Thank you !! That's exactly what I was searching for ! 
Now I have one more question about the vr_head_orientation_quaternion variable.
vr_head_orientation_quaternion is a quaternion, so it has a r, a x, a y and a z attribute. Could you (or someone else) explain me what is the r attribute ?
I guess the x, y and z are attributes of a tm_vector3d, but I don't know why there is a tm_vector3d here.
I mean, maybe the r attribute is the rotation around the vector, but why only one vector ?
Have a nice day,
Dalfrak