A watchful eye is coming to San Francisco's bar scene, but not to keep you in check. It just wants to check you out.

A new app launched this weekend that will scan the faces of patrons in 25 bars across the city to determine their ages and genders.

Would-be customers can then check their smartphones for real-time updates on the crowd size, average age and men-to-women mix to decide whether the scene is to their liking.

The Austin, Texas-based makers of SceneTap say the app doesn't identify specific individuals or save personal information.

But in a city known for its love of both libations and civil liberties, a backlash erupted even before the first cameras were switched on from bar-goers who said they would boycott any venue with SceneTap.