User`s guide

5 Build Virtual Reality Worlds
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aircraft operation in space). For such navigation to be successful, the scene's author must
define the NavigationInfo parameters correctly.
Coordinate System Used
VRML uses a Cartesian coordinate system with axes defined so that:
• +x points right
• +y points up
• +z points out of the screen
To avoid transforming object axes into the VRML system later on, export CAD models
using an identical coordinate system whenever possible. If your CAD tool uses a different
coordinate system, and it does not allow you to change it for the exported objects,
make sure to note the difference between the systems so that you can implement axes
transformations in your model later.
Also, make a note of the orientation of the parts in the coordinate system. For instance, if
a vehicle model is exported so that it points towards the +x axis on a road in the virtual
world, then the road should also point towards the +x direction, and the model of vehicle
dynamics should also use the x coordinate.
When the CAD tool allows you to animate parts and assemblies, reset their positions to
the initial state before the export.
Assembly Hierarchy
How assembly of parts are exported depends on the structure of the model, which usually
comes in two forms:
• All parts are independent from each other, or objects in the scene are independent
from each other at the same level of the scene hierarchy. The exported VRML file has
a flat structure, with all part coordinates defined in global coordinates.
• Parts follow some kind of hierarchy defined in the CAD tool. The exported VRML file
will use this hierarchy via the VRML Transform-children mechanism, to create
a nested structure. In this case, part coordinates are usually defined in the part's
parent local coordinate system.
For example, a robot can be exported with the following object hierarchy, in which
each part's coordinates are defined in the parent's local coordinate system: