Augmented reality app Blippar already has a huge database of everyday objects and images that it can recognize. But now, the app is also learning to recognize faces as well.

Facial recognition is nothing new of course--you may already be using it to unlock your smartphone. And searchable databases of images are not all that new either.

But put the two together, and you have something that could be incredibly useful, and possibly a little invasive. It works like this: Blippar already has an inventory of 70,000 celebrities and counting, drawn from the most popular Wikpedia entries, the CEO explained to CNN. Use your smartphone to scan an image of one, say on a magazine cover, and it will identify that celebrity and provide information about his or her previous projects. In the future, the company plans to use published content and news, combined with verified Facebook and Twitter accounts (which usually belong to high-profile people) to automatically upload more faces and create a much large database of public figures.

In other words, that phenomenon when you see an actor you just know you've seen before on television or in a picture but you can't quite place who it is and it's driving you up the wall? That won't ever be a problem again. The same goes for celebrities you see on the street or in a restaurant that you can't quite recognize--although you might want to be discreet about scanning them.

But then, celebrities are accustomed to being recognized by strangers. It's a bit different for ordinary citizens, but Blippar is planning to include all of us in its database as well, or at least as many of us as are willing to upload our own faces to the app, along with such info as our Twitter and Spotify accounts, favorite movies and books, and so on.

That raises the intriguing prospect that you could use the app to jog your memory when you just can't come up with the name of a friend you haven't seen in a while, or can't remember where you know someone from. (I personally am terrible at remembering faces so I'd probably use this feature a lot.)

There's a dark side.

Of course, it could be used all kinds of other ways as well. You might see someone on the street you find attractive and use the app to find out who that person is so you can try to connect on social media, or even stalk that person if he or she has "checked in" at a particular location or listed a workplace. If you see your child, or your partner, talking to someone you don't know, you could use it to learn that person's identity as well. If you get into an argument with a stranger on the street, you could use it to find out who that stranger is.

You may think this is brilliant, or disquieting, or both. But really, the only new thing about Blippar is that it's putting its impressive facial recognition powers into users' hands. Google and Facebook are two other companies--and not the only two--that use facial recognition algorithms routinely in various ways. Facebook in particular uses facial recognition algorithms on its database of more than a trillion images to guess who people are in photographs and suggest tags.

There's only one problem with all this: It may not be legal, at least not everywhere. Facial features are biometric data, increasingly used to authenticate identities in all sorts of settings, and maybe use of that data should belong only to you. That's how Illinois sees it, and three Illinois residents are currently suing Facebook under the state's Biometric Information Privacy Act. If they win, legal experts predict a class-action flood will follow.

What would that mean for Blippar? Unknown. In the meantime, you might as well grab the app. You never know when you might see a famous person you can't quite recognize sitting at the next table.