Rigid Body Skeleton Marker Set

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

Important Note

• Please note that Motive: Body license is required to access and use the rigid body skeleton Marker Sets. This is not supported with Motive: Tracker license.

• Rigid body skeleton Marker Sets are intended for real-time applications only. Exporting into BVH/FBX file is not supported.

Overview

6 Rigid Body Skeleton in Motive.

The Rigid Body Skeleton Marker Sets 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 Marker Sets 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 instructions 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.

Rigid Body Placements

Q : Skeleton is not tracking

Step 1. Prepare the 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 rigid bodies onto the actor

Placement	Description
Right/Left Hand	Place a rigid body on top of each hand. (2)
Right/Left Foot	Place a rigid body on top of each foot. (2) This is needed only for the 6 Rigid Body skeleton markerset.
Chest (or Hip)	Attach a rigid body 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 HMD clip to attach active markers onto the HMD. If not using an HMD 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)

Anterior view of the actor with rigid bodies

Posterior view of the actor with rigid bodies

Creating the Skeleton

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

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.

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 HMD clip or a rigid body attached to the head. Only one of them needs to be created.

HMDHead

or HMDs using the active HMD 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.

1. Open the Builder 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.

Head

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.

Step 5: Set skeleton name.

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.

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 Builder 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:

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

Pivot Point Locations: Pivot: Head (when HMD is not used). Pivot: Left and Right Hand, Pivot: Left and Right Foot, Pivot: Hip (or replaced by Chest rigid body).

Step 7: Double-check the created skeleton

After the skeleton has been created, confirm tracking of the skeleton. If any of the skeleton segments seems to be misaligned, double-check the position of the attached rigid bodies and corresponding pivot points.

• Chest RB Pivot: When using the Chest rigid body for the torso-tracking. The length of the abdomen segment gets solved by referencing the location of the chest rigid body pivot point. If the created skeleton has an abnormally long or short abdomen segment, double-check and adjust the height of the chest pivot point.

• Neck: If the pivot orientation of the Chest rigid body doesn't align with the head rigid body, the neck of the created skeleton is may appear to be bent. Make sure the chest rigid body's y-axis is pointed directly up towards the Head/HMD rigid body pivot.

Troubleshooting

Q : Skeleton is not tracking

A: 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:

• 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

Q : HMD head is facing backward/sideways

A: 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

A: 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

A: 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.