Unfortunately, there is a war going on between humans and 'bots. Software
'bots are attempting to generate massive numbers of computer accounts
which are then sold in bulk to spammers. Spammers use these accounts to
inundate emails and discussion boards. Meanwhile humans are trying to
simply create an account and don't want to spend a lot of time proving
that they are not a program.

Typically we use CAPTCHAs -- we present an image of some distorted text
and then ask the applicant to type in the letters. As image processing gets
more sophisticated, these letter sequences tend to get longer and more
distorted, sometimes to the point where humans fail too.

So we switched the game. We show an image, say an airplane, but it
is randomly rotated and we ask the applicant to rotate it to "up." This
is generally hard for computers but easy for people. Well, for the most
part.

Since computers are good at faces, skies, text, etc. we sift
through our database of images running state-of-the-art up detectors to
remove those images. But of the images that remain, some are too hard
for people to figure out. What is up for a plate or a piece of
abstract art?

So here is where it gets interesting. We show people several images, one
of which is a "candidate" and we see how people do. If everyone rotates
it the same way, it is a keeper. If there is a lot of variation, we
discard it. As extra credit it turns out that even if the original image were
taken at an angle, it does not matter, since people, in large numbers,
socially adjust the CAPTCHA.

Read the full paper here (posted with the permission of WWW'09).