Optitrack : Skeleton tutorial


How to create a skeleton model and start tracking his movments

  • Creation: 09/29/2020
  • Update: 09/29/2020
Motive proposes his own auto-labelled specific markersets for tracking human motion. This is called the skeleton mode. Contrary to a rigid body, a skeleton is an assembly of multiple semi-rigid body segments. Thus, it requires additional calculations to correctly identify and label 3D markers in the space.

Pre-defined skeleton markerset templates are already available on Motive. There, you will find a collection of marker labels and their specific position on a subject. This tutorial will show you how to create and use a skeleton asset in Motive.

NOTE : make sure to have the accurate licence : Motive/Body, which is the only one with the skeleton features.

Create the markerset :

For more information about the markers and their properties, make sure to check the Introduction to skeletons and rigid bodies article.

Once you have properly attached your markers on the subject, open the Builder pane. Go to the skeleton creation feature and select the markerset you want to use from the drop-down menu. Make sure you provide adequate data (number of markers required, ,…) and check the location of each marker on the avatar that will show up in the Builder pane.

NOTE : The skeleton mode has two calibration poses : T-pose or A-pose. You will have to choose one of them and carefully follow the figure to place your markers at corresponding locations of the subject.

When it comes to skeleton markers, you will find different types of markers :
  •  Joint markers : these are quite tricky to place because you will have to try to minize marker movements during motions. When plaing them, ask the subject to flex and extend the joint (eg knee) a few times and palpate the joint to locate the corresponding axis. Once you find the proper orientation, attach the markers along the axis where the likelihood of skin movements is the lowest.
  • Segment markers : these are all the markers on the skeleton body that are not placed around a joint. Remember : asymmetry is key (cf the Introduction to skeletons and rigid bodies article). It will enable Motive to distinguish properly the right side from the left side of the avatar while processing the data.

Tip : Segment markers that can be slightly moved on the same segment are highlighted on the 3D avatar on the Builder pane.

Create the skeleton :

Note : Motive offers two types of poses :

  • T-pose : default pose on Motive. It requires straight posture, straight back, head looking forward, both arm stretched forming a « T » shape. Arms and legs must be straight and feet are aligned, parallel to each other ;
  •  A-pose : often used for subjects who can’t lift their arms. Here, arms are opened at approximately 40 degrees from the body, creating an A-shape. There are threee different types of A-pose : palms down, palms forward and elbow bent. With the palms forward pose, make sure not to over rotate the arm. When it comes to the elbow bent pose, the forearms need to be pointing towards the front, palms facing downwards and both forearms aligned.

Creation step by step :

  1.  Open the Builder pane
  2.  Select the markerset previously created on the Marker set drop-down : a skeleton avatar will appear where the markers need to be placed on the subject
  3.  According to the layout you can now see on the avatar, place the markers on the subject : make sure to be as precise as possible
  4. Remember : highlighted markers (in magenta) are those you can place in a slightly different position within the same segment.
  5.  Use the 3D viewport to double check your markers : number, placements,… and make sure Motive detects all of them. Select the missing ones manually.
  6.  Label the markers with the Label pane : you can either select the default labels assigned by the markerset template or do it manually
  7.  Select the creation pose setting : T-shape or A-shape and make sure the subject stands in this pose
  8.  Click on Create and confirm all the skeleton segments and markers : if they are not at expected location, delete, adjust your take and then create again.

Tip : if you want to modify the default properties of a skeleton, go to the Application Settings pane.

Modify the skeleton :

Adding markers :

  1.  Select Assign Markers to Skeleton from the Perspective view pane
  2.  Select the skeleton segment you want to add markers onto
  3.  CTRL + LEFT-CLICK on the marker to associate it to the selected segment
  4.  Select Reconstruct & Auto-label (right click) to update your take
Note : Added markers will automatically labeled SegmentName_# in relation to the associated segments. You can modify them manually if necessary.

Removing markers :

  1.  Select Assign Markers to Skeleton and Assigned marker selection from the Perspective view pane
  2.  Select the skeleton segment you want to modify
  3.  Select Assigned marker or Solved marker
  4.  Delete the assocation by pressing Delete while both the marker and the segment are selected
  5.  Select YES when Motive will ask if you want to un-assign the current selected marker from the skeleton
  6.  Select Reconstruct & Auto-label (right click) to update your take

Recalibration :

If needed, you can recalibrate the skeleton using existing skeleton inforamtion/ markerset. It will refresh expected markers location on the assets.

Be careful : this feature only works with skeleton templates that have no added markers ! You only have to select the markers from the Perspective view and click Recalibrate from markers in the Asset pane > Skeleton context menu or the Perpective view pane.

Marker colors & sticks :

To make the Perspective view pane more functional, you can change the skeleton marker colors. It will enable clearer identification of skeleton segments and individual marker labels. Go to Application Settings and enable the Marker Sticks and Marker Colors.