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<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN"><html><head> <meta content="text/html; charset=ISO-8859-1" http-equiv="content-type"> <title>Java 3D API - Reusing Scene Graphs</title></head><body><h2>Reusing Scene Graphs</h2><p>Java 3D provides application programmerswith two different means for reusing scene graphs. First, multiplescene graphs can share a common subgraph. Second, the node hierarchy ofa common subgraph can be cloned, while still sharing large componentobjects such as geometry and texture objects. In the first case,changes in the shared subgraph affect all scene graphs that refer tothe shared subgraph. In the second case, each instance is unique-achange in one instance does not affect any other instance.</p><h2>Sharing Subgraphs</h2><p>An application that wishes to share a subgraph from multiple placesina scene graph must do so through the use of the <a href="../Link.html">Link</a>leaf node and anassociated <a href="../SharedGroup.html">SharedGroup</a> node. TheSharedGroup node serves as the root ofthe shared subgraph. The Link leaf node refers to the SharedGroup node.It does not incorporate the shared scene graph directly into its scenegraph.</p><p>A SharedGroup node allows multiple Link leaf nodes to share itssubgraph as shown in <a href="#Figure_1">Figure1</a> below.<br></p><p><a name="Figure_1"></a><img style="width: 500px; height: 476px;" alt="Sharing a Subgraph" title="Sharing a Subgraph" src="SceneGraphSharing1.gif"></p><ul> <font size="-1"><b><i>Figure 1</i> – Sharing a Subgraph</b></font></ul><h2>Cloning Subgraphs</h2><p>An application developer may wish to reuse a common subgraph withoutcompletely sharing that subgraph. For example, the developer may wishto create a parking lot scene consisting of multiple cars, each with adifferent color. The developer might define three basic types of cars,such as convertible, truck, and sedan. To create the parking lot scene,the application will instantiate each type of car several times. Thenthe application can change the color of the various instances to createmore variety in the scene. Unlike shared subgraphs, each instance is aseparate copy of the scene graph definition: Changes to one instance donot affect any other instance.</p><p>Java 3D provides the <a href="../Node.html#cloneTree%28%29"><code>cloneTree</code></a>method for thispurpose. The <code>cloneTree</code>method allows the programmer to change some attributes (NodeComponentobjects) in a scene graph, while at the same time sharing the majorityof the scene graph data-the geometry.</p><h3>References to Node ComponentObjects</h3><p>When <code>cloneTree</code> reaches a leaf node,there are two possible actions for handling the leaf node'sNodeComponent objects (such as Material, Texture, and so forth). First,the cloned leaf node can reference the original leaf node'sNodeComponent object-the NodeComponent object itself is not duplicated.Since the cloned leaf node shares the NodeComponent object with theoriginal leaf node, changing the data in the NodeComponent object willeffect a change in both nodes. This mode would also be used for objectsthat are read-only at run time.</p><p>Alternatively, the NodeComponent object can be duplicated, in whichcase the new leaf node would reference the duplicated object. This modeallows data referenced by the newly created leaf node to be modifiedwithout that modification affecting the original leaf node.</p><p><a href="#Figure_2">Figure2</a>shows two instances of NodeComponent objects that are shared and oneNodeComponent element that is duplicated for the cloned subgraph.</p><p><a name="Figure_2"></a><img style="width: 499px; height: 287px;" alt="Referenced and Duplicated NodeComponent Objects" title="Referenced / Duplicated NodeComponens" src="SceneGraphSharing2.gif"></p><p></p><ul> <font size="-1"><b><i>Figure 2</i> – Referenced and DuplicatedNodeComponent Objects</b></font></ul><h3>References to Other SceneGraph Nodes</h3>Leaf nodes that contain references to other nodes(for example, Light nodes reference a Group node) can create a problemfor the <code>cloneTree</code> method. After the <code>cloneTree</code>operation is performed, the reference in the cloned leaf node willstill refer to the node in the original subgraph-a situation that ismost likely incorrect (see <a href="#Figure_3">Figure3</a>).<p>To handle these ambiguities, a callback mechanism is provided.