Java 3D API - Scene Graph Overview

From 54f1415fa01061f0bcc5126605713b9a391cec46 Mon Sep 17 00:00:00 2001 From: phil Date: Fri, 29 Dec 2017 22:53:47 +1300 Subject: Some excellent doc files added --- doc-files/SceneGraphOverview.html | 226 ++++++++++++++++++++++++++++++++++++++ 1 file changed, 226 insertions(+) create mode 100644 doc-files/SceneGraphOverview.html (limited to 'doc-files/SceneGraphOverview.html') diff --git a/doc-files/SceneGraphOverview.html b/doc-files/SceneGraphOverview.html new file mode 100644 index 0000000..f1616df --- /dev/null +++ b/doc-files/SceneGraphOverview.html @@ -0,0 +1,226 @@ + + + + + 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. +

Directed Acyclic Graph

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.
+

Viewing a Scene Graph +

Figure 2 – Viewing a Scene Graph

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