I know that my avatar is based on my email address, but how can I predict the kinds of shapes and the color of my avatar based on my email address?
What characteristics (characters, I assume) of the hash equal what shapes, colors etc?
The way Gravatar works, the hash of your email address is used to form the parameters to their image generating function. These are passed to a special URL which returns an image:
The default Gravatars for StackExchange are Identicon images (notice the
Edit: the source code for Identicon can be found here: http://downloads.wordpress.org/plugin/wp-identicon.zip