Java 3D API - Scene Graph Overview

## Scene Graph Basics

A scene graph consists of Java 3D objects, called nodes, arranged in a tree structure. The user creates one or more scene subgraphs and attaches them to a virtual universe. The individual connections between Java 3D nodes always represent a directed relationship: parent to child. Java 3D restricts scene graphs in one major way: Scene graphs may not contain cycles. Thus, a Java 3D scene graph is a directed acyclic graph (DAG). See Figure 1.

Java 3D refines the Node object class into two subclasses: Group and Leaf node objects. Group node objects group together one or more child nodes. A group node can point to zero or more children but can have only one parent. The SharedGroup node cannot have any parents (although it allows sharing portions of a scene graph, as described in "Reusing Scene Graphs"). Leaf node objects contain the actual definitions of shapes (geometry), lights, fog, sounds, and so forth. A leaf node has no children and only one parent. The semantics of the various group and leaf nodes are described in subsequent chapters.

## Scene Graph Structure

A scene graph organizes and controls the rendering of its constituent objects. The Java 3D renderer draws a scene graph in a consistent way that allows for concurrence. The Java 3D renderer can draw one object independently of other objects. Java 3D can allow such independence because its scene graphs have a particular form and cannot share state among branches of a tree.

### Spatial Separation

The hierarchy of the scene graph encourages a natural spatial grouping on the geometric objects found at the leaves of the graph. Internal nodes act to group their children together. A group node also defines a spatial bound that contains all the geometry defined by its descendants. Spatial grouping allows for efficient implementation of operations such as proximity detection, collision detection, view frustum culling, and occlusion culling.

Figure 1 – A Java 3D Scene Graph Is a DAG (Directed Acyclic Graph)

### State Inheritance

A leaf node's state is defined by the nodes in a direct path between the scene graph's root and the leaf. Because a leaf's graphics context relies only on a linear path between the root and that node, the Java 3D renderer can decide to traverse the scene graph in whatever order it wishes. It can traverse the scene graph from left to right and top to bottom, in level order from right to left, or even in parallel. The only exceptions to this rule are spatially bounded attributes such as lights and fog.

This characteristic is in marked contrast to many older scene graph-based APIs (including PHIGS and SGI's Inventor) where, if a node above or to the left of a node changes the graphics state, the change affects the graphics state of all nodes below it or to its right.

The most common node object, along the path from the root to the leaf, that changes the graphics state is the TransformGroup object. The TransformGroup object can change the position, orientation, and scale of the objects below it.

Most graphics state attributes are set by a Shape3D leaf node through its constituent Appearance object, thus allowing parallel rendering. The Shape3D node also has a constituent Geometry object that specifies its geometry-this permits different shape objects to share common geometry without sharing material attributes (or vice versa).

### Rendering

The Java 3D renderer incorporates all graphics state changes made in a direct path from a scene graph root to a leaf object in the drawing of that leaf object. Java 3D provides this semantic for both retained and compiled-retained modes.

## Scene Graph Objects

A Java 3D scene graph consists of a collection of Java 3D node objects connected in a tree structure. These node objects reference other scene graph objects called node component objects. All scene graph node and component objects are subclasses of a common SceneGraphObject class. The SceneGraphObject class is an abstract class that defines methods that are common among nodes and component objects.

Scene graph objects are constructed by creating a new instance of the desired class and are accessed and manipulated using the object's `set` and `get` methods. Once a scene graph object is created and connected to other scene graph objects to form a subgraph, the entire subgraph can be attached to a virtual universe---via a high-resolution Locale object-making the object live. Prior to attaching a subgraph to a virtual universe, the entire subgraph can be compiled into an optimized, internal format (see the `BranchGroup.compile()` method).

An important characteristic of all scene graph objects is that they can be accessed or modified only during the creation of a scene graph, except where explicitly allowed. Access to most `set` and `get` methods of objects that are part of a live or compiled scene graph is restricted. Such restrictions provide the scene graph compiler with usage information it can use in optimally compiling or rendering a scene graph. Each object has a set of capability bits that enable certain functionality when the object is live or compiled. By default, all capability bits are disabled (cleared). Only those `set` and `get` methods corresponding to capability bits that are explicitly enabled (set) prior to the object being compiled or made live are legal.

## Scene Graph Superstructure Objects

Java 3D defines two scene graph superstructure objects, VirtualUniverse and Locale, which are used to contain collections of subgraphs that comprise the scene graph. These objects are described in more detail in "Scene Graph Superstructure."

### VirtualUniverse Object

A VirtualUniverse object consists of a list of Locale objects that contain a collection of scene graph nodes that exist in the universe. Typically, an application will need only one VirtualUniverse, even for very large virtual databases. Operations on a VirtualUniverse include enumerating the Locale objects contained within the universe.

### Locale Object

The Locale object acts as a container for a collection of subgraphs of the scene graph that are rooted by a BranchGroup node. A Locale also defines a location within the virtual universe using high-resolution coordinates (HiResCoord) to specify its position. The HiResCoord serves as the origin for all scene graph objects contained within the Locale.

A Locale has no parent in the scene graph but is implicitly attached to a virtual universe when it is constructed. A Locale may reference an arbitrary number of BranchGroup nodes but has no explicit children.

The coordinates of all scene graph objects are relative to the HiResCoord of the Locale in which they are contained. Operations on a Locale include setting or getting the HiResCoord of the Locale, adding a subgraph, and removing a subgraph.

## Scene Graph Viewing Objects

Java 3D defines five scene graph viewing objects that are not part of the scene graph per se but serve to define the viewing parameters and to provide hooks into the physical world. These objects are Canvas3D, Screen3D, View, PhysicalBody, and PhysicalEnvironment. They are described in more detail in the "View Model" document.

### Canvas3D Object

The Canvas3D object encapsulates all of the parameters associated with the window being rendered into. When a Canvas3D object is attached to a View object, the Java 3D traverser renders the specified view onto the canvas. Multiple Canvas3D objects can point to the same View object.

### Screen3D Object

The Screen3D object encapsulates all of the parameters associated with the physical screen containing the canvas, such as the width and height of the screen in pixels, the physical dimensions of the screen, and various physical calibration values.

### View Object

The View object specifies information needed to render the scene graph. Figure 2 shows a View object attached to a simple scene graph for viewing the scene.

The View object is the central Java 3D object for coordinating all aspects of viewing. All viewing parameters in Java 3D are directly contained either within the View object or within objects pointed to by a View object. Java 3D supports multiple simultaneously active View objects, each of which can render to one or more canvases.

### PhysicalBody Object

The PhysicalBody object encapsulates all of the parameters associated with the physical body, such as head position, right and left eye position, and so forth.

### PhysicalEnvironment Object

The PhysicalEnvironment object encapsulates all of the parameters associated with the physical environment, such as calibration information for the tracker base for the head or hand tracker.

Figure 2 – Viewing a Scene Graph