Scene graphs are fundamental data structures for hierarchical scene modeling. The generalized scene graph overcomes various limitations of current scene graph architectures such as support for different 3D rendering systems, integration of multi-pass rendering, and declarative modeling of scenes. The main idea is to separate scene specification from scene evaluation. To specify scenes, scene graph nodes are arranged and equipped with rendering objects, e.g., shapes, attributes, and algorithms. To evaluate scenes, the contents of scene graphs nodes, the rendering objects, are evaluated by rendering engines, which use the algorithm objects to interpret shapes and attributes. Using generalized scene graphs, most real-time rendering techniques for OpenGL and several 3D rendering systems can be integrated in a single scene representation without loosing control over or limiting individual strengths of rendering systems.

types are non-instantiable, i.e. it is not possible to have an object which has an abstract type as its immediate type. Instead, any behaviour of an abstract type must be accessed through instances of some subtype. 3.5 Inheritance Where subtyping is a relationship between object types, inheritance is a mechanism for re-using the definitions of one object type in the description of another. Informally, if object type T inherits from S, T acquires all of the state components and operations of S; it may also introduce state and operations of its own. If a PREMO object type T inherits from an object type S, T is defined to be a subtype of S. Multiple inheritance, where an object type can inherit from more than one other object type, is supported in the PREMO object model. While it is useful to be able to combine features from various object types, in practice multiple inheritance introduces the potential for conflict between names (of states or operations) inherited from different sourc...