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7. Registration
Although different usage areas of AR have different problems, the main issue is generally the
registration problem. The objects of the virtual and the real world must be perfectly aligned at
all times or the illusion of coexistence will fail. The same problems exist in virtual reality as
well, but due to the total immersion they are not as serious as in augmented reality. The
virtual reality is helped by the fact that the visual sense is the strongest of our senses and can
override the others in case of conflict. For example if we are in a totally immersed virtual
environment and turn our head 20 degrees and the eyes register 19 degrees the visual sense
will override the sense of balance and accept that we have actually turned 19 degrees. If this
error would happen in AR it would be visually apparent that we have turned 20 degrees and
therefore unacceptable. Research shows [39] that the human eye has a resolving power of a
small fraction of a degree. So in order to obtain perfect registration one needs to build a
system that has higher resolution than the human sensory system. Although this kind of
system is not likely to appear in the near future most applications are usable at much lower
resolutions due to the fact that the human brain automatically compensates for small errors in
order to understand what it perceives. If the visual errors are kept at a sub pixel level we will
actually never be able to detect them at all.
Tracking
AR requires technology that can accurately measure the position and orientation of a user in
the environment, referred to as tracking. Although tracking can be applied to the whole body
current research concentrates on tracking head movements. This section will try to overview
the basic principles of tracking position and orientation instead of individual systems.
For tracking to work effectively in Augmented Reality it should be accurate and at interactive
speed. This overview uses the six principles used in [55]: time of flight (TOF), spacial scan,
inertial sensing, mechanical linkages, phase-difference sensing and direct field sensing.
7.1. Time of flight
7.1.1. Ultrasonic
The time of flight principle relies on measuring the time of propagation of acoustic signals
between points, assuming that the propagation speed is constant. The most common
frequency used is in the ultrasonic range, typically around 40 kHz, to prevent the user from
hearing it. By using three emitters and three receivers, the position and orientation of the
target can be calculated using triangulation
Reference
Target
Figure 13 Ultrasonic tracker
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