Is this even possible? (Rotation question)

Question

Hi! I'm new to Java 3D... What I am trying to do is to rotate one transform group around another transform group. i.e. get a moon spinning the earth does anyone know how to do this? I am posting my code below.... I've searched the forums already and alas found no answer :( I've tried everything as you can see from the bottom of the code where all the "garbage" is. The actual earth has been commented out so that I can see the moon. Any help would be greatly appreiciated!! Thank you in advance! -JET
import java.util.*;
import java.awt.*;
import java.awt.event.*;
import javax.swing.JFrame;
import java.awt.GraphicsConfiguration;
import javax.media.j3d.*;
import javax.vecmath.*;
import com.sun.j3d.utils.universe.*;
import com.sun.j3d.utils.geometry.*;
import com.sun.j3d.utils.image.TextureLoader;
class System2 extends JFrame // implements ActionListener
     //a SimpleUniverse is used for this program
     System2() //constructor executes Program
          JFrame view = new JFrame("Program");
          view.setSize(800,600);
          GraphicsConfiguration config = SimpleUniverse.getPreferredConfiguration();
          Canvas3D c = new Canvas3D(config);
          Container cp = view.getContentPane();
          cp.add(c);
          SimpleUniverse u = new SimpleUniverse(c);          
          BranchGroup run = createSceneGraph();
          u.addBranchGraph(run);
          u.getViewingPlatform( ).setNominalViewingTransform( );
          view.setVisible(true);
     public BranchGroup createSceneGraph()
          //define Colors
          Color3f yellow = new Color3f(0.9f, 0.8f, 0.0f);
          Color3f bgColor = new Color3f(0.05f, 0.05f, 0.2f);
          Color3f white      = new Color3f(1.0f,1.0f,1.0f);     
          Color3f nullCol = new Color3f(0.0f,0.0f,0.0f);
          Color3f earthDiff = new Color3f(0.49f,0.34f,0);
          Color3f earthSpec = new Color3f(0.89f,0.79f,0);
          // Create a bounds for the background and lights
          BoundingSphere bounds = new BoundingSphere(new Point3d(0.0,0.0,0.0), 100.0);
          //create BranchGroup
          BranchGroup root = new BranchGroup();
          // Set up the background
          Background bg = new Background(bgColor);
          bg.setApplicationBounds(bounds);
          //root.addChild(bg);
          //add first TransformGroup
          TransformGroup earthTrans = new TransformGroup();
          Transform3D earth3d = new Transform3D();
          earthTrans.setTransform(earth3d);
          earthTrans.setCapability(TransformGroup.ALLOW_TRANSFORM_WRITE);
          root.addChild(earthTrans);
          //Creating a Sphere for the Earth          
          //sets the appearance of the earth adds a texture image
          Appearance earthApp = new Appearance();
          earthApp.setColoringAttributes(new ColoringAttributes( yellow,ColoringAttributes.NICEST));
          PolygonAttributes polyAt=new PolygonAttributes(PolygonAttributes.POLYGON_FILL,
                                        PolygonAttributes.CULL_NONE,0f);
          earthApp.setPolygonAttributes(polyAt);
          Material earthMat = new Material(new Color3f(1f,1f,1f)
          ,new Color3f(0.25f,0.25f,0.25f)//Shadows color
          ,new Color3f(1.0f,1.0f,0.8f)//Sun's light
          ,new Color3f(1f,1f,1f)
          ,128);
          //earthMat.setAmbientColor(0,0,0.5f);
          //earthMat.setDiffuseColor(1,1,1);
          //earthMat.setShininess(1000);
          //Material(white,white,earthDiff,earthSpec,100f);
          earthMat.setLightingEnable(true);
          TextureLoader picload = new TextureLoader("earth.jpg",this);
          Texture earthpic = picload.getTexture();
          earthpic.setMagFilter(Texture.BASE_LEVEL_LINEAR | Texture.NICEST);
          earthpic.setMinFilter(Texture.BASE_LEVEL_LINEAR | Texture.NICEST);
          earthApp.setMaterial(earthMat);
          earthApp.setTexture(earthpic);
          TextureAttributes texAttr = new TextureAttributes();
          texAttr.setTextureMode(TextureAttributes.MODULATE);
          earthApp.setTextureAttributes(texAttr);
          //adding lights to the sphere
          System.out.println("directional lights");
          Vector3f direction = new Vector3f(-1.0f,-0.5f,0.1f);
          DirectionalLight dirLight = new DirectionalLight(true,white,direction);
          dirLight.setInfluencingBounds(bounds);
          root.addChild(dirLight);     
          //creates the earth
          Sphere earth = new Sphere(0.5f,Sphere.GENERATE_TEXTURE_COORDS | Sphere.GENERATE_NORMALS,100,earthApp);
          earthTrans.addChild(earth);
          System.out.println("doing rotation");     
          //Setting rotation of Earth
          Alpha rotEarth = new Alpha(-1, 10000);
          Transform3D yAxis = new Transform3D();
          //Setting the rotation in a clock wise direction (could have used default constructor)
          RotationInterpolator rotator = new RotationInterpolator(rotEarth, earthTrans, yAxis,
