Sloppy rotation (solved)

Troubleshooting General Help
1

Hello,



I have problems with pointing a tube towards an object, i.e. a sphere.

Sample Code goes here:


      Sphere s1 = new Sphere("test", new Vector3f(-1, -1, -1), 30, 30, 0.2f);
      Sphere s2 = new Sphere("test2", new Vector3f(1, 1, 1), 30, 30, 0.2f);
      Tube t = new Tube("test3", 0.1f, 0, new Vector3f(2, 2, 2).length());
      
      Quaternion q = new Quaternion();
      q.fromAngles(FastMath.PI / 4f, 0, -FastMath.PI / 4f);
      // q.fromAngleAxis(FastMath.PI / 4f, new Vector3f(1, 0, -1));
      t.setLocalRotation(q);

      rootNode.attachChild(s1);
      rootNode.attachChild(s2);
      rootNode.attachChild(t);
      cam.setLocation(new Vector3f(0, 0, 5));




The result looks like this:


I would expect the ends of the tube to be exactly in the middle of the two spheres. fromAngleAxis() leads to the same result. The application I have in mind is a 3d molecule viewer.

Hope someone can help me. Many thanks.
2

I can't really help, because my math skills are pretty much non existent :slight_smile:

I created a grid, to make sure the spheres are correctly placed, maybe it helps other to see whats wrong.

3

You see it if you first rotate into one direction (45

4

3D Rotations and quaternions make my hair standup, so this was a good exercise in rotation for me too. Following code uses the angle-axis method, where' we're aligning vector u (tube's direction) with some vector v (the vector between the two sphere's).


      Sphere s1 = new Sphere("test", new Vector3f(-1, -1, -1), 30, 30, 0.2f);
      Sphere s2 = new Sphere("test2", new Vector3f(1, 1, 1), 30, 30, 0.2f);
      Tube t = new Tube("test3", 0.1f, 0, new Vector3f(2, 2, 2).length());
      
      //Find the vector between the two sphere's
      Vector3f v = s2.getCenter().subtract(s1.getCenter());
      v.normalizeLocal();
      
      //Note that the tube is is upright, if you use it's local
      //rotation it's direction will be <0,0,1> and the finished
      //rotation won't look correct
      Vector3f u = new Vector3f(0,1,0);
      
      //Do the cross product of both vectors to find the axis of rotation
      Vector3f axis = u.cross(v).normalizeLocal();
      
      //Find the rotation angle
      float angle = FastMath.acos(u.dot(v));

      //Setup the quaternion
      Quaternion q = new Quaternion();
      q.fromAngleAxis(angle, axis);
      t.setLocalRotation(q);

      rootNode.attachChild(s1);
      rootNode.attachChild(s2);
      rootNode.attachChild(t);
      cam.setLocation(new Vector3f(0, 0, 5));



And the rotation:


5

Great, that's exactly what I needed. I'm not sure what I've done wrong, though. Never mind.

Thanks a lot!!



Can I mark this topic as solved, somehow?

6

I guess just edit the your first post's title

7

Hey Starnick, I don't require this type of info now, but I read your post and the math after a bit actually made sense. I do appreciate your post and hopefully if I run into rotations etc… in the future I will be able to look back at this for some guidance. Thanks.



Btw, the axis vector seems to be the normal of the plane being rotated on or a vector for where the rotation is hinged on?



Thanks.

8
msj121 said:

Hey Starnick, I don't require this type of info now, but I read your post and the math after a bit actually made sense. I do appreciate your post and hopefully if I run into rotations etc... in the future I will be able to look back at this for some guidance. Thanks.

Btw, the axis vector seems to be the normal of the plane being rotated on or a vector for where the rotation is hinged on?

Thanks.


Yeah, something like that. Some reference:

http://en.wikipedia.org/wiki/Axis_angle
9

Oh okay, it is the normal of the plane, pretty much. Thanks for the link, I appreciate it.