</p><p><a name="Figure_3"></a><img style="width: 499px; height: 240px;" alt="References to Other Scene Graph Nodes" title="References to Other Nodes" src="SceneGraphSharing3.gif"></p><ul> <font size="-1"><b><i>Figure 3</i> – References to Other Scene GraphNodes</b></font></ul><p>A leaf node that needs to update referenced nodes upon being duplicatedby a call to <code>cloneTree</code> must implement the <code>updateNodeReferences</code>method. By using this method, the cloned leaf node can determine if anynodes referenced by it have been duplicated and, if so, update theappropriate references to their cloned counterparts.</p><p>Suppose, for instance, that the leaf node Lf1 in <a href="#Figure_3">Figure3</a> implemented the <code>updateNodeReferences</code> method. Onceall nodes had been duplicated, the <code>clone-Tree</code> methodwould then call each cloned leaf's node <code>updateNodeReferences</code>method. When cloned leaf node Lf2's method was called, Lf2 could ask ifthe node N1 had been duplicated during the <code>cloneTree</code>operation. If the node had been duplicated, leaf Lf2 could then updateits internal state with the cloned node, N2 (see <a href="#Figure_4">Figure4</a>).</p><p><a name="Figure_4"></a><img style="width: 499px; height: 190px;" alt="Updated Subgraph after updateNodeReferences Call" title="Subgraph after updateNodeReferences" src="SceneGraphSharing4.gif"></p><p></p><ul> <font size="-1"><b><i>Figure 4</i> – Updated Subgraph afterupdateNodeReferences Call</b></font></ul><p>All predefined Java 3D nodes will automatically have their <code>updateNodeReferences</code>method defined. Only subclassed nodes that reference other nodes needto have this method overridden by the user.</p><h3>Dangling References</h3>Because <code>cloneTree</code> is able to startthe cloning operation from any node, there is a potential for creatingdangling references. A dangling reference can occur only when a leafnode that contains a reference to another scene graph node is cloned.If the referenced node is not cloned, a dangling reference situationexists: There are now two leaf nodes that access the same node (<a href="#Figure_5">Figure5</a>). A dangling reference is discovered when a leaf node's <code>updateNodeReferences</code>method calls the <code>getNewNodeReference</code> method and thecloned subgraph does not contain a counterpart to the node being lookedup.<p><a name="Figure_5"></a><img style="width: 499px; height: 232px;" alt="Dangling Reference" title="Dangling Reference" src="SceneGraphSharing5.gif"></p><p></p><ul> <font size="-1"><b><i>Figure 5</i> – Dangling Reference: Bold NodesAre Being Cloned</b></font></ul><p>When a dangling reference is discovered, <code>cloneTree</code> canhandle it in one of two ways. If <code>cloneTree</code> is calledwithout the <code>allowDanglingReferences</code> parameter set to <code>true</code>,a dangling reference will result in a <code>DanglingReferenceException</code>being thrown. The user can catch this exception if desired. If <code>cloneTree</code>is called with the <code>allowDanglingReferences</code> parameter setto <code>true</code>, the <code>update-NodeReferences</code> methodwill return a reference to the same object passed into the <code>getNewNodeReference</code>method. This will result in the <code>cloneTree</code> operationcompleting with dangling references, as in <a href="#Figure_5">Figure5</a>.</p><h3>Subclassing Nodes</h3>All Java 3D predefined nodes (for example, Interpolators and LODnodes)automatically handle all node reference and duplication operations.When a user subclasses a Leaf object or a NodeComponent object, certainmethods must be provided in order to ensure the proper operation of <code>cloneTree</code>.<p>Leaf node subclasses (for example, Behaviors) that contain any usernode-specific data that needs to be duplicated during a <code>cloneTree</code>operation must define the following two methods:</p><pre><b>Node cloneNode(boolean forceDuplicate);<br>void duplicateNode(Node n, boolean forceDuplicate)<br></b></pre>The <code>cloneNode</code> method consists of three lines:<pre><hr><br><code>UserSubClass usc = new UserSubClass();<br>usc.duplicateNode(this, forceDuplicate);</code><br>return usc;<br><br><hr></pre>The <code>duplicateNode</code> method must first call <code>super.duplicateNode</code>before duplicating any necessary user-specific data or setting anyuser-specific state.