Advanced Building Simulation

228 Malkawi
9.2.2.1 Registration
As mentioned earlier, registration requirements and needs are different for various
immersive environments. In completely immersed environments, our eyes do not
notice slight errors since we are not “in” the real scene; but in augmented environments,
slight errors are noticed instantly due to the known phenomenon of our brain—
“visual capture” (Welch and Robert 1978), where visual information overrides all
other senses. In other words, the registration error of a Virtual Environment results in
visual–kinesthetic and visual–proprioceptive conflicts rather than a visual–visual conflict
as seen in the case of an AR system (Pausch et al. 1992). The sensitivity of human
eyes to detect registration errors is extremely high (registration errors range from 0.1
to 1.8mm for position and 0.05� to 0.5� for orientation) and this poses a great challenge
in creating augmented environments without registration error. In AR, the range
of space is limited by the sensor technology used. In general, registration errors are
mainly due to sensors tracking, display configuration and viewing parameters. These
issues will be disussed in the following sections.
SENSORS TRACKING
Sensors track the movement of an object or viewer in terms of position and orientation.
Most commercial sensors have the capability to track six degrees of freedom
(DOF) at any given time-interval. These sensors use different technologies such as
mechanical, electromagnetic, ultrasonic, inertial, and optical. Each of these sensor
technologies has limitations. Mechanical sensors are bounded by the connected
device such as the BOOM. Electromagnetic sensors are prone to distortion due to
metal present in the environment and propagate a high degree of error that changes
with the distance between the sensors and the magnetic transmitter; ultrasonic sensors
suffer from noise and temperature; the inertial sensors drift with time and cannot
determine position (Figure 9.12). Although the optical sensors are comparatively
Figure 9.12 Outdoor AR-battlefield augmented reality system (inertial GPS technology) (Courtesy:
The Advanced Information Technology Branch of Information Technology Division at the
Naval Research Laboratory, US Navy). (See Plate XIV.)