Identicon

An identicon is a picture computed from a string. Hash the identifier (a username, an email, an account id), spend the bits on a small pattern and a hue, and you have a mark that is stable, unique enough to tell rows apart, and free of any decision the user had to make. Nothing is stored and nothing is uploaded: the same handle recomputes to the same pattern on every server, in every session, forever.

That is the whole distinction from its neighbours. An avatar is the slot, and the picture in it usually came from the person. An identicon is what fills that slot before a picture exists, which is why it reads as a fallback rather than as a portrait, and why a wall of them in an avatar group still lets you follow one person down a list of commits. Initials are the other common filler and they are not the same thing: initials carry meaning from the name, an identicon carries none, which is a feature when the identifier is an id nobody should have to read.

Don Park coined the word in 2007 for small patterns generated from a visitor’s IP address, as a way of giving anonymous commenters a recognisable face. GitHub made the form famous a few years later: five columns by five rows of blocks, mirrored down the middle so the shape reads as a deliberate emblem rather than as noise, tinted with a hue taken from the same hash. Because the left three columns are mirrored into the right two, a 5 by 5 identicon only spends fifteen bits on its pattern, which is plenty to separate the handful of people on one screen and nowhere near enough to be an identity.

Treat it as a placeholder with manners. It should be recognisable but not authoritative, never used as a security signal (two accounts can and do collide), and it should give way the moment a real picture arrives. Give it an accessible name from the identifier it was made from, not from the shape: the pattern means nothing to anyone who cannot see it.