Joint positions and orientations are given in the real world coordinate system.The origin of the system is at the sensor.  +X points to the right of the, +Y points up, and +Z points in the direction of increasing depth.  The coordinate frame is shown in the figure above.
关节的位置和方向依据于现实世界的坐标系统。坐标原点定位在Sensor。X轴的正方向是人体面对Sensor的右方。Y轴的正方向是朝上。Z轴的正方向是深度值增加的方向。（综合而看，这是一个典型的右手坐标系）

Joint positions are measured in units of mm.
关节的位置是以毫米为单位来衡量的。

Joint orientations are given as a 3x3 rotation (orthonormal) matrix.  This represents a rotation between the joint's local coordinates and the world coordinates.  
The first column is the direction of the joint's +X axis given as a 3-vector in the world coordinate system. 
The second column is the +Y axis direction, and the third column is the +Z axis direction.  
Our “neutral pose” is the T-pose shown in the figure above.  In this pose, each joint's orientation is aligned with the world coordinate system.  That is, its orientation is the identity matrix.
关节的方向依据于一个3x3的旋转（正交）矩阵。这个矩阵描绘的是关节的局部坐标和世界坐标之间的旋转。
矩阵的第一列代表的是关节X轴的正方向，一个在世界坐标系里的 3-Vector.
矩阵的第二列代表的是关节Y轴的正方向，矩阵的第三列代表的是关节Z轴的正方向。
我们的"neutral pose"是一个T-姿势，如上图所示。在这个姿势中，每个关节的方向是一致的与世界坐标系。也就是说，它的方向是单位矩阵。
 

Known issues 已知问题

1. Arm tracking is less stable when the arm is close to other body parts, especially the torso. If both arms are close to the torso, as well as to each other, they might get mixed up. 
在手臂贴近身体的其它部位时（特别是躯干），手臂跟踪的稳定性会较差。如果双臂非常接近躯干以及对方的话，那么它们也可能会混淆在一起。

2. Leg tracking is still somewhat unstable and noisy. It works better when the user  stands with legs separated.  Fast motions and complex kicks or crouches might cause the tracking to fail. 
腿的跟踪一直有一些不稳定和干扰。当用户把双腿分开站立时，跟踪的效果会更好。快速的运动和复杂的踢或蹲可能会导致跟踪失败。

3. Pose tracking may also become somewhat unstable if the head is not visible. 
如果头是不可见的话，那么姿态跟踪也会变得不稳定。

4. Arms and legs in extremely stretched positions (i.e. near the limits of human flexibility) might be lost by the tracker. 
当手臂和腿处于极限拉伸的位置时（比如达到了人体灵活性的极限），可能会丢失跟踪。

5. If the skeleton is stuck in a faulty pose, or stuck facing the opposite direction, then returning to a “simple” pose (arms away from torso so the sensor can see them and legs separated) should help resolve it. 
如果骨骼卡住在一个错误的姿态，或者面向相反的方向，那么请回到一个简单的姿势（如：手臂远离躯干，双腿分开站立。）这些会帮助解决被卡住的问题。

6. In general, very fast motions may cause tracking failure. 
在一般情况下，速度非常快的运动可能会导致跟踪失败。

7. In some cases, overall tracking might be bad. Re-calibrating the user may resolve the problem. 
在某些情况下，整体跟踪的效果可能不太好。那么重新"Calibrating"会解决问题。


来自：http://www.cnblogs.com/TravelingLight/archive/2011/04/18/2019887.html