Next time you look at a sign or billboard be aware it might be looking back at you.

Now, some entrepreneurs have introduced technology to solve that problem. They are equipping billboards with tiny cameras that gather details about passers-by — their gender, approximate age and how long they looked at the billboard. These details are transmitted to a central database.

Behind the technology are small start-ups that say they are not storing actual images of the passers-by, so privacy should not be a concern. The cameras, they say, use software to determine that a person is standing in front of a billboard, then analyze facial features (like cheekbone height and the distance between the nose and the chin) to judge the person’s gender and age. So far the companies are not using race as a parameter, but they say that they can and will soon.

The goal, these companies say, is to tailor a digital display to the person standing in front of it — to show one advertisement to a middle-aged white woman, for example, and a different one to a teenage Asian boy.

This is a step in the direction of websites like Amazon where they show you products based on which products you've previously purchased or viewed. The image processing computers behind the cameras do not identify you personally today. But they probably will in the future.

What will you be able to do about it? Get onto a web site that provides genetic engineering services and come up with some instructions to feed into your home bioreactor to modify your stem cells to give them orders to reshape your face. Then the cameras won't recognize you.