory
psychomotor coordination under monaural and binaural listening conditions. Journal of the
Acoustical Society of America, vol. 82, 1987, pp. 1637-1645.
Rosinski, R. R.; Mulholland, T.; Degelman, D.; and Faber, J.: Picture perception: an analysis of
compensation. Perception and Psychophysics, vol. 28, 1980, pp. 512-525.
Sedgwick, H. A.: Space perception in Handbook of Perception and Human Performance vol. 1.,
K. R. Boff, L. Kaufman, J. P. Thomas eds, New York: Wiley, vol. 21, no. 19-21, 1986.
209
Reference Direction
Above
Below
Ahead
Figure 1. A schematic illustration of the direction judgement task. The subject adjusted the angles
and e shown on the dials at the fight until they appeared equal to the azimuth angle W and the eleva-
tion angle e of the target cube relative to reference at the center. Dotted lines, labels and arrows did
not appear on the map display.
Mean Pointing, l_rrQr
? 6O
40
v
201100 , I_P' ,_, _1,, ,
- -50 0 50
Azimuth (deg.)
• i
lOO
Figure 2. A schematic illustration of the experimental arrangement by which the subject indicated
the visual direction at which he would expect 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 average error arcs in a rectangular projection of the
forward sphere when both experimental conditions are combined. Each arrow represented the aver-
age pitch and yaw error in visual direction to a point at the tall of the arrow.
210
90
70
50
'_' 210'
t_
"_ 10-
Mean Egocentric Direction Error
Curson On
-10
-30
-,_)0 - ! • ! - !
-70 -50 -30 -10
N-5
i - ! • i -
10 3o 5O 70
Azimuth (deg)
Figure 3. The data in the figure represent the average error arcs in a rectangular projection of the
forward sphere for the condition in which the head driven cursor was turned on. Each arrow repre-
sents the average pitch and yaw error in visual direction to a point at the tail of the arrow.
Mean Egocentric Direction Error
Cursor Off
go
7o
$o
30
• tO-
>
u.i .lO
• o
o, 'r
N:,,S
• i • i - i I - i - i
-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 projection of the
forward sphere for the condition in which the head driven cursor was turned off. Each arrow repre-
sents the average pitch and yaw error in visual direction to a point at the tail of the arrow.
211
Reference Dlrection
/-
2
Eye
Figure 5. Circular plots for perspective displays in which subjects indicated target azimuth 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.
212


Visual direction as a metric of virtual space

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