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1992) explores
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over the viewpoint in a manner which is similar to
directing a cameraman to make a movie shot . Thus
the director might request a cameraman to track a
certain moving object, pan from object A to object B,
or zoom in to a close-up. An even more ambitious
approach is to have an autonomous smart camera
which positions the viewpoint automatically to allow
the user to concentrate on some other task (Phillips, et
al, 1992).
2) World in Hand and Mechanical Metaphors.
In the "World in Hand Metaphor" the viewpoint is
changed by moving the object. Thus, to look at a
displayed scene from the right hand side, the scene is
rotated clockwise (as viewed from the top), like an
object on a turntable. If a 6DF input device, such as the
Polhemus™, or a SpaceBall™, is available then
rotations and translations can be carried out
simultaneously. As a method for changing the
viewpoint the world in hand metaphor works well for
single, reasonably compact objects. However, when
the environment to be explored is a landscape or
enclosed interior space, picking it up and moving it
does not seem natural (it has poor cognitive
affordances). There also exists a problem selecting the
center of rotation. Especially when moving through an
interior the metaphor clashes with the user's perception
of being enclosed and the linkage of scene motion to
hand motion is incongruous and difficult to grasp
(Ware and Osborne, 1990).
The World in hand metaphor is almost the opposite of
the eyeball in hand metaphor; instead of moving his or
her viewpoint the user imagines moving the object.
Useful variants on the metaphor are such devices as the
virtual turntable (Evans, et al, 1981), stirrer, or virtual
sphere (Chen, et al, 1988) which transform the input
from a mouse or digitizing tablet to give the feeling of
direct manipulation of a graphical object. These
devices tend to be easy to learn but not very flexible.
Often they must be carefully customized to provided
the range of movements required by the application.
We are using a virtual turntable in an interface to a
DEM visualization system for satellite imagery.
3) Functions and Smart functions.
It is a common practice to control the viewpoint using
common graphics library functions such as scale,
translate and rotate. These functions may be controlled
directly by the mouse or indirectly via sliders. We
include this kind of interface for completeness although
it does not embody the use of a consistent metaphor.
A much more interesting use of (non metaphorical)
functions is the point of focus zoom developed by
MacKinlay, et al (1990). The implemented a zoom
which corresponds to moving the viewpoint to the
surface, halving the distance for each unit time. The
also evaluated some complex compound viewpoint
movements, for example, to zoom in on a particular
489
surface point and at the same time rotate so as to place
the surface at right angles to the viewing direction.
4) Head Coupling and the Virtual Reality (VR)
metaphor
The VR metaphor involves coupling the perspective
image to the user's head position so that, for example,
to look at the far side of an object the user must walk
around the object (Sutherland, 1968). This method
uses the interface of everyday life but its affordances
are highly restrictive, it only allows viewpoint
manipulation within the range of head movements. To
allow for greater flexibility it must be combined with
other methods. It most naturally fits with the use of
mechanical widgets (such as a virtual turntable)
because they can be implemented as objects in the
virtual world (Connor, et al. 1992). It is also possible
to implement a localized form of VR using a
conventional workstation an coupling the viewpoint to
the measured eye position (Ware, et al, 1993) as shown
in Figure 2.
Head Tracking
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Glasses
Virtual Objects
Figure 2. By coupling the perspective viewpoints for the two
eyes to the measured head position of the observer and using
stereo glasses it is possible to create a highly realistic
localized virtual image.
3) Flying and virtual vehicle control
Flight simulators have the rather inconvenient
affordances of flying vehicles, problems such as
stalling when the velocity is low are hindrances to
arbitrary control of viewpoints. We have done
extensive work towards making an easy-to-use velocity
control interface for exploring 3D graphical
environments. This which makes no attempt to model
real flight dynamics but is designed to give a great
flexibility in the control of viewpoint movements
through environments which mostly consist of
scientific data representations (Ware and Osborne,
1990). Predictive feedback can greatly enhance the
user's sense of control (Chapman and Ware, 1990). The
form of predictor that we have developed is illustrated
in Figure 3. We use this system in making movies to
convey information to others.