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were significant differences (at the 95% confidence level) in the mean values of
the error rate and response time measures as a function of three main effects:
viewing mode; type of motion; and surface characteristic. The main effect of the
(cube or sphere) object shape was not significant with respect to error rate nor
response time.
The mean error rate across all experimental trials was 11.04%. The mean
overall response time per image pair was 13.01 seconds. Subjects viewing image
pairs in stereo made fewer errors, and responded more quickly, than did sub-
jects viewing the image pairs in mono. Moreover, subjects controlling the object
motion of the right-hand object image were more accurate, although they took
longer to make their object comparison decisions, than did the subjects who did
not control this motion. Furthermore, subjects viewing wire frame images took
longer to complete trials than did subjects viewing solid objects, although they
were no more nor less accurate in their object matching judgments. Complete
quantitative results will be provided by requesting same from the lead author of
this chapter.
3.2
Experiment #2:
Object Positioning and Resizing in Virtual Worlds
Method.
The second experiment involved object positioning and resizing tasks
in 2D and 3D 'virtual worlds' [19]. In the positioning task, subjects were pre-
sented with object images arranged to outline the vertices of larger, symmetrical
figures, either cubes or octahedrons. The task was to reposition the misplaced
vertex object in the x, y and z dimensions, as quickly and accurately as possible,
so as to complete the symmetrical arrangement (see Figs. 5 and 6). In the resiz-
ing task, the vertex objects were correctly positioned, but one of the objects was
either larger or smaller than the remaining objects (see Fig.. 7). The task was to
resize this object, as quickly and accurately as possible, to make it correspond
with the uniform size of the other objects. In both the positioning and resizing
tasks, subjects used a spaceball input device to manipulate (i.e. move or resize)
the target objects. A 'spaceball' is a six-degrees-of-freedom input device that has
a large round, graspable ball mounted on a flat base. In the positioning task,
the spaceball was programmed such that 'pushing' or 'pulling' the ball forward,
backward, up or down, or in any direction caused the selected object on the
screen to move in that corresponding direction. In the resizing task, the space-
ball was programmed such that 'pushing' forward, or 'pulling' backward, caused
the selected object to increase, or decrease in size, respectively.
Design.
The positioning and resizing virtual world experiment also used a
within-subjects, repeated measures design, manipulating the independent vari-
ables: shadows (on, off); number of shadow-casting light sources (one, two);
viewing mode (stereo, mono); and scene background (flat plane, room, 'stair-
step' plane). The number of shadow casting light sources was a condition nested
within the shadows on condition. The dependent variables included error magni-
tude and response time. Error magnitude for the positioning task was defined as
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