In light of the results in the thesis, we can also begin to gain
insights into issues such as isomorphism (direct manipulation)
versus tool-using in 6 DOF manipulation. As discussed in Chapter
1, there is a continuum between ideal isomorphism and indirect
tools. This concept is illustrated in Figure 6.2. Although many
attributes of an input device may influence its directness (its
location on the isomorphism - tool continuum), the most dominating
factor is the transformation from the control space to
display space. The more mathematically complex this transformation
is, the more indirect the input technique is. As shown in Figure
6.2, input techniques with first order (rate control) or higher
order control dynamics are indirect tools. With these techniques,
one or more integrals are involved in the mathematical mapping
from the control space (user's control actions) to the display
space (cursor movements). The elastic (EGG) and the isometric
(Spaceball) devices used in Experiment 1, 2 and 3 in rate control
mode are examples of indirect tools.
Figure 6.2 Isomorphism - tool continuum: A taxonomy of
classifying input devices according to directness of transformation
from control space to display space
Moving to the left of Figure 6.2, input devices become more direct. For position control techniques, the mathematical transformation from the control space to the display space is a multiplication, which is simpler than integration. Among position control techniques, absolute devices, such as a 2 DOF digitising tablet or the 6DOF Fball in Experiment 4 are more direct than relative devices, such as a 2 DOF mouse and the 6 DOF glove used in Experiments 1 and 4. Relative devices require a clutch mechanism to engage and disengage the link between control actions and cursor movements. For a mouse, for example, lifting it from mouse pad will disengage the linkage between control and display.
Another factor that affects the directness of position control techniques is the control-display (C-D) ratio. When the C-D ratio is 1, the multiplication operation is reduced to an assignment (copying) operation, which makes the input control more direct than when the C-D ratio is not 1.
There is still another factor that makes some absolute position input techniques more direct than the others: the orientation or location offset between the control space and the display space. Both a touch-screen and a tablet are absolute position control devices but the latter has an offset between the display and the control space in orientation (about 90_ in pitch) and in location (about 20 - 40 cm in the vertical and/or in the horizontal axes). A touch screen interface is therefore more direct than a tablet interface. In the experiments presented in this thesis, all input techniques had a translation offset between the control space and the display space, but no orientation offset. 6DOF techniques without offset can conceivably be implemented, particularly in immersive virtual environments in which the display space (where the user looks) and the control space (where the user moves her limbs) can completely overlap with each other.
It should be noted that to the left of Figure 6.2 there are input devices that are even more direct. These are the position control devices with force-reflecting capabilities. The ultimate isomorphic input controller is one that allows force feedback in all directions, to recreate what we would feel when manipulating real 3D objects directly with our bare hands. In other words, the ultimate isomorphic interfaces are completely "transparent" to the user.
It is important to note that there are both advantages and disadvantages
to techniques on each end of the isomorphism - tool continuum.
In daily life, we prefer to perform many tasks with our bare hands.
Even with a glove, the small "transformation" between
the hand and the actual manipulation may be undesirable on some
occasions. On the other hand, we do frequently use various tools,
sometimes as simple as rulers, wrenches, screwdrivers, etc., for
precision, for power and for overcoming some of our other physical
limitations. In general, more isomorphic (more direct) designs
are more intuitive and require less learning. Such devices are
needed for applications where an explicit learning period is perhaps
not available, such as commercial video games where users should
be able to walk-up and play immediately. The disadvantages with
such isomorphic designs lie in possible fatigue, coarseness of
the control action and anatomical limitations of the human limb.
In contrast, less direct, tool-like devices may take more time
to learn but may be more efficient in terms of reduced fatigue,
smoother motions and fewer physical limitations of the human limb.
Such designs are clearly more suitable for tasks of long duration,
such as in teleoperation.