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Fig. 7.
This is an example of an initial virtual world resizing task trial over a “stair-
step” scene background with objects casting no shadows. The task was to resize the
larger sphere so as to match the size of the remaining spheres, some of which were
displaced in depth. The spheres in this example outline the vertices of an octahedron.
the Euclidean summation of the three directional errors in the x, y, and z dimen-
sions (i.e. error magnitude = (
)
1
/
2
). This metric constitutes the
exact distance of the repositioned object from its correct position in three dimen-
sional space. Error magnitude for the resizing task was defined as the absolute
value of the error in length of either the radius (in task trials using spheres as
vertex objects) or the diagonal (in task trials using cubes or tetrahedrons as ver-
tex objects) of the resized object. Trial completion time was the period of time,
measured in milliseconds, from when the scene first appeared until the subject
pushed a button on the spaceball, causing the next scene to appear. Written in
the C programming language and OpenGL, repeated measures of the dependent
variables were automatically recorded by the task trial software. There were a
total of 288 trials per subject, 144 trials for each task. All subjects viewed the
same 288 scenes, although the presentation order varied. The volunteer subject
population consisted of 14 male and 16 female professional employees of GSFC.
One half of all trials randomly presented shadows on, and the remaining
trials presented shadows off. In the shadows on condition, one half of the trials
randomly used one light source to produce the shadows, and the remaining
trials used two light sources. One half of all trials were viewed in stereo, and the
remaining trials were viewed in mono. One third of the background conditions
randomly used a flat plane for the scene background; one third used the room;
and the remaining third used the 'stair-step' plane.
In all positioning and resizing task trials, the location of the misplaced or
incorrectly sized vertex was randomly located with respect to the other vertices.
In addition, light sources were consistently positioned 'from above' and at various
angles from the zenith. Moreover, in trials with two light sources, their angle of
separation varied from trial to trial. Thus, the relative locations and separation
of the light sources changed from trial to trial.
2
x
2
y
2
z
e
+
e
+
e
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