Edit

Azure Kinect body tracking joints

  • Article

Azure Kinect body tracking can track multiple human bodies at the same time. Each body includes an ID for temporal correlation between frames and the kinematic skeleton. The number of bodies detected in each frame can be acquired using k4abt_frame_get_num_bodies().

Joints

Joint position and orientation are estimates relative to the global depth sensor frame of reference. The position is specified in millimeters. The orientation is expressed as a normalized quaternion.

Joint coordinates

The position and orientation of each joint form its own right-handed joint coordinate system. All joint coordinate systems are absolute coordinate systems in the depth camera 3D coordinate system.

Note

The choice of flipped axis orientation for corresponding joints across the two sides of the body is intended to simplify mirror movement e.g. raise both arms by +20 degrees, which is common with commercial avatars, game engines, and rendering software.

Joint coordinates

Legend: | x-axis = red | y-axis = green | z-axis = blue |

Note

The visual output of the k4abt_simple_3d_viewer.exe tool is mirrored.

Joint hierarchy

A skeleton includes 32 joints with the joint hierarchy flowing from the center of the body to the extremities. Each connection (bone) links the parent joint with a child joint. The figure illustrates the joint locations and connection relative to the human body.

Joint hierarchy

The following table enumerates the standard joint connections.

Index Joint name Parent joint
0 PELVIS -
1 SPINE_NAVAL PELVIS
2 SPINE_CHEST SPINE_NAVAL
3 NECK SPINE_CHEST
4 CLAVICLE_LEFT SPINE_CHEST
5 SHOULDER_LEFT CLAVICLE_LEFT
6 ELBOW_LEFT SHOULDER_LEFT
7 WRIST_LEFT ELBOW_LEFT
8 HAND_LEFT WRIST_LEFT
9 HANDTIP_LEFT HAND_LEFT
10 THUMB_LEFT WRIST_LEFT
11 CLAVICLE_RIGHT SPINE_CHEST
12 SHOULDER_RIGHT CLAVICLE_RIGHT
13 ELBOW_RIGHT SHOULDER_RIGHT
14 WRIST_RIGHT ELBOW_RIGHT
15 HAND_RIGHT WRIST_RIGHT
16 HANDTIP_RIGHT HAND_RIGHT
17 THUMB_RIGHT WRIST_RIGHT
18 HIP_LEFT PELVIS
19 KNEE_LEFT HIP_LEFT
20 ANKLE_LEFT KNEE_LEFT
21 FOOT_LEFT ANKLE_LEFT
22 HIP_RIGHT PELVIS
23 KNEE_RIGHT HIP_RIGHT
24 ANKLE_RIGHT KNEE_RIGHT
25 FOOT_RIGHT ANKLE_RIGHT
26 HEAD NECK
27 NOSE HEAD
28 EYE_LEFT HEAD
29 EAR_LEFT HEAD
30 EYE_RIGHT HEAD
31 EAR_RIGHT HEAD

Next steps

Body tracking index map