Optitrack : Introduction to Motive/Body

Tutorial

Introduction, history and state of the art for human motion capture

Dates
  • Creation: 01/03/2021
  • Update: 01/16/2021
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Motion capture, also called mocap is a technic that enables you to record positions and movements from an asset (solid or human body) in order to convert these data into synthesis images on a computer (3D model, avatar,… )

First, it was developed and applied in the military world, for flying drones for example. With time, mocap appeared to be such a powerful tool, appliable to human motions. It enabled huge progresses in the healthcare world, with rehabilitation for example, traumatology,… as well as performance measurements in sports.

History of motion capture :

The first ancestor of mocap is considered to be the chronophotography, back to 1870. However, the most significant century for human motion capture improvements is the XXth century, especially its second half.


State of the art :

Here at DVIC, we use a specific software for motion capture : Motive. Two licences are available, the first one is used for drones. Many tutorials are already available on the dedicated drone page.

The second one, which is the subject of this article, is used for human movement : Motive/Body. A lot of studies have already been conducted to test the accuracy and the efficiency of this software. It turns out that OptiTrack is a software with many advantages.

Fist of all, it is very precise. In a sudy of the Ohio State University (1), dynamic 3D errors of OptiTrack motion capture systems in a large capture volume (>100 m3) were tested. They used 42 OptiTrack Prime 41 cameras with a capture volume of 135 m3 and compared it to measurements provided by a ThorLabs linear motion stage : 97% of the capture area had an error below 200 μm, which is quite satisfying.
The conclusion is that the OptiTrack optical motion capture system is more than sufficient for measuring full-body human kinematics with skin-mounted markers in a large capture volume (>100 m3). It turns out that the uncertainty is more likely due to the placement of the skin-mounted markers rather than the camera system itself.

Secondly, the convenience and portability of the system were tested, especially in analysis protocoles of pelvis and lower limb motion for impaired children (2). Therefore, the protocol had to ensure the child collaboration by being quick and the least cumbersome possible.
By using the OptiTrack system, the protocol became feasible and applicable to children. It allows quite a precise 3D anatomical-based measurements of segment and joint motion while being very handy.
This study reveals the practical aspect of the Optitrack system which will make our experiments at the Lab easier.

Finally, if we compare OptiTrack to other software available on the market, it is among the best rated when it comes to biomechanical applications (3). Hence, it is well referenced by six major literature databases : PubMed, PubMed Central, ScienceDirect, IEE Xplore, PLOS and Web Of Science. 85 articles were included : 4 of them validated the OptiTrack systems and 81 utilized it for biomechanical analyses.
They offer a broad collection of biomechanical applications of OptiTrack motion capture systems and insist on the accuracy of the system.

How does Motive Body work?

Similarly to motion capture applied to drones, you will find the same material:

  1. Passive reflective markers : these are called “passive” because they don’t emit any light or radiation, and neither do they collect any information. They are just here to reflect light emitted from the cameras and are placed at strategic points such as joints for example.
  2. Infrared cameras : to perform motion capture, you will need at least 3 cameras. When it comes to human movements, markers can easily overlap and therefore get swapped during the recording. Hence, the more cameras, the better.
  3. Non reflective suit/ velcro suit : this suit will enable you to attach your markers to the subject. You can choose to do it directly onto the skin but it might be more difficult to secure your markers and prevent them from falling and/or moving. Opt for a non reflective piece of clothe to avoid any extraneous reflections.
  4. Set-up environment : Choose an empty space, free from light reflections so the cameras will only detect markers. 

To get to know more about Motive/Body and learn how to use it, here are some articles that might be helpful:

  • Skeletons and rigid bodies overview
https://dvic.devinci.fr/resource/tutorial/optitrack-obj-overview/
  • How to do your own markers and suit for Optitrack ?
https://dvic.devinci.fr/resource/tutorial/optitrack-diy-suit/
  • Skeleton tutorial
https://dvic.devinci.fr/resource/tutorial/optitrack-skeleton/


References :

(1) : Alexander M.Aurand, Jonathan S.Dufour, Willial S.Marras, Accuracy map of an optical motion capture system with 42 or 21 cameras in a large measurement volume,  Journal of Biomechanics, Volume 58, 14 June 2017, Pages 237-240

(2) : Gabriele Paolini, Stefania Ingrosso, Roberto Nativo, Maria Grazia Benedetti, A new anatomically based protocol for gait analysis in children, Alberto Leardini, Zimi Sawacha, 2007 Oct;26(4):560-71, doi: 10.1016

(3) : Gergely Nagymaté, Rita M. Kiss, Application of OptiTrack motion capture systems in human movement analysis: A systematic literature review