Preventing a box rotating when its node rotates?

Troubleshooting General Help
1

The subject says it all really - I have a box attached to a node, and would like its rotation to remaing fixed even if the node rotates.  I thought it might be simple with something like this:-



box.setLocalRotation(box.getParent().getLocaRotation().mult(-1))



…but that doesn't seem to work.

2

lock transforms on the box should do it…

3
ncomp said:

lock transforms on the box should do it....


Without having actually experimented with this, I think that this solution would also prevent the box from moving within the node. This could be a problem, if the functionality you want includes being able to move the box, but not rotate it.

If this is the case, I suggest running your game and printing the rotation during start-up (i.e. the rotation you'll want the box to maintain). Then, in your update method, do something like this:


//during initGame()
cuberot = myCube.getLocalRotation().clone(); //cuberot is a global Quaternion object

//during update()
if(!myCube.getLocalRotation().equals(cuberot))
     myCube.setLocalRotation(cubeRot.clone());



Which would be an effective solution for locking only rotations.
4

But fixing the local rotation won't stop the world rotation changing when the parent rotates.



I'm curious as to how the original solution doesn't work.



Here's a SimpleGame test I modified to try it, I used inverse() instead of multiplying by -1.:



public class RotationEx extends SimpleGame {

   Quaternion q = null;
   Box b2 = null;
   Box b = null;
   Node n = null;
   
   @Override
   protected void simpleInitGame() {
   
      b = new Box("Box", new Vector3f( 0, -1.0f, -0.5f), new Vector3f( 5.0f, 1.0f, 0.5f));
      b.setLocalTranslation( 0, 0, 0);
      b2 = new Box("Box", new Vector3f( -2.5f, -2.5f, -2.5f), new Vector3f( 2.5f, 2.5f, 2.5f));
      b2.setLocalTranslation(5.0f, 0, 0);
      
      q = new Quaternion();
      
      n = new Node ("centreOfRotation");
      n.attachChild(b);
      n.attachChild(b2);
      
      rootNode.attachChild(n);
      wireState.setEnabled(true);
   }
   
   protected void simpleUpdate() {
      q.fromAngleNormalAxis(1.0f * tpf, new Vector3f(0,1.0f,0) );
      n.getLocalRotation().multLocal(q);

      b2.setLocalRotation( b2.getParent().getLocalRotation().inverse() );
   }

   /**
    * @param args
    */
   public static void main(String[] args) {
      RotationEx demo = new RotationEx();
      demo.setConfigShowMode(ConfigShowMode.AlwaysShow);
      demo.start();
   }

}



EDIT: Made it JME2 ;p
5

my bad…i thought you wanted the box's transform to stay fixed at all time…

apologies…

6

Alric, that does it perfectly!  Thanks a lot.  Looking at the JME sourcecode, inverse() does a lot more than just multiplies it by -1, and it does the trick.


7

Isn't this pretty much what a billboard node does ?

8

Not exactly - a billboard doesn't rotate relative to the camera, but it is actively rotated in the world, to face the camera.