Impostor - is a form of Level Of Detail group node which allows both switching
between children depending on distance from eye point and image caching.
The principle behind Imposters is that they cache an image of real geometry and then the image is drawn
in subsequent frames instead of the real geometry. Its a bit like a
Billboard *but* is updated at runtime and w.r.t view point. By drawing
just the texture mapped quad you can cut down scene complexity and
improve performance.
For more details have a look at:
http://grail.cs.washington.edu/projects/hic/
The OSG doesn't implement exactly the same technique as above, but its
should be a good starting place. The OSG's impostors are much less
intrusive since you don't need to restructure your whole scene to use
them.
All you need to do to use Impostors is to set up the visible
range values for each LOD child of the Impostor, as per osg::LOD,
and set an Impostor threshold to tell the renderer at what distance
the Impostor's image caching should cut in. The osg::CullVisitor
automatically handles all the setting of pre-rendering stages to
calculate the required ImpostorSprites (which encapsulates the image
cache and quad), and updates them as the view point changes. If you
use osg::SceneView/CullVisitor all the complexity of supporting
Impostor will be nicely hidden away.
TODO:
Various improvements are planned for the Impostor-
1) Estimation of how many frames an ImpostorSprite will be reused, if
it won't be used more often than a minimum threshold then do not create
ImpostorSprite - use the real geometry.
2) Sharing of texture memory between ImpostorSprites.
3) Simple 3D geometry for ImpostorSprite's rather than Billboarding.
4) Shrinking of the ImpostorSprite size to more closely fit the underlying
geometry.
![[more]](icon1.gif)
Impostor()
enum CenterMode