6.4 Extracting Information from a scene

So far you have learned that you can access any node in a existing VRML scene that has a DEFed name. You can actually do more. Since you can inspect exposed field values, you can also retrieve children nodes. We are going to demo this in the next example. It will retrieve all the Viewpoints from a Group node called ``Viewpoints'' in a VRML scene. The Java applet then generates navigation nodes inserted into the applet. Note that this example is still pretty useless. Useful things have a tendency to become quite complex.

Example 6.4.1   EAI: Retrieving children nodes  

Contents:	getEventOut, children nodes
HTML page:	eai/navigate/navigate-dyn.html
JAVA code:	eai/navigate/NavigateDyn.java
Directory:	(dir)

DEF Viewpoints Group {
   children [
      DEF Entry Viewpoint {
         position       0 1 15
         description    "Entry"
      }
      
      DEF AcrossTheRoom Viewpoint {
    ............
      } 
      
    ............
     ]
}

The init() method is fairly standard, so we will not comment it. Now let's see how we exact these nodes: We first declare an array variable to store the nodes:

  Node[] VPList ;

We extract the children in the initScene() method:

  // Get the Group node called Viewpoints
  Node Viewpoints = browser.getNode("Viewpoints");
  // Extract the list of children
  VPList = ((EventOutMFNode) 
       (Viewpoints.getEventOut("children_changed"))).getValue();

Note how we cast the result of getEventOut to EventOutMFNode, i.e. it will return a list of Nodes. And that's it. Now in order to be able to rebind the user to a Viewpoint, we are interested in getting handles on the set_bind field for each found viewpoint. The following lines of code relate to that:

   nVPs = VPList.length;
   set_VPList = new EventInSFBool[nVPs];
   ......
     for (i=0; i < nVPs ; i++) {

        // Get the handles for the Viewpoints set_bind Event fields

        set_VPList[i] = (EventInSFBool)VPList[i].getEventIn("set_bind");
      }

In the same loop we also generate the Java press buttons which we also store in an array (look at the source code) All we finally need to do is to implement the action() method that will handle user events and bind the user to a new ViewPoint. It's very much the same technology we discussed in section 6.3.1 as well as others. The only difference is that buttons are not stored in variables but in an array called VPButtons:

  /** Handle actions from AWT widgets */
  public boolean action(Event event, Object what) {
    
    if (event.target instanceof Button) {
      Button b = (Button) event.target;
      
      for (int i=0; i < nVPs ; i++) {
        if (b == VPButtons[i]) {
          // get the corresponding VP
          EventInSFBool set_bindVP = set_VPList[i];
          set_bindVP.setValue(true);
          System.out.println("action(): set_bound " + set_bindVP);
        }  }
    }
    return true;  }
}

This example assumed that the only children of the ``Viewpoints'' node were Viewpoints. In other words, our code would break if there were other types of children nodes.

The next example deals with this issue. The applet below adds a ``tour'' button that will move the user throught all viewpoints with some pause in between (Jumping is not nice I know, but this will be addressed somewhere else).

Example 6.4.2   EAI: Retrieving children nodes with errors  

Contents:	getEventOut(), getType(), children nodes
HTML page:	eai/navigate/navigate-tour.html
JAVA code:	eai/navigate/NavigateDyn.java
Directory:	(dir)

Basically we just had to add a few lines to the initScene() method. The Node.getType() methode will return a string with the VRML Node name which you then can use to identify the kind of node you have retrieved.

      for (int i=0; i < nVPCs ; i++) {
        String type_found = VPCandidates[i].getType();
        if (!type_found.equals("Viewpoint")) {
          System.out.println("WARNING! initScene: Ignored node of type " + type_found);
        }
        else {
          // got a ViewPoint
          VPList[nVPs] = VPCandidates[i];
          nVPs = nVPs+1;
        }
      }

We will also add here some comments about a revisited version of the Tiny3D applet that extracts node definitions]

Example 6.4.3   EAI: Tiny3d-2  

Java 1.1.x (Netscape 4x PATCHED):	eai/tiny3D-2/tiny3D-2.html
Java 1.0.x (Netscape 3x,4x):	eai/tiny3D-2-java1.0.2/tiny3D-2.html