E4 : Engineering has an article on people counters. I'd almost forgot about this but Faith and I were at a supermarket in Europe last year (can't recall which country) and while I waited at the front for her to get something I noticed a computer display by the registers. A closer examination found that it was tracking the number of people in the store and graphing it against time. The store had a single "chokepoint" entrance and staring straight down at the aisle of incoming people was a camera. This computer showed a small live video window and as a shape was identified as a human a little X overlayed it and the counter click up. Nothing like some of the face recognition that's being attempted.

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