Game Development Reference
In-Depth Information
LOD
(2,40)
Geode 1
45000t
Geode 2
20000t
Geode 3
1000t
Figure 14.19 Representation of the levels of detail in the tree structure of a 3D data file
14.5.2 Levels of details
The technique of levels of details consists of reducing the number of polygons of an
object displayed on the screen depending on the manner in which it is perceived by the
user: An object that is far away or small requires fewer details than a closer or bigger
object. It is so for two main reasons:
The centre of interest of the image is generally at the centre of the screen and is of
a decent size;
The image is made up of a series of discreet pixels; the details whose projection is
less than the size of the pixel thus cannot be represented.
Using levels of detail in an application requires having a certain number of models of
the same object at different precisions and a function to determine the model to be
used. In the standard 3D data files representing 3D models in tree structures, a level
of detail (LOD) node is presented as shown in figure 14.19.
If it is possible to model the same object three times with different precisions, it
is certainly more useful to make a model only once with a good precision and then
to use the automatic decimation tools. This calculation is generally done off-line, but
can also be done online. The function of choosing the level of model to be shown is
calculated online.
14.5.2.1 Transition command
There are several criteria for transition from one level of detail to another. The main
parameter used is of course the distance between the virtual camera and the object
considered. A decision table that gives the level of detail to be used depending on this
distance is used. This technique is not sufficient since it does not consider the size of
the objects: A large object far from the camera may require more details than a small
object closer to the camera. In general, the level of detail required for an object depends
more on the object's size on the screen than its actual size. Thus, the angle from which
the object is seen is equally important.
Other than these criteria, there are certain other details to be taken into account.
J. Nouvel (1998) explains them in the case of a fighter pilot in his flight simulator:
Relative importance of the object in its environment; A tree at the centre of a
plain can be a landmark for the flight, whereas it is completely insignificant in the
middle of a forest;
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