6 Rigid Body Skeleton Markerset

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New in Motive 2.0

Page Scope

This page provides instructions on how to set up and use the 6-Rigid Body skeleton markerset for full-body skeleton tracking.

Overview

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The 6 Rigidbody Skeleton markerset allows users to use just rigid bodies to establish full body skeleton tracking. A total of six rigid bodies are attached at specific locations of the subject and Motive solves the entire skeleton through inverse kinematics. The rigid bodies are attached to the head, left-hand, right-hand, hip, left-foot, and right-foot, and then the pivot point of those rigid bodies needs be translated over to the corresponding joint centers.

Creating 6RB skeleton markerset

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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.