BillboardsA billboard is a (typically flat) shape that always faces the user by rotating automatically around a user-specified axis.
It has a wide variety of uses, including help messages, text annotations, and status displays, but is also widely used to add scenery such as people and trees to models in a CPU efficient manner.
Using a billboard to represent, say, a tree or plant is usually much more efficient than using a 'real' 3D model of a tree.
Note that if there are a large number of billboards in a virtual world then the amount of work done to do the rotation calculations may slow the rendering down too much. Another way to represent a tree or other similar object efficiently is to take the shape that would otherwise have been used as a billboard and add a duplicate of it, intersecting the original at 90 degrees to it.
In VRML, a billboard is a kind of grouping node that automatically rotates it's children to face the user.
