DASL Wiki

Author: Keitaro Nishimura Email: nishik1@unlv.nevada.edu
Date: Last modified on 04/25/17
Keywords: Optitrack, Motion Capture, Setup, Tutorial

The picture above depicts the 4 camera setup of the Optitrack Motion Capture System which allows you to capture the position and orientation data of an object within the capture field over. This tutorial will show you how to setup the camera's for your specific project and needs, calibrate the cameras, and how to capture and export position data of an arbitrary rigid object.

This tutorial's motivation is to teach the reader how to set up their own motion capture space. Readers of this tutorial are assumed to have the following background and interests:

*Know how to follow direction
*Interests in capturing the position data of a moving object over a set time

The rest of the tutorial will be made up of three sections:

• Hardware Setup
• Camera Calibration
• Data Acquisition

For this section, the reader will first need to confirm that they have all the parts listed in the bellow. Please find the desktop computer labeled “Motion PC” for the Arena Motion Capture Software. Reference the links to find out what each part should look like.

The lab should have all of the necessary parts listed above in one place as shown below.

Please note that you will also need:

• Carpet or something to cover the floor if it is reflective
• A ladder tall enough to reach to top of the 2m tripod comfortable

This section will guide the reader step by step how to setup their own space.
1. Decide on the space and configuration.

• Depending on how much space is available or the capture volume needed, the camera configuration and space needed will change drastically. Below are some examples of camera configurations for 6, 12, and 16 cameras. The closer the cameras are to the tracked subject the higher the accuracy, but shrinks the capture volume, while placing the cameras farther away enlarges the capture volume while lowering accuracy. Also the area within the volume the object is can change the setup. If you need to capture data at close to the ground having cameras that point at the ground is beneficial. This can be achieved by pointing all the cameras at the ground. But if you also need to capture data points above ground level, having two rows of cameras would be the best setup (one at the top and one at the middle of the tripods). The capture volume and accuracy needed will also vary the number of cameras needed. Although adding more cameras will raise the accuracy of the data if the cameras are placed close enough to where the images from them are similar, the benefit of the additional camera is negligible and will only add a load to the software.
• For example, in the picture of the current system (04/25/17) there are only 4 cameras in each corner of the capture volume close together. This is because it will be mainly capturing still objects close to ground level. This makes having a large volume extraneous and in turn extra cameras as well. All of the cameras are pointed at the ground floor to maximize the accuracy. Although it would be possible to add a second row of cameras the added benefits would be negligible and shrink the already small capture volume further.

2. Setting up the Tripods

• Once the space and configuration have been chosen evaluate the area. Is it free of potential obstacles to the cameras? If not move them out of the space. Is the floor reflective? If it is, use carpets or something similar (eg wood pallets) to cover the floor of the capture volume. Light reflecting off the floor will add noise to the data and make calibration impossible.
• Then depending on how many cameras and configuration take out the number of tripods needed. Place the cameras on the top of the tripods using the clamps to secure them. For the top row make sure that the cameras are mounted vertically (similar to a mobile phone video). Do not worry about the camera angle, for now, this will be fixed during calibration. Then extend the tripod to their maximum heights making sure to secure each part with the locking pin. If your configuration merits a second row place them an the mid point of the tripod with the clamps, this time mounting them horizontally (similar to a DSLR or point and shoot camera).
• Once the tripods are set up move them into your desired configuration.

3. Setting up the Optitrack Hubs

• Once the cameras are all in place connect usb camera cables (the usb end is red) to each camera.
• Take out the Optitrack hubs and based on the number and length place them around the perimeter of your capture volume. You can not have a usb extender between the camera and the hub. Introducing an extender will throw off the synchronization(timing) of that camera. making it impossible to calibrate and get accurate data from that camera. However, there is no need to connect the same number of cameras to each hub. It can however help the software equally distribute the load for For our current setup we are using two hubs on the sides with two cameras pluged into each.