Databases Reference
In-Depth Information
6
Spatio-Temporal LoD
Our proposed frame grouping can decrease the number of representative 3D arrow
icons as visualization of the movement of the camera. For longer duration, too
many 3D arrow icons will appear in a virtual space. It is necessary to provide
users with some tools or equipment for setting intended time and duration easily
as the condition of selecting spatial objects from spatial databases. The number of
polygons of all 3D arrow icons must be below a constant number, for example,
10,000, in order to walk through a virtual space smoothly.
LoD (Levels of Detail)
is the main rule for realizing walk-through based on
spatial databases. The rule is used for selecting spatial objects from spatial databases
by spatial queries, and for constructing a virtual space dynamically. We extend the
LoD in order to take
time
into account for calculating the distance between a user
and objects. The time extension for LoD leads us to
time walk-through
.
6.1
LoD
LoD is determined depending on the positions of a camera and these of some
objects in a virtual space. For example, a frame group is visualized as multiple
arrows, one arrow, only point, and none, depending on the distance between the
camera of the frame group and the viewpoint of a user (Figure 9). When a user
sees the camera at a near position, the precise representation of its movement is
adopted. If a user is far from the camera, the representation of its movement
becomes rough. This way of controlling the quality of spatial data is natural and
effective for practical walk-through.
Figure 9: LoD for space and time scale distance
