Searching images for portraits

Building my search engine drew me to try to index other things than raw text: images, videos, news, etc. It has been a while now since I worked on the stuff, and I recently said to myself that it's a shame not to do something useful and creative with the bunch of images that I gathered years ago.
I forgot that I had more than 10 millions images from all over the web. It would indeed be a shame not to do something with that.

Following what was done at the time, and not having so much time to look back at all the possibilities, I thought about what anyone was already doing: search by tone color, sizes, etc. Things (more or less) easily technically done, but most of the time with very little value for the user.

And then, a simple thought came to mind: what about allowing search by portraits ? As a first step towards search by shapes and objects.

It's rather simple to do nowadays: you probably have stumbled upon one of those software for webcams which track faces: that's essentially the kind of software  we need to find a portrait in an image.
If there's a face, and it's big enough, it must be a portrait.

So, the first big step is being able to recognize a face on an image. That's simple enough thanks to a great library which can (among a lot of other things) do that: OpenCV. For that task, cvHaarDetectObjects is what we are looking for (it's for C, the exact function/method name which applies will vary according to your programming language of choice).

And to make things even more simple for a quick and dirty evaluation of the thing, there's for example a very basic PECL extention for PHP, PHP-Facedetect, which does exactly what we need.
The configure script being broken as the time of this writing and the git pull request waiting to be accepted, you can check out the patched extension I wrote.

That should be enough ground and ideas to get you started ! Share your ideas ! :)