Scene graph structure of the framework components. The Java 3D library offers a set of classes that are organized in a directed acyclic graph. The relationships of the nodes in the graph determine how they are rendered in the 3D scene. Our framework, built on top of Java 3D, consists of a chain of global TransformGroup (TG) nodes that represent the view's zoom, rotation and panning (top), and which affect the rendering of all subtree nodes. Subtree nodes contain the image volumes. Other nodes include BranchGroup (BG) nodes, capable of holding several subtrees (each representing the internal state and data of an image volume or mesh; lower right). Switch (S) nodes are immediate parents of data nodes, and are used to toggle their visibility state.