a made-up example might be a customer at best buy spending time in both the tv and pc departments and then having an ad pop up on a nearby screen for an intel NUC and/or apple tv (making the assumption you want a tv-connected computer). the ad didn't need to know any personal characteristics about you other than limited, directly observable behavior in the store.

i'm sure people who focus on this every day (i.e., not me) can provide better examples.

You'd have to recognize the face and track it prior to displaying the ad.

if you went that route, you could get away with simpler person recognition (height, width, clothing color, accessories, etc.). or you could just track phones and other devices via wifi/bluetooth (and presumably uwb in the near future).

but you could also just have salespeople on the floor doing the recognition & ad placement part of it, for a lower-tech solution.

i'd actually bet trained salespeople would have better hit rates over algorithmic solutions, where access to large amounts of dislocated data is the main advantage (at the cost of our privacy).