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What is the "precision greyscale mode" option that is available in the arena calibration software?

Camera Processing modes
These options select the in-camera video pre-processing mode.

Segment : Standard marker detection. Provides a balance between precision and USB bandwidth use.

HW Object: Most efficient marker detection. Provides more efficient USB bandwidth utilization, but delivers less marker information to Arena's 3D reconstruction engine than the other modes.

Precision Greyscale: Most precise marker detection. Calculates marker locations using full grayscale data for each marker, provides greater accuracy than other modes. It requires more USB bandwidth and is not recommended for any cameras which share a USB EHCI controller with a V100 operating in Scene A/V mode. It is also recommended that cameras operating in this mode have unwanted light sources carefully blocked/masked during calibration in order to prevent excessive USB bus traffic.

What is "Scene A/V" mode?

I guess the real question is if I have a 12 camera setup, with 3 cameras per hub, and 2 hubs connected to two USB 2.0 EHCI controllers, which settings should I use for the best results?

For a introduction to Scene A/V, check out its FAQ entries.

http://www.naturalpoint.com/optitrack/s ... .html#q501

If you want to use a V100 for Scene video in your setup it will work best if you can dedicate one of the EHCI's to that camera, then put the rest of the cameras on the other EHCI. That isn't the most ideal scenario, but it should work. Alternately, you could install an extra USB 2.0 card in your system to gain an extra EHCI controller.

So, assuming I don't want Scene video, what would be the best setting to use? Segment, HW Object, or Precision Greyscale

If Precision Greyscale works on your system without causing dropped frames (you can check after trajectorization), then using it will get you a little more precision.