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1992) explores 
he user control 
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 
og 
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tereo 
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.