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I think the communication of the licencing model could be more up front and improved greatly. Reading something like "For the full, unlimited SDK package please fill this form" on the download page always crushes conversion rate and adds resitence where you don't want any.

Just adding a "Pricing" or "Licencing" page directly linked from the developer page would solve that problem. The licencing is more than fair and transparent, so why hide it?

They always uses a white background. Is it working well with a non white background? Generally the detection is more complicated with a different background.

I just tried Go Dance (an ios game that uses the Extreme Reality engine) with artwork in the background and it worked well enough.

Free and/or open source equivalents?

don't think there are any (but if you know a good one, do post!) And this one is based of Rev-Share so it`s pretty simple business model even for indie-games.

http://opencv.org/

definitely not as plug and play a solution as the sdk in question, but certainly an option.

Extreme Motion SDK – Add motion experiences to existing applications