BoundingVolume

A BoundingVolume is a solid enclosing all the geometry of a node and its children that is used for culling and collision detection. (If the BoundingVolume for a node is not visible, there is no need to render that node.) Acron will generate “bounds” for each node automatically by creating bounding volumes.

Acron defines the “bounds” of a node to be a geometric bounding volume, of an arbitrary shape (but usually a sphere) that is no smaller than its enclosed geometry. The get_bounds() method returns an acceptable bounding volume. It is indeed no smaller than the enclosed geometry. This makes it suitable for use in culling operations and so on. To check the size of the bounding volume, use show_bounds().

Note that this BoundingVolume is not the smallest possible. For instance, a sphere of radius 1 has a BoundingSphere with radius 1.73205. Acron doesn’t bother going through all the trouble it would take to compute a tight spherical bounds, because the loose bounds that it computes is good enough for Acron’s needs. The extra performance gain you’d get for having a tighter culling bounds isn’t worth the effort it would take to compute it.

Although it doesn’t use it, Acron can create a tighter bounding box. This “tight” bounding box is the smallest axis-aligned box that is no smaller than its enclosed geometry. Thus, it satisfies its definition as a “tight” bounds, because you will not find a tighter bounding volume that is also a box. You can retrieve the bounding box using the get_tight_bounds() method. This box can be shown with show_tight_bounds().

Further tweaking of the bounding volume used must be done manually

  • If all you care about is Acron’s usage of bounding boxes, you can create a BoundingVolume for the node you want and tell Acron to use that one with set_final(1).

    node->set_bounds(new BoundingVolume(...));
node->set_final(true);

    This will tell Acron to stop calculating bounds and use the one you gave it instead.

  • If you want to have a node with a manual bounding box set for your own nefarious purposes, set the bounds at the bottom: on the Geom within a GeomNode. This will propagate upwards, assuming there are no other nodes with bounding volumes above it.