The Spaceball. A Device for Controlling the Viewpoint.
The Spaceball is a pressure sensitive device, in the form of a large
softball (Fig. D.1). The Spaceball is sensitive to both torque and direct
force. It has 6 degrees of freedom, allowing movement in the X, Y and Z axis
as well as the three rotations, Pitch, Yaw and Roll. It is generally used to
control the translation and rotation of objects in interactive 3D environments.
In this study, it is used in an unconventional way because the object it
controls is the viewpoint.
In the interest of minimizing confusion while using the tool, and to best
simulate the normal movement of the eyes while walking, the degrees of freedom
of the Spaceball had to be constrained. Movement was constrained to the
forward direction. Participants were prevented from backing up because
preliminary studies showed that people tend to back-up much more often in
virtual environments than in real life. This factor was a source of
confounding error. By prohibiting the backward movement, participants had to
turn around when they wished to leave a space. This was closer to real life
behavior. It should be noted that this constraint on movement came at the cost
of increased difficulty in navigation. People often find themselves close to
flat surface which fill their entire field of view with one indistinguishable
polygon color. At these moments, it is very difficult to know the evolution of
one's position. Being able to back up is essential for not getting lost while
navigating in virtual environments.
In the non-tracked conditions, the rotation of the viewpoint was constrained to
pitch and yaw. In the Tracked condition, the rotation of the viewpoint was
controlled by the Polhemus tracker.
The rate of movement was fixed to an approximate walking speed (1.5
meters/second). Although the Spaceball is sensitive to varying pressure,
preliminary studies showed that when given a chance, people tend to go as fast
as they can all the time. By controlling the speed, individual differences in
rate of movement would not be a confounding factor.
The Spaceball was selected as a movement device because it was the only one
which could be used in all three simulation conditions. While there are better
adapted position tracked devices available, such as the dataglove and the wand,
none of these devices would have made any sense in the Monitor condition which
requires a space stabilized device.
The Spaceball was clumsy in the Monitor and Fixed conditions because it
controlled both the translation and rotation of the view point. It was clumsy
in the Tracked condition because it was fixed to the table. This is
particularly apparent when the participants are coming back south to the
entrance of the museum, and it lead to rather uncomfortable postures.
Particularly true to the Tracked condition, using the Spaceball also had the
effect of "grounding" participants to the laboratory. Traditional virtual
interface movement devices are not firmly attached to the real space, and the
only physical contact participants have with the laboratory is through their
feet. As a result, they often lose all sense of their orientation to the lab,
and when they remove the eyephones, many express surprise as to the direction
they are facing. Having the table and the fixed Spaceball mounted on it
greatly increased peoples' sense direction.
Fig. D.2 - The VPL Eyephones (customized).
The Eyephone and Head Position Tracking Device.
The Eyephones (Head Mounted Displays) are stereoscopic color video
displays placed inside goggles. Black rubber sidings completely block out any
incoming light outside of the video display (Fig. D.2).
The Head Tracking device is the Polhemus. The receptor is the little cube
located at the top of the eyephones. The position of the receptor relative to
the actual location of the eyes is corrected for in the programming.
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