PositionPathInterpolator woes...

[Boy, I sure hope this will be one of those things that, after being 
stuck for several days, once I ask, the answer will be obvious to me  
<G>]

I've got my scene all set up, and things are working quite well, but I'm 
trying to get some of my objects (which are represented as textured 
spheres) to "rise up" from under my terrain.  The tool I'm using is the 
PositionPathEnterpolator, but I clearly don't understand what the docs 
are trying to tell me.

Here's the code that's giving me trouble:

// This says "do this once, over 5 seconds", right?
         Alpha    alpha = new Alpha (1, 5000);
// 2 positions...
         float [] knots = { 0.0f, 1.0f };
// start 1.5 below ground (the TG with which we started)
         Point3f     start = new Point3f (0, -1.5f, 0);
// End at the TG I passed in
         Point3f     end      = new Point3f (0, 0, 0);
         Point3f []  pos      = { start, end };
// the xform representing the Y axes, where the
// movement should take place
         Transform3D xform = new Transform3D();
         xform.set (new Vector3d (0, 1, 0));
         PositionPathInterpolator interp =
                new PositionPathInterpolator (alpha, tg, xform, knots, 
pos);
// Make it nice & big, so I know *THAT* isn't the problem...
         interp.setSchedulingBounds (new BoundingSphere 
(PigUtils.getPointFromTransGroup (tg), 1000));

         tg.addChild (interp);


For background, here's the branch-object constructor that includes the 
code from above:

// PigObject extends BranchGroup and is a sphere
//    with a pig-face texture on it.
// The TG passed-in is described below.

   public   PigObject (TransformGroup tg)
   {
      super();

      setName ("pig (BG)");

      Sphere   sphere   = new Sphere ((float) pigSize,
                        Sphere.GENERATE_NORMALS | 
Primitive.GENERATE_TEXTURE_COORDS,
                        pigAppearance());

      setCapability (BranchGroup.ALLOW_DETACH);
      setCapability (Group.ALLOW_CHILDREN_WRITE);

      tg.addChild (sphere);

// If I set this to "if (true)"... I see no pigs.  Ever.
// With the "trouble code" (above) out, all works just fine.
      if (false)
      {
         // Get pigs to "fade in" from below the ground.
// Troublesome code, from above, goes here
      }

      addChild (tg);
   }

Here's an example of how I create a pig object, in the first place -- 
pretty straightforward stuff:

      double         pigX     = xx * dist;
      double         pigZ     = zz * dist;
      double         pigY     = terrain.getAltitude (pigX, pigZ) + 
(PigObject.pigSize * 0.66);

      PigObject   thePig   = new PigObject (PigUtils.createLocationTG 
(pigX, pigY, pigZ));
      pigGroup.addChild (thePig);

and, lastly, here're my utility routines, used above.

   public   static   TransformGroup createLocationTG (double xx, double 
yy, double zz)
   {
      Transform3D    pos      = new Transform3D ();
      pos.setTranslation (new Vector3d (xx, yy, zz));

        TransformGroup  trans    = new TransformGroup (pos);
        trans.setCapability (TransformGroup.ALLOW_TRANSFORM_READ);
        trans.setCapability (TransformGroup.ALLOW_TRANSFORM_WRITE);
      
      return (trans);
   }


   public   static   Point3d     getPointFromTransGroup (TransformGroup 
tg)
   {
      Transform3D    xform = new Transform3D();
      tg.getTransform (xform);

      Vector3f    loc      = new Vector3f();
      xform.get (loc);
      
      return (new Point3d (loc));
   }


All of this other code works, if I just don't-include my interpolator, 
above -- so I'm pretty sure the problem is with my [mis-]use of the 
interpolator.

Any ideas?

Thanks!

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