How is the user id image generated on SO?

Question

I am a little curious about the cute little kaleidoscopic images associated with each user on this site. How are those generated? Possibilities are:

1. A list of images is already there in some folder and it is chosen randomly.
2. The image is generated whenever a user registers.

In any case, I am more interested in what kind of algorithm is used to generate such images.

Answer 1

It's called an Identicon. If you entered and e-mail, it's a based on a hash of your e-mail address. If you didn't enter an e-mail, it's based on your IP address.

Jeff posted some .NET code to generate IP based Identicons.