Java3D - 是否有可能有一个跟踪移动对象的查看平台

Question

I have a very simple Java3D application, it contains two cubes, of which one of them orbits the other.

Like so:

As you can see, the default position for the viewing platform is from a 'kind of' birds eye view.

The code that looks like so

TransformGroup cameraTG = u.getViewingPlatform().
                   getViewPlatformTransform();
Vector3f translate = new Vector3f(); 
Transform3D T3D = new Transform3D();
translate.set( 0.0f, 0.0f, 20.0f);
T3D.setTranslation(translate);
cameraTG.setTransform(T3D);

My question is, is it possible to set the viewing platform to track the larger cube (the cube that rotates around the smaller square). Or more straight forward, is it possible for the viewing platform to rotate around a given body?

More information: My goal is to have a miniature solar system, containing the Sun, Earth and moon. And I would like to view it from the point of view of the earth (almost like a view from the ISS)

Any help or pointers would be fantastic. Please feel free to ask for more information if needed.

Answer 1

You have a nice example here http://java3d.nl/Tutorials/Java/Java3d/Controlthecamera_12.php

The original http://java3d.nl website is no longer available, but you can still use the cached version from the Internet Archive: https://web.archive.org/web/20131218022035/http://java3d.nl/Tutorials/Java/Java3d/Controlthecamera_12.php

         this.camera = this.universe.getViewingPlatform().getViewPlatformTransform();

         //Add things to the universe
         this.root = new BranchGroup();
         this.root.addChild(new ColorCube(0.2));
         this.universe.addBranchGraph(root);

My idea is like this:

BranchGroup b=new BranchGroup();
b.addChild(cube);
b.addChild(camera);

then in a loop where you rotate the block:

while(true) {
  ... b.getChild()......... etc
  apply transform
}

or more specifically

for(j=0; j<group.numChildren(); j++) {
    Node ni=group.getChild(j);
    if(ni instanceof TransformGroup) {
      Transform3D tdi=new Transform3D();
      TransformGroup tgi=(TransformGroup)group.getChild(j);
      tgi.getTransform(tdi);
      Transform3D rotation = new Transform3D();
      rotation.rotX(Math.toRadians(0.001*i));
      tdi.mul(rotation);
      tgi.setTransform(tdi);
    }
}

Answer 2

There is a simpler solution. Instaed of trying to control the viewer gaze using angles and rotation, you can merely say what 3D point to look at. In the ViewingTransform, you can call "lookAt(...)":

        TransformGroup viewingTransformGroup = simpleUniv.getViewingPlatform().getViewPlatformTransform();
    Transform3D viewingTransform = new Transform3D();
    Point3d eye = viewersLocation;
    Point3d center = gazePoint;
    Vector3d up = new Vector3d(0,1,0); //assumes  +y-axis points up
    viewingTransform.lookAt(eye, center, up);
    viewingTransform.invert();
    viewingTransformGroup.setTransform(viewingTransform);