Main Page → Skeleton Tracking → 6 RigidBody Skeleton Markerset
The Rigid Body Skeleton markersets allows users to use rigid bodies to establish skeleton tracking. Rigid bodies are attached to head, torso, both hands, and both feet (6 rigid body skeleton only). Then, using the tracking information, Motive solves the entire skeleton through inverse kinematics. There are total three types of rigid body skeleton markersets in Motive:
4 Rigid Body Skeleton is used for upper body tracking only, and 6 Rigid Body Skeleton is used for the entire full-body skeleton tracking. For the 6 Rigid Body Skeleton, two additional rigid bodies are attached to both feet, but the basic insturctions for creating the skeleton is the same. You can use either the passive retro-reflective markers or the active pucks to produce the rigid body skeleton.
This is needed only for the 6 Rigid Body skeleton markerset.
Let's first start with defining a rigid body for the chest rigid body. Open up the Builder pane, make sure the Rigid Bodies option is selected at the bottom, and access the Create tab. This will bring up options for defining rigid bodies in Motive.
In the 3D viewport, select all of the markers on the chest rigid body, and you should be able to see the selected markers in the Builder pane. Name the rigid body as Chest, and click create. If you are attaching the rigid body on the lower back, you will name the rigid body Hip instead.
Only either one of the Chest or the Hip rigid body is needed for the 6 rigid body skeleton.
Once the chest or hip rigid body has been created, the next step is to position and orient its pivot point at the appropriate location. You can use the Gizmo tool to easily translate the rigid body pivot points.
For the chest rigid body, the pivot point must be placed at the center of the torso, approximately at the heart center; which is in between the spine and the bottom end of the sternum. For the hip rigid body, the pivot point must be placed at the center of the hip bone.
After positioning the pivot point, adjust the orientation of the rigid body pivot also by using the rotate Gizmo tool. For creating 6 Rigid Body skeleton, the +z axis must direct towards the front of the actor.
Enabling the Orientation setting under the rigid body properties will reveal the orientation of the selected rigid body.
Now that the chest, or hip, rigid body has been set up, next step is to create a rigid body for the head. This can be either the active CV1 clip or an rigid body attached to the head. Only one of them needs to be created.
For HMDs using the active CV1 clip, you can use the HMD tool in the Builder pane to easily create a rigid body. Make sure to choose either the +Z-forward or the +X-forward orientation, and the rigid body must be named HMDHead for +Z-forward orientation or HMDHeadX for +X-forward orientation.
The process of creating a head rigid body is similar to the steps for creating the Chest rigid body. You can use the Builder pane to define a rigid body named Head.
Once the rigid body is defined, use the Gizmo tool to translate the pivot point to at the center of the actor's head near the neck joint. Then, likewise, orient the rigid body so that +z axis is directing towards the front.
Now that the head and torso rigid body has been prepared, we can start defining the skeleton. Open the Builder pane and select desired Rigid Body Skeleton from the marker set drop-down menu, and assign a name for the skeleton that will be created.
Ask the subject to strike a calibration pose (e.g. T-Pose). Then select the two rigid bodies and press Create on the Skeleton pane. This action will automatically define rigid bodies for the remaining rigid bodies on the hand/foot, and also place the pivot points at the proper location automatically. All of the rigid bodies will have the prefix with the name given to the skeleton (see screenshot below), and the skeleton will be created and tracked.
Rigid Body Names:
After the skeleton has been created, confirm tracking of the skeleton. If any of the skeleton segment seems to be misaligned, double-check the position of the attached rigid bodies and corresponding pivot points.
› Q : Skeleton is not tracking
If the skeleton, or certain parts of the skeleton, appears to be not being tracked at all, make sure the rigid bodies are named properly. The rigid body must be named with the following syntax:
› Q : HMD head is facing backward/sideways
If the head or HMD is facing backward or sideways, it indicates that it's orientation is not properly set. Please double check the position/orientation of the rigid body head to make sure they are set properly.
› Q : Skeleton bones are flipped
When this happens, double check the orientation of each rigid body. All rigid body must be named properly and their z-axis must be facing the forward direction.
› Q : Shoulder/hip bone is too high up
When this happens, it indicates that the position of the HMD (or head) and the Chest (or hip) rigid body is not accurately placed. The 6 rigid body skeleton gets calculated from pivot points of these two rigid bodies, so if there is an obvious misalignment of the created skeleton bones, double-check and make sure these two pivot points are set up properly.
Step 1) Prepare 6 rigid bodies with markers.
Step 2) Attach rigid bodies at the following locations:
Step 3) Make sure the rigid bodies are securely attached.
First of all, define rigid body assets for all of the attached rigid bodies.
Name the rigid bodies with the following syntax:
Hip/Chest Rigid Body
VR: Using HMD for head tracking
Now you need to position the rigid body pivot points. Use the Gizmo tool and translate the rigid body pivot points so that each pivot point is located at its respective parent joint axis. Pivot point of the head rigid body must be placed over the head/neck joints, the hip rigid body must have its pivot point at the pelvis center, pivot point of the left/right hand rigid bodies must be placed over the wrist joints, and the left/right foot rigid bodies must have its pivot points over the ankle joints, as shown in the image below. If a Chest rigid body is used in place of the Hip rigid body, its pivot point location need to be plated at the heart center, solar plexus.
This is easier to set up by switching one of the cameras to a grayscale video mode and using the Reference View pane to monitor where the pivot point is placed within each respective joints.
VR: HMD Rigid Bodies
Orient the rigid body pivot points so that all of its z-axis is facing in forward-direction. This can be done easily using the Rigid Body pane. Ask the subject to face towards the positive-z direction (determined by the global coordinate system) and use the Edit tab on the Rigid Body pane to reset the orientations of the each rigid body. This will align the rigid body orientation with the global coordinate axis, making the z-axis direct forward.
Open the Skeleton pane and select the 6 Rigidbody Skeleton markerset. Name the skeleton that matches the prefix given to the rigid bodies.
Ask the subject to strike a calibration pose (e.g. T-Pose). Then select all six rigid bodies and press "Create" on the Skeleton pane. Created skeleton will search for rigid bodies with corresponding names for tracking. If all of the rigid bodies have been named correctly and the pivot points have been positioned and oriented properly, the skeleton will be tracked.
After the skeleton has been created, confirm tracking of the skeleton. If any of the skeleton segment seems to be misaligned, double-check the position of the attached active puck and the rigid body pivot points.