Realtime interactive tools allow the user to navigate through a model environment at their own pace and to go wherever they want to go. Console games are a good example of interactive model environments. Objects in the scene can be set up for interactivity as well- the user can be allowed to switch between alternatives in the design, to query objects for information, or even to move elements around in the scene.
Because the scene is being rendered in realtime, interactive models require higher end hardware for playback, primarily newer video displays with a good amount of video RAM. The models must also be built with some attention to model size and texture map needs. Object in the scene must be built with "optimization" of geometry being a major concern.
Recent hardware improvements, most notably in video display cards, has made realtime models a much more reasonable alternative for presenting design visualization. Standard computers, even laptops, can now be used to present relatively large computer models.
Optimization for Realtime
There are some basic approaches to building and optimizing geometry in a 3D model. The first and easiest is to eliminate any surfaces that will not be visible to viewers of the model- the bottoms of primitives like boxes and cylinders, and any faces that are "inside" objects, such as the inside of structures. Curved objects should be constructed with as few segments as possible along the curved surfaces as feasible. It is often a tradeoff between smooth surfaces and reasonable playback speeds. It will also depend on how close the viewer is expected to get to model elements.
When surfacing road designs in Geopak, the designer has to conscious of the segmentation along the centerline between the 3D sections. This segmentation is controlled by the Minimum Rule Spacing setting under FILE>PREFERENCES on the 3D Modeling Dialog in Geopak
This number controls the number of segments between sections, so if the sections are close together, or the road is relatively straight, this number can be lower. Curves and longer sections require more segmentation.
The VRML format is discussed in section 4.1, as is U3D. There are several high-end interactive authoring environments, such as Anark, EonReality, MultiGen. Most of these require a significant investment, and use custom, proprietary viewers. All of them allow a high level of control over interaction with the camera, and objects in the scene. All of them have access to scripting languages, and the ability to link with external data sources.
Issues with density of geometry will be similar for any realtime application, as with VRML and U3D.