Biomedical Engineering Reference
In-Depth Information
Fig. 7.19
Relationships between coordinate systems in a typical VE system
Display Center is a fixed position in the display from which measurements of user
position are reported. In Fig. 7.18 the display center is the center of the floor.
User Ground Center is a calculated position relative to the Display Center where
the user “is.” Typically this is the centroid of his two feet or just the point directly
below his head.
The relationships between these coordinate systems are represented in Fig. 7.19 ,
which is an abstract scene graph. The lines represent parent-child relationships
between coordinate systems. Each line also represents the transformation from one
coordinate system to another. We see that the Display Center is a child of the World
Coordinates. The position of the Tracker Base is known relative to the Display Center.
This will usually be a one-time calibration at the installation of the tracking system.
We can then see that Hand Tracker and Head Tracker coordinates are known relative
to the Tracker Base. These are the values reported in real-time by the tracking sys-
tem. We can thus see that by concatenating the Display Center to Tracker Base and
Tracker Base to Hand Tracker transformations, we can find the location of the Hand
Tracker relative to the Display Center. The relative position of the Head from the
Head Tracker and Hand from the Hand Tracker are also calibrated and fixed. Given
that all these transformations are known (the double lines in the figure and the large
dashed line), we can then calculate the positions of the Head and Hand relative to
the Display Center. However, a common convention, especially when implementing
travel techniques is to introduce a User Ground Center coordinate system as defined
above. Thus the three light dashed lines must be updated whenever the tracking
system reports new positions.
 
 
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