Passive markers

A project log for Optical motion capture system

Tracking human motion for digital animation.

Robert GowansRobert Gowans 01/07/2018 at 13:091 Comment

In previous logs I wrote about active markers which were powered by an LED and a coin cell. In this log we'll look at the options I explored when trying to build my own passive markers. Note that professional markers are readily available and can be purchased online, for a hefty price as with everything in the motion capture industry.

A marker consists of a core, a coating and a base. The core can be of various sizes and densities, it's useful to have a softer core that can be squished if an actor impacts a surface. The higher resolution the cameras, the smaller the markers can be, and really small markers can be used to track facial and finger movements. The coating is often a highly reflective tape that is hand applied to the cores. A very good choice for the tape is 3M 7610 due to its super high reflectivity and flexibility. The base of the marker allows it to be attached to the subject in various ways.

I thought it would be easy to find some sort of soft core, but it was really difficult to find something consistently sized and appropriate for tape adhesion. In the end I went with 3D printing as it was cheap, quick and controlling size is simple. I couldn't get my hands on 3M 7610 so I experimented with different tapes and settled on some that has good flexibility and reflectivity (First marker in the above photo). Below is a shot from the camera feed of the above 3 markers. Clearly the tapes have very different reflective properties.

The combination of a 15mm diameter 3D printed core and the reflective tape gives very promising results. In a future log I'll discuss marker detection.


thomasschreiter wrote 10/16/2018 at 16:22 point

you can also buy a warning vest with 3M reflective fabric strips and cut off the fabric and stick it on the balls.

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