The Background leaf node defines a solid background color and a background image that are used to fill the window at the beginning of each new frame. The background image may be null. It optionally allows background geometry---which is pre-tessellated onto a unit sphere and is drawn at infinity---to be referenced. It also specifies an application region in which this background is active. A Background node is active when its application region intersects the ViewPlatform's activation volume. If multiple Background nodes are active, the Background node that is "closest" to the eye will be used. If no Background nodes are active, then the window is cleared to black.
The set of nodes that can be added to a BranchGroup associated with a Background node is limited. All Group nodes except ViewSpecificGroup are legal in a background geometry branch graph. The only Leaf nodes that are legal are Shape3D (except OrientedShape3D), Morph, Light, and Fog. The presence of any other Leaf node, including OrientedShape3D, or of a ViewSpecificGroup node will cause an IllegalSceneGraphException to be thrown. Note that Link nodes are not allowed; a background geometry branch graph must not reference shared subgraphs. NodeComponent objects can be shared between background branches and ordinary (non-background) branches or among different background branches, however.
Light and Fog nodes in a background geometry branch graph do not affect nodes outside of the background geometry branch graph, and vice versa. Light and Fog nodes that appear in a background geometry branch graph must not be hierarchically scoped to any group node outside of that background geometry branch graph. Conversely, Light and Fog nodes that appear outside of a particular background geometry branch graph must not be hierarchically scoped to any group node in that background geometry branch graph. Any attempt to do so will be ignored.
The influencing bounds of any Light or Fog node in a background geometry branch graph is effectively infinite (meaning that all lights can affect all geometry objects nodes within the background geometry graph, and that an arbitrary fog is selected). An application wishing to limit the scope of a Light or Fog node must use hierarchical scoping.
Picking and collision is ignored for nodes inside a background geometry branch graph.