Snow Bells is a small game that I made for my family for christmas.
Here is a short video :
http://vimeo.com/18272807
Each player has a unique sound when they hit a snow-flake, so the more poeple play, the more complex the music becomes.
I’ve used the openNI library to identify players (it generates an array of pixels containing the player IDs 
) I’ve used openCV - blob detection to draw the contours.
the source is not super clean, but works (for Code::Blocks, Windows) :
http://www.binaura.net/bnc/temp/OF/ofxONI-1.2.zip
http://www.binaura.net/bnc/temp/OF/snowBells-copy.zip
You can download the windows executables from my site:
http://www.binaura.net/bnc/blog/comments.php?y=10&m=12&entry=entry101229-184305
Here is a topic that might help you installing the drivers:
http://forum.openframeworks.cc/t/openni-skeleton-tracking/5125/0
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