                    0.0f,(float) Math.PI*2.0f);
          rotator.setSchedulingBounds(bounds);          
          earthTrans.addChild(rotator);
          //end Earth
          //Start Moon
          TransformGroup moonTrans = new TransformGroup();
          Transform3D moon3d = new Transform3D();     
          moonTrans.setTransform(moon3d);
          moonTrans.setCapability(TransformGroup.ALLOW_TRANSFORM_WRITE);
          root.addChild(moonTrans);
          Appearance moonApp = new Appearance();
          Material moonMat = new Material(new Color3f(1f,1f,1f)
          ,new Color3f(0.25f,0.25f,0.25f)//Shadows color
          ,new Color3f(1.0f,1.0f,0.8f)//Sun's light
          ,new Color3f(1f,1f,1f)
          ,128);
          moonApp.setMaterial(moonMat);
          moonApp.setColoringAttributes(new ColoringAttributes( white,ColoringAttributes.NICEST));
          Sphere moon = new Sphere(0.2f,Sphere.GENERATE_NORMALS,moonApp);
          moonTrans.addChild(moon);
          Alpha rotMoon = new Alpha(-1, 4000);
          yAxis = new Transform3D();
          //Setting the rotation in a clock wise direction (could have used default constructor)
          RotationInterpolator moonSpin = new RotationInterpolator(rotMoon, moonTrans, yAxis,
                    0.0f,(float) Math.PI*2.0f);
          moonSpin.setSchedulingBounds(bounds);
          Transform3D axisOfTranslation = new Transform3D();
          Alpha transAlpha = new Alpha(-1,
                    Alpha.INCREASING_ENABLE |
                    Alpha.DECREASING_ENABLE,
                    0, 0,
                    5000, 0, 0,
                    5000, 0, 0);
          axisOfTranslation.rotY(-Math.PI/2.0);
          PositionInterpolator translator =
                         new PositionInterpolator(transAlpha,
                         moonTrans,
                         axisOfTranslation,
                         2.0f, 3.5f);
          translator.setSchedulingBounds(bounds);
          moonTrans.addChild(translator);               
          moonTrans.addChild(moonSpin);
          root.compile();
          return root;
     public static void main (String [] args)
          System2 sys = new System2();
          TransformGroup moonTrans = new TransformGroup();
          Transform3D moon3d = new Transform3D();     
          moonTrans.setTransform(moon3d);
          moonTrans.setCapability(TransformGroup.ALLOW_TRANSFORM_WRITE);
          root.addChild(moonTrans);
          TransformGroup sunTrans = new TransformGroup();
          Transform3D sun3d = new Transform3D();     
          sunTrans.setTransform(sun3d);
          sunTrans.setCapability(TransformGroup.ALLOW_TRANSFORM_WRITE);
          root.addChild(sunTrans);
          //creates the Sun that is used for Directional lighting
          Appearance sunApp = new Appearance();
          sunApp.setColoringAttributes(new ColoringAttributes( yellow,ColoringAttributes.NICEST));
          Sphere sun = new Sphere(0.2f,sunApp);
          sunTrans.addChild(sun);
          //positions the sun
          Alpha posSun = new Alpha(-1,Alpha.INCREASING_ENABLE |
                    Alpha.DECREASING_ENABLE,
                    0, 0,
                    5000, 0, 0,
                    5000, 0, 0);
          PositionInterpolator sunPos = new PositionInterpolator(posSun, sunTrans);
          sunPos.setSchedulingBounds(bounds);
          sunTrans.addChild(sunPos);
          //root.addChild(aLgt);
          AmbientLight a = new AmbientLight(white);     
          a.setInfluencingBounds(bounds);
          root.addChild(a);
          System.out.println("point light");
          PointLight pl = new PointLight(true, white,new Point3f(1f,0f,1f),new Point3f(1.0f,0f,0f));
          pl.setInfluencingBounds(bounds);
          root.addChild(pl);
Transform3D Trafo_1 = new Transform3D();
          Transform3D Trafo_2 = new Transform3D();
          Trafo_1.rotX(0.5);
          Trafo_2.rotY(0.5);
          Trafo_2.mul(Trafo_1);
          RotationInterpolator moonEarth = new RotationInterpolator(rotMoon, moonTrans, Trafo_2,
                    0.0f,(float) Math.PI*2.0f);
          //Billboard moonEarth = new Billboard(moonTrans, Billboard.ROTATE_ABOUT_POINT, new Point3f(0f,0f,0f));

Answer 1

Yes it is possible. Build a scene graph where the TransformGroup of the moon is a child of the TransformGroup of the earth.
TG1 __________ Earth Shape3D
|
|____ TG2 ___ Moon Shape3D
In this way the overal transformation for the moon is the concatenation of TG2 and TG1. The transformation for the moon TG2 has to be given relative to the earth (i.e. the middle of the earth is the center of the local coordinate system for the moon).
If you want the earth spinning then build the following graph:
TG1___ TG3 ___ Earth
|
|____ TG2 ___ Moon
Put the transformation of the earth position in TG1. Put the spinning transformation of the earth in TG3 and the position transformation of the moon relative to the earth in TG2. Thus the spinning of the earth will not interfere with the position of the moon.