In virtual reality, it is important to ensure that the application will run fast in real-time. Hence as few polygons as possible need to be displayed at any time. At the same time, the objects should have sufficient detail to be interesting. The "Level of Detail" tool offers a mechanism to do both. It offers the ability to create several versions of the same object, and specify that the object should switch from one version to another depending on how far the eye is from the object. This way, when the object is far away, it uses a very coarse representation (perhaps even nothing!), and as the eye comes closer, it gets more and more detailed.
Figure 17: The "Level of Detail" tool is used in VR, to create multiple representations for the same 3D object, ranging from the highest detail to the lowest. The "Level of Detail" tool is used in conjunction with "Complexity" and other modeling tools.
Object Name
This allows you to change the name of the object for which multiple levels of detail are created.
Mode
In "Edit" mode, you are always dealing with a single level, irrespective of the eye point. "Idle" mode shows how the object will be displayed in the final output - including transitions from one level to the next.
Level
By default, one level is created. This is called the "highest" level of detail. If you move the slider from "1" to "2", by default, it makes a copy of the previous level. You can then use the "Complexity" tool, or use any modeling tool to reduce the complexity of the second level of detail. After specifying the required levels of detail, you can also specify that the last level of detail be "empty" (that is, nothing is displayed, when the object is far away).
Eye Range
The next step is to set the distances at which the object transitions
from one level to the next. By default, the system takes the current view, and doubles the distance for each subsequent level of detail. You can fine-tune the range for each level, by moving the Level slider to choose the particular level, zooming in the main viewer to where you want the transition to happen, and press "Set to View". Experts in 3D geometry can even type in values in text field!
Insert
This is used to insert a new level before the current level. By default, the current level is copied, and the eye range is set to half that of the current level.
Set Selected
This is used to replace the geometry of the current level by another object in the scene that has been selected (perhaps after loading from another file). This is useful when you don't want to derive each level of detail from the same geometry, but load separate pre-defined objects instead.
Remove
This removes the current level of detail.
Close
This closes the "Level of Detail" tool.
If you now bring up "Scene - Object List", you will notice that a hierarchy (similar to "Edit - Group") has been created, with the several levels of detail appearing as "children" of the parent object. When you zoom in and out, you will see different children appearing at different times. When you select the object in the viewer, the child that is displayed at that moment is selected. But when you then transform the child using the 3D manipulator, the entire object (including all other levels of detail) move as a group.
Hint: As a rule, always "WWW Inline" the first one or two levels of detail. This saves the VR browser from having to load the most detailed geometries if the eye never gets near the object. Select the specific level, and use "Model - WWW Inline".