<p>NodeComponent subclasses that contain any user node-specific datamust define the following two methods:</p><pre><b>NodeComponent cloneNodeComponent();<br>void duplicateNodeComponent(NodeComponent nc, boolean forceDuplicate);<br></b></pre>The <code>cloneNodeComponent</code> method consists of three lines:<pre><hr><br><code>UserNodeComponent unc = new UserNodeComponent();<br>unc.duplicateNodeComponent(this, forceDuplicate);</code><br>return un;<br><br><hr></pre>The <code>duplicateNodeComponent</code> must first call <code>super.duplicateNodeComponent</code>and then can duplicate any user-specific data or set any user-specificstate as necessary.<h3>NodeReferenceTable Object</h3>The NodeReferenceTable object is used by a leaf node's <code>updateNodeReferences</code>method called by the <code>cloneTree</code>operation. The NodeReferenceTable maps nodes from the original subgraphto the new nodes in the cloned subgraph. This information can than beused to update any cloned leaf node references to reference nodes inthe cloned subgraph. This object can be created only by Java 3D.<h3>Example: User Behavior Node</h3>The following is an example of a user-defined Behavior object to showproperly how to define a node to be compatible with the <code>cloneTree</code>operation.<hr><pre>class RotationBehavior extends Behavior {<br> TransformGroup objectTransform;<br> WakeupOnElapsedFrames w;<br></pre><pre> Matrix4d rotMat = new Matrix4d();<br> Matrix4d objectMat = new Matrix4d();<br> Transform3D t = new Transform3D();<br></pre><pre><i> // Override Behavior's initialize method to set up wakeup<br> // criteria<br></i></pre><pre> public void initialize() {<br></pre><pre><i> // Establish initial wakeup criteria<br></i></pre><pre> wakeupOn(w);<br> }<br></pre><pre><i> // Override Behavior's stimulus method to handle the event<br></i></pre><pre> public void processStimulus(Enumeration criteria) {<br></pre><pre><i> // Rotate by another PI/120.0 radians<br></i></pre><pre> objectMat.mul(objectMat, rotMat);<br> t.set(objectMat);<br> objectTransform.setTransform(t);<br></pre><pre><i> // Set wakeup criteria for next time<br></i></pre><pre> wakeupOn(w);<br> }<br></pre><pre><i> // Constructor for rotation behavior.<br></i></pre><pre> public RotationBehavior(TransformGroup tg, int numFrames) {<br> w = new WakeupOnElapsedFrames(numFrames);<br> objectTransform = tg;<br></pre><pre><i> objectMat.setIdentity();<br></i></pre><pre><i> // Create a rotation matrix that rotates PI/120.0<br> // radians per frame<br> rotMat.rotX(Math.PI/120.0);<br></i></pre><pre><i> // Note: When this object is duplicated via cloneTree,<br> // the cloned RotationBehavior node needs to point to<br> // the TransformGroup in the just-cloned tree. <br> }<br></i></pre><pre><i> // Sets a new TransformGroup.<br></i></pre><pre> public void setTransformGroup(TransformGroup tg) {<br> objectTransform = tg;<br></pre><pre><i> }<br></i></pre><pre><i> // The next two methods are needed for cloneTree to operate<br> // correctly.<br> // cloneNode is needed to provide a new instance of the user<br> // derived subclass.<br></i></pre><pre> public Node cloneNode(boolean forceDuplicate) {<br></pre><pre><i> // Get all data from current node needed for<br> // the constructor<br> int numFrames = w.getElapsedFrameCount();<br></i></pre><pre> RotationBehavior r =<br> new RotationBehavior(objectTransform, numFrames);<br> r.duplicateNode(this, forceDuplicate);<br> return r;<br> }<br></pre><pre><i> // duplicateNode is needed to duplicate all super class<br> // data as well as all user data.<br></i></pre><pre> public void duplicateNode(Node originalNode, boolean <br> forceDuplicate) {<br> super.duplicateNode(originalNode, forceDuplicate);<br></pre><pre><i> // Nothing to do here - all unique data was handled<br> // in the constructor in the cloneNode routine.<br> }<br></i></pre><pre><i> // Callback for when this leaf is cloned. For this object<br> // we want to find the cloned TransformGroup node that this<br> // clone Leaf node should reference.<br></i></pre><pre> public void updateNodeReferences(NodeReferenceTable t) {<br></pre><pre><i> super.updateNodeReferences(t);<br></i></pre><pre><i> // Update node's TransformGroup to proper reference<br></i></pre><pre> TransformGroup newTg =<br> (TransformGroup)t.getNewObjectReference(<br> objectTransform);<br> setTransformGroup(newTg);<br> }<br>}<br></pre></body></html>⌨️ 快捷键说明
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