Introduction

This is "work in progress" at the moment.

• A web browser which supports JDK1.1 (i.e. Internet Explorer 4.0, Netscape 4.0)
• A VRML plug-in (such as SGI's CosmoPlayer)

I've found that VRML + java support on Windows is better than that on Unix systems, but the area is in a state of flux at the moment, and not everything works quite how it should.

PROTO SPHERE [
	exposedField SFVec3f translation 0 0 0
	exposedField SFInt32 numbeads 1
] {
  Transform {
    translation IS translation
    children [...]
  }
}

Once the handle to the browser has been obtained, you can call getNode("NodeName") to get a pointer to the VRML nodes in the scene named using DEF.

Once you've got a Node, you access its eventIns, its eventOuts and exposedFields using e.g. node.getEventIn("startTime").

Once you've got an EventIn, you can set its values using eventin.setValue(123);

The code to do all this is outlined in the next section.

import vrml.external.field.*;
import vrml.external.Node;
import vrml.external.Browser;
import vrml.external.exception.*;

  Browser browser=null;
  boolean error=true;

  /** Connect to browser */
  public void initBrowser() { 
    pause(1000);
    browser = Browser.getBrowser(this);
    if (browser == null) {
	error = true;
    } else  {
	error = false;
    }
  }

  /** Pause thread for t millisecs */
  void pause(int t) {
    try {Thread.currentThread().sleep(t);} 
    catch (InterruptedException e) {
      System.out.println("Rudely interrupted");
    }
  }

The basic function to get a DEFed node by name is:-

  public Node getNode(String name) {
    Node n = null;
    if (!error) {
      try {n = browser.getNode(name); }
      catch (InvalidNodeException ne) { 
	  System.out.println("Failed to get node:" + ne);   
	  error = true; }
    }
    return n;
  }

  public EventIn getEventIn(Node n, String ename) {
    EventIn e = null;
    if (!error) {
    if (n!=null) {
	try {
 	  e = n.getEventIn(ename);
 	} catch (Exception ex) { 
	  System.out.println("Couldn't get event "+ename+":"+ex);
	}
    }}
    return e;
  }

  EXTERNPROTO CPU [
	exposedField SFInt32 state 
	exposedField SFVec3f translation] "cpu.wrl"
  DEF Node0 CPU { state 0 translation -3 4 0 }

  Node node0 = getNode("Node0");
  EventIn  EventInSFInt32 set_state = 
    (EventInSFInt32) getEventIn(node0,"set_state");
  set_state.setValue(1);

A Sample Simjava/3D Simulation

The problem is that the Java program and the VRML browser operate asynchronously and each may take an unpredictable time to load java classes, html and wrl worlds. Trying to connect to any objects (or the browser) before they are ready usually results in a crash. This problem is made worse by the cacheing of class files and everything else which can lead to intermittant problems.

The browser's `Reload' or `Refresh' buttons don't refetch the .class files - you need to hold Shift- or Ctrl- to force a refetch of class files each time you recompile the java source.

If the browser does crash on Windows NT, it can be helpful to switch java logging on and check the javalog.txt file in the Winnt\java folder to read its last words.

• When Java3D is stable, it might be more convenient for building and manipulating worlds than the VRML interface. At time of writing (April '98) it is only supported by JDK1.2.
• The method described above for building the VRML world by hand, and connecting with it using DEFs is not very good for large regular connections of simulation objects. The reason it was chosen is that I found creating vrml on the fly from Java using CreateVrmlFromString and CreateVrmlFromUrl to be slow and unreliable, and they returned incompletely loaded nodes whose fields couldn't be accessed (unless you added a 'sleep' time delay). You may have better luck with this approach.

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