Hardware for Homebrew Motion Capture?

goruka asks: "We are a small garage game development and 3D animation group, and as such, we try to develop by reducing costs as much as we can. Recently, it came to our mind that we could setup and develop a home-brew motion capture system by using three consumer USB web-cams to motion track bright objects attached to the body. However, we don't know which web-cam models can: capture at a decent frame rate (25fps) and resolution; are supported and easily programmed under GNU/Linux, since we'd like to later release our software as open source; and lastly, won't cost us a fortune. What are your experiences with such devices?"

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cheap homebrew infra-red mocap rig

[Robbat2]

Couple of things in here, from researching the field with a university research lab to see about buying commercial gear, I have a lot of suggestions.

- For your camera, look for cheap used DV cameras on ebay. Not super high res, but lots of them 3 ain't going to cut it, consider at least 9 (high/low from each of the cardinal directions, and on top [might want a few for different sectors]) - occlusion is an absolute bitch of a problem.
- This will provide reliable time-synced data, and NOT max out your USB bus.
- USB cannot provide you with images from 3 cameras with the same timesync, it's just not capable of such behavior.
- Firewire has a longer length limit on the cables, which is a big help for your work.

- Cheap PCI firewire cards - two should be enough, this will give you 6 seperate firewire busses, and put you at the limit of your PCI bus.
- Find filters that fit said cameras, and are opaque to visible light, but transparent to infrared.
- Rig up really bright infra-red lighting, ideally with a low quantity of visible light output.
- Go to an burgler alarm supply place, and buy infra-red reflective tape - I leart this tip from the EA guys a couple of years back, the 'official' reflective tape from 3M costs too much, and is a pain to order, but alarm places stock stuff that works even better, and is cheaper to boot.
- Buy really small polystrene balls, and cover with infra-red tape. On one small part of the ball, put the hook side of a velcro dot. These are reusable now, avoiding problem with tape waste. You can also clean them easily to keep them very reflective.
- For your subjects, get them to wear any clothing that velcro will hook reliably onto (pretty easy choice)
- Place the reflective balls on either side of every joint, spaced not more than 90 degrees apart - eg your elbow should have 8 balls.

Using infra-red helps reduce the data-set size way down, and also lets you use the cameras in monochrome for capturing, greatly reducing the data-set size.

From working with several commercial mocap rigs, I'll say that the calibration routines are extremely important. You need to accurately map the entire volume that you wish to capture in. Depending on space available to you, consider building a simple frame or using a lighting rig to attach the cameras to.

I will repeat again, occlusion is an absolute killer problem. From visting the EA facilities in Burnaby BC to specifically research their systems (I was working with a university research lab at the time), they estimated that they lost 2 hours of production a day to occlusion problems during mocap shoots.
Your system must be capable of tracking all the balls, all of the time. If it loses one, it's almost impossible to pick it up again properly during a runtime - you'd need to recode the relative location of that ball before it gave you useful data again.