pi_tracker/Skeleton.msg message interpretation

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Hello all,

I am using using ROS Electric, and pi_tracker package for skeletal tracking. The launch file "skeleton.launch" publishes "/skeleton" topic, which is a "pi_tracker/Skeleton.msg" message. Using "rosbag -record", I recorded the "/skeleton" topic. The message definition is:

Header header
int32 user_id
string[] name
float32[] confidence
geometry_msgs/Vector3[] position
geometry_msgs/Quaternion[] orientation

And dumping the /skeleton topic to a txt file, the output is like:

------------------------------------------
header: 
  seq: 1
  stamp: 
    secs: 1363464929
    nsecs: 445623850
  frame_id: openni_depth_frame
user_id: 1
name: ['head', 'neck', 'torso', 'left_shoulder', 'left_elbow', 'left_hand', 'right_shoulder', 'right_elbow', 'right_hand', 'left_hip', 'left_knee', 'left_foot', 'right_hip', 'right_knee', 'right_foot']
confidence: [1.0, 1.0, 1.0, 1.0, 1.0, 1.0, 1.0, 1.0, 1.0, 1.0, 0.0, 0.0, 1.0, 0.0, 0.0]
position: 
  - 
    x: -0.377875793457
    y: 0.0851003723145
    z: 2.09605566406
  - 
    SIMILAR FOR OTHER 14 JOINTS
orientation: 
  - 
    x: 0.213657463007
    y: 0.188112339353
    z: 0.0245842250896
    w: 0.958310941594
  - 
    SIMILAR FOR OTHER 14 JOINTS
------------------------------------------

Now, my question is, if I want to know the position of the joint 'HEAD', 1. How can I get the position? Means, what is the relation between 'position' and 'orientation' field? 2. What is the unit of (x,y,z)? 3. And, if I only want the (x,y) co-ordination in the image coordinate, how can I convert it to only 2-D (x,y) coordinate?

Thanks in advance.