Databases Reference
In-Depth Information
Figure 3: A screen image of a VWDB prototype
Language. This language is different from a traditional virtual world definition
language provided by VR systems in the sense that the former should describe a
schema level definition of a virtual world, while the latter is enough to describe its
instance level definition. For example, a “scene graph,” which is known in VR to
describe a virtual world, is an instance level description of the virtual world, yet it
is not a schema level description of the virtual world. The schema level information
is necessary because database systems use it to process queries and updates
efficiently, i.e., to avoid exhaustive searches of databases to find targeted objects.
Another point we should consider in designing a schema definition language is
that the VWDB objects are different from the objects in traditional object-oriented
database systems in the sense that the former are usually “three-dimensional spatial
objects” while the latter are not. Actually, a VWDB object has two different kinds
of attributes as follows:
1.
Spatial attributes such as the position, direction, and orientation of a VWDB
object in a VWDB virtual database space.
2.
Non-spatial attributes such as name, weight, color, texture, and so on.
Notice that, corresponding to the two types of attributes, two different types of
domains are defined, and these are spatial domains and non-spatial domains,
respectively.
Another feature of the VWDB database is that a set of objects in a VWDB
database conforms to a certain class hierarchy that is well defined in the object-
oriented paradigm. In VWDB, we call a set of similar objects a “category”. In
