Biomedical Engineering Reference
In-Depth Information
be stationary. Therefore some sort of virtual travel technique is necessary. Given
what we know about sensors and displays, it is first important to identify what we
know about the user, and thus what a travel technique actually moves.
We already noted that viewpoint control in the real world is a consequence of
movements of various parts of the body. We also note that sometimes the user's
head is tracked, and sometimes it is not. If the user stands inside a CAVE™ -like
device, then they can physically move a little bit, but may utilize a joystick to move
longer distances. Thus in many systems there are effectively two travel techniques: a
short-range physical travel technique and a long-range virtual travel technique. This
is unlike a standard desktop-style metaphor where the whole of the viewpoint motion
control is under a single control metaphor (e.g., a joystick or mouse plus keyboard
combination).
In order to understand how these two types of movement work together, we need to
describe the various coordinate systems involved, and the relationships among them.
A logical separation which is reified in some implementations is to separate a
display center coordinate
system from a tracker coordinate system. To explain this
we need to delve into one potential way of structuring coordinate systems within a
display system. An example is shown in Fig.
7.18
. This depicts an SSD-type system
but the same coordinate systems would usually exist in other display types.
•
Tracker Base is the fixed origin of the tracking system coordinate system. Typically
this is centered on a physical base unit or some physical component of the tracking
system that is static. The tracking unit reports positions relative to this.
•
Head Tracker is the relative position of the tracking unit attached to the head.
Note that it is not exactly the same as the position of the head, but it is commonly
assumed that the tracking unit is in a fixed position relative to the user's head (e.g.,
on the glasses or on a cap).
•
Hand Tracker is the relative position of the tracking unit attached to the hand.
Again this is not exactly the same as the hand position as it is assumed there is a
known offset between the two.
Fig. 7.18
Coordinate systems within a SSD-type display
