Is there a name for the default avatars that are generated for users who do not assign one themselves and register using an authentication mode other than Google or Facebook?
Not the avatar of the blank user, but the one made up of shards, shapes, and colours:
Is there an online generator for something like that?
They're called Identicons.
If you do not upload your own image or register for your account using an authentication mode other than Google or Facebook, then Stack Exchange uses Gravatar and specifies Identicons as the default image:
How the URL is constructed, with:
When you're not registered (when Stack Exchange does not know your email address), this is based on your last known IP address so might change when your IP address changes
For new users and those who click the "Identicon" in their profile, Stack Exchange adds a salt if the user does not have a Gravatar account, to make it a bit more secure. This only applies to new profiles created or identicons selected after November 2013 on the main sites (not on Area 51).
Original blog post by Don Park, the Identicons inventor; their implementation in Java is available still.
Blog post by the creators of a PHP implementation of Identicons as used by Gravatar.
wp_identicon still the exact algorithm used by Gravatar?
Commented
Feb 12 at 19:25
They are called Identicons. The Gravatar identicon image is based off of an MD5 hash of your email address. Here is an identicon implmentation if you are using .NET. There are other implementations listed in the Wikipedia article as well.
Important Update You may also be interested in Unicornicons.
Checkout Coding Horror, it has an algorithm based on IP Address similar to SO.
From Wikipedia:
An Identicon is a visual representation of a hash value, usually of the IP address, serving to identify a user of a computer system. The original Identicon is a 9-block graphic, which has been extended to other graphic forms by third parties some of whom have used MD5 instead of the IP address as the identifier. In summary, an Identicon is a privacy protecting derivative of each user's IP address built into a 9-block image and displayed next the user's name. A visual representation is thought to be easier to compare than one which uses only numbers and more importantly, it maintains the person's privacy. The Identicon graphic is unique since it's based on the users IP, but it is not possible to recover the IP by looking at the Identicon.
As regards showing them automatically with Gravatar, here is the answer I posted here:
The 'random' colorful gravatars are displayed because this query string parameter is being added to every gravatar source url: d=identicon
This is done so that if the user doesn't have a gravatar image associated with his email, this 'random' image is displayed, instead of the default blue gravatar image.
The following displays the 'default' blue image because the parameter is not included:
Yet, the same url with the d=identicon parameter included, shows this:
PS: This is the url used for the example: https://www.gravatar.com/avatar/94d093eda664addd6e450d7e9881bcad?s=32&d=identicon&r=PG
Btw, these images (called Identicons) are not really random, but are generated based on a Hash of your IP address.
