How to add a tag overlay to a photo in iOS aka Facebook

I was wondering if anyone had an idea as to how the people tagging feature works on facebooks iPhone app i.e. in the app you can touch the photo and then associate that touch-point with a fac开发者_StackOverflowebook friend. Specifically I was wondering whether this is just as simple as associating co-ordinates on the image with a data object (facebook friend in this case) using the iPhone or whether they are doing some smarter image recognition in the background to workout what other areas of the photo also may belong to that person i.e. is does the tag extend beyond the point touched on the screen. If the latter is the case is anyone familiar with the techniques used?

Thanks in advance

Dave

I don't think they are using face recognition algorithms on the iphone, since that is processor consuming specially if you have hundreds of friends. If you want to do a face recognition and you have faces of people that you want to search in you should do it on the server, so after you take or import a photo, you should send it to your server where you search for the face and return a JSON with points for faces and data for matched users. Then do your UI to present it on the screen for the user.

Edit

If you want to use face recogingitioning on iphone try this: Face recoginition iOS