Rigid Body Skeleton Markerset

Main Page → Skeleton Tracking → 6 RigidBody Skeleton Markerset

Page Scope

This page provides instructions on how to set up and the Rigid Body skeleton markersets for skeleton tracking.

Overview

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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) and Motive solves the entire skeleton through inverse kinematics.

Creating 6RB skeleton markerset

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Active Puck Placements

Step 1. Prepare Total 5 Active Pucks and 1 Active HMD clip. Total 6 rigid bodies.

Step 2. Attach the pucks at the following locations of the actor's body. Make they are attached securely

Placement	Description
Right/Left Hand	Place an active puck on top of each hand. (2)
Right/Left Foot	Place an active puck on top of each foot. (2) This is needed only for the 6 Rigid Body skeleton markerset.
Chest (or Hip)	Attach an active puck at mid-spine on the back. If using a VR backpack PC, this can be attached on top of the backpack PC. This rigid body can also be replaced by a hip rigid body, and in that case, the rigid body needs to be placed slightly above the center of the hip bone. (1)
HMD (or Head)	Use the active CV1 HMD clip to attach active markers onto the HMD. If not using an CV1 clip, you will need to manually placed the markers on the HMD. If not using an HMD, place the rigid body on the back of the head. (1)

• File:6RB PlacementPosteriorView.png

  Posterior view of the actor with rigid bodies
• File:6RB PlacementAnteriorView.png

  Posterior view of the actor with rigid bodies

Creating the Skeleton

Step 1: Create a rigid body for the puck attached to Chest/Hip

Let's first start with defining a rigid body for the chest puck. Open up the Trackables 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 8 markers on the chest puck, and you should be able to see the selected markers in the Trackables pane. Name the rigid body as Chest, and click create. If you are attaching the active puck 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.

Step 2: Position the pivot point of the Chest/Hip rigid body

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.

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 the actor's body.

Step 3: Orient the pivot point of the Chest/Hip rigid body

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.

Step 4: Create the HMDhead/Head rigid body

Now that the chest, or hip, is set up, next step is to set up the head rigid body. This can be either the HMD (HMDHead) if you are using the active CV1 clip, or simply an active puck attached to the head (Head). Only one of them needs to be created.

HMDHeadHead

To create the HMD rigid body, you can use the HMD calibration tool in the Trackables pane to easily create a rigid body for the active CV1 clip. Make sure to choose either the +Z-forward or the +X-forward orientation when creating the rigid body, and it must be named HMDHead for +Z-forward orientation or HMDHeadX for +X-forward orientation.

1. First of all, make sure Motive is configured for tracking active markers.
2. Open the Builder pane under View tab and click Rigid Bodies.
3. Under the Type drop-down menu, select HMD. This will bring up the options for defining an HMD rigid body.
4. If the selected marker matches one of the Active clips, it will indicate which type of Active Clip is being used.
5. Under the Orientation drop-down menu, select the desired orientation of the HMD. The orientation used for streaming to Unity is +Z forward and Unreal Engine is +X forward, or you can also specify the expected orientation axis on the client plugin side.
6. Hold the HMD at the center of the tracking volume where all of the active markers are tracked well.
7. Select the 8 active markers in the 3D viewport.
8. Click Create. An HMD rigid body will be created from the selected markers and it will initiate the calibration process.
9. During calibration, slowly rotate the HMD to collect data samples in different orientations.
10. Once all necessary samples are collected, the calibrated HMD rigid body will be created.

The process of creating a head rigid body from an active puck is similar to the steps for creating the Chest rigid body. You can use the Trackables 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.

Step 5: Set skeleton name.

Now that the head and torso rigid body has been prepared, we can start defining the skeleton. Open the Trackables 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.

Creating 6RB skeleton with skeleton name Actor

Step 6: Create Rigid Body Skeleton.

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 active pucks 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: In order for the rigid body skeleton to work, it is important that all of the rigid bodies have name prefix that matches the skeleton name. If one of the rigid body doesn't seem to be contributing to the skeleton solve, please check these names.