## Tracking Tools + VRPN

araa
Posts: 3
Joined: Fri Apr 08, 2011 7:05 am

### Tracking Tools + VRPN

Hello,

I'm using Tracking Tools 2.3.1 (Right handed system) & VRPN. I would like to obtain the position of a target point relative to the pivot point of my rigid body. First, I tried using the offsets of Tracking Tools and it worked. Now, Iï¿½m using the quaternion and the position of the pivot point (VRPN) but, sometimes, the position of the target point is extremely bad.

The equations which I use are the following ones:

[Xtarget] = R1*[Xpivot]+R2*[Ypivot]+R3[Zpivot]+[XOffset]
[Ytarget] = R4*[Xpivot]+R5*[Ypivot]+R6[Zpivot]+[YOffset]
[Ztarget] = R7*[Xpivot]+R8*[Ypivot]+R9[Zpivot]+[ZOffset]

R1 = 1 - 2*(t.quat[1]*t.quat[1]+t.quat[2]*t.quat[2]);
R2 = 2*(t.quat[0]*t.quat[1]-t.quat[3]*t.quat[2]);
R3 = 2*(t.quat[0]*t.quat[2]+t.quat[3]*t.quat[1]);
R4 = 2*(t.quat[0]*t.quat[1]+t.quat[3]*t.quat[2]);
R5 = 1 - 2*(t.quat[0]*t.quat[0]+t.quat[2]*t.quat[2]);
R6 = 2*(t.quat[1]*t.quat[2]-t.quat[3]*t.quat[0]);
R7 = 2*(t.quat[0]*t.quat[2]-t.quat[3]*t.quat[1]);
R8 = 2*(t.quat[1]*t.quat[2]+t.quat[3]*t.quat[0]);
R9 = 1 - 2*(t.quat[0]*t.quat[0]+t.quat[1]*t.quat[1]);

What's the problem?

Thanks,

araa

beckdo
NaturalPoint Employee
Posts: 520
Joined: Tue Jan 02, 2007 2:02 pm

### Re: Tracking Tools + VRPN

I think I understand. You have a point in space that you want in a coordinate system based on the rigid body instead of the world coordinate system.

araa
Posts: 3
Joined: Fri Apr 08, 2011 7:05 am

### Re: Tracking Tools + VRPN

Sorry, what I wanted to say is that both the pivot and target points belong to the same rigid body but Tracking Tools (through VRPN) just give the position of the pivot point. I also have the offsets between both points. I would like to obtain the position of the target point in the world coordinate system.

beckdo
NaturalPoint Employee
Posts: 520
Joined: Tue Jan 02, 2007 2:02 pm

### Re: Tracking Tools + VRPN

If you want to avoid the math, you can make the target point the pivot point of the rigid body.

araa
Posts: 3
Joined: Fri Apr 08, 2011 7:05 am

### Re: Tracking Tools + VRPN

The target point is not a reflective marker. We would like not to use the offsets of Traking Tools.

Currently, we are not sure if the problem of having positions of the target point extremely bad comes from the equations above or from Tracking Tools. When we move the rigid body slowly and we plot the values of each term of the quaternion separately, we have seen some peaks which correspond with extremely bad target point positions.

NaturalPoint - Mike
Posts: 1896
Joined: Tue Feb 01, 2011 8:41 am
Location: Corvallis, OR

### Re: Tracking Tools + VRPN

Could you send a CSV of data that exibits these traits to support@naturalpoint.com? If you have specific frame numbers that could greatly help.

maxlem
Posts: 5
Joined: Wed Nov 24, 2010 3:37 pm

### Re: Tracking Tools + VRPN

I have a similar need. I would like to set the pivot point to a precise position in the rigid body. A very simple solution (software wise) would be to allow to set the pivot point at an offset on a line between two markers.

Example :

- create a tracker with 3 or more markers
- select two points among tracker's markers
- create pivot on the line between the two selected markers