Hi,

I am subscribing to the /tf data from openni_tracker. Here is my program where I am using lookupTransform() function to get the data published over /tf.

#include <ros/ros.h>
#include <tf/transform_listener.h>
#include <stdio.h>

using namespace std;

int main(int argc, char** argv){
  ros::init(argc, argv, "my_tf_listener");

  ros::NodeHandle node;
  tf::TransformListener listener;

  ros::Rate rate(10.0);

  while (node.ok()){
    tf::StampedTransform transform;
    try{
      listener.lookupTransform("/openni_depth_frame", "/left_elbow_1", ros::Time(0), transform);  
      }
    catch (tf::TransformException ex){
         ROS_ERROR("%s",ex.what());
         }
    double left_elbow_1_x = transform.getOrigin().x();  
    double left_elbow_1_y = transform.getOrigin().y();
    double left_elbow_1_z = transform.getOrigin().z();

    cout <<"left_elbow_1_x " <<left_elbow_1_x<<endl;
    cout <<"left_elbow_1_y " <<left_elbow_1_y<<endl;
    cout <<"left_elbow_1_z " <<left_elbow_1_z<<endl;

    try{
      listener.lookupTransform("/openni_depth_frame", "/right_elbow_1", ros::Time(0), transform);
    }
    catch (tf::TransformException ex){
      ROS_ERROR("%s",ex.what());
    }

    double right_elbow_1_x = transform.getRotation().x(); 
    double right_elbow_1_y = transform.getRotation().y(); 
    double right_elbow_1_z = transform.getRotation().z();
    double right_elbow_1_w = transform.getRotation().w();

    cout <<"right_elbow_1_x " <<right_elbow_1_x<<endl;
    cout <<"right_elbow_1_y " <<right_elbow_1_y<<endl;
    cout <<"right_elbow_1_z " <<right_elbow_1_z<<endl;
    cout <<"right_elbow_1_w " <<right_elbow_1_w<<endl;

    rate.sleep();
    }
  return 0;
}

I know tf::StampedTransform::getOrigin() returns the origin vector translation between the two /tf messages used in lookupTransform function. Similarly I have an idea that tf::StampedTransform::getRotation() returns a quaternion representing the rotation between the two /tf messages used in lookupTransform function. Here is an excerpt of the output I get after running the program.

left_elbow_1_x 2.35472
left_elbow_1_y 0.146672
left_elbow_1_z 0.299355
right_elbow_1_x 0.331682
right_elbow_1_y -0.58186
right_elbow_1_z 0.308364
right_elbow_1_w 0.675527

Here are my questions:

1. I am finding it hard to understand these output numbers. Can someone explain or direct me to sources I can consult to relate these numbers to actual physical movements or rotations?
2. I googled but couldn't find what unit these numbers represent. cm, mm or what?
3. Any other resource that can help me extract meaningful data from these numbers would be appreciated.

Cheers!