I would like to know how the reference system of each joint is defined. For example, it seems that x-axis of left shoulder is on the line connecting left shoulder to left elbow and directed towards the elbow, is it right? How are other axis oriented? Is there a general rule for all joints?
Please refer to this post: Local Joint Coordinate & Rotation Matrix problem
Thanks, but actually it did not answer to my question, I wonder if there are some convention for joint orientation, something like Denavit Hartenberg convention for example
Did you watch the video? As it clearly illustrates the directions.
Left hand: orientations (red lines in the video) are directed along the bone along the X axis.
Right hand: orientations (red lines) are directed in the opposite direction to the bone along the X axis.
Spine and legs: orientations (green lines) are so that the Y axis is parallel to the direction of bones.
Ok, thank you I watched the video, I have just a last question. Considering the right hand-wrist-elbow-shouder system, if x-axis is along the bone, then how is y-z oriented? Is y the axis of rotation for the joint? It seems to me that Nuitrack doesn’t use right-handed coordinates, am I right?
If a user is standing in a T-Pose in front of a sensor, the orientation of all joints corresponds to the orientation of a sensor. If you look from the point of view of the sensor, X would be directed to the right, Y - up, and Z - from the sensor towards the user.