Rigid Body Skeleton Markerset

Main Page → Skeleton Tracking → 6 RigidBody Skeleton Markerset

Scope

This page provides step-by-step instructions for creating the rigid body based skeleton markersets in Motive.

Overview

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6 Rigid Body Skeleton in Motive.

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:

• 6 Rigid Body Skeleton
• 4 Rigid Body Skeleton
• 4 Rigid Body Skeleton + Active Fingers

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.

Using Active Pucks / HMD Clips

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Active pucks and HMD clips are easier to use.

Active Puck Placements

Step 1. Prepare the active marker rigid bodies

• For 6 rigid body skeleton, total 6 rigid bodies are needed: head (1), torso (1), both hands (2), and both feet (2)
• For 4 rigid body skeleton, total 4 rigid bodies are needed: head (1), torso (1), both hands (2)

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, 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 active puck attached to the head. Only one of them needs to be created.

HMDHeadHead

For HMDs using the active CV1 clip, you can use the HMD tool in the Trackables 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.

1. Open the Trackables pane under View tab and click Rigid Bodies.
2. Under the Type drop-down menu, select HMD. This will bring up the options for defining an HMD rigid body.
3. Under the Orientation drop-down menu, select the desired orientation of the HMD.
4. Hold the HMD at the center of the tracking volume where all of the active markers are tracked well.
5. Select the 8 active markers in the 3D viewport.
6. Name the rigid body as HMDHead for +Z-forward orientation or HMDHeadX for +X-forward orientation.
7. Click Create. An HMD rigid body will be created from the selected markers and it will initiate the calibration process.
8. During calibration, slowly rotate the HMD to collect data samples in different orientations.
9. 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.

Using Passive Marker Rigid Bodies

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Marker Placements

Step 1) Prepare 6 rigid bodies with markers.

Step 2) Attach rigid bodies at the following locations:

• Head (or HMD): Place the rigid body back of the head. This can be replaced by an HMD.
• Hip (or Chest): Place the rigid body on the back, slightly above the hip bone. This can be replaced by a Chest rigid body.
• Right Hand / Left Hand
• Right Foot / Left Foot

Step 3) Make sure the rigid bodies are securely attached.

Creating the Skeleton

Example rigid body names for Actor skeleton.

Step 1: Create rigid bodies

First of all, define rigid body assets for all of the attached rigid bodies.

Step 2: Name the rigid bodies

Name the rigid bodies with the following syntax:

• Head: (Skeleton Name)Head OR (Skeleton Name)HMDHead OR (Skeleton Name)HMDHeadX
• Torso: (Skeleton Name)Hip OR (Skeleton Name)Chest
• Left hand: (Skeleton Name)LHand
• Right hand: (Skeleton Name)RHand
• Left foot: (Skeleton Name)LFoot
• Right foot: (Skeleton Name)RFoot

Hip/Chest Rigid Body If placing a rigid body on the waist is not possible (e.g. when using backpack PCs). The Hip rigid body can be replaced with a Chest rigid body.

VR: Using HMD for head tracking In order to replace the head rigid body with the HMD rigid body for creating 6RB skeleton, the HMD rigid body must be named differently because their pivot point position and orientation would be slightly different. Streaming to Unity (z-forward): (Skeleton Name)HMDHead Streaming to UE4 (x-forward): (Skeleton Name)HMDHeadX

Step 3: Position each pivot point to corresponding joint center

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 Pivot point of the HMD rigid bodies don't need to be adjusted for 6RB SKL. They just need to be calibrated using the HMD Calibration feature and named as noted in step 2.

Pivot Point Locations: Pivot: Head. Pivot: Left and Right Hand, Pivot: Left and Right Foot, Pivot: Hip (or replaced by Chest rigid body).

Step 4: Adjust the orientation axis

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.

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  Reseting rigid body orientation
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  RHand rigid body with pivot point placed on the joint center with z-axis forward.

Step 5: Set skeleton name.

Open the Skeleton pane and select the 6 Rigidbody Skeleton markerset. Name the skeleton that matches the prefix given to the rigid bodies.

Creating 6RB skeleton with skeleton name Actor

Step 6: Create the skeleton

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.