After I place a Kinect 2 and stand in front of it, I moved my arm up in front and down (e.g. forward flexion).
Then, I found my y-coordinate of the wrist joint that changes large (0.17) to small (0.11) and to around (0.16). I found this strange, because in the Kinect guide, positive y-axis indicates upward direction. https://msdn.microsoft.com/en-us/library/hh973078.aspx
it seems like we should have larger value of wrist-y coordinate when we place a arm up direction. I am getting the opposite results. Anyone can comment on this?
Q. Are we supposed to get decreasing y-value of wrist when is moving upward direction?
Q. If not, can anyone have any ideas why this happens?
Q. In addition, I found my the other side of wrist (left) has negative value. Can anyone comment why left side of wrist has negative value?
That happens because of the Kinect v2 reference system. The center of the sensor corresponds to (0,0,0) - x,y,z.
If you make movements to the left or right in the X and Y axis, it is logical to have negative values.
The origin (x=0, y=0, z=0) is located at the center of the IR sensor on Kinect X grows to the sensor’s left Y grows up (note that this direction is based on the sensor’s tilt) Z grows out in the direction the sensor is facing 1 unit = 1 meter
https://msdn.microsoft.com/en-us/library/dn785530.aspx
Hope this helps